Showing 1–50 of 454 results for author: Chang, C

Enhanced Superconductivity in Multilayer FeSe Films by Simplified Molecular Beam Epitaxy

Authors: Maria Hilse, Hemian Yi, Zhe Chen, Jessica L. Thompson, Kalana D. Halanayake, Danielle Reifsnyder Hickey, Seong H. Kim, Cui-Zu Chang, Nitin Samarth, Roman Engel-Herbert

Abstract: Multi-unit-cell (UC) \b{eta}-FeSe films grown on SrTiO3(100) continue to attract attention because of the significant enhancement in the superconducting transition temperature (Tc) compared to that in bulk FeSe. In prior reports of molecular beam epitaxy (MBE)-grown \b{eta}-FeSe/SrTiO3(100), elaborate growth protocols have been used to achieve enhanced Tc, leading to a general belief that careful… ▽ More Multi-unit-cell (UC) \b{eta}-FeSe films grown on SrTiO3(100) continue to attract attention because of the significant enhancement in the superconducting transition temperature (Tc) compared to that in bulk FeSe. In prior reports of molecular beam epitaxy (MBE)-grown \b{eta}-FeSe/SrTiO3(100), elaborate growth protocols have been used to achieve enhanced Tc, leading to a general belief that careful pre-treatment of the SrTiO3 substrate and post-growth annealing in ultrahigh vacuum (UHV) are essential. Here, we report a greatly simplified protocol for the MBE growth of superconducting multi-UC \b{eta}-FeSe films on SrTiO3(100), eliminating the need for careful substrate pre-treatment and post-growth UHV annealing while still achieving an enhanced Tc. With appropriate capping, epitaxial films with 14 UC thickness exhibit a zero-resistance transition temperature Tc ~ 20 K in ex situ electrical transport measurements. The MBE optimization process is guided by the growth-parameter dependencies of film morphology and structural properties, as characterized by reflection high-energy electron diffraction, X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy. △ Less

Submitted 25 August, 2026; originally announced August 2026.

Comments: 20 pages, 4 figures, 1 table

Modelling the onset and evolution of immiscible viscous fingering in porous media

Authors: Paulo L. K. Caetano Chang, Kundan Kumar, Arne Skauge, Kenneth S. Sorbie

Abstract: The simulation of viscous fingering in porous media is of direct relevance to displacement processes in petroleum engineering and hydrogeology. Building on recent work proposing a modelling approach for well-defined fingers at very adverse viscosity ratios, we investigate the physical mechanisms behind viscous fingering and the modelling requirements for capturing the finger scales and saturation… ▽ More The simulation of viscous fingering in porous media is of direct relevance to displacement processes in petroleum engineering and hydrogeology. Building on recent work proposing a modelling approach for well-defined fingers at very adverse viscosity ratios, we investigate the physical mechanisms behind viscous fingering and the modelling requirements for capturing the finger scales and saturation patterns observed in experiments. We simulate and match a viscous fingering experiment at a viscosity ratio of $μ_{o}/μ_{w}{=}2000$, discussing the physical significance of each modelling step. Linear stability analysis is used to characterize the early-stage instability of the displacement. Subsequent numerical simulations show that, for the simulated finger scales to match the experiment, the most unstable wavelength at onset must be several times smaller than the desired finger width---so that, after accounting for shielding and merging in the nonlinear regime, the fingers remain thin. Small-scale channelling effects are also required to disrupt the trailing stable region commonly observed in simulations of viscous fingering in nearly homogeneous media. Finally, we show that including a weakly oil-wet capillary pressure function enables our model to capture the bypassed oil observed in the experiment. △ Less

Submitted 18 August, 2026; originally announced August 2026.

MSC Class: 76Dxx; 76S05; 76Exx

A piston-like polymer stochastic heat engine

Authors: Yi-Jui Chiu, Cheng-Hung Chang

Abstract: Colloidal stochastic engines are often regarded as microscopic analogues of macroscopic pis ton cylinder heat engines. However, although they share some underlying physical principles, such systems remain far from being direct force generators from a practical perspective. Motivated by this limitation, the present study introduces a polymer-based stochastic engine that more closely mimics the oper… ▽ More Colloidal stochastic engines are often regarded as microscopic analogues of macroscopic pis ton cylinder heat engines. However, although they share some underlying physical principles, such systems remain far from being direct force generators from a practical perspective. Motivated by this limitation, the present study introduces a polymer-based stochastic engine that more closely mimics the operation of piston cylinder engines. In this setup, heat is converted into work through a cyclic process in which a polymer is pulled into and out of a narrow channel under varying tem peratures. The work performed by the engine can be directly obtained from the cyclic trajectory in the force position diagram, analogous to the pressure volume diagram in traditional heat engines. Despite its much higher number of degrees of freedom compared to colloidal engines, the polymer en gine nevertheless follows several characteristic features observed in such systems. Numerical results demonstrate consistency with universal low-dissipation bounds for e ciency, recovery of Carnot e ciency under regeneration, and low-dissipation scaling of work and power. △ Less

Submitted 11 August, 2026; originally announced August 2026.

Comments: 16 pages, 4 figures

Interface-Confined Superconductivity with Thickness-Independent Superfluid Stiffness in (Pb,Sn)Te/FeTe Bilayers

Authors: Zi-Jie Yan, Hongtao Rong, Yiyuan Luo, Yufei Zhao, Pu Xiao, Zihao Wang, Lok-Kan Lai, Annie G. Wang, Zhiyuan Xi, Yanxing Li, Xiaoyu Wei, Ke Wang, Binghai Yan, Chih-Kang Shih, Cui-Zu Chang

Abstract: Interface-induced superconductivity in FeTe-based heterostructures provides a promising route toward topological superconductivity, yet the roles of the neighboring layers topology, symmetry, and electronic structure remain unresolved. In this work, we employ molecular beam epitaxy to grow Pb1-xSnxTe/FeTe bilayers and use angle-resolved photoemission spectroscopy to track the evolution of the Pb1-… ▽ More Interface-induced superconductivity in FeTe-based heterostructures provides a promising route toward topological superconductivity, yet the roles of the neighboring layers topology, symmetry, and electronic structure remain unresolved. In this work, we employ molecular beam epitaxy to grow Pb1-xSnxTe/FeTe bilayers and use angle-resolved photoemission spectroscopy to track the evolution of the Pb1-xSnxTe layer from a trivial insulator to a topological crystalline insulator hosting multiple Dirac surface states. Electrical transport measurements reveal robust superconductivity throughout the entire composition range, with a nearly constant superconducting transition temperature of ~12 K despite substantial changes in the electronic structure and topology of Pb1-xSnxTe. Double-coil mutual-inductance measurements further reveal comparable superfluid stiffness across the topological phase transition and nearly thickness-independent superfluid stiffness despite large variations in the constituent-layer thicknesses, demonstrating that superconductivity is confined near the interface. These results establish that superconductivity in FeTe-based heterostructures is largely insensitive to the topology, crystal symmetry, and detailed electronic structure of the neighboring layer, supporting a primary origin in modifications to the FeTe layer. The coexistence of interface-confined superconductivity and tunable multiple Dirac surface states in Pb1-xSnxTe/FeTe bilayers provides a versatile platform for exploring topological superconductivity and interactions among multiple Majorana zero modes. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Comments: 30 pages, 4 figures. Comments are very much welcome

High-Q superconducting microwave resonators using MBE titanium nitride

Authors: Anand Ithepalli, Haoran Lu, Eegene Clara Chung, Xiangqin Wang, Amit Rohan Rajapurohita, Keun-Yeol Park, Celesta S. Chang, Peter McMahon, Huili Grace Xing, David Muller, Valla Fatemi, Debdeep Jena

Abstract: Using molecular beam epitaxy, we have realized thin films of titanium nitride (TiN) on c-plane sapphire that exhibit the lowest observed full-width at half maximum X-ray rocking curve width of 18 arcsec. Though the (111) oriented TiN exhibits an abrupt and crystalline interface with sapphire, for the first time we observe sub-surface defects in the sapphire substrate, which nucleate structural def… ▽ More Using molecular beam epitaxy, we have realized thin films of titanium nitride (TiN) on c-plane sapphire that exhibit the lowest observed full-width at half maximum X-ray rocking curve width of 18 arcsec. Though the (111) oriented TiN exhibits an abrupt and crystalline interface with sapphire, for the first time we observe sub-surface defects in the sapphire substrate, which nucleate structural defects in the epitaxial TiN layer. Using quarter-wavelength coplanar waveguide (CPW) resonators in a 3 \textmu m/6 \textmu m/3 \textmu m gap/strip/gap lines in a hanger geometry, we find the internal quality factor of the TiN resonators to be $>10^{6}$ in the single-photon $\langle n \rangle \sim 1$ limit at 5.8 GHz and 10 mK, rising to $>20 \times 10^{6}$ at $\langle n \rangle \sim 10^{6}$. The results are of high interest for applications of superconducting TiN in several areas, and provide a path towards epitaxial Josephson junctions with crystalline barriers in the future for high coherence qubits. △ Less

Submitted 19 July, 2026; v1 submitted 16 July, 2026; originally announced July 2026.

Comments: 8 pages and 5 figures

Strain-tuned orbital-dependent electronic correlations in FeTe thin films

Authors: Hyunjee Song, Sangjae Lee, Keun-Yeol Park, Jaehyun Park, Suyoung Lee, Yeonjae Lee, Jinyoung Kim, Jaeung Lee, Celesta S. Chang, Younsik Kim, Changyoung Kim

Abstract: Iron chalcogenides exhibit rich phenomena which are governed by orbital-dependent electronic interactions and strong electronic correlation. In particular, many studies have explored orbital selectivity in FeTe through Se doping. Here, applying tensile strain to thin films allows us to precisely control the system without other impurities that may arise from chemical doping to investigate the emer… ▽ More Iron chalcogenides exhibit rich phenomena which are governed by orbital-dependent electronic interactions and strong electronic correlation. In particular, many studies have explored orbital selectivity in FeTe through Se doping. Here, applying tensile strain to thin films allows us to precisely control the system without other impurities that may arise from chemical doping to investigate the emergent behaviors in FeTe. Using angle-resolved photoemission spectroscopy, we observe a spectral weight transfer between $d_{\rm xy}$ and $d_{\rm z^{2}}$ orbitals, evidence of an orbital-selective Mott phase (OSMP). Beyond OSMP, we reveal hitherto unobserved strain-induced effects, distinct from chemical doping. The evolution of $d_{\rm xz}$ orbital demonstrates how electron hopping mechanism plays an important role in defining the electronic properties of the system. Our findings highlight a direct correlation between epitaxial strain and the evolution of electronic structures in FeTe. △ Less

Submitted 21 June, 2026; originally announced June 2026.

Comments: 5 pages, 4 figures

The Kondo effect in ferromagnetic quantum critical CeRh$_6$Ge$_4$

Authors: Martin Sundermann, Joe D. Thompson, Eric D. Bauer, Chun Fu Chang, Sheng-Huai Chen, Chang-Yang Kuo, Liu Hao Tjeng, Getrud Zwicknagl, Andrea Severing

Abstract: The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh$_6$Ge$_4$ has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4\textit{f} electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4\textit{f… ▽ More The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh$_6$Ge$_4$ has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4\textit{f} electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4\textit{f} shell in CeRh$_6$Ge$_4$ using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4\textit{f} with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystal-electric-field (CEF) sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the non crossing approximation (SIAM/NCA). The Kondo effect mixes higher lying crystal-field states into a resulting multiorbital ground state with 4\textit{f} occupancy \textit{n}$_f$\,$\sim$\,0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh$_6$Ge$_4$. △ Less

Submitted 10 June, 2026; originally announced June 2026.

Comments: 9 pages, 7 Figures, accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 113, 245149 (2026)

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

Authors: Mario Ulises González-Rivas, Chun-Fu Chang, Martin Bluschke, Jessica Freese, Peter Bencok, Ronny Sutarto, Teak D. Boyko, Robert J. Green, George A. Sawatzky, Liu Hao Tjeng, Alannah M. Hallas

Abstract: High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in… ▽ More High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways. △ Less

Submitted 25 May, 2026; originally announced May 2026.

Imaging Surface Magnetization in Altermagnetic MnTe Films

Authors: Ling-Jie Zhou, Senlei Li, Zi-Jie Yan, Yufei Zhao, Hongtao Rong, Zelong Xiong, Yiran Zhao, Pu Xiao, Lok Kan Lai, Hyeonhu Bae, Haoyu Liu, Chao-Xing Liu, Binghai Yan, Cui-Zu Chang, Hailong Wang, Chunhui Rita Du

Abstract: Altermagnets with pronounced spin-splitting band structure, unconventional magnetic and crystal symmetries, and exotic magneto-transport properties have received immense interest in cutting-edge spintronics, materials science, and condensed matter physics research. Microscopic imaging of spontaneous magnetic domains and phases in altermagnets constitutes an important step for investigating their u… ▽ More Altermagnets with pronounced spin-splitting band structure, unconventional magnetic and crystal symmetries, and exotic magneto-transport properties have received immense interest in cutting-edge spintronics, materials science, and condensed matter physics research. Microscopic imaging of spontaneous magnetic domains and phases in altermagnets constitutes an important step for investigating their underlying material properties, mechanisms, and spin behaviors. Taking advantage of scanning-probe quantum microscopy, here we report nanoscale quantum sensing of a prototypical altermagnet candidate $α$-MnTe. We visualize evanescent magnetization and the associated magnetic domains in epitaxial MnTe films, which allows external magnetic fields to control the intrinsic altermagnetic order and configurations. By evaluating a series of MnTe films with different thicknesses down to the atomic scale, we further present evidence for the interfacial origin of the observed weak magnetization and show its correlation with the anomalous Hall effect in MnTe film. Our results advance the current understanding of emergent altermagnetism, providing insights into future material design of altermagnet-integrated spintronic devices. △ Less

Submitted 24 May, 2026; originally announced May 2026.

Designing Magnetic Topological Insulator Trilayers for Highly-Efficient Spin-Orbit Torque Switching

Authors: Ling-Jie Zhou, Deyi Zhuo, Han Tay, Zi-Jie Yan, Pu Xiao, Xiaoda Liu, Bomin Zhang, Cui-Zu Chang

Abstract: Spin-orbit torque (SOT) enables efficient electrical control of magnetization, offering a pathway towards low-power spintronic devices. Magnetic topological insulators (TIs), with spin-momentum-locked surface states and intrinsic ferromagnetism, provide a unique platform for realizing SOT switching of edge current chirality in quantum anomalous Hall (QAH) insulators. In this work, we employ molecu… ▽ More Spin-orbit torque (SOT) enables efficient electrical control of magnetization, offering a pathway towards low-power spintronic devices. Magnetic topological insulators (TIs), with spin-momentum-locked surface states and intrinsic ferromagnetism, provide a unique platform for realizing SOT switching of edge current chirality in quantum anomalous Hall (QAH) insulators. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI trilayers with controlled layer thicknesses on heat-treated SrTiO3(111) substrates. Electrical transport measurements reveal that SOT-driven magnetization reversal and the associated switching of edge current chirality are governed by the SrTiO3(111) substrate-induced charging effect, which generates an asymmetric chemical-potential alignment between the top and bottom magnetic TI layers. Furthermore, we demonstrate that the switching polarity and efficiency can be tuned through heterostructure design, gate voltage, and in-plane magnetic field, consistent with SOT symmetry. These findings identify chemical potential asymmetry as the origin of the large SOT switching ratio in magnetic TI trilayers and establish a route for electrical control of edge current chirality in QAH insulators. This work advances the understanding of SOT switching mechanism in magnetic topological materials and paves the way for next-generation, energy-efficient QAH-based logic and memory devices. △ Less

Submitted 20 May, 2026; originally announced May 2026.

Comments: 20 pages and 4 figures. Comments are very welcome

Journal ref: Nano Lett. 26, 7833-7840 (2026)

Designer Quantum States in Magnetic Topological Insulator Multilayers

Authors: Deyi Zhuo, Han Tay, Cui-Zu Chang

Abstract: Magnetic topological insulators (TIs) provide a highly tunable platform for engineering quantum states that emerge from the interplay between topology and magnetism. In this review article, we summarize experimental progress over the past decade in designing magnetic TI multilayers by molecular beam epitaxy (MBE). By treating magnetically doped and undoped TI layers as topological Legos, we discus… ▽ More Magnetic topological insulators (TIs) provide a highly tunable platform for engineering quantum states that emerge from the interplay between topology and magnetism. In this review article, we summarize experimental progress over the past decade in designing magnetic TI multilayers by molecular beam epitaxy (MBE). By treating magnetically doped and undoped TI layers as topological Legos, we discuss how layer thickness, magnetic doping, heterostructure architecture, and stacking sequence can be used to control magnetic exchange gaps, interlayer coupling, and the Chern number C with atomic-layer precision. We first briefly review the realization of the C = 1 quantum anomalous Hall (QAH) effect in uniformly Cr-doped (Bi,Sb)2Te3 films in 2013 and uniformly V-doped (Bi,Sb)2Te3 films in 2015. We then discuss how Cr-doped and undoped (Bi,Sb)2Te3 layers can be combined to realize the C = 1 QAH effect in magnetically modulation-doped trilayers, including its extension into the three-dimensional (3D) regime. Next, we review the development of high-C QAH states, engineered plateau phase transitions, mesoscopic QAH devices, and electrical switching of chiral edge-current chirality. Finally, we discuss the realizations of axion insulator and C = 1/2 parity anomaly states in asymmetric magnetic TI trilayers. These advances establish magnetic TI multilayers as a versatile materials platform for creating new designer quantum states, including synthetic Weyl semimetal and QAH metal phases, and for probing the topological magnetoelectric effect in thick axion insulators and 3D QAH insulators. △ Less

Submitted 20 May, 2026; originally announced May 2026.

Comments: 38 pages and 8 figures. Comments are very welcome. An invited review for the 50th Anniversary Issue of the Journal of Magnetism and Magnetic Materials

Rapid Atmospheric Vapor Deposition of H:In2O3 Transparent Conducting Oxide Thin Films

Authors: Xiaoyu Guo, Hae-Jun Seok, Eilidh L. Quinn, Matthew K Sharpe, Callum. D. McAleese, Yi-Teng Huang, Xinjuan Li, Kexue Li, Chia-Yu Chang, Yongjie Wang, John O'Sullivan, Katie L. Moore, Caterina Ducati, Ruy Sebastian Bonilla, Han-Ki Kim, Abderrahime Sekkat, Robert L. Z. Hoye

Abstract: Transparent conducting oxides (TCOs) are essential for the optoelectronics industry, but there is a critical gap in cost-effective methods to rapidly deposit low sheet resistance, high transmittance films without damaging delicate materials, including emerging soft semiconductors like metal-halide perovskites. In this work, atmospheric pressure chemical vapor deposition (AP-CVD) is used to synthes… ▽ More Transparent conducting oxides (TCOs) are essential for the optoelectronics industry, but there is a critical gap in cost-effective methods to rapidly deposit low sheet resistance, high transmittance films without damaging delicate materials, including emerging soft semiconductors like metal-halide perovskites. In this work, atmospheric pressure chemical vapor deposition (AP-CVD) is used to synthesise H:In2O3 films with 7.20+/-0.01 Ohm/sq sheet resistance (0.50+/-0.06 mOhm.cm resistivity) and transmittance up to 89% in the near-infrared (NIR), surpassing commercial sputter-deposited indium tin oxide. The growth rate is 40x higher than atomic layer deposition (ALD), and the AP-CVD films are fully processed under atmospheric conditions at only 140 C. Comparison of secondary ion mass spectrometry and time-of-flight elastic recoil detection analysis with changes in carrier concentration indicate that H dopants are introduced from the water oxidant. There is an increase in mobility form 40+/-10 cm2/Vs to 160+/-30 cm2/Vs when changing from O2 to H2O as the oxidant, which is attributed to H dopants passivating oxygen vacancies that act as carrier scattering centers. This work establishes AP-CVD as a promising method for manufacturing high figure-of-merit TCOs in a rapid, scalable and cost-effective manner, using mild growth conditions compatible with thermally-sensitive materials. △ Less

Submitted 15 May, 2026; originally announced May 2026.

Comments: 20 pages, 3 figures

A renormalization-group inspired lattice-based framework for piecewise generalized linear models

Authors: Joshua C. Chang

Abstract: We formally introduce a class of models inspired by renormalization group (RG) theory, built on additive hierarchical expansions analogous to those appearing in functional ANOVA and mixed-effects models. Like ReLU convolutional neural networks, they are almost everywhere locally linear; unlike ReLU networks, their partition structure is explicit, interpretable, and easy to modify or constrain. In… ▽ More We formally introduce a class of models inspired by renormalization group (RG) theory, built on additive hierarchical expansions analogous to those appearing in functional ANOVA and mixed-effects models. Like ReLU convolutional neural networks, they are almost everywhere locally linear; unlike ReLU networks, their partition structure is explicit, interpretable, and easy to modify or constrain. In these models, one defines a multidimensional lattice partition of the input space and uses it to scaffold variations in regression parameters. Each dimension of the lattice corresponds to an attribute by which the statistics of the problem may vary. The parameters are themselves expressed in the form of an expansion, where each term captures variations relative to a lower (coarser) interaction scale. These models admit multiple equivalent interpretations: as piecewise GLMs, as hierarchical mixed-effects regressions, or as regression trees with structured parameter sharing. Since RG motivates the design of these models, we use techniques from statistical physics -- specifically replica analysis -- to study their generalization properties. Specifically, we analyze the behavior of the Watanabe-Akaike Information Criterion (WAIC) as a proxy for generalization loss. This analysis yields two practical results: (i) guidance on the lattice design as a function of dataset size and predictor dimensionality; and (ii) a principled scaling law for the regularization prior when adding higher-order terms to the expansion so that one can increase model complexity without an expected increase in generalization loss. We evaluate the methodology on public datasets and find performance competitive against both blackbox methods and other intrinsically interpretable approaches. △ Less

Submitted 6 May, 2026; originally announced May 2026.

Comments: Under review

Nitrogen doping induced metal-insulator transition with iso-symmetric character in rutile VO2

Authors: Baichen Lin, Shanquan Chen, Yubo Zhang, Yangyang Si, Haoliang Huang, Chuanrui Huo, Frans Munnik, Yongqi Dong, Lu You, Jian Shao, Yu-Chieh Ku, Nguyen Nhat Quyen, Aryan Keshri, Zhenlin Luo, Weiwei Zhao, Chun-Fu Chang, Chih-Wei Luo, Sujit Das, Shiqing Deng, Chang-Yang Kuo, Zuhuang Chen

Abstract: Metal-insulator transitions (MITs) in correlated oxides offer immense potential for next-generation Mottronic devices. However, their integration into practical applications is often hindered by the coupling of MITs with symmetry-lowering structural phase transitions, which limits switching speed and endurance. In this study, we engineered an iso-symmetric MIT on average in epitaxial rutile VO2 th… ▽ More Metal-insulator transitions (MITs) in correlated oxides offer immense potential for next-generation Mottronic devices. However, their integration into practical applications is often hindered by the coupling of MITs with symmetry-lowering structural phase transitions, which limits switching speed and endurance. In this study, we engineered an iso-symmetric MIT on average in epitaxial rutile VO2 thin films via an in-situ nitrogen doping strategy. Nitrogen incorporation effectively suppresses V-V dimerization, enabling an iso-symmetric MIT, while preserving the original crystal symmetry. Furthermore, in-operando time-resolved optical reflectivity measurements revealed a shortened switching time in nitrogen-doped films, highlighting their enhanced performance. Our findings provide critical insights into the underlying mechanisms of MITs and introduce anion doping as a powerful tool for tailoring phase transitions in strongly correlated electron systems. This approach opens new avenues for the development of high-performance electronic and photonic devices. △ Less

Submitted 27 April, 2026; originally announced April 2026.

Comments: 17 pages, 4 figures, accpeted by Newton

UCd$_{11}$: A strongly localized 5$f^3$ material

Authors: Martin Sundermann, Naoki Ito, Daisuke Takegami, Chun-Fu Chang, Sheng-Huai Chen, Chang-Yang Kuo, Simone G. Altendorf, Andrei Gloskovskii, Hlynur Gretarsson, Eric D. Bauer, Jan Kuneš, Liu Hao Tjeng, Andrea Severing, Atsushi Hariki

Abstract: UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (P… ▽ More UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (PES) data of UCd$_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd$_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd$_{11}$ is a highly localized uranium 5$f^3$ system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5$f$ behavior. △ Less

Submitted 18 April, 2026; originally announced April 2026.

Journal ref: Phys. Rev. Research 8, 023008 (2026)

Classification of 2D Fermionic Systems with a $\mathbb Z_2$ Flavor Symmetry

Authors: Chi-Ming Chang, Jin Chen, Fengjun Xu

Abstract: We classify superfusion categories describing two-dimensional fermionic systems equipped with the universal fermion-parity symmetry, implemented by a topological defect line (TDL) $Z$, and an additional $\mathbb{Z}_2$ flavor symmetry generated by a $W$ TDL. Depending on whether $W$ is m-type or q-type, its fusion rules lead to three distinct classes, and solving the super-pentagon equations yields… ▽ More We classify superfusion categories describing two-dimensional fermionic systems equipped with the universal fermion-parity symmetry, implemented by a topological defect line (TDL) $Z$, and an additional $\mathbb{Z}_2$ flavor symmetry generated by a $W$ TDL. Depending on whether $W$ is m-type or q-type, its fusion rules lead to three distinct classes, and solving the super-pentagon equations yields 16 consistent superfusion categories. These are labeled by invariants $(ν_W,ν_Z,ν_{WZ})$, which determine the $\mathbb{Z}_8$ anomaly classes of the symmetries generated by $W$, $Z$, and $WZ$. We also provide explicit realizations using multiple Majorana fermions and comment on implications for fermionic CFTs and gapped phases. △ Less

Submitted 10 April, 2026; originally announced April 2026.

Comments: 17 pages+4 appendices

Report number: USTC-ICTS/PCFT-26-24

Stoichiometric FeTe is a Superconductor

Authors: Zi-Jie Yan, Zihao Wang, Bing Xia, Stephen Paolini, Ying-Ting Chan, Nikalabh Dihingia, Hongtao Rong, Pu Xiao, Kalana D. Halanayake, Jiatao Song, Veer Gowda, Danielle Reifsnyder Hickey, Weida Wu, Jiabin Yu, Peter J. Hirschfeld, Cui-Zu Chang

Abstract: Iron-based superconductors are a fascinating family of materials in which multiple electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans sup… ▽ More Iron-based superconductors are a fascinating family of materials in which multiple electronic bands and strong antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans superconductivity. In this work, we employ molecular beam epitaxy to grow FeTe films and perform post-growth annealing under a Te flux. By performing spin-polarized scanning tunneling microscopy and spectroscopy, we demonstrate that the AFM order in as-grown FeTe films is induced by interstitial Fe atoms that disrupt the ideal 1:1 stoichiometry. Remarkably, the removal of these interstitial Fe atoms through Te annealing yields stoichiometric FeTe films that show no AFM order and instead exhibit robust superconductivity with a critical temperature of ~13.5K. This superconducting state is further confirmed by the observation of Cooper pair tunneling, zero electrical resistance, and the Meissner effect. Therefore, our results demonstrate that stoichiometric FeTe is inherently a superconductor, overturning a long-held view that it is an AFM metal. This work clarifies the origin of superconductivity in FeTe-based heterostructures and demonstrates the importance of stoichiometry control in understanding the competition between AFM and superconductivity in iron-based superconductors. △ Less

Submitted 17 March, 2026; originally announced March 2026.

Comments: 39 pages, 4 main figures, and 10 extended data figures. Accepted by Nature. Comments are very welcome

Journal ref: Nature 652, 342(2026)

Single-Crystal Growth and Magnetic, Electronic Properties of the FCC Antiferromagnet Ba_2CoMoO_6

Authors: A. R. N. Hanna, M. M. Ferreira-Carvalho, S. H. Chen, C. F. Chang, C. Y. Kuo, A. T. M. N. Islam, R. Feyerherm, L. H. Tjeng, B. Lake

Abstract: This work presents a comprehensive investigation of the structural, magnetic, and electronic properties of the double perovskite Ba$_2$CoMoO$_6$ (BCMO). Single crystals were grown via floating-zone and Czochralski techniques and characterized using a set of complementary methods. X-ray diffraction analysis confirmed that BCMO crystallizes in a face-centered cubic structure with space group… ▽ More This work presents a comprehensive investigation of the structural, magnetic, and electronic properties of the double perovskite Ba$_2$CoMoO$_6$ (BCMO). Single crystals were grown via floating-zone and Czochralski techniques and characterized using a set of complementary methods. X-ray diffraction analysis confirmed that BCMO crystallizes in a face-centered cubic structure with space group $Fm\bar{3}m$. Magnetic susceptibility measurements reveal antiferromagnetic ordering below $T_\mathrm{N} = 20.1(1)$~K, with a spin-flop transition at $μ_0 H = 2.65$~T. Heat-capacity measurements and entropy analysis, $ΔS \approx 0.95\,R\ln 2$, are consistent with a $J_\mathrm{eff} = \tfrac{1}{2}$ effective ground state for Co$^{2+}$ ions. X-ray absorption spectroscopy at the Co~$L_{2,3}$ edges provides insight into the local electronic structure, revealing spin-orbit and crystal-field splitting effects; cluster-model calculations constrained by the XAS spectra yield a Landé $g$ factor $g = 4.52$, consistent with a spin-orbit-entangled $J_\mathrm{eff} = \tfrac{1}{2}$ ground state. Surface photovoltage spectroscopy reveals a strong optical response with a prominent feature at $2.65$~eV. These findings advance the understanding of face-centered cubic lattice antiferromagnets with strong spin-orbit coupling and suggest the technological promise of BCMO for spintronic and energy-conversion applications. △ Less

Submitted 16 March, 2026; originally announced March 2026.

Suppression of Spectral Gap and Flat Bands on a Cuprate Superconductor Side-Surface

Authors: Gabriele Domaine, Mihir Date, Sydney K. Y. Dufresne, Natalie Lehmann, Daiyu Geng, Tohru Kurosawa, Amit Kumar, Jiaju Wang, Tianlun Yu, Chien-Ching Chang, Swosti P. Sarangi, Ding Pei, Yiran Liu, Julia Küspert, Shigemi Terakawa, Markel Pardo Almanza, Jiabao Yang, Izabela Biało, Matthew D. Watson, Timur K. Kim, Stephen M. Hayden, Kritika Singh, Banabir Pal, Matteo Minola, Johan Chang , et al. (5 additional authors not shown)

Abstract: Side surfaces of cuprate superconductors are expected to display a suppressed $d$-wave order parameter and zero-energy topological flat bands with a large density of states, making them susceptible to symmetry broken orders. Yet such surfaces have never been investigated with momentum-resolved, surface-sensitive probes, because high-temperature superconductors rarely cleave along them. Using focus… ▽ More Side surfaces of cuprate superconductors are expected to display a suppressed $d$-wave order parameter and zero-energy topological flat bands with a large density of states, making them susceptible to symmetry broken orders. Yet such surfaces have never been investigated with momentum-resolved, surface-sensitive probes, because high-temperature superconductors rarely cleave along them. Using focused-ion-beam milling to define a controlled breaking point, we expose pristine (110) side surfaces of overdoped La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.22$) suitable for angle-resolved photoemission. We observe the suppression of the superconducting spectral gap within our energy resolution ($\sim 4~\mathrm{meV}$), and surprisingly, the expected zero-energy flat band peak is also suppressed, despite the high topographic quality of the surface. Self-consistent Bogoliubov--de~Gennes calculations show that the measured geometric roughness of the cleaved surface is too weak to eliminate these modes. The calculations further demonstrate that bulk inhomogeneities characteristic of high-temperature superconductors, modelled as moderate Anderson-type disorder, can broaden the flat-band states beyond detectability. Our results provide the first momentum-resolved view of the electronic structure on a cuprate side surface and reveal disorder as the key factor currently preventing appearance of flat bands and their associated correlated orders. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Moire Engineering of Cooper-Pair Density Modulation States

Authors: Zihao Wang, Bing Xia, Stephen Paolini, Zi-Jie Yan, Pu Xiao, Jiatao Song, Veer Gowda, Hongtao Rong, Di Xiao, Xiaodong Xu, Weida Wu, Ziqiang Wang, Cui-Zu Chang

Abstract: Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work,… ▽ More Cooper-pair density modulation (CPDM) states are superconducting phases in which the order parameter varies periodically in real space without breaking translational symmetry. Recently, moire superlattices in layered materials have emerged as powerful platforms for engineering charge density with tunable lattice symmetry, offering a new route to creating and controlling CPDM states. In this work, we demonstrate moire-induced CPDM states in a bilayer heterostructure formed by epitaxially stacking one quintuple layer (1 QL) of topological insulator Sb2Te3 on a six-unit-cell (6 UC) antiferromagnetic FeTe layer. Scanning tunneling microscopy and spectroscopy (STM/S) measurements reveal a moiré superlattice formed between the hexagonal Te lattice of Sb2Te3 and the square Te lattice of FeTe, which spatially modulates the two superconducting gaps of the 1 QL Sb2Te3/6 UC FeTe bilayer. Our Josephson STM/S measurements provide direct real-space imaging of the CPDM states with a wavelength corresponding to the periodicity of the moire superlattice. By substituting Sb2Te3 with Bi2Te3, we achieve control over both the periodicity and magnitude of the CPDM states. Our work demonstrates an epitaxial strategy for synthesizing moire superlattices from materials with different crystal symmetries and reveals a new mechanism for engineering CPDM states in designer bilayer heterostructures. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Comments: 36 pages, 4 main figures, and 10 Extended Data figures. Accepted by Nature. Comments are very welcome

Journal ref: Nature 652, 335 (2026)

Magnonic spontaneous oscillation induced by parametric pumping

Authors: Yi Li, Carissa Kiehl, Jinho Lim, Cliff Abbott, Pratap K. Pal, Alex J. Szymczak, Juliang Li, Ralu Divan, Clarence L. Chang, Charudatta Phatak, Dmytro A. Bozhko, Axel Hoffmann, Valentine Novosad

Abstract: Spontaneous dynamic systems have attracted significant attention for their rich underlying physics such as phase-locking and synchronization. In this work, we report a new mechanism of generating magnetic spontaneous oscillation via parametric pumping. By applying a pump tone to excite propagating spin waves in a yttrium iron garnet delay line, four-wave mixing converts the pump mode into two phas… ▽ More Spontaneous dynamic systems have attracted significant attention for their rich underlying physics such as phase-locking and synchronization. In this work, we report a new mechanism of generating magnetic spontaneous oscillation via parametric pumping. By applying a pump tone to excite propagating spin waves in a yttrium iron garnet delay line, four-wave mixing converts the pump mode into two phase-autonomous propagating magnon modes, i.e. a spontaneous mode with nearly twice the wavenumber of the pump mode and an idler mode with nearly zero wavenumber. This allows us to reliably generate ultrasharp spin wave dynamics with broad frequency tunability from the pump and magnetic field. We show that the spontaneous mode can be phase-locked to a probe tone, similar to an auto-oscillator. Furthermore, the spontaneous dynamics can be used to implement a high-gain magnonic parametric amplifier with a gain up to 40 dB. Our results open a new avenue for studying nonlinear magnonics and synchronization physics in propagating magnon geometry and for developing new magnonic devices. △ Less

Submitted 14 February, 2026; originally announced February 2026.

Surface-State-Driven Anomalous Hall Effect in Altermagnetic MnTe Films

Authors: Ling-Jie Zhou, Zi-Jie Yan, Hongtao Rong, Yufei Zhao, Pu Xiao, Lok-Kan Lai, Zhiyuan Xi, Ke Wang, Tibendra Adhikari, Ganesh P. Tiwari, Zhong Lin, Pascal Manue, Fabio Orlandi, Dmitry Khalyavin, Alexander J. Grutter, Chao-Xing Liu, Binghai Yan, Cui-Zu Chang

Abstract: Altermagnets have recently emerged as a new class of magnetic materials that combine compensated magnetic order with spin-split electronic band structures. In this work, we employ molecular beam epitaxy to grow MnTe thin films with controlled thickness on InP(111)A substrates. By performing angle-resolved photoemission spectroscopy measurements, we observe a large spin splitting of ~230 meV for bu… ▽ More Altermagnets have recently emerged as a new class of magnetic materials that combine compensated magnetic order with spin-split electronic band structures. In this work, we employ molecular beam epitaxy to grow MnTe thin films with controlled thickness on InP(111)A substrates. By performing angle-resolved photoemission spectroscopy measurements, we observe a large spin splitting of ~230 meV for bulk bands well below the Fermi level and identify surface states that cross the Fermi level. Electrical transport measurements reveal that a robust anomalous Hall (AH) effect persists down to 2 K and an AH sign reversal occurs near 175 K. By systematically tuning film thickness, growth conditions, and interfacial structure, we demonstrate that the AH response in MnTe films originates from the Berry curvature of surface states rather than from bulk bands. Our first-principles calculations reveal that this surface-state-driven AH effect is imprinted by the bulk altermagnetic order and remains unchanged for terminations with opposite Mn magnetic orientations. Our results establish a unique surface transport probe of bulk altermagnetism, demonstrate interface engineering as an effective route to generate and control the AH effect in altermagnets, and provide a unified understanding of the AH response in altermagentic MnTe films. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Comments: 28 pages, 5 figures, comments are very much welcome

Interplay of Quantum Size Effect and Tensile Strain on Surface Morphology of Sn(100) Islands

Authors: Bing Xia, Xiaoyin Li, Hongyuan Chen, Bo Yang, Jie Cai, Stephen Paolini, Zihao Wang, Zi-Jie Yan, Hao Yang, Xiaoxue Liu, Liang Liu, Dandan Guan, Shiyong Wang, Yaoyi Li, Canhua Liu, Hao Zheng, Cui-Zu Chang, Feng Liu, Jinfeng Jia

Abstract: The quantum size effect (QSE) and strain effect are two key factors influencing the surface morphology of thin films, which can increase film surface roughness through QSE-induced thickness oscillation and strain-induced island formation, respectively. Surface roughness usually manifests in the early stages of film growth and diminishes beyond a critical thickness. In this work, we employ molecula… ▽ More The quantum size effect (QSE) and strain effect are two key factors influencing the surface morphology of thin films, which can increase film surface roughness through QSE-induced thickness oscillation and strain-induced island formation, respectively. Surface roughness usually manifests in the early stages of film growth and diminishes beyond a critical thickness. In this work, we employ molecular beam epitaxy (MBE) to grow Sn(100) islands with varying thickness N on bilayer graphene-terminated 6H-SiC(0001) substrates. Scanning tunneling microscopy and spectroscopy measurements reveal an inverse surface roughness effect that highlights the interplay of QSE and misfit strain in shaping the surface morphology of Sn(100) islands. For N =< 10, the islands exhibit flat surfaces, while for N >= 26, the island surfaces become corrugated and patterned. For the intermediate range, i.e., 12 =< N =<24, both flat and patterned surfaces coexist, with the percentage coverage of the patterned surface oscillating as a function of N. By performing density functional theory calculations, we demonstrate that the unusual surface pattern evolution in our MBE-grown Sn(100) islands is a result of the interplay between QSE-induced surface roughing and tensile strain-induced smoothening effect. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Comments: 23 pages, 5 figures, comments are very much welcome

Journal ref: ACS Nano 20, 8289 (2026)

Direct Evidence of a Near-Ideal Jeff = 1/2 Ground State in Triangular-Lattice Na2BaCo(PO4)2

Authors: M. M. Ferreira-Carvalho, S. H. Chen, Y. C. Ku, Anagha Jose, Ryan Morrow, C. Y. Kuo, C. F. Chang, Z. Hu, M. W. Haverkort, L. H. Tjeng

Abstract: We investigated the local Co 3d electronic structure of Na2BaCo(PO4)2 using polarization-dependent X-ray absorption spectroscopy (XAS) in combination with full multiplet cluster calculations. We employed the line-fitting inverse partial fluorescence yield (IPFY) technique to obtain accurate XAS spectra from strong insulating materials. Our combined experimental and theoretical analysis reveals a v… ▽ More We investigated the local Co 3d electronic structure of Na2BaCo(PO4)2 using polarization-dependent X-ray absorption spectroscopy (XAS) in combination with full multiplet cluster calculations. We employed the line-fitting inverse partial fluorescence yield (IPFY) technique to obtain accurate XAS spectra from strong insulating materials. Our combined experimental and theoretical analysis reveals a very small effective trigonal distortion of only 11 meV in the CoO6 octahedra, indicating a close to ideal condition to render a ground state with the Jeff = 1/2 character. With our cluster model we were also able to simulate magnetic susceptibility measurements along different directions in the crystal. These findings highlight Na2BaCo(PO4)2 as a promising platform for exploring exotic magnetic phenomena associated with Jeff = 1/2 ground states on triangular lattices. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Thermal Stability and Phase Transformation of Conductive $α$-$(\mathrm{Al}_{x}\mathrm{Ga}_{1-x})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ Heterostructures on Sapphire Substrates

Authors: Botong Li, Shisong Luo, Jaeheon Jung, Bobby G. Duersch, Cheng Chang, Lucas Lau, Zonghao Zhang, Jianhua Li, Hunter Ellis, Imteaz Rahaman, Roy Byung Kyu Chung, Kai Fu, Yuji Zhao

Abstract: Thermal stability and phase transformation of conductive $α$-$(\mathrm{Al}_{0.16}\mathrm{Ga}_{0.84})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ heterostructures on sapphire substrates were investigated using in situ high-temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Conductive $α$-… ▽ More Thermal stability and phase transformation of conductive $α$-$(\mathrm{Al}_{0.16}\mathrm{Ga}_{0.84})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ heterostructures on sapphire substrates were investigated using in situ high-temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Conductive $α$-$(\mathrm{Al}_{0.16}\mathrm{Ga}_{0.84})_{2}\mathrm{O}_{3}/\mathrm{Ga}_{2}\mathrm{O}_{3}$ heterostructures with fluorine (F) doping were grown by mist chemical vapor deposition on sapphire substrates, achieving a Hall mobility of $28~\mathrm{cm^{2}\,V^{-1}\,s^{-1}}$ and an electron concentration of $1.4\times10^{20}~\mathrm{cm^{-3}}$. The heterostructures exhibited thermal stability up to approximately $550$--$ △ Less

Submitted 7 February, 2026; originally announced February 2026.

Comments: 13 pages; 4 figures

IEPDYN: Integral-equation formalism of population dynamics

Authors: Kento Kasahara, Ryo Okabe, Chia-en A. Chang, Toshifumi mori, Nobuyuki Matubayasi

Abstract: We propose the integral-equation formalism of population dynamics (IEPDYN) to describe the population dynamics of distinct configurational states. According to classical reaction dynamics theory, the probability density associated with a given state obeys the Liouville equation, including influx from and efflux to neighboring states. By introducing a Markov approximation for the crossing of bounda… ▽ More We propose the integral-equation formalism of population dynamics (IEPDYN) to describe the population dynamics of distinct configurational states. According to classical reaction dynamics theory, the probability density associated with a given state obeys the Liouville equation, including influx from and efflux to neighboring states. By introducing a Markov approximation for the crossing of boundaries separating the states, tractable integral equations governing the state populations are derived. Once the time-dependent quantities appearing in these equations are evaluated, the population dynamics on long timescales can be obtained. Because these quantities depend only on a few states in the local neighborhood of a given state, they can be computed using a set of short-timescale molecular dynamics (MD) simulations. The IEPDYN method is formulated in continuous time and therefore does not rely on a coarse-grained timescale (lag time). Consequently, kinetic quantities obtained from IEPDYN are free from lag-time dependence, which has been discussed as a limitation in other approaches. We apply the IEPDYN method to the binding and unbinding kinetics of CH$_4$/CH$_4$, Na$^+$/Cl$^-$, and 18-crown-6-ether (crown ether)/K$^+$ in water. For both kinetics, the time constants estimated from the IEPDYN method are almost comparable to those obtained from brute-force MD simulations. The required timescale of each MD trajectory in the IEPDYN method is approximately two orders of magnitude shorter than that in the brute-force MD approach in the crown ether/K$^+$ system. This reduction in the trajectory timescale enables applications to complex binding and unbinding systems whose characteristic timescales are far beyond those directly accessible by brute-force MD simulations. △ Less

Submitted 23 March, 2026; v1 submitted 14 January, 2026; originally announced January 2026.

Comments: Main text: 18 pages, 8 figures, SI: 3 pages, 2 figures

Electrically Controllable Flat Band in Two-Dimensional Electron Gases under Nonuniform Magnetic Fields

Authors: You-Ting Huang, Chao-Cheng Kaun, Ching-Hao Chang

Abstract: Flat bands underlie a diverse range of quantum phenomena, from strongly correlated phases to superconductivity. We theoretically establish that a two-dimensional electron gas under a linear magnetic-field gradient and a transverse electric field exhibits electrically tunable flat bands. When the electric field magnitude is tuned to a value within a discrete sequence, these bands become strictly di… ▽ More Flat bands underlie a diverse range of quantum phenomena, from strongly correlated phases to superconductivity. We theoretically establish that a two-dimensional electron gas under a linear magnetic-field gradient and a transverse electric field exhibits electrically tunable flat bands. When the electric field magnitude is tuned to a value within a discrete sequence, these bands become strictly dispersionless. By providing exact classical and quantum solutions, we demonstrate that these states are high-order Landau levels associated with drift-compensated cyclotron orbits of carriers arising from the synergy between the magnetic-field gradient and the electric field. These electrically controllable Landau levels exhibit quantized Hall conductance and a strongly enhanced density of states. Our results provide a new route for flat-band creation, magnetoelectric band engineering, and quantized Hall currents controlled via source-drain voltage. △ Less

Submitted 4 August, 2026; v1 submitted 8 January, 2026; originally announced January 2026.

Comments: Preprint. To be presented at APS

On the Cocycle Structure of the Boltzmann Distribution

Authors: Chuan-Tsung Chan, Chan-Yi Chang, Zhong-Tang Wu

Abstract: Based on a cocycle structure, we identify a new derivation of the Boltzmann distribution for finite energy-level systems from the maximal entropy principle (MEP). Our approach does not rely on the method of the Lagrange multiplier, and it provides a more transparent way to understand the dependence on the energy levels of the temperature $T = 1/β$ for the equilibrium distribution. Finally, we make… ▽ More Based on a cocycle structure, we identify a new derivation of the Boltzmann distribution for finite energy-level systems from the maximal entropy principle (MEP). Our approach does not rely on the method of the Lagrange multiplier, and it provides a more transparent way to understand the dependence on the energy levels of the temperature $T = 1/β$ for the equilibrium distribution. Finally, we make two curious observations associated with our derivations. △ Less

Submitted 28 December, 2025; originally announced December 2025.

Comments: 7 pages, no figure

Boundary-Bulk Interplay in Nonlinear Topological Transport

Authors: Deyi Zhuo, Xiaoda Liu, Huu-Thong Le, Annie G. Wang, Han Tay, Bomin Zhang, Ling-Jie Zhou, Binghai Yan, Chao-Xing Liu, Cui-Zu Chang

Abstract: Nonlinear transport has emerged as a powerful approach to probe the quantum geometry of electronic wavefunctions, such as Berry curvature and quantum metric, in topological materials. While nonlinear responses governed by bulk quantum geometry and band topology are well understood, the role of boundary modes (e.g., edge, surface, and hinge states) in nonlinear transport of topological materials re… ▽ More Nonlinear transport has emerged as a powerful approach to probe the quantum geometry of electronic wavefunctions, such as Berry curvature and quantum metric, in topological materials. While nonlinear responses governed by bulk quantum geometry and band topology are well understood, the role of boundary modes (e.g., edge, surface, and hinge states) in nonlinear transport of topological materials remains largely unexplored. In this work, we demonstrate boundary-bulk interplay in nonlinear transport, including second-harmonic Hall and nonreciprocal longitudinal responses, in molecular beam epitaxy-grown magnetic topological insulator heterostructures. We find that the nonlinear transport is maximized when the sample is tuned slightly away from the well-quantized states, including the quantum anomalous Hall and axion insulator states. The sign and amplitude of the nonlinear transport depend on electrode configuration, magnetic order, and carrier type, establishing boundary mode transport as the dominant contributor. These findings, supported by symmetry analysis and nonlinear Landauer-Büttiker formalism, demonstrate that nonlinear transport in topological materials is governed by the interplay between boundary and bulk states. We further derive a universal relation between different lead voltages from electrode geometry symmetry, which allows us to distinguish nonlinear boundary transport from bulk contributions. Our work highlights the critical role of electrodes in nonlinear transport, which is absent in nonlinear optics, and establishes boundary modes as a key origin of the giant nonlinear response in nearly bulk-insulating topological materials. This insight opens new opportunities for engineering nonlinear transport through boundary-bulk interplay in future device applications of topological materials. △ Less

Submitted 7 December, 2025; originally announced December 2025.

Comments: 38 pages, 4 figures and 10 extended data figures

Journal ref: Nat. Commun. (2026)

Ligand Engineering for Precise Control of Ultrathin CsPbI3 Nanoplatelet Superlattices for Efficient Light-Emitting Diodes

Authors: Jongbeom Kim, Woo Hyeon Jeong, Junzhi Ye, Allison Nicole Arber, Vikram, Donghan Kim, Yi-Teng Huang, Yixin Wang, Dongeun Kim, Dongryeol Lee, Chia-Yu Chang, Xinyu Shen, Sung Yong Bae, Ashish Gaurav, Akshay Rao, Henry J. Snaith, M. Saiful Islam, Bo Ram Lee, Myoung Hoon Song, Robert L. Z. Hoye

Abstract: Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI6 octahedra across the thickness, which are challenging to synthesis… ▽ More Strongly-confined perovskite nanoplatelets (PeNPLs) offer opportunities not found in conventional isotropic nanocubes, especially in producing linearly polarized light, as well as enhancing outcoupling through control over the transition dipole moment. But this requires ultrathin nanoplatelets with three or fewer monolayers of PbI6 octahedra across the thickness, which are challenging to synthesise uniformly, and their luminescence is strongly affected by surface defects. Together, these limit the performance of ultrathin PeNPLs in light-emitting diodes (LEDs). Here, we address these challenges with an ancillary ligand engineering strategy. We demonstrate that ligands with phosphoryl functional groups strongly bind to the perovskite surface, while having an organic backbone that is not sterically bulky ensures high ligand density. By modulating nucleation and growth, these ancillary ligands lead to monodisperse PeNPLs that stack more uniformly when self-assembled into superlattices, with suppressed agglomeration. As a result, from edge-up PeNPL superlattices, we achieve enhanced degree of polarization, while from face-down PeNPL superlattices, we achieve enhanced outcoupling that results in LEDs with 13.1% external quantum efficiency, the highest reported for ultrathin PeNPL LEDs. This work establishes ancillary ligand-induced synthesis as a decisive route to achieve uniform nanoplatelets with robust orientation control, enabling full utilization of the multifunctionality of anisotropic PeNPLs. △ Less

Submitted 23 January, 2026; v1 submitted 14 November, 2025; originally announced November 2025.

Comments: 25 pages, 4 figures, 1 table

High Mobility Multiple-Channel AlScN/GaN Heterostructures

Authors: Aias Asteris, Thai-Son Nguyen, Chuan F. C. Chang, Chandrashekhar Savant, Pierce Lonergan, Huili Grace Xing, Debdeep Jena

Abstract: Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy, and demonstrate the lowest sheet resistance am… ▽ More Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy, and demonstrate the lowest sheet resistance among AlScN-based systems reported to date. Assorted schemes of GaN/AlN interlayers are first introduced in single-channel structures between AlScN and GaN to improve conductivity, increasing electron mobility up to $1370$ cm$^{2}$/V$\cdot$s at 300 K and $4160$ cm$^{2}$/V$\cdot$s at 77 K, reducing the sheet resistance down to 170 $Ω/\square$ and 70 $Ω/\square$ respectively. These improvements are then leveraged in multi-channel heterostructures, reaching sheet resistances of 65 $Ω/\square$ for three channels and 45 $Ω/\square$ for five channels at 300 K, further reduced to 21 $Ω/\square$ and 13 $Ω/\square$ at 2 K, respectively, confirming the presence of multiple 2DEGs. Structural characterization indicates pseudomorphic growth with smooth surfaces, while partial barrier relaxation and surface roughening are observed at high scandium content, with no impact on mobility. This first demonstration of ultra-low sheet resistance multi-channel AlScN/GaN heterostructures places AlScN on par with state-of-the-art multi-channel Al(In)N/GaN systems, showcasing its capacity to advance existing and enable new high-speed, high-power electronic devices. △ Less

Submitted 13 November, 2025; originally announced November 2025.

Comments: 10 pages, 10 figures

Quasiperiodicity-induced bulk localization with self similarity in non-Hermitian systems

Authors: Yu-Peng Wang, Chuo-Kai Chang, Ryo Okugawa, Chen-Hsuan Hsu

Abstract: We analyze the localization behavior in a non-Hermitian system subject to a quasiperiodic onsite potential. We characterize localization transitions using multiple quantitative indicators, including inverse participation ratio (IPR), eigenstate fractal dimension (EFD), extended eigenstate ratio (EER), and spectral survival ratio. Despite the breaking of self-dual symmetry due to non-Hermiticity, o… ▽ More We analyze the localization behavior in a non-Hermitian system subject to a quasiperiodic onsite potential. We characterize localization transitions using multiple quantitative indicators, including inverse participation ratio (IPR), eigenstate fractal dimension (EFD), extended eigenstate ratio (EER), and spectral survival ratio. Despite the breaking of self-dual symmetry due to non-Hermiticity, our results reveal the existence of a critical potential strength, with its value increasing linearly with the nearest-neighbor antisymmetric hopping term. On the other hand, the inclusion of longer-range hopping not only enriches the topological properties but also gives rise to novel localization phenomena. In particular, it induces the emergence of mobility edges, as evidenced by both IPR and EFD, along with distinct features in the spectrum fractal dimension, which we extract using the box-counting method applied to the complex energy spectrum. Additionally, we uncover self-similar structures in various quantities, such as EER and complex eigenvalue ratio, as the potential strength varies. These findings highlight important aspects of localization and fractal phenomena in non-Hermitian quasiperiodic systems. △ Less

Submitted 17 December, 2025; v1 submitted 31 October, 2025; originally announced November 2025.

Comments: 16 pages, 21 figures. Revised version accepted for publication in Phys. Rev. Research

Journal ref: Phys. Rev. Research 7, 043353 (2025)

Wireless Sensing of Temperature, Strain and Crack Growth in 3D-Printed Metal Structures via Magneto-Responsive Inclusions

Authors: Connor G. McMahan, Chia-Ming Chang, Raymond Nguyen, Souren Soukiazian, David A. Smith, Tobias Schaedler, David Shahan

Abstract: This study demonstrates the first realization of wireless strain, temperature and crack growth sensing within 3D-printed metallic structures using standard electromagnetic inspection hardware. This establishes a path toward need-based maintenance for parts operating in harsh environments driven by accurate, real-time damage assessments instead of relying on regularly scheduled maintenance teardown… ▽ More This study demonstrates the first realization of wireless strain, temperature and crack growth sensing within 3D-printed metallic structures using standard electromagnetic inspection hardware. This establishes a path toward need-based maintenance for parts operating in harsh environments driven by accurate, real-time damage assessments instead of relying on regularly scheduled maintenance teardowns. To this end, we encapsulate and embed magnetoelastic and thermomagnetic materials during additive manufacturing. Mechanical and thermal stimuli affect the magnetic permeability of the embedded materials, which modulates the flux through a coil placed on or near the part's surface. We demonstrate strain sensing accurate to +/-27x10-6 and temperature sensing accurate to +/-0.75 oC. We highlight these sensors' capabilities by detecting the onset of plasticity and fatigue-driven crack growth thousands of cycles before critical failure. △ Less

Submitted 6 February, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Comments: 14 pages, 10 figures

Non-Hermitian topological superconductivity with symmetry-enriched spectral and eigenstate features

Authors: Chuo-Kai Chang, Kazuma Saito, Nobuyuki Okuma, Hsien-Chung Kao, Chen-Hsuan Hsu

Abstract: We investigate a one-dimensional superconducting lattice that realizes all internal symmetries permitted in non-Hermitian systems, characterized by nonreciprocal hopping, onsite dissipation, and $s$-wave singlet pairing in a Su-Schrieffer-Heeger-type structure. The combined presence of pseudo-Hermiticity and sublattice symmetry imposes constraints on the energy spectra. We identify parameter regim… ▽ More We investigate a one-dimensional superconducting lattice that realizes all internal symmetries permitted in non-Hermitian systems, characterized by nonreciprocal hopping, onsite dissipation, and $s$-wave singlet pairing in a Su-Schrieffer-Heeger-type structure. The combined presence of pseudo-Hermiticity and sublattice symmetry imposes constraints on the energy spectra. We identify parameter regimes featuring real spectra, purely imaginary spectra, complex flat bands, and Majorana zero modes, the latter emerging when a uniform transverse magnetic field suppresses the non-Hermitian skin effect. We show that a uniform onsite dissipation is essential for stabilizing the zero modes, whereas a purely staggered dissipation destroys the topological superconductivity. Through Hermitianization, we construct a spectral winding number as a topological invariant and demonstrate its correspondence with the gap closing conditions and appearance of the Majorana zero modes, allowing us to establish topological phase diagrams. Moreover, we reveal nontrivial correlations between the particle-hole and spin components of left and right eigenstates, enforced by chiral symmetry, pseudo-Hermiticity, and their combination. Our results highlight how non-Hermiticity, sublattice structure, and superconductivity together enrich symmetry properties and give rise to novel topological phenomena. △ Less

Submitted 15 January, 2026; v1 submitted 27 September, 2025; originally announced September 2025.

Comments: 25 pages, 22 figures; revised version accepted for publication in Phys. Rev. Research

Journal ref: Phys. Rev. Research 8, 013167 (2026)

Navigating entanglement via Ruderman-Kittel-Kasuya-Yosida exchange: Snake, bouncing, boundary-residing, pulse, and damping-stabilized time-frozen trajectories

Authors: Son-Hsien Chen, Seng Ghee Tan, Ching-Ray Chang

Abstract: Entanglement dynamics are fundamental to quantum technologies, yet navigating their temporal profiles (trajectories) remains challenging. Here, we propose a scalable solid-state platform based on RKKY exchange, where two spin qubits couple to a central spin qudit that oscillatorily spin-polarizes the surrounding conduction electrons. We introduce the exchange-time integral (ETI), which maps the sp… ▽ More Entanglement dynamics are fundamental to quantum technologies, yet navigating their temporal profiles (trajectories) remains challenging. Here, we propose a scalable solid-state platform based on RKKY exchange, where two spin qubits couple to a central spin qudit that oscillatorily spin-polarizes the surrounding conduction electrons. We introduce the exchange-time integral (ETI), which maps the spatial motion of the qubits to a time-dependent exchange interaction and serves as an effective "trajectory clock" governing the system evolution. We focus specifically on entanglement trajectories initially near the entanglement-unentanglement boundary, with the distance to this boundary quantified by concurrence extended to include negative values. By alternating the sign changes of the exchange, implemented through vibrational motion of qubits, the ETI enables programmable entanglement trajectories. For in-phase and antiphase vibrations, including scenarios with controlled stopping at the RKKY exchange-free nodes, we identify distinctive trajectories: snake (repeatedly crossing the boundary), bouncing (immediately reversing upon reaching the boundary), boundary-residing (remaining at the transition point), and pulse (controllable entanglement intervals). The vibration phase creates asymmetric shifts to the trajectories. The proposed device offers built-in error correction against dephasing by utilizing both ferromagnetic and antiferromagnetic regimes. Out-of-phase vibrations drive trajectories away from the boundary, accessing larger entanglement values but with irregular/unsteady final states. To stabilize these trajectories, we introduce a damping mechanism. Our framework offers a systematic method for navigating and engineering entanglement dynamics in quantum systems, with potential applications in quantum computation, cryptography, and metrology. △ Less

Submitted 30 December, 2025; v1 submitted 20 September, 2025; originally announced September 2025.

Comments: 15 pages, 9 figures, two tables

Evidence for Half-Quantized Chiral Edge Current in a C = 1/2 Parity Anomaly State

Authors: Deyi Zhuo, Bomin Zhang, Humian Zhou, Han Tay, Xiaoda Liu, Zhiyuan Xi, Chui-Zhen Chen, Cui-Zu Chang

Abstract: A single massive Dirac surface band is predicted to exhibit a half-quantized Hall conductance, a hallmark of the C = 1/2 parity anomaly state in quantum field theory. Experimental signatures of the C = 1/2 parity anomaly state have been observed in semi-magnetic topological insulator (TI) bilayers, yet whether it supports a half-quantized chiral edge current remains elusive. Here, we observe a rob… ▽ More A single massive Dirac surface band is predicted to exhibit a half-quantized Hall conductance, a hallmark of the C = 1/2 parity anomaly state in quantum field theory. Experimental signatures of the C = 1/2 parity anomaly state have been observed in semi-magnetic topological insulator (TI) bilayers, yet whether it supports a half-quantized chiral edge current remains elusive. Here, we observe a robust half-quantized Hall conductance plateau in a molecular beam epitaxy (MBE)-grown asymmetric magnetic TI trilayer under specific in-plane magnetic field regimes, corresponding to the C = 1/2 parity anomaly state. Within this state, both nonlocal and nonreciprocal transport signals are greatly enhanced, which we identify as direct evidence for a half-quantized chiral edge current localized at the boundary of the top gapped surface. Our numerical simulations demonstrate that this half-quantized chiral edge channel is the essential carrier of the observed half-quantized Hall conductance plateau, analogous to the quantized chiral edge channel in the C = 1 quantum anomalous Hall state. Our results provide experimental evidence for the half-quantized chiral edge transport in a C = 1/2 parity anomaly state. This work establishes asymmetric magnetic TI trilayers as a platform for probing single Dirac fermion physics and paves the way to explore a series of exciting phenomena in the C = 1/2 parity anomaly state, including the topological magnetoelectric effect and quantized magneto-optical response. △ Less

Submitted 18 September, 2025; originally announced September 2025.

Comments: 19 pages, 4 figures, comments are welcome

Journal ref: Phys. Rev. Lett. 136, 016601 (2026)

Accurate bootstrap bounds from optimal interpolation

Authors: Cyuan-Han Chang, Vasiliy Dommes, Petr Kravchuk, David Poland, David Simmons-Duffin

Abstract: We develop new methods for approximating conformal blocks as positive functions times polynomials, with applications to the numerical bootstrap. We argue that to obtain accurate bootstrap bounds, conformal block approximations should minimize a certain error norm related to the asymptotics of dispersive functionals. This error norm can be made small using interpolation nodes with an appropriate op… ▽ More We develop new methods for approximating conformal blocks as positive functions times polynomials, with applications to the numerical bootstrap. We argue that to obtain accurate bootstrap bounds, conformal block approximations should minimize a certain error norm related to the asymptotics of dispersive functionals. This error norm can be made small using interpolation nodes with an appropriate optimal density. The optimal density turns out to satisfy a kind of force-balance equation for charges in one dimension, which can be solved using standard techniques from large-N matrix models. We also describe how to use optimal density interpolation nodes to improve condition numbers inside the semidefinite program solver SDPB. Altogether, our new approximation scheme and improvements to condition numbers lead to more accurate bootstrap bounds with fewer computational resources. They were crucial in the recent bootstrap study of stress tensors in the 3d Ising CFT. △ Less

Submitted 26 May, 2026; v1 submitted 17 September, 2025; originally announced September 2025.

Comments: 37 pages, 13 figures, 1 table; v2: JHEP version

Report number: CALT-TH 2025-031

Room temperature reactive sputtering deposition of titanium nitride with high sheet kinetic inductance

Authors: Juliang Li, Peter S. Barry, Tom Cecil, Marharyta Lisovenko, Volodymyr Yefremenko, Gensheng Wang, Serhii Kruhlov, Goran Karapetrov, Clarence Chang

Abstract: Superconducting thin films with high intrinsic kinetic inductance $L_{k}$ are important for high-sensitivity detectors, enabling strong coupling in hybrid quantum systems, and enhancing nonlinearities in quantum devices. We report the room-temperature reactive sputtering of titanium nitride thin films with a critical temperature $T_{c}$ of \SI{3.8}{K} and a thickness of \SI{27}{nm}. Fabricated int… ▽ More Superconducting thin films with high intrinsic kinetic inductance $L_{k}$ are important for high-sensitivity detectors, enabling strong coupling in hybrid quantum systems, and enhancing nonlinearities in quantum devices. We report the room-temperature reactive sputtering of titanium nitride thin films with a critical temperature $T_{c}$ of \SI{3.8}{K} and a thickness of \SI{27}{nm}. Fabricated into resonators, these films exhibit a sheet kinetic inductance $L_{k, \square}$ of 394~$\textrm{pH}/\square$, as inferred from resonant frequency measurements. %from this film and measure quality factors of $4\times 10^{4}$; these quality factors are likely limited by the low resistivity wafer. X-ray diffraction analysis confirms the formation of stoichiometric TiN, with no residual unreacted titanium. The films also demonstrate a characteristic sheet resistivity of 475~$Ω/\square$, yielding an impedance an order of magnitude higher than conventional 50~$Ω$ resonators. This property could enhance microwave single\textendash photon coupling strength by an order of magnitude, offering transformative potential for hybrid quantum systems and quantum sensing. Furthermore, the high $L_{k}$ enables Kerr nonlinearities comparable to state\textendash of\textendash the\textendash art quantum devices. Combined with its relatively high $T_{c}$, this thin film presents a promising platform for superconducting devices, including amplifiers and qubits operating at higher temperatures. △ Less

Submitted 17 September, 2025; originally announced September 2025.

Trigonal distortion in the Kitaev candidate honeycomb magnet BaCo2(AsO4)2

Authors: M. M. Ferreira-Carvalho, S. Rößler, C. F. Chang, Z. Hu, S. M. Valvidares, P. Gargiani, M. W. Haverkort, Prashanta K. Mukharjee, P. Gegenwart, A. A. Tsirlin, L. H. Tjeng

Abstract: We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound BaCo$_2$(AsO$_4$)$_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quant… ▽ More We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound BaCo$_2$(AsO$_4$)$_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quantitative analysis. Our experimental results revealed a significant linear dichroic signal, indicating strong trigonal distortion in the CoO$_{6}$ octahedra in BaCo$_2$(AsO$_4$)$_2$. We performed a detailed analysis of the experimental XAS and XMCD spectra using a full-multiplet configuration-interaction cluster model. This analysis unveiled that the $t_{2g}$ hole density is predominantly localized in the $a_{1g}$ orbital. Through XMCD sum rules and theoretical calculations, we quantified both the spin and orbital magnetic moments. Our study demonstrates that the local electronic structure of the CoO$_{6}$ octahedra displays an effective trigonal distortion of approximately $-0.114$ eV. This distortion is larger than the Co $3d$ spin-orbit coupling constant, emphasizing the crucial impact of local structural distortions on the electronic and magnetic properties of BaCo$_2$(AsO$_4$)$_2$. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Journal ref: Phys. Rev. B 112, 125135 (2025)

Orbital Hybridization-Induced Ising-Type Superconductivity in a Confined Gallium Layer

Authors: Hemian Yi, Yunzhe Liu, Chengye Dong, Yiheng Yang, Zi-Jie Yan, Zihao Wang, Lingjie Zhou, Dingsong Wu, Houke Chen, Stephen Paolini, Bing Xia, Bomin Zhang, Xiaoda Liu, Hongtao Rong, Annie G. Wang, Saswata Mandal, Kaijie Yang, Benjamin N. Katz, Lunhui Hu, Jieyi Liu, Tien-Lin Lee, Vincent H. Crespi, Yuanxi Wang, Yulin Chen, Joshua A. Robinson , et al. (2 additional authors not shown)

Abstract: In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper pair wavefunctions and induce novel forms of unconventional superconductivity. In this work, we employ a plasma-free, carbon buffer layer-assisted confinement epitaxy method to synthesize trilayer gallium (Ga) sandwiched between a graphene layer and a 6H-SiC(0001)… ▽ More In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper pair wavefunctions and induce novel forms of unconventional superconductivity. In this work, we employ a plasma-free, carbon buffer layer-assisted confinement epitaxy method to synthesize trilayer gallium (Ga) sandwiched between a graphene layer and a 6H-SiC(0001) substrate, forming an air-stable graphene/trilayer Ga/SiC heterostructure. In this confined light-element Ga layer, we demonstrate interfacial Ising-type superconductivity driven by atomic orbital hybridization between the Ga layer and the SiC substrate. Electrical transport measurements reveal that the in-plane upper critical magnetic field u0Hc2,|| reaches ~21.98T at T=400 mK, approximately 3.38 times the Pauli paramagnetic limit (~6.51T). Angle-resolved photoemission spectroscopy (ARPES) measurements combined with theoretical calculations confirm the presence of split Fermi surfaces with Ising-type spin textures at the K and K' valleys of the confined Ga layer strongly hybridized with SiC. Moreover, by incorporating finite relaxation time induced by impurity scattering into an Ising-type superconductivity model, we reproduce the entire temperature-dependent u0Hc2,|| phase diagram. This work establishes a new strategy to realize unconventional pairing wavefunctions by combining quantum confinement and interfacial hybridization effects in superconducting thin films. It also opens new avenues for designing scalable superconducting quantum electronic and spintronic devices through interfacial engineering. △ Less

Submitted 6 September, 2025; originally announced September 2025.

Comments: 40 pages, 4 figures and 10 extended data figures, comments are welcome

Journal ref: Nature Mater. (2026)

Gate-Tunable Ambipolar Josephson Current in a Topological Insulator

Authors: Bomin Zhang, Xiaoda Liu, Junjie Qi, Ling-Jie Zhou, Deyi Zhuo, Han Tay, Hongtao Rong, Annie G. Wang, Zhiyuan Xi, Chao-Xing Liu, Chui-Zhen Chen, Cui-Zu Chang

Abstract: Dirac surface states in a topological insulator (TI) with proximity-induced superconductivity offer a promising platform for realizing topological superconductivity and Majorana physics. However, in TIs, the Josephson effect is usually observed in regimes where transport is dominated by either substantial bulk conduction channels or unipolar surface states. In this work, we demonstrate gate-tunabl… ▽ More Dirac surface states in a topological insulator (TI) with proximity-induced superconductivity offer a promising platform for realizing topological superconductivity and Majorana physics. However, in TIs, the Josephson effect is usually observed in regimes where transport is dominated by either substantial bulk conduction channels or unipolar surface states. In this work, we demonstrate gate-tunable ambipolar Josephson current in lateral Josephson junction (JJ) devices based on bulk-insulating (Bi,Sb)2Te3 thin films grown by molecular beam epitaxy (MBE). For thinner films, the supercurrent exhibits pronounced gate-tunable ambipolar behavior and is significantly suppressed as the chemical potential approaches the Dirac point, yet persists across it. In contrast, thicker films exhibit a much weaker ambipolar response. Moreover, we find that the supercurrent becomes significantly less resilient to external magnetic fields when the chemical potential is tuned near the Dirac point in both thickness regimes. Our numerical simulations demonstrate the ambipolar behavior of these TI JJ devices and attribute the asymmetric supercurrent observed in thicker TI films to the coexistence of Dirac surface states and bulk conduction channels. The demonstration of gate-tunable ambipolar Josephson transport in MBE-grown TI films paves the way for realizing Dirac-surface-state-mediated topological superconductivity and establishes a foundation for future exploration of electrically tunable Majorana modes. △ Less

Submitted 6 September, 2025; originally announced September 2025.

Comments: 26 pages, 4 figures, comments are welcome

Journal ref: Nano Lett. 25, 17416 (2025)

Breakdown of the Kirchhoff's law of thermal radiation by a spatiotemporally modulated nonreciprocal metasurface

Authors: Anatoly Efimov, Chun-Chieh Chang, Simo Pajovic, Wilton J. M. Kort-Kamp, Dongsung Kim, Hou-Tong Chen, Diego A. R. Dalvit, Abul K. Azad

Abstract: Kirchhoff's law of thermal radiation, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, fundamentally limits the efficiency of photonic systems by enforcing reciprocal energy exchange between source and detector. Breaking this reciprocity is particular… ▽ More Kirchhoff's law of thermal radiation, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, fundamentally limits the efficiency of photonic systems by enforcing reciprocal energy exchange between source and detector. Breaking this reciprocity is particularly important for advancing photonic devices for energy conversion, radiative cooling, and mid-infrared sensing and imaging. Driven by the growing need for photonic platforms to overcome reciprocity constraints, we present the first demonstration of spatiotemporally modulated nonreciprocal metasurfaces operating at mid-infrared frequencies suitable for the violation of the Kirchhoff's law at room temperature. We fabricate a graphene-based integrated photonic structure and experimentally demonstrate nonreciprocal reflection from a metasurface modulated at gigahertz frequencies. We develop a theoretical framework to relate nonreciprocal scattering under spatiotemporal modulation with unequal absorptivity and emissivity for violation of the spectral directional Kirchhoff's law. Our experiment and theory imply effective decoupling of absorption and emission channels by breaking time-reversal symmetry at thermal wavelengths. △ Less

Submitted 12 October, 2025; v1 submitted 30 August, 2025; originally announced September 2025.

Comments: Final version submitted for publication

Bending nanoribbon to induce large anisotropic magnetoconductance

Authors: Ponder Liu, Hao-Cheng Hung, You-Ting Huang, Jia-Cheng Li, Carmine Ortix, Ching-Hao Chang

Abstract: When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a s… ▽ More When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a strong geometry-induced anisotropic magnetoconductance (GAMC) in perpendicularly bent nanoribbon, which can reach up to 100\%. Moreover, the GAMC can be further enhanced to 200\%, 300\%, or even higher by either further bending or tuning the bending angle. The potential of this phenomenon for practical applications is demonstrated by its stable anisotropy, which remains consistent across a wide range of Fermi energies, can be observed even at weak magnetic fields and room temperature, and occurs in various systems such as two-dimensional electron gas (2DEG) and graphene. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Emergent cavity-QED dynamics along the edge of a photonic lattice

Authors: Enrico Di Benedetto, Xuejian Sun, Marcel A. Pinto, Luca Leonforte, Chih-Ying Chang, Vincent Jouanny, Léo Peyruchat, Pasquale Scarlino, Francesco Ciccarello

Abstract: We investigate qubits coupled to the boundary of a two dimensional photonic lattice that supports dispersionless edge modes, unlike conventional edge modes that sustain propagating photons. As a case study, we consider a honeycomb lattice (photonic graphene) of coupled resonators with a zigzag edge, where the edge modes form a flat band defined only over a restricted region of momentum space. We s… ▽ More We investigate qubits coupled to the boundary of a two dimensional photonic lattice that supports dispersionless edge modes, unlike conventional edge modes that sustain propagating photons. As a case study, we consider a honeycomb lattice (photonic graphene) of coupled resonators with a zigzag edge, where the edge modes form a flat band defined only over a restricted region of momentum space. We show that light matter interactions are effectively captured by a dissipative cavity QED model, wherein the emitter coherently couples to a fictitious cavity mode emerging as a superposition of edge modes. This mode has support on only one sublattice and, most notably, displays an unconventional power law localization around the qubit, yet remaining normalizable in the thermodynamic limit, with a spatial range that can be tuned by introducing lattice anisotropy We predict occurrence of vacuum Rabi oscillations and efficient state transfer between distant emitters. An experimental demonstration using superconducting circuits is proposed. △ Less

Submitted 25 May, 2026; v1 submitted 17 July, 2025; originally announced July 2025.

Comments: 29 pages, 8 figures

Enhanced Stability and Linearly Polarized Emission from CsPbI$_3$ Perovskite Nanoplatelets through A-site Cation Engineering

Authors: Woo Hyeon Jeong, Junzhi Ye, Jongbeom Kim, Rui Xu, Xinyu Shen, Chia-Yu Chang, Eilidh L. Quinn, Myoung Hoon Song, Peter Nellist, Henry J. Snaith, Yunwei Zhang, Bo Ram Lee, Robert L. Z. Hoye

Abstract: The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI$_3$. Herein, we address this limitation by alloying FA into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nonco… ▽ More The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI$_3$. Herein, we address this limitation by alloying FA into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nonconfined nanocubes, FA incorporation in nanoplatelets requires meticulous control over the reaction conditions, given that nanoplatelets are obtained in kinetically-driven growth regimes instead of thermodynamically-driven conditions. Through in-situ photoluminescence (PL) measurements, we find that excess FA leads to uncontrolled growth, where phase-impurities and nanoplatelets of multiple thicknesses co-exist. Restricting the FA content to up to 25% Cs substitution enables monodisperse PeNPLs, and increases the PL quantum yield (from 53% to 61%), exciton lifetime (from 18 ns to 27 ns), and stability in ambient air (from ~2 days to >7 days) compared to CsPbI$_3$. This arises due to hydrogen bonding between FA and the oleate and oleylammonium ligands, anchoring them to the surface to improve optoelectronic properties and stability. The reduction in non-radiative recombination, improvement in the nanoplatelet aspect ratio, and higher ligand density lead to FA-containing PeNPLs more effectively forming edge-up superlattices, enhancing the PL degree of linear polarization from 5.1% (CsPbI$_3$) to 9.4% (Cs$_{0.75}$FA$_{0.25}$PbI$_3$). These fundamental insights show how the stability limitations of PeNPLs could be addressed, and these materials grown more precisely to improve their performance as polarized light emitters, critical for utilizing them in next-generation display, bioimaging and communications applications. △ Less

Submitted 28 May, 2025; originally announced May 2025.

Comments: 19 pages, 5 figures

Interlayer Coupling-Induced Quantum Phase Transition in Quantum Anomalous Hall Multilayers

Authors: Ling-Jie Zhou, Deyi Zhuo, Ruobing Mei, Yi-Fan Zhao, Kaijie Yang, Ruoxi Zhang, Zijie Yan, Han Tay, Moses H. W. Chan, Chao-Xing Liu, Cui-Zu Chang

Abstract: A quantum phase transition arises from competition between different ground states and is typically accessed by varying a single physical parameter near absolute zero temperature. The quantum anomalous Hall (QAH) effect with high Chern number C has recently been achieved in magnetic topological insulator (TI) multilayers. In this work, we employ molecular beam epitaxy to synthesize a series of mag… ▽ More A quantum phase transition arises from competition between different ground states and is typically accessed by varying a single physical parameter near absolute zero temperature. The quantum anomalous Hall (QAH) effect with high Chern number C has recently been achieved in magnetic topological insulator (TI) multilayers. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI penta-layers by varying the thickness of the middle magnetic TI layer, designated as m quintuple layers. Electrical transport measurements demonstrate a quantum phase transition between C = 1 and C = 2 QAH states. For m 1 and m 2, the sample exhibits the well-quantized C = 1 and C = 2 QAH states, respectively. For 1 m 2, we observe a monotonic decrease in Hall resistance from h/e2 to h/2e2 with increasing m, accompanied by a peak in the longitudinal resistance. The quantum phase transition between C = 1 and C = 2 QAH states is attributed to the weakening of the interlayer coupling between the top and the bottom C = 1 QAH layers. Our findings provide a scalable strategy for engineering QAH devices with a tunable Chern number. This approach enables precise control and enhanced functionality in chiral edge current-based electronic devices. △ Less

Submitted 25 August, 2025; v1 submitted 30 April, 2025; originally announced May 2025.

Comments: 22 pages and 4 figures. Comments are welcome

Journal ref: Phys. Rev. B 111, L201304 (2025)

Selective Oxidation and Cr Segregation in High-Entropy Oxide Thin Films

Authors: Le Wang, Krishna Prasad Koirala, Shuhang Wu, Jueli Shi, Hsin-Mei Kao, Andrew Ho, Min-Ju Choi, Dongchen Qi, Anton Tadich, Mark E. Bowden, Bethany E. Matthews, Hua Zhou, Yang Yang, Chih-hung Chang, Zihua Zhu, Chongmin Wang, Yingge Du

Abstract: High-entropy oxides (HEOs) offer exceptional compositional flexibility and structural stability, making them promising materials for energy and catalytic applications. Here, we investigate Sr doping effects on B-site cation oxidation states, local composition, and structure in epitaxial La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 thin films. X-ray spectroscopies reveal that Sr doping preferentially prom… ▽ More High-entropy oxides (HEOs) offer exceptional compositional flexibility and structural stability, making them promising materials for energy and catalytic applications. Here, we investigate Sr doping effects on B-site cation oxidation states, local composition, and structure in epitaxial La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 thin films. X-ray spectroscopies reveal that Sr doping preferentially promotes Cr oxidation from Cr3+ to Cr6+, partially oxidizes Co and Ni, while leaving Mn4+ and Fe3+ unchanged. Atomic-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy shows pronounced Cr segregation, with Cr exhibiting depletion at the film-substrate interface and enrichment at the film surface, along with the formation of a partially amorphous phase in heavily Sr-doped samples. This segregation is likely driven by oxidation-induced migration of smaller, high-valence Cr cations during the growth. These findings underscore the critical interplay between charge transfer, local strain, and compositional fluctuations, providing strategies to control surface composition and electronic structure in HEOs for more robust electrocatalyst design. △ Less

Submitted 21 March, 2025; originally announced March 2025.

Complex Magnetic Ordering in Candidate Topological Superconductors

Authors: Purnima P. Balakrishnan, Hemian Yi, Zi-Jie Yan, Wei Yuan, Andreas Suter, Christopher J. Jensen, Pascal Manuel, Fabio Orlandi, Takayasu Hanashima, Christy J. Kinane, Andrew J. Caruana, Brian B. Maranville, Zaher Salman, Thomas Prokscha, Cui-Zu Chang, Alexander J. Grutter

Abstract: The search for chiral topological superconductivity in magnetic topological insulator (TI)-FeTe heterostructures is a key frontier in condensed matter physics, with potential applications in topological quantum computing. The combination of ferromagnetism, superconductivity, and topologically nontrivial surface states brings together the key elements required for chiral Majorana physics. In this w… ▽ More The search for chiral topological superconductivity in magnetic topological insulator (TI)-FeTe heterostructures is a key frontier in condensed matter physics, with potential applications in topological quantum computing. The combination of ferromagnetism, superconductivity, and topologically nontrivial surface states brings together the key elements required for chiral Majorana physics. In this work, we examine the interplay between magnetism and superconductivity at the interfaces between FeTe and a series of TI overlayers. In superconducting MnBi$_2$Te$_4$/FeTe, any interfacial suppression of antiferromagnetism must affect at most a few nanometers. On the other hand, (Bi,Sb)$_2$Te$_3$/FeTe layers exhibit near-total suppression of antiferromagnetic ordering. Ferromagnetic Cr$_x$(Bi,Sb)$_{2-x}$Te$_3$ (CBST)/FeTe bilayers exhibit net magnetization in both CBST and FeTe layers, with evidence of interactions between superconductivity and ferromagnetism. These observations identify magnetic TI/FeTe interfaces as an exceptionally robust platform to realize chiral topological superconductivity. △ Less

Submitted 14 March, 2025; originally announced March 2025.

Comments: Main text: 9 pages, 3 figures, 1 table. Supplementary materials: 18 pages, 17 figures

Gate-Tunable Spin-to-Charge Conversion in Topological Insulator-Magnetic Insulator Heterostructures at Room Temperature

Authors: Wenxuan Sun, Yequan Chen, Ruijie Xu, Wenzhuo Zhuang, Di Wang, Long Liu, Anke Song, Guozhong Xing, Yongbing Xu, Rong Zhang, Cui-Zu Chang, Xuefeng Wang

Abstract: Over the past decade, topological insulators have received enormous attention for their potential in energy-efficient spin-to-charge conversion, enabled by strong spin-orbit coupling and spin-momentum locked surface states. Despite extensive research, the spin-to-charge conversion efficiency, usually characterized by the spin Hall angle (θSH), remains low at room temperature. In this work, we empl… ▽ More Over the past decade, topological insulators have received enormous attention for their potential in energy-efficient spin-to-charge conversion, enabled by strong spin-orbit coupling and spin-momentum locked surface states. Despite extensive research, the spin-to-charge conversion efficiency, usually characterized by the spin Hall angle (θSH), remains low at room temperature. In this work, we employed pulsed laser deposition to synthesize high-quality ternary topological insulators (Bi0.1Sb0.9)2Te3 thin films on magnetic insulator Y3Fe5O12. We find that the value of θSH reaches ~0.76 at room temperature and increases to ~0.9 as the Fermi level is tuned to cross topological surface states via electrical gating. Our findings provide an innovative approach to tailoring the spin-to-charge conversion in topological insulators and pave the way for their applications in energy-efficient spintronic devices. △ Less

Submitted 26 February, 2025; originally announced February 2025.

Comments: 19 pages, 11 figures, 1 table

Journal ref: Adv. Funct. Mater. 35, 2501880 (2025)

A Metal-Insulator Transition of the Buried MnO2 Monolayer in Complex Oxide Heterostructure

Authors: Heng-Jui Liu, Jheng-Cyuan Lin, Yue-Wen Fang, Jing-Ching Wang, Bo-Chao Huang, Xiang Gao, Rong Huang, Philip R. Dean, Peter D. Hatton, Yi-Ying Chin, Hong-Ji Lin, Chien-Te Chen, Yuichi Ikuhara, Ya-Ping Chiu, Chia-Seng Chang, Chun-Gang Duan, Qing He, Ying-Hao Chu

Abstract: Functionalities in crystalline materials are determined by 3-dimensional collective interactions of atoms. The confinement of dimensionality in condensed matter provides an exotic research direction to understand the interaction of atoms, thus can be used to tailor or create new functionalities in material systems. In this study, a 2-dimensional transition metal oxide monolayer is constructed insi… ▽ More Functionalities in crystalline materials are determined by 3-dimensional collective interactions of atoms. The confinement of dimensionality in condensed matter provides an exotic research direction to understand the interaction of atoms, thus can be used to tailor or create new functionalities in material systems. In this study, a 2-dimensional transition metal oxide monolayer is constructed inside complex oxide heterostructures based on the theoretical predictions. The electrostatic boundary conditions of oxide monolayer in the heterostructure is carefully designed to tune the chemical, electronic, and magnetic states of oxide monolayer. The challenge of characterizing such an oxide monolayer is overcome by a combination of transmission electron microscopy, x-ray absorption spectroscopy, cross-sectional scanning tunneling microscopy, and electrical transport measurements. An intriguing metal-insulator transition associated with a magnetic transition is discovered in the MnO2 monolayer. This study paves a new route to understand the confinement of dimensionality and explore new intriguing phenomena in condensed matters. △ Less

Submitted 31 January, 2025; originally announced January 2025.

Comments: 23 pages, 6 figures

Journal ref: Adv. Mater., 28: 9142-9151(2016)