Showing 1–50 of 314 results for author: Chang, J

Extreme Strain Controlled Correlated Metal-Insulator Transition in the Altermagnet CrSb

Authors: Cong Li, Mengli Hu, Jianfeng Zhang, Magnus H. Berntsen, Francesco Scali, Dibya Phuyal, Chun Lin, Wanyu Chen, Johan Chang, Oliver J. Clark, Timur K. Kim, Jacek Osiecki, Craig Polley, Balasubramanian Thiagarajan, Zhilin Li, Tao Xiang, Oscar Tjernberg

Abstract: Correlated flat bands and altermagnetism are two important directions in quantum materials, centred respectively on interaction-dominated phases and symmetry-enforced spin-textured states, yet both derive from lattice symmetry and orbital hybridization. This common origin implies that extreme crystal distortion, by narrowing bandwidths, enhancing correlations and reshaping the symmetries of alterm… ▽ More Correlated flat bands and altermagnetism are two important directions in quantum materials, centred respectively on interaction-dominated phases and symmetry-enforced spin-textured states, yet both derive from lattice symmetry and orbital hybridization. This common origin implies that extreme crystal distortion, by narrowing bandwidths, enhancing correlations and reshaping the symmetries of altermagnetic spin splittings, could unify flat-band and altermagnetic physics in a single material; in practice, however, achieving such large distortions in a crystalline altermagnet is a formidable challenge. Here we combine a dedicated strain device with a tailored single-crystal mounting scheme to impose a highly tensile strain gradient in bulk CrSb, a prototypical altermagnet, creating a near-surface layer in which the in-plane lattice is strongly distorted relative to the weakly strained bulk, while the average bulk distortion remains small. Angle-resolved photoemission reveals a reversible regime at moderate strain, where a deeper flat-band feature, attributed to a strain-gradient-driven suppression of Cr-Sb hybridization, coexists with a correlation-enhanced Cr 3d flat band, and an irreversible regime at larger strain where partial bond decoupling drives a predominantly insulating spectral response. Density-functional calculations show that an orbital-selective altermagnetic spin texture persists across this correlated regime despite strong bandwidth renormalisation. These results define a strain-symmetry-correlation map for CrSb and establish extreme tensile strain as a route to co-engineer flat-band tendencies and spin-textured, zero-net-moment correlated states in altermagnets, pointing toward strain-adaptive, spin-selective Mott filtering and related device concepts. △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: Main text: 21 pages, 4 figures; SI: 43 pages, 30 figures, which are too large to be included. Comments are welcome

Formation of bosonic $^{23}$Na$^{41}$K Feshbach molecules

Authors: Sungjun Lee, Younghoon Lim, Jongyeol Kim, Jaeryeong Chang, Jee Woo Park

Abstract: Ultracold Feshbach molecules are a crucial intermediate step for the creation of quantum degenerate gases of strongly dipolar molecules. After coherent transfer to the rovibrational ground state, these dimers can realize stable dipolar gases with strong, tunable long-range interactions. Here, we report the creation of bosonic $^{23}$Na$^{41}$K Feshbach molecules by radio-frequency (RF) association… ▽ More Ultracold Feshbach molecules are a crucial intermediate step for the creation of quantum degenerate gases of strongly dipolar molecules. After coherent transfer to the rovibrational ground state, these dimers can realize stable dipolar gases with strong, tunable long-range interactions. Here, we report the creation of bosonic $^{23}$Na$^{41}$K Feshbach molecules by radio-frequency (RF) association. An RF pulse applied on the molecular side of an interspecies Feshbach resonance at 73.6(1)~G associates up to $1.1(1)\times10^4$ molecules from a thermal mixture of $^{23}$Na and $^{41}$K atoms. Measurements of the binding energy reveal a broad resonance width of 5.1(2)~G, facilitating robust control over interspecies interactions. The molecule lifetime in the presence of background atoms exceeds 2~ms, extending to 7~ms after removal of $^{23}$Na. These results constitute a key step toward the production of ultracold $^{23}$Na$^{41}$K ground state molecules for the exploration of novel many-body phenomena in strongly dipolar Bose gases. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 8 pages, 6 figures

Impact of Electron Correlations on Infinite-Layer Cuprates and Nickelates

Authors: Xunyang Hong, Yuetong Wu, Ying Chan, Sze Tung Li, I. Biało, L. Martinelli, A. Drewanowski, Qiang Gao, Xiaolin Ren, Xingjiang Zhou, Zhihai Zhu, A. Galdi, D. G. Schlom, K. M. Shen, J. Choi, M. Garcia Fernandez, Ke-Jin Zhou, N. B. Brookes, H. M. Rønnow, Qisi Wang, J. Chang

Abstract: Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their inte… ▽ More Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and magnetic excitations and extract their dispersions along high-symmetry directions. Using a single-band Hubbard model and including higher-order exchange interactions, we derive the correlation factor $U/t$ for both compounds. A key finding is that despite a smaller Coulomb repulsion $U$, PrNiO$_2$ exhibits a correlation strength that is 20% stronger than that of its isostructural cuprate counterpart SrCuO$_2$. This indicates that a moderation of the correlation strength may further optimize superconductivity in nickelates. △ Less

Submitted 12 November, 2025; originally announced November 2025.

Comments: Supplementary information available upon request

Topological Metal-Insulator Transition within the Ferromagnetic state

Authors: Ola Kenji Forslund, Chin Shen Ong, Moritz M. Hirschmann, Nicolas Gauthier, Hiroshi Uchiyama, Christian Tzschaschel, Daniel G. Mazzone, Romain Sibille, Antonio M. dos Santos, Masafumi Horio, Elisabetta Nocerino, Nami Matsubara, Deepak John Mukkattukavil, Konstantinos Papadopoulos, Kazuya Kamazawa, Kazuhiko Ikeuchi, Hidenori Takagi, Masahiko Isobe, Jun Sugiyama, Johan Chang, Yasmine Sassa, Olle Eriksson, Martin Månsson

Abstract: A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 u… ▽ More A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact. △ Less

Submitted 10 November, 2025; originally announced November 2025.

Quantum Tomography of Suspended Carbon Nanotubes

Authors: Jialiang Chang, Nicholas Pietrzak, Cristian Staii

Abstract: We present an all-mechanical protocol for coherent control and full quantum-state reconstruction of the fundamental flexural mode of a suspended carbon nanotube (CNT). Calibrated impulses from a nearby atomic force microscope (AFM) tip serve a dual role: they implement mechanical pi/2 rotations for Ramsey interferometry and realize phase-space displacements for Wigner function tomography via displ… ▽ More We present an all-mechanical protocol for coherent control and full quantum-state reconstruction of the fundamental flexural mode of a suspended carbon nanotube (CNT). Calibrated impulses from a nearby atomic force microscope (AFM) tip serve a dual role: they implement mechanical pi/2 rotations for Ramsey interferometry and realize phase-space displacements for Wigner function tomography via displaced-parity sampling. The same actuator thus unifies control and tomography while avoiding optical heating and eliminating on-chip microwave drive lines at the resonator. We derive explicit control pulse sequences and a master-equation description that map measured signals onto the energy-relaxation and phase-coherence times, as well as onto parity-based quantum signatures, including negative regions of the Wigner function. The approach is compatible with several readout modalities: direct AFM deflection, dispersive coupling to a Cooper-pair box, and dispersive microwave cavity probing. Together, these techniques provide complete access to populations, coherence, and parity within a single device architecture. This minimal scheme provides a practical route to all-mechanical quantum control and state-resolved characterization of decoherence in mesoscopic mechanical systems. △ Less

Submitted 22 September, 2025; v1 submitted 1 September, 2025; originally announced September 2025.

Comments: 11 pages, 3 figures

Magnetic Excitations of a Half-Filled Tl-based Cuprate

Authors: I. Biało, Q. Wang, J. Küspert, X. Hong, L. Martinelli, O. Gerguri, Y. Chan, K. von Arx, O. K. Forslund, W. R. Pudełko, C. Lin, N. C. Plumb, Y. Sassa, D. Betto, N. B. Brookes, M. Rosmus, N. Olszowska, M. D. Watson, T. K. Kim, C. Cacho, M. Horio, M. Ishikado, H. M. Rønnow, J. Chang

Abstract: Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of those interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compo… ▽ More Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of those interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl$_2$Ba$_5$Cu$_4$O$_{10+x}$ (Tl2504). Using high-resolution resonant inelastic x-ray scattering (RIXS), we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Comments: 10 pages, 5 figures, 2 tables

Differentiation of Site-Specific Symmetry Breaking Orders in Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{6+y}$

Authors: L. Martinelli, S. Rüdiger, I. Bialo, J. Oppliger, F. Igoa Saldana, M. v. Zimmermann, E. Weschke, R. Arpaia, J. Chang

Abstract: Solid matter is classified through symmetry of ordering phenomena. Experimentally, this approach is straightforward, except when distinct orderings occur with identical or almost identical symmetry breaking. Here we show that the cuprate system Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{6+y}$ hosts two distinct orderings with almost identical translational symmetry breaking. Only when applying site-sensitive… ▽ More Solid matter is classified through symmetry of ordering phenomena. Experimentally, this approach is straightforward, except when distinct orderings occur with identical or almost identical symmetry breaking. Here we show that the cuprate system Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{6+y}$ hosts two distinct orderings with almost identical translational symmetry breaking. Only when applying site-sensitive resonant elastic x-ray scattering (REXS), charge ordering can be conclusively differentiated from a super-lattice structure. These two orderings occur with almost the same in-plane symmetry but manifest at different atomic sites and display different temperature dependence. Differentiating these orders provides an important clue to the anomalous behavior of PrBa$_2$Cu$_3$O$_7$ within the 123-series of high-temperature superconductors. We conclude that the symmetry breaking at the Pr-site is unfavorable for superconducting pairing. △ Less

Submitted 28 July, 2025; originally announced July 2025.

Comments: Supplemental Material available upon request

Persistent paramagnons in high-temperature infinite-layer nickelate superconductors

Authors: Yujie Yan, Ying Chan, Xunyang Hong, S. Lin Er Chow, Zhaoyang Luo, Yuehong Li, Tianren Wang, Yuetong Wu, Izabela Biało, Nurul Fitriyah, Saurav Prakash, Xing Gao, King Yau Yip, Qiang Gao, Xiaolin Ren, Jaewon Choi, Ganesha Channagowdra, Jun Okamoto, Xingjiang Zhou, Zhihai Zhu, Liang Si, Mirian Garcia-Fernandez, Ke-Jin Zhou, Hsiao-Yu Huang, Di-Jing Huang , et al. (3 additional authors not shown)

Abstract: The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind… ▽ More The recent discovery of high-temperature superconductivity in hole-doped SmNiO$_2$, exhibiting the record-high transition temperature $T_c$ among infinite-layer (IL) nickelates, has opened a new avenue for exploring design principles of superconductivity. Experimentally determining the electronic structure and magnetic interactions in this new system is crucial to elucidating the mechanism behind the enhanced superconductivity. Here, we report a Ni $L$-edge resonant inelastic x-ray scattering (RIXS) study of superconducting Sm-based IL nickelate thin films Sm$_{1-x-y-z}$Eu$_x$Ca$_y$Sr$_z$NiO$_2$ (SECS). Dispersive paramagnonic excitations are observed in both optimally and overdoped SECS samples, supporting a spin-fluctuation-mediated pairing scenario. However, despite the two-fold enhancement of $T_c$ in the Sm-based nickelates compared to their Pr-based counterparts, the effective exchange coupling strength is reduced by approximately $20\%$. This behavior contrasts with hole-doped cuprates, where magnetic interactions correlate positively with $T_c$, highlighting essential differences in their superconducting mechanisms. △ Less

Submitted 24 July, 2025; originally announced July 2025.

Comments: Supplementary Information available upon request

Distinct Uniaxial Stress and Pressure Fingerprint of Superconductivity in the 3D Kagome Lattice Compound CeRu2

Authors: O. Gerguri, D. Das, V. Sazgari, H. X. Liu, C. Mielke III, P. Kràl, S. S. Islam, J. N. Graham, V. Grinenko, R. Sarkar, T. Shiroka, J. -X. Yin, J. Chang, R. Thomale, H. H. Klauss, R. Khasanov, Y. Shi, H. Luetkens, Z. Guguchia

Abstract: The exploration of tunable superconductivity in strongly correlated electron systems is a central pursuit in condensed matter physics, with implications for both fundamental understanding and potential applications. The Laves phase CeRu$_{2}$, a pyrochlore compound, exhibits a three-dimensional (3D) Kagome lattice type geometry giving rise to flat bands and degenerate Dirac points, where band stru… ▽ More The exploration of tunable superconductivity in strongly correlated electron systems is a central pursuit in condensed matter physics, with implications for both fundamental understanding and potential applications. The Laves phase CeRu$_{2}$, a pyrochlore compound, exhibits a three-dimensional (3D) Kagome lattice type geometry giving rise to flat bands and degenerate Dirac points, where band structure features intertwine with strong multi-orbital interaction effects deriving from its correlated electronic structure. Here, we combine muon spin rotation ($μ$SR), uniaxial in-plane stress, and hydrostatic pressure to probe the superconducting state of CeRu$_{2}$. Uniaxial stress up to 0.22 GPa induces a dome-shaped evolution of the critical temperature $T_{\rm c}$, with an initial plateau, successively followed by enhancement and suppression without any structural phase transition. Stress is further found to drive a crossover from anisotropic to isotropic $s$-wave pairing. In contrast, hydrostatic pressure up to 2.2 GPa leaves $T_{\rm c}$ largely unchanged but alters the superfluid density from exponential to linear behavior at low temperatures, indicative of nodal superconductivity under hydrostatic pressure. Taken together, these results indicate that CeRu$_{2}$ occupies an ideal position in parameter space, enabling highly responsive and multifold tunability of superconductivity in this three-dimensional correlated electronic system. This warrants further quantitative analysis of the interplay between lattice geometry, electronic correlations, and pairing symmetry. △ Less

Submitted 13 July, 2025; originally announced July 2025.

Comments: 9 pages, 4 Figures

Discovery of High-Temperature Charge Order and Time-Reversal Symmetry-Breaking in the Kagome Superconductor YRu3Si2

Authors: P. Kràl, J. N. Graham, V. Sazgari, I. Plokhikh, A. Lukovkina, O. Gerguri, I. Bialo, A. Doll, L. Martinelli, J. Oppliger, S. S. Islam, K. Wang, M. Salamin, H. Luetkens, R. Khasanov, M. v. Zimmermann, J. -X. Yin, Ziqiang Wang, J. Chang, B. Monserrat, D. Gawryluk, F. O. von Rohr, S. -W. Kim, Z. Guguchia

Abstract: The identification of high-temperature unconventional charge order and superconductivity in kagome quantum materials is pivotal for deepening our understanding of geometrically frustrated and correlated electron systems, and for harnessing their exotic properties in future quantum technologies. Here, we report the discovery of a remarkably rich phase diagram in the kagome superconductor YRu$_{3}$S… ▽ More The identification of high-temperature unconventional charge order and superconductivity in kagome quantum materials is pivotal for deepening our understanding of geometrically frustrated and correlated electron systems, and for harnessing their exotic properties in future quantum technologies. Here, we report the discovery of a remarkably rich phase diagram in the kagome superconductor YRu$_{3}$Si$_{2}$, uncovered through a unique combination of muon spin rotation ($μ$SR), magnetotransport, X-ray diffraction (XRD), and density functional theory (DFT) calculations. Our study reveals the emergence of a charge-ordered state with a propagation vector of (1/2, 0, 0), setting a record onset temperature of 800 K for such an order in a kagome system and for quantum materials more broadly. In addition, we observe time-reversal symmetry (TRS) breaking below $T_{2}^{*}$ ${\simeq}$ 25 K and field-induced magnetism below $T_{1}^{*}$ ${\simeq}$ 90 K, indicating the presence of a hidden magnetic state. These transitions are mirrored in the magnetoresistance data, which show a clear onset at ${\sim}$ $T_{1}^{*}$ and a pronounced increase below ${\sim}$ $T_{2}^{*}$, ultimately reaching a maximum magnetoresistance of 45${\%}$. Band structure calculations identify two van Hove singularities (VHSs) near the Fermi level, one of which resides within a flat band, suggesting a strong interplay between electronic correlations and emergent orders. At low temperatures, we find bulk superconductivity below $T_{\rm c}$ = 3.4 K, characterized by a pairing symmetry with either two isotropic full gaps or an anisotropic nodeless gap. Together, our findings point to a coexistence of high-temperature charge order, tunable magnetism, and multigap superconductivity in YRu$_{3}$Si$_{2}$, positioning it as a compelling platform for exploring correlated kagome physics. △ Less

Submitted 9 July, 2025; originally announced July 2025.

Comments: 13 pages, 7 Figures

From Pixels to Camera: Scaling Superconducting Nanowire Single-Photon Detectors for Imaging at the Quantum-Limit

Authors: Jun Gao, Jin Chang, Bruno Lopez Rodriguez, Iman Esmaeil Zadeh, Val Zwiller, Ali W. Elshaari

Abstract: Superconducting nanowire single-photon detectors (SNSPDs) have emerged as essential devices that push the boundaries of photon detection with unprecedented sensitivity, ultrahigh timing precision, and broad spectral response. Recent advancements in materials engineering, superconducting electronics integration, and cryogenic system design are enabling the evolution of SNSPDs from single-pixel dete… ▽ More Superconducting nanowire single-photon detectors (SNSPDs) have emerged as essential devices that push the boundaries of photon detection with unprecedented sensitivity, ultrahigh timing precision, and broad spectral response. Recent advancements in materials engineering, superconducting electronics integration, and cryogenic system design are enabling the evolution of SNSPDs from single-pixel detectors toward scalable arrays and large-format single-photon time tagging cameras. This perspective article surveys the rapidly evolving technological landscape underpinning this transition, focusing on innovative superconducting materials, advanced multiplexed read-out schemes, and emerging cryo-compatible electronics. We highlight how these developments are set to profoundly impact diverse applications, including quantum communication networks, deep-tissue biomedical imaging, single-molecule spectroscopy, remote sensing with unprecedented resolution, and the detection of elusive dark matter signals. By critically discussing both current challenges and promising solutions, we aim to articulate a clear, coherent vision for the next generation of SNSPD-based quantum imaging systems. △ Less

Submitted 30 May, 2025; originally announced May 2025.

Unraveling the storage mechanism of Na$_{3}$V$_{2-x}$Ni$_x$(PO$_4$)$_2$F$_3$/C cathodes for sodium-ion batteries through electrochemical, {\it operando} X-ray diffraction and microscopy studies

Authors: Simranjot K. Sapra, Jeng-Kuei Chang, Rajendra S. Dhaka

Abstract: The storage mechanism and diffusion kinetics of Na$_{3}$V$_{2-x}$Ni$_{x}$(PO$_{4}$)$_{2}$F$_{3}$/C ($x=$ 0--0.07) cathodes are investigated through electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV). All the samples are prepared through the facile pH-assisted sol-gel route and crystallize in the P4$_{2}$/mnm symmetry. The… ▽ More The storage mechanism and diffusion kinetics of Na$_{3}$V$_{2-x}$Ni$_{x}$(PO$_{4}$)$_{2}$F$_{3}$/C ($x=$ 0--0.07) cathodes are investigated through electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV). All the samples are prepared through the facile pH-assisted sol-gel route and crystallize in the P4$_{2}$/mnm symmetry. The optimal doping of Ni ($x=$ 0.05) exhibits superior specific capacities of 119 and 100 mAh g$^{-1}$ at 0.1 C and 10 C rates, respectively, along the excellent capacity retention of 78\% after 2000 cycles at 10 C rate with nearly 100\% Coulombic efficiency. The apparent diffusion coefficient values are found to be in the range of 10$^{-9}$--10$^{-10}$ cm$^{2}$ s$^{-1}$ through detailed analysis of CV and GITT. Moreover, we report the reversible structural evolution and morphological changes during charging and discharging under non-equilibrium conditions through the {\it operando} X-ray diffraction and the {\it in-situ} synchrotron based transmission X-ray microscopy, respectively. Further, to understand the stability mechanism and obtain precise polarization values, we performed the distribution of relaxation times (DRT) analysis using the EIS data. The structure and morphology are found to be stable after long cycling. △ Less

Submitted 4 May, 2025; originally announced May 2025.

Comments: submitted

Journal ref: Energy Storage Materials, 81, 104478 (2025)

Energy Dispersion, Superconductivity and Magnetic Fluctuations in Stacked Altermagnetism Materials

Authors: Jun Chang, Hantao Lu, Jize Zhao, Hong-Gang Luo, Yang Ding

Abstract: Recently, altermagnetism (AM) has emerged as a new category of magnetism, alongside conventional antiferromagnetism (AFM) and ferromagnetism (FM). In an AM, superconductivity (SC) is faced with a dilemma that the spin-polarized bands, induced by the broken time reversal (T ) symmetry, dominantly supports spin-triplet pairing. In contrast, AM spin fluctuations routinely facilitate spin-singlet pair… ▽ More Recently, altermagnetism (AM) has emerged as a new category of magnetism, alongside conventional antiferromagnetism (AFM) and ferromagnetism (FM). In an AM, superconductivity (SC) is faced with a dilemma that the spin-polarized bands, induced by the broken time reversal (T ) symmetry, dominantly supports spin-triplet pairing. In contrast, AM spin fluctuations routinely facilitate spin-singlet pairing as in AFM. Consequently, unconventional SC is either absent or weak in AM materials. Here, we propose that stacking 2D AM materials could resolve this dilemma. Stacked 2D materials have yielded a variety of new electronic properties by altering the symmetries inherent in the monolayer. In a 2D anisotropic Hubbard model, we investigate the general energy dispersions of both single-layer and stacked AM materials. We demonstrate that AM sheet stacking can alter the original symmetries, consequently affecting the energy dispersion. The interlayer magnetic coupling enhances the low q magnetic fluctuations. T symmetry is restored in the AA stacking with an antiferromagnetic interlayer coupling, and then both the energy dispersion and pairing interaction are in favor of spin-singlet SC. The ferromagnetic interlayer coupling in the AB stacking not only recovers T symmetry but also supports spin-triplet pairing. It is further anticipated that twisted bilayer AM sheets could exhibit additional novel electronic properties, including topology, flat bands, and collective excitations. Our work illustrates that stacking sheets of AM materials could open up a unique research domain in exploring novel quantum phenomena and offer a fertile ground for potential electronic applications. △ Less

Submitted 1 April, 2025; v1 submitted 16 March, 2025; originally announced March 2025.

Comments: 8 pages,3 figures

Journal ref: PhysRevB.111.104432(2025)

Concurrent Multifractality and Anomalous Hall Response in the Nodal Line Semimetal Fe$_3$GeTe$_2$ Near Localization

Authors: Subramanian Mathimalar, Ambikesh Gupta, Yotam Roet, Stanislaw Galeski, Rafal Wawrzynczak, Mikel Garcia-Diez, Iñigo Robredo, Praveen Vir, Nitesh Kumar, Walter Schnelle, Karin von Arx, Julia Küspert, Qisi Wang, Johan Chang, Yasmine Sassa, Ady Stern, Felix von Oppen, Maia G. Vergniory, Claudia Felser, Johannes Gooth, Nurit Avraham, Haim Beidenkopf

Abstract: Topological states of matter exhibit unique protection against scattering by disorder. Different topological classes exhibit distinct forms and degrees of protection. Here, we investigate the response of the ferromagnetic nodal line semimetal Fe$_3$GeTe$_2$ to disorder and electronic interactions. By combining global magneto-transport with atomic-scale scanning tunneling spectroscopy we find a sim… ▽ More Topological states of matter exhibit unique protection against scattering by disorder. Different topological classes exhibit distinct forms and degrees of protection. Here, we investigate the response of the ferromagnetic nodal line semimetal Fe$_3$GeTe$_2$ to disorder and electronic interactions. By combining global magneto-transport with atomic-scale scanning tunneling spectroscopy we find a simultaneous onset of diverse phenomena below a common temperature scale of about 15 K: A crossover from metallic to insulating temperature dependence of the longitudinal resistivity, saturation of the anomalous Hall conductivity to its maximal value, formation of a sharp zero-bias dip in the tunneling density of state, and emergence of multi-fractal structure of the electronic wavefunction peaking at the Fermi energy. These concurrent observations reflect the emergence of a novel energy scale possibly related to the opening of a gap in the nodal line band of Fe$_3$GeTe$_2$. Our study provides overarching insight into the role of disorder, electronic interactions and Berry curvature in setting the micro- and macro-scale responses of topological semimetals. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Observation of collective charge excitations in a cuprate superconductor

Authors: Xunyang Hong, Yujie Yan, L. Martinelli, I. Biało, K. von Arx, J. Choi, Y. Sassa, S. Pyon, T. Takayama, H. Takagi, Zhenglu Li, M. Garcia-Fernandez, Ke-Jin Zhou, J. Chang, Qisi Wang

Abstract: Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolut… ▽ More Emergent symmetry breakings in condensed matter systems are often intimately linked to collective excitations. For example, the intertwined spin-charge stripe order in cuprate superconductors is associated with spin and charge excitations. While the collective behavior of spin excitations is well established, the nature of charge excitations remains to be understood. Here we present a high-resolution resonant inelastic x-ray scattering (RIXS) study of charge excitations in the stripe-ordered cuprate La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_4$. The RIXS spectra consist of both charge and phonon excitations around the charge ordering wave vector. By modeling the momentum-dependent phonon intensity, the charge-excitation spectral weight is extracted for a wide range of energy. As such, we reveal the highly dispersive nature of the charge excitations, with an energy scale comparable to the spin excitations. Since charge order and superconductivity in cuprates are possibly driven by the same electronic correlations, determining the interaction strength underlying charge order is essential to establishing a comprehensive microscopic model of high-temperature superconductivity. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Comments: Supplementary Information available upon request

Anatomy of anomalous Hall effect due to magnetic fluctuations

Authors: Ola Kenji Forslund, Xiaoxiong Liu, Soohyeon Shin, Chun Lin, Masafumi Horio, Qisi Wang, Kevin Kramer, Saumya Mukherjee, Timur Kim, Cephise Cacho, Chennan Wang, Tian Shang, Victor Ukleev, Jonathan S. White, Pascal Puphal, Yasmine Sassa, Ekaterina Pomjakushina, Titus Neupert, Johan Chang

Abstract: The anomalous Hall {\color{black} e}ffect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic… ▽ More The anomalous Hall {\color{black} e}ffect (AHE) has emerged as a key indicator of time-reversal symmetry breaking (TRSB) and topological features in electronic band structures. Absent of a magnetic field, the AHE requires spontaneous TRSB but has proven hard to probe due to averaging over domains. The anomalous component of the Hall effect is thus frequently derived from extrapolating the magnetic field dependence of the Hall response. We show that discerning whether the AHE is an intrinsic property of the field free system becomes intricate in the presence of strong magnetic fluctuations. {\color{black}As a study case,} we use the Weyl semimetal PrAlGe, where TRSB can be toggled via a ferromagnetic transition, providing a transparent view of the AHE's topological origin. Through a combination of thermodynamic, transport and muon spin relaxation measurements, we contrast the behaviour below the ferromagnetic transition temperature to that of strong magnetic fluctuations above. Our results {\color{black}on PrAlGe provide general insights into the} interpretation of anomalous Hall signals in systems where TRSB is debated, such as families of Kagome metals or certain transition metal dichalcogenides. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 8 pages, 5 figures

Dynamic Competition between Cooper-Pair and Spin-Density-Wave Condensation

Authors: B. Decrausaz, M. Pikulski, O. Ivashko, N. B. Christensen, J. Choi, L. Udby, Ch. Niedermayer, K. Lefmann, H. M. Rønnow, J. Mesot, J. Ollivier, T. Kurosawa, N. Momono, M. Oda, J. Chang, D. G. Mazzone

Abstract: Quantum matter phases may co-exist microscopically even when they display competing tendencies. A fundamental question is whether such a competition can be avoided through the elimination of one phase while the other one condenses into the ground state. Here, we present a high-resolution neutron spectroscopy study of the low-energy spin excitations in the high-temperature superconductor La1.855Sr0… ▽ More Quantum matter phases may co-exist microscopically even when they display competing tendencies. A fundamental question is whether such a competition can be avoided through the elimination of one phase while the other one condenses into the ground state. Here, we present a high-resolution neutron spectroscopy study of the low-energy spin excitations in the high-temperature superconductor La1.855Sr0.145CuO4. In the normal state, we find low-energy magnetic fluctuations at incommensurate reciprocal lattice positions where spin-density-wave order emerges at lower Sr concentration or at high magnetic fields. While these spin excitations are largely suppressed by the emergence of the superconducting spin gap, some low-energy magnetic fluctuations persist deep inside the superconducting state. We interpret this result in terms of a dynamic competition between superconductivity and magnetism, where superconductivity impedes the condensation of low-energy magnetic fluctuations through the formation of magnetically-mediated Cooper pairs. △ Less

Submitted 25 September, 2025; v1 submitted 29 January, 2025; originally announced January 2025.

Predictive Modeling and Uncertainty Quantification of Fatigue Life in Metal Alloys using Machine Learning

Authors: Jiang Chang, Deekshith Basvoju, Aleksandar Vakanski, Indrajit Charit, Min Xian

Abstract: Recent advancements in machine learning-based methods have demonstrated great potential for improved property prediction in material science. However, reliable estimation of the confidence intervals for the predicted values remains a challenge, due to the inherent complexities in material modeling. This study introduces a novel approach for uncertainty quantification in fatigue life prediction of… ▽ More Recent advancements in machine learning-based methods have demonstrated great potential for improved property prediction in material science. However, reliable estimation of the confidence intervals for the predicted values remains a challenge, due to the inherent complexities in material modeling. This study introduces a novel approach for uncertainty quantification in fatigue life prediction of metal materials based on integrating knowledge from physics-based fatigue life models and machine learning models. The proposed approach employs physics-based input features estimated using the Basquin fatigue model to augment the experimentally collected data of fatigue life. Furthermore, a physics-informed loss function that enforces boundary constraints for the estimated fatigue life of considered materials is introduced for the neural network models. Experimental validation on datasets comprising collected data from fatigue life tests for Titanium alloys and Carbon steel alloys demonstrates the effectiveness of the proposed approach. The synergy between physics-based models and data-driven models enhances the consistency in predicted values and improves uncertainty interval estimates. △ Less

Submitted 24 January, 2025; originally announced January 2025.

MSC Class: I.2.8

Emergence of Giant Magnetic Chirality during Dimensionality Crossover of Magnetic Materials

Authors: Dae-Yun Kim, Yun-Seok Nam, Younghak Kim, Kyoung-Whan Kim, Gyungchoon Go, Seong-Hyub Lee, Joon Moon, Jun-Young Chang, Ah-Yeon Lee, Seung-Young Park, Byoung-Chul Min, Kyung-Jin Lee, Hyunsoo Yang, Duck-Ho Kim, Sug-Bong Choe

Abstract: Chirality, an intrinsic preference for a specific handedness, is a fundamental characteristic observed in nature. In magnetism, magnetic chirality arises from the anti-symmetric Dzyaloshinskii-Moriya interaction in competition with the symmetric Heisenberg exchange interaction. Traditionally, the anti-symmetric interaction has been considered minor relative to the symmetric interaction. In this st… ▽ More Chirality, an intrinsic preference for a specific handedness, is a fundamental characteristic observed in nature. In magnetism, magnetic chirality arises from the anti-symmetric Dzyaloshinskii-Moriya interaction in competition with the symmetric Heisenberg exchange interaction. Traditionally, the anti-symmetric interaction has been considered minor relative to the symmetric interaction. In this study, we demonstrate an observation of giant magnetic chirality during the dimensionality crossover of magnetic materials from three-dimensional to two-dimensional. The ratio between the anti-symmetric and symmetric interactions exhibits a reversal in their dominance over this crossover, overturning the traditional consideration. This observation is validated theoretically using a non-local interaction model and tight-binding calculation with distinct pairing schemes for each exchange interaction throughout the crossover. Additional experiments investigating the asphericity of orbital moments corroborate the robustness of our findings. Our findings highlight the critical role of dimensionality in shaping magnetic chirality and offer strategies for engineering chiral magnet states with unprecedented strength, desired for the design of spintronic materials. △ Less

Submitted 6 January, 2025; originally announced January 2025.

Comments: 21 pages, 4 figures

Epitaxial Sr-doped nickelate perovskite thin films and Ruddlesden-Popper phases grown by magnetron sputtering

Authors: Changhwan Kim, Min Young Jung, Yeong Gwang Khim, Kyeong Jun Lee, Young Jun Chang, Seo Hyoung Chang

Abstract: Sr-doped nickelate, Nd1-xSrxNiO3 (NSNO), perovskite thin films and Ruddlesden-Popper (RP) phases are actively investigated because of their physical properties, such as the metal-insulator transition and superconductivity. However, achieving epitaxial growth of NSNO perovskite and RP phase films in a sputtering system is challenging compared to pulsed laser deposition and molecular beam epitaxy, d… ▽ More Sr-doped nickelate, Nd1-xSrxNiO3 (NSNO), perovskite thin films and Ruddlesden-Popper (RP) phases are actively investigated because of their physical properties, such as the metal-insulator transition and superconductivity. However, achieving epitaxial growth of NSNO perovskite and RP phase films in a sputtering system is challenging compared to pulsed laser deposition and molecular beam epitaxy, due to the difficulty in stabilizing nickel oxidation states and minimizing structural defects. Here, we used an off-axis radio frequency (RF) magnetron sputtering to fabricate epitaxial NSNO perovskite and RP phase thin films on SrTiO3 (001) substrates, systematically controlling the growth temperatures. We investigated the thermal stability of the perovskite phase and the structural and electronic characteristics of the RP phase films. These findings provide valuable insights into the synthesis of nickelate RP phase films using RF magnetron sputtering, paving the way for scalable thin films fabrication technologies. △ Less

Submitted 30 December, 2024; originally announced December 2024.

Comments: 13 pages, 4 figures

Strange metal from spin fluctuations in a cuprate superconductor

Authors: D. J. Campbell, M. Frachet, V. Oliviero, T. Kurosawa, N. Momono, M. Oda, J. Chang, D. Vignolles, C. Proust, D. LeBoeuf

Abstract: Strange metals challenge our understanding of charge transport in metals. Here, we investigate how the strange metal phase of La$_{2-x}$Sr$_x$CuO$_4$ is impacted by a field-induced glassy antiferromagnetic state. Using magnetic fields above 80 T, we discover a strong enhancement of the normal state magnetoresistance when entering the spin glass phase. We demonstrate that the spin glass causes insu… ▽ More Strange metals challenge our understanding of charge transport in metals. Here, we investigate how the strange metal phase of La$_{2-x}$Sr$_x$CuO$_4$ is impacted by a field-induced glassy antiferromagnetic state. Using magnetic fields above 80 T, we discover a strong enhancement of the normal state magnetoresistance when entering the spin glass phase. We demonstrate that the spin glass causes insulating-like upturns in the resistivity inside the pseudogap phase, which resolves the origin of the ''metal-insulator'' crossover. In addition the strange metal phase appears at low temperatures over an extended range of magnetic fields where magnetic moments fluctuate, and disappears when these moments freeze out. We conclude that the transport properties of the strange metal phase are controlled by magnetic fluctuations persisting at the lowest temperatures. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Journal ref: Nat. Phys. (2025)

Pressure induced transition from chiral charge order to time-reversal symmetry-breaking superconducting state in Nb-doped CsV$_3$Sb$_5$

Authors: J. N. Graham, S. S. Islam, V. Sazgari, Y. Li, H. Deng, G. Janka, Y. Zhong, O. Gerguri, P. Kral, A. Doll, I. Bialo, J. Chang, Z. Salman, A. Suter, T. Prokscha, Y. Yao, K. Okazaki, H. Luetkens, R. Khasanov, Z. Wang, J. -X. Yin, Z. Guguchia

Abstract: The experimental realisation of unconventional superconductivity and charge order in kagome systems \textit{A}V$_3$Sb$_5$ is of critical importance. We conducted a highly systematic study of Cs(V$_{1-x}$Nb$_x$)$_3$Sb$_5$ with $x$=0.07 (Nb$_{0.07}$-CVS) by employing a unique combination of tuning parameters such as doping, hydrostatic pressure, magnetic fields, and depth, using muon spin rotation,… ▽ More The experimental realisation of unconventional superconductivity and charge order in kagome systems \textit{A}V$_3$Sb$_5$ is of critical importance. We conducted a highly systematic study of Cs(V$_{1-x}$Nb$_x$)$_3$Sb$_5$ with $x$=0.07 (Nb$_{0.07}$-CVS) by employing a unique combination of tuning parameters such as doping, hydrostatic pressure, magnetic fields, and depth, using muon spin rotation, AC susceptibility, and STM. We uncovered tunable magnetism in the normal state of Nb$_{0.07}$-CVS, which transitions to a time-reversal symmetry (TRS) breaking superconducting state under pressure. Specifically, our findings reveal that the bulk of Nb$_{0.07}$-CVS (at depths greater than 20 nm from the surface) experiences TRS breaking below $T^*=40~$K, lower than the charge order onset temperature, $T_\mathrm{CO}$ = 58 K. However, near the surface (within 20 nm from the surface), the TRS breaking signal doubles and onsets at $T_\mathrm{CO}$, indicating that Nb-doping decouples TRS breaking from charge order in the bulk but synchronises them near the surface. Additionally, Nb-doping raises the superconducting critical temperature $T_\mathrm{C}$ from 2.5 K to 4.4 K. Applying hydrostatic pressure enhances both $T_\mathrm{C}$ and the superfluid density by a factor of two, with a critical pressure $p_\mathrm{cr}$ ${\simeq}$ 0.85 GPa, suggesting competition with charge order. Notably, above $p_\mathrm{cr}$, we observe nodeless electron pairing and weak internal fields below $T_\mathrm{C}$, indicating broken TRS in the superconducting state. Overall, these results demonstrate a highly unconventional normal state with a depth-tunable onset of TRS breaking at ambient pressure, a transition to TRS-breaking superconductivity under low hydrostatic pressure, and an unconventional scaling between $T_\mathrm{C}$ and the superfluid density. △ Less

Submitted 27 November, 2024; originally announced November 2024.

Probing enhanced superconductivity in van der Waals polytypes of V$_x$TaS$_2$

Authors: Wojciech R. Pudelko, Huanlong Liu, Francesco Petocchi, Hang Li, Eduardo Bonini Guedes, Julia Küspert, Karin von Arx, Qisi Wang, Ron Cohn Wagner, Craig M. Polley, Mats Leandersson, Jacek Osiecki, Balasubramanian Thiagarajan, Milan Radović, Philipp Werner, Andreas Schilling, Johan Chang, Nicholas C. Plumb

Abstract: Layered transition metal dichalcogenides (TMDs) stabilize in multiple structural forms with profoundly distinct and exotic electronic phases. Interfacing different layer types is a promising route to manipulate TMDs' properties, not only as a means to engineer quantum devices, but also as a route to explore fundamental physics in complex matter. Here we use angle-resolved photoemission (ARPES) to… ▽ More Layered transition metal dichalcogenides (TMDs) stabilize in multiple structural forms with profoundly distinct and exotic electronic phases. Interfacing different layer types is a promising route to manipulate TMDs' properties, not only as a means to engineer quantum devices, but also as a route to explore fundamental physics in complex matter. Here we use angle-resolved photoemission (ARPES) to investigate a strong layering-dependent enhancement of superconductivity in TaS$_2$, in which the superconducting transition temperature, $T_c$, of its $2H$ structural phase is nearly tripled when insulating $1T$ layers are inserted into the system. The study is facilitated by a novel vanadium-intercalation approach to synthesizing various TaS$_2$ polytypes, which improves the quality of ARPES data while leaving key aspects of the electronic structure and properties intact. The spectra show the clear opening of the charge density wave gap in the pure $2H$ phase and its suppression when $1T$ layers are introduced to the system. Moreover, in the mixed-layer $4Hb$ system, we observe a strongly momentum-anisotropic increase in electron-phonon coupling near the Fermi level relative to the $2H$ phase. Both phenomena help to account for the increased $T_c$ in mixed $2H$/$1T$ layer structures. △ Less

Submitted 7 November, 2024; v1 submitted 20 September, 2024; originally announced September 2024.

Flexible Trilayer Cellulosic Paper Separators engineered with BaTiO$_3$ ferroelectric fillers for High Energy Density Sodium-ion Batteries

Authors: Simranjot K. Sapra, Mononita Das, M. Wasim Raja, Jeng-Kuei Chang, Rajendra S. Dhaka

Abstract: We design a full cell configuration having Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$ as cathode and pre-sodiated hard carbon as an anode with Cellulosic Paper Separators and compare the electrochemical performance of these ceramic-impregnated polymer-coated cellulose paper separators with commercial glass fiber separator. Notably, the paper-based multilayer separators provide desirable characteristics such… ▽ More We design a full cell configuration having Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$ as cathode and pre-sodiated hard carbon as an anode with Cellulosic Paper Separators and compare the electrochemical performance of these ceramic-impregnated polymer-coated cellulose paper separators with commercial glass fiber separator. Notably, the paper-based multilayer separators provide desirable characteristics such as excellent electrolyte wettability, thermal stability up to 200\degree C, and ionic conductivity, which are essential for the efficient operation of SIBs. The cellulose separator is coated by a layer of polyvinylidene fluoride polymer, followed by a second layer of styrene butadiene rubber (SBR) polymer in which ferroelectric fillers BaTiO$_{3}$ are integrated, which interacts with the polymer hosts through Lewis acid-base interactions ion and improves the conduction mechanism for the Na$^{+}$ ions. The final lamination is performed by varying the SBR concentrations (0.5, 0.75, and 1.0 w/v\%). The incorporated polymer matrices improve the flexibility, adhesion and dispersion of the nanoparticles and affinity of the electrolyte to the electrode. The morphology of the paper separators shows the uniform interconnected fibers with the porous structure. Interestingly, we find that the paper separator with 0.75 w/v\% content of SBR exhibit decreased interfacial resistance and improved electrochemical performance, having retention of 62\% and nearly 100\% Coulombic efficiency up to 240 cycles, as compared to other concentrations. Moreover, we observe the energy density around 376 Wh kg$^{-1}$ (considering cathode weight), which found to be comparable to the commercially available glass fiber separator. Our results demonstrate the potential of these multilayer paper separators towards achieving sustainability and safety in energy storage systems. △ Less

Submitted 10 September, 2024; originally announced September 2024.

Comments: submitted

Decoupling of Static and Dynamic Charge Correlations revealed by Uniaxial Strain in a Cuprate Superconductor

Authors: L. Martinelli, I. Biało, X. Hong, J. Oppliger, C. Lin, T. Schaller, J. Küspert, M. H. Fischer, T. Kurosawa, N. Momono, M. Oda, D. V. Novikov, A. Khadiev, E. Weschke, J. Choi, S. Agrestini, M. Garcia-Fernandez, Ke-Jin Zhou, Q. Wang, J. Chang

Abstract: We use uniaxial strain in combination with ultra-high-resolution Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K- and copper L3-edges to study the excitations stemming from the charge ordering wave vector in La1.875Sr0.125CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain… ▽ More We use uniaxial strain in combination with ultra-high-resolution Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K- and copper L3-edges to study the excitations stemming from the charge ordering wave vector in La1.875Sr0.125CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain an in-plane four-fold symmetry. As such, we find that both phonon and charge excitations are decoupled entirely from the strength of static charge ordering. The almost isotropic character of charge excitations is indicative of a quantum critical behaviour and remains a possible source for the strange metal properties found in the normal state of cuprate superconductors. △ Less

Submitted 24 February, 2025; v1 submitted 21 June, 2024; originally announced June 2024.

Comments: 9 pages, 6 figures

Absence of bulk charge density wave order in the normal state of UTe$_2$

Authors: Caitlin S. Kengle, Jakub Vonka, Sonia Francoual, Johan Chang, Peter Abbamonte, Marc Janoschek, P. F. S. Rosa, Wolfgang Simeth

Abstract: A spatially modulated superconducting state, known as pair density wave (PDW), is a tantalizing state of matter with unique properties. Recent scanning tunneling microscopy (STM) studies revealed that spin-triplet superconductor UTe$_2$ hosts an unprecedented spin-triplet, multi-component PDW whose three wavevectors are indistinguishable from a preceding charge-density wave (CDW) order that surviv… ▽ More A spatially modulated superconducting state, known as pair density wave (PDW), is a tantalizing state of matter with unique properties. Recent scanning tunneling microscopy (STM) studies revealed that spin-triplet superconductor UTe$_2$ hosts an unprecedented spin-triplet, multi-component PDW whose three wavevectors are indistinguishable from a preceding charge-density wave (CDW) order that survives to temperatures well above the superconducting critical temperature, $T_{c}$. Whether the PDW is the mother or a subordinate order remains unsettled. Here, based on a systematic search for bulk charge order above $T_{c}$ using resonant elastic X-ray scattering (REXS), we show that the structure factor of charge order previously identified by STM is absent in the bulk within the sensitivity of REXS. Our results invite two scenarios: either the density-wave orders condense simultaneously at $T_{c}$ in the bulk, in which case PDW order is likely the mother phase, or the charge modulations are restricted to the surface. △ Less

Submitted 6 August, 2024; v1 submitted 20 June, 2024; originally announced June 2024.

Sensing Spin Wave Excitations by Spin Defects in Few-Layer Thick Hexagonal Boron Nitride

Authors: Jingcheng Zhou, Hanyi Lu, Di Chen, Mengqi Huang, Gerald Q. Yan, Faris Al-matouq, Jiu Chang, Dziga Djugba, Zhigang Jiang, Hailong Wang, Chunhui Rita Du

Abstract: Optically active spin defects in wide band-gap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of "hard" and "soft" condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by boron vacancy $V_B^-$… ▽ More Optically active spin defects in wide band-gap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of "hard" and "soft" condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y3Fe5O12 (YIG) by boron vacancy $V_B^-$ spin defects contained in few-layer thick hexagonal boron nitride nanoflakes. We show that the ferromagnetic resonance and parametric spin excitations can be effectively detected by $V_B^-$ spin defects under various experimental conditions through optically detected magnetic resonance measurements. The off-resonant dipole interaction between YIG magnons and $V_B^-$ spin defects is mediated by multi-magnon scattering processes, which may find relevant applications in a range of emerging quantum sensing, computing, and metrology technologies. Our results also highlight the opportunities offered by quantum spin defects in layered two-dimensional vdW materials for investigating local spin dynamic behaviors in magnetic solid-state matters. △ Less

Submitted 1 May, 2024; originally announced May 2024.

Discovery of Giant Unit-Cell Super-Structure in the Infinite-Layer Nickelate PrNiO$_2$

Authors: J. Oppliger, J. Küspert, A. -C. Dippel, M. v. Zimmermann, O. Gutowski, X. Ren, X. J. Zhou, Z. Zhu, R. Frison, Q. Wang, L. Martinelli, I. Biało, J. Chang

Abstract: Spectacular quantum phenomena such as superconductivity often emerge in flat-band systems where Coulomb interactions overpower electron kinetics. Engineering strategies for flat-band physics is therefore of great importance. Here, using high-energy grazing-incidence x-ray diffraction, we demonstrate how in-situ temperature annealing of the infinite-layer nickelate PrNiO$_2$ induces a giant superla… ▽ More Spectacular quantum phenomena such as superconductivity often emerge in flat-band systems where Coulomb interactions overpower electron kinetics. Engineering strategies for flat-band physics is therefore of great importance. Here, using high-energy grazing-incidence x-ray diffraction, we demonstrate how in-situ temperature annealing of the infinite-layer nickelate PrNiO$_2$ induces a giant superlattice structure. The annealing effect has a maximum well above room temperature. By covering a large scattering volume, we show a rare period-six in-plane (bi-axial) symmetry and a period-four symmetry in the out-of-plane direction. This giant unit-cell superstructure likely stems from ordering of diffusive oxygen. The stability of this superlattice structure suggests a connection to an energetically favorable electronic state of matter. As such, our study provides a new pathway - different from Moiré structures - to ultra-small Brillouin zone electronics. △ Less

Submitted 27 April, 2024; originally announced April 2024.

Comments: Main: 7 pages, 4 figures. Supplementary: 2 pages, 3 figures

Journal ref: Commun Mater 6, 3 (2025)

Direct Visualization of a Disorder Driven Electronic Smectic Phase in Nonsymmorphic Square-Net Semimetal GdSbTe

Authors: Balaji Venkatesan, Syu-You Guan, Jen-Te Chang, Shiang-Bin Chiu, Po-Yuan Yang, Chih-Chuan Su, Tay-Rong Chang, Kalaivanan Raju, Raman Sankar, Somboon Fongchaiya, Ming-Wen Chu, Chia-Seng Chang, Guoqing Chang, Hsin Lin, Adrian Del Maestro, Ying-Jer Kao, Tien-Ming Chuang

Abstract: Electronic liquid crystal (ELC) phases are spontaneous symmetry breaking states believed to arise from strong electron correlation in quantum materials such as cuprates and iron pnictides. Here, we report a direct observation of a smectic phase in a weakly correlated nonsymmorphic square-net semimetal GdSbxTe2-x. Incommensurate smectic charge modulation and intense local unidirectional nanostructu… ▽ More Electronic liquid crystal (ELC) phases are spontaneous symmetry breaking states believed to arise from strong electron correlation in quantum materials such as cuprates and iron pnictides. Here, we report a direct observation of a smectic phase in a weakly correlated nonsymmorphic square-net semimetal GdSbxTe2-x. Incommensurate smectic charge modulation and intense local unidirectional nanostructure, which coexist with Dirac fermions across Fermi level, are visualized by using spectroscopic imaging - scanning tunneling microscopy. As materials with highly mobile carriers are mostly weakly correlated, the discovery of such an ELC phase are anomalous and raise questions on the origin of their emergence. Specifically, we demonstrate how chemical substitution generates these symmetry breaking phases before the system undergoes a charge density wave (CDW) - orthorhombic structural transition. Our results highlight the importance of impurities in realizing ELC phases and present a new material platform for exploring the interplay among quenched disorder, Dirac fermions and electron correlation. △ Less

Submitted 22 July, 2025; v1 submitted 29 February, 2024; originally announced February 2024.

Journal ref: npj Quantum Mater. 10, 56 (2025)

Charge orders with distinct magnetic response in a prototypical kagome superconductor LaRu$_{3}$Si$_{2}$

Authors: C. Mielke III, V. Sazgari, I. Plokhikh, S. Shin, H. Nakamura, J. N. Graham, J. Küspert, I. Bialo, G. Garbarino, D. Das, M. Medarde, M. Bartkowiak, S. S. Islam, R. Khasanov, H. Luetkens, M. Z. Hasan, E. Pomjakushina, J. -X. Yin, M. H. Fischer, J. Chang, T. Neupert, S. Nakatsuji, B. Wehinger, D. J. Gawryluk, Z. Guguchia

Abstract: The kagome lattice has emerged as a promising platform for hosting unconventional chiral charge order at high temperatures. Notably, in LaRu$_{3}$Si$_{2}$, a room-temperature charge-ordered state with a propagation vector of ($\frac{1}{4}$,~0,~0) has been recently identified. However, understanding the interplay between this charge order and superconductivity, particularly with respect to time-rev… ▽ More The kagome lattice has emerged as a promising platform for hosting unconventional chiral charge order at high temperatures. Notably, in LaRu$_{3}$Si$_{2}$, a room-temperature charge-ordered state with a propagation vector of ($\frac{1}{4}$,~0,~0) has been recently identified. However, understanding the interplay between this charge order and superconductivity, particularly with respect to time-reversal-symmetry breaking, remains elusive. In this study, we employ single crystal X-ray diffraction, magnetotransport, and muon-spin rotation experiments to investigate the charge order and its electronic and magnetic responses in LaRu$_{3}$Si$_{2}$ across a wide temperature range down to the superconducting state. Our findings reveal the emergence of a charge order with a propagation vector of ($\frac{1}{6}$,~0,~0) below $T_{\rm CO,2}$ ${\simeq}$ 80 K, coexisting with the previously identified room-temperature primary charge order ($\frac{1}{4}$,~0,~0). The primary charge-ordered state exhibits zero magnetoresistance. In contrast, the appearance of the secondary charge order at $T_{\rm CO,2}$ is accompanied by a notable magnetoresistance response and a pronounced temperature-dependent Hall effect, which experiences a sign reversal, switching from positive to negative below $T^{*}$ ${\simeq}$ 35 K. Intriguingly, we observe an enhancement in the internal field width sensed by the muon ensemble below $T^{*}$ ${\simeq}$ 35 K. Moreover, the muon spin relaxation rate exhibits a substantial increase upon the application of an external magnetic field below $T_{\rm CO,2}$ ${\simeq}$ 80 K. Our results highlight the coexistence of two distinct types of charge order in LaRu$_{3}$Si$_{2}$ within the correlated kagome lattice, namely a non-magnetic charge order ($\frac{1}{4}$,~0,~0) below $T_{\rm co,1}$ ${\simeq}$ 400 K and a time-reversal-symmetry-breaking charge order below $T_{\rm CO,2}$. △ Less

Submitted 28 February, 2024; v1 submitted 25 February, 2024; originally announced February 2024.

Comments: 10 pages, 5 figures

Uniaxial strain tuning of charge modulation and singularity in a kagome superconductor

Authors: Chun Lin, Armando Consiglio, Ola Kenji Forslund, Julia Kuspert, M. Michael Denner, Hechang Lei, Alex Louat, Matthew D. Watson, Timur K. Kim, Cephise Cacho, Dina Carbone, Mats Leandersson, Craig Polley, Thiagarajan Balasubramanian, Domenico Di Sante, Ronny Thomale, Zurab Guguchia, Giorgio Sangiovanni, Titus Neupert, Johan Chang

Abstract: Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for such singular electronic states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the giant respo… ▽ More Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for such singular electronic states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the giant responses induced by compressive and tensile strains on the charge-density-wave (CDW) order parameter and high-order van Hove singularity (HO-VHS) in CsV3Sb5. We observe a tripling of the CDW gap magnitudes with ~1% strain, accompanied by the changes of both energy and mass of the saddle-point fermions. Our results reveal an anticorrelation between the unconventional CDW order parameter and the mass of a HO-VHS, and highlight the role of the latter in the superconducting pairing. The giant electronic responses uncover a rich strain tunability of the versatile kagome system in studying quantum interplays under lattice perturbations. △ Less

Submitted 4 December, 2024; v1 submitted 25 February, 2024; originally announced February 2024.

Journal ref: Nat. Commun. 15, 10466 (2024)

Two-Dimensional Phase-Fluctuating Superconductivity in Bulk-Crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$

Authors: C. S. Chen, J. Küspert, I. Biało, J. Mueller, K. W. Chen, M. Y. Zou, D. G. Mazzone, D. Bucher, K. Tanaka, O. Ivashko, M. v. Zimmermann, Qisi Wang, Lei Shu, J. Chang

Abstract: We present a combined growth and transport study of superconducting single-crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$. Evidence of two-dimensional superconductivity with significant phase fluctuations of preformed Cooper pairs preceding the superconducting transition is reported. This result is based on three key observations. (1) The resistive superconducting transition temperature $T_c$ (defined by… ▽ More We present a combined growth and transport study of superconducting single-crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$. Evidence of two-dimensional superconductivity with significant phase fluctuations of preformed Cooper pairs preceding the superconducting transition is reported. This result is based on three key observations. (1) The resistive superconducting transition temperature $T_c$ (defined by resistivity $ρ\rightarrow 0$) increases with increasing disorder. (2) As $T\rightarrow T_c$, the conductivity diverges significantly faster than what is expected from Gaussian fluctuations in two and three dimensions. (3) Non-Ohmic resistance behavior is observed in the superconducting state. Altogether, our observations are consistent with a temperature regime of phase-fluctuating superconductivity. The crystal structure with magnetic ordering tendencies in the NdO$_{0.5}$F$_{0.5}$ layers and (super)conductivity in the BiS$_2$ layers is likely responsible for the two-dimensional phase fluctuations. As such, NdO$_{0.5}$F$_{0.5}$BiS$_2$ falls into the class of unconventional ``laminar" bulk superconductors that include cuprate materials and 4Hb-TaS$_2$. △ Less

Submitted 24 February, 2024; v1 submitted 30 January, 2024; originally announced January 2024.

Tuning of Charge Order by Uniaxial Stress in a Cuprate Superconductor

Authors: Laure Thomarat, Frank Elson, Elisabetta Nocerino, Debarchan Das, Oleh Ivashko, Marek Bartkowiak, Martin Månsson, Yasmine Sassa, Tadashi Adachi, Martin v. Zimmermann, Hubertus Luetkens, Johan Chang, Marc Janoschek, Zurab Guguchia, Gediminas Simutis

Abstract: Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, w… ▽ More Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La(2-x)BaxCuO4, around the x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at approximately 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to the critical stress, above which they stay constant. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller 'islands'. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place. △ Less

Submitted 24 January, 2024; originally announced January 2024.

Journal ref: Communications Physics 7, 271 (2024)

DPA-2: a large atomic model as a multi-task learner

Authors: Duo Zhang, Xinzijian Liu, Xiangyu Zhang, Chengqian Zhang, Chun Cai, Hangrui Bi, Yiming Du, Xuejian Qin, Anyang Peng, Jiameng Huang, Bowen Li, Yifan Shan, Jinzhe Zeng, Yuzhi Zhang, Siyuan Liu, Yifan Li, Junhan Chang, Xinyan Wang, Shuo Zhou, Jianchuan Liu, Xiaoshan Luo, Zhenyu Wang, Wanrun Jiang, Jing Wu, Yudi Yang , et al. (18 additional authors not shown)

Abstract: The rapid advancements in artificial intelligence (AI) are catalyzing transformative changes in atomic modeling, simulation, and design. AI-driven potential energy models have demonstrated the capability to conduct large-scale, long-duration simulations with the accuracy of ab initio electronic structure methods. However, the model generation process remains a bottleneck for large-scale applicatio… ▽ More The rapid advancements in artificial intelligence (AI) are catalyzing transformative changes in atomic modeling, simulation, and design. AI-driven potential energy models have demonstrated the capability to conduct large-scale, long-duration simulations with the accuracy of ab initio electronic structure methods. However, the model generation process remains a bottleneck for large-scale applications. We propose a shift towards a model-centric ecosystem, wherein a large atomic model (LAM), pre-trained across multiple disciplines, can be efficiently fine-tuned and distilled for various downstream tasks, thereby establishing a new framework for molecular modeling. In this study, we introduce the DPA-2 architecture as a prototype for LAMs. Pre-trained on a diverse array of chemical and materials systems using a multi-task approach, DPA-2 demonstrates superior generalization capabilities across multiple downstream tasks compared to the traditional single-task pre-training and fine-tuning methodologies. Our approach sets the stage for the development and broad application of LAMs in molecular and materials simulation research. △ Less

Submitted 16 August, 2024; v1 submitted 24 December, 2023; originally announced December 2023.

Journal ref: npj Comput Mater 10, 293 (2024)

Spin fluctuations sufficient to mediate superconductivity in nickelates

Authors: Paul Worm, Qisi Wang, Motoharu Kitatani, Izabela Biało, Qiang Gao, Xiaolin Ren, Jaewon Choi, Diana Csontosová, Ke-Jin Zhou, Xingjiang Zhou, Zhihai Zhu, Liang Si, Johan Chang, Jan M. Tomczak, Karsten Held

Abstract: Infinite-layer nickelates show high-temperature superconductivity, and the experimental phase diagram agrees well with the one simulated within the dynamical vertex approximation (D$Γ$A). Here, we compare the spin-fluctuation spectrum behind these calculations to resonant inelastic X-ray scattering experiments. The overall agreement is good. This independent cross-validation of the strength of spi… ▽ More Infinite-layer nickelates show high-temperature superconductivity, and the experimental phase diagram agrees well with the one simulated within the dynamical vertex approximation (D$Γ$A). Here, we compare the spin-fluctuation spectrum behind these calculations to resonant inelastic X-ray scattering experiments. The overall agreement is good. This independent cross-validation of the strength of spin fluctuations strongly supports the scenario, advanced by D$Γ$A, that spin-fluctuations are the mediator of the superconductivity observed in nickelates. △ Less

Submitted 13 December, 2023; originally announced December 2023.

Comments: 13 pages, 8 figures

Dual-species Bose-Einstein condensates of $^{23}$Na and $^{41}$K with tunable interactions

Authors: Jaeryeong Chang, Sungjun Lee, Yoonsoo Kim, Younghoon Lim, Jee Woo Park

Abstract: We report the creation of dual-species Bose-Einstein condensates (BECs) of $^{23}$Na and $^{41}$K. Favorable background scattering lengths enable efficient sympathetic cooling of $^{41}$K via forced evaporative cooling of $^{23}$Na in a plugged magnetic trap and an optical dipole trap. The $1/e$ lifetime of the thermal mixture in the stretched hyperfine state exceeds 5 s in the presence of backgro… ▽ More We report the creation of dual-species Bose-Einstein condensates (BECs) of $^{23}$Na and $^{41}$K. Favorable background scattering lengths enable efficient sympathetic cooling of $^{41}$K via forced evaporative cooling of $^{23}$Na in a plugged magnetic trap and an optical dipole trap. The $1/e$ lifetime of the thermal mixture in the stretched hyperfine state exceeds 5 s in the presence of background scattering. At the end of evaporation, we create dual BECs in the immiscible phase, with about $3\times10^5$ $^{23}$Na atoms surrounding $5\times10^4$ $^{41}$K atoms. To further enable the tuning of the interspecies interaction strength, we locate multiple Feshbach resonances at magnetic fields up to 100 G. The broadest $s$-wave resonance located at 73.4(3) G features a favorable width of 1.8(2) G. This work sets the stage for the creation of ultracold gases of strongly dipolar bosonic $^{23}$Na$^{41}$K molecules as well as the exploration of many-body physics in bosonic $^{23}$Na-$^{41}$K mixtures. △ Less

Submitted 13 December, 2023; originally announced December 2023.

Comments: 6 pages, 4 figures

Resolving the Orbital Character of Low-energy Excitations in Mott Insulator with Intermediate Spin-orbit Coupling

Authors: K. von Arx, P. Rothenbühler, Qisi Wang, J. Choi, M. Garcia-Fernandez, S. Agrestini, Ke-Jin Zhou, A. Vecchione, R. Fittipaldi, Y. Sassa, M. Cuoco, F. Forte, J. Chang

Abstract: Multi-band Mott insulators with moderate spin-orbit and Hund's coupling are key reference points for theoretical concept developments of correlated electron systems. The ruthenate Mott insulator Ca$_{2}$RuO$_{4}$ has therefore been intensively studied by spectroscopic probes. However, it has been challenging to resolve the fundamental excitations emerging from the hierarchy of electronic energy sc… ▽ More Multi-band Mott insulators with moderate spin-orbit and Hund's coupling are key reference points for theoretical concept developments of correlated electron systems. The ruthenate Mott insulator Ca$_{2}$RuO$_{4}$ has therefore been intensively studied by spectroscopic probes. However, it has been challenging to resolve the fundamental excitations emerging from the hierarchy of electronic energy scales. Here we apply state-of-the-art resonant inelastic x-ray scattering to probe deeper into the electronic excitations found in Ca$_{2}$RuO$_{4}$. In this fashion, we probe a series of spin-orbital excitations at low energies and resolve the level splitting of the intra-$t_{2g}$ structure due to spin-orbit coupling and crystal field splitting. Most importantly, the low-energy excitations exhibit strong orbital character. Such direct determination of relevant electronic energy scales is important, as it sharpens the target for theory developments of Mott insulators' orbital degree of freedom. △ Less

Submitted 7 December, 2023; originally announced December 2023.

Engineering Phase Competition Between Stripe Order and Superconductivity in La$_{1.88}$Sr$_{0.12}$CuO$_4$

Authors: J. Küspert, I. Biało, R. Frison, A. Morawietz, L. Martinelli, J. Choi, D. Bucher, O. Ivashko, M. v. Zimmermann, N. B. Christensen, D. G. Mazzone, G. Simutis, A. A. Turrini, L. Thomarat, D. W. Tam, M. Janoschek, T. Kurosawa, N. Momono, M. Oda, Qisi Wang, J. Chang

Abstract: Unconventional superconductivity often couples to other electronic orders in a cooperative or competing fashion. Identifying external stimuli that tune between these two limits is of fundamental interest. Here, we show that strain perpendicular to the copper-oxide planes couples directly to the competing interaction between charge stripe order and superconductivity in La$_{1.88}$Sr$_{0.12}$CuO… ▽ More Unconventional superconductivity often couples to other electronic orders in a cooperative or competing fashion. Identifying external stimuli that tune between these two limits is of fundamental interest. Here, we show that strain perpendicular to the copper-oxide planes couples directly to the competing interaction between charge stripe order and superconductivity in La$_{1.88}$Sr$_{0.12}$CuO$_4$ (LSCO). Compressive $c$-axis pressure amplifies stripe order within the superconducting state, while having no impact on the normal state. By contrast, strain dramatically diminishes the magnetic field enhancement of stripe order in the superconducting state. These results suggest that $c$-axis strain acts as tuning parameter of the competing interaction between charge stripe order and superconductivity. This interpretation implies a uniaxial pressure-induced ground state in which the competition between charge order and superconductivity is reduced. △ Less

Submitted 15 August, 2024; v1 submitted 6 December, 2023; originally announced December 2023.

Journal ref: Commun Phys 7, 225 (2024)

Magnon interactions in a moderately correlated Mott insulator

Authors: Qisi Wang, S. Mustafi, E. Fogh, N. Astrakhantsev, Z. He, I. Biało, Ying Chan, L. Martinelli, M. Horio, O. Ivashko, N. E. Shaik, K. von Arx, Y. Sassa, E. Paris, M. H. Fischer, Y. Tseng, N. B. Christensen, A. Galdi, D. G. Schlom, K. M. Shen, T. Schmitt, H. M. Rønnow, J. Chang

Abstract: Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO$_2$, single- and bi-magnon d… ▽ More Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO$_2$, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La$_2$CuO$_4$ indicate that quantum fluctuations are much stronger in SrCuO$_2$ suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO$_2$ - due to strong quantum fluctuations - is a unique starting point for the exploration of novel magnetic ground states. △ Less

Submitted 26 June, 2024; v1 submitted 28 November, 2023; originally announced November 2023.

Journal ref: Nature Communications 15, 5348 (2024)

Design and performance of an ultrahigh vacuum spectroscopic-imaging scanning tunneling microscope with a hybrid vibration isolation system

Authors: Pei-Fang Chung, Balaji Venkatesan, Chih-Chuan Su, Jen-Te Chang, Hsu-Kai Cheng, Che-An Liu, Henry Yu, Chia-Seng Chang, Syu-You Guan, Tien-Ming Chuang

Abstract: A spectroscopic imaging-scanning tunneling microscope (SI-STM) allows the atomic scale visualization of surface electronic and magnetic structure of novel quantum materials with high energy resolution. To achieve the optimal performance, low vibration facility is required. Here, we describe the design and the performance of an ultrahigh vacuum STM system supported by a hybrid vibration isolation s… ▽ More A spectroscopic imaging-scanning tunneling microscope (SI-STM) allows the atomic scale visualization of surface electronic and magnetic structure of novel quantum materials with high energy resolution. To achieve the optimal performance, low vibration facility is required. Here, we describe the design and the performance of an ultrahigh vacuum STM system supported by a hybrid vibration isolation system that consists of a pneumatic passive and a piezoelectric active vibration isolation stages. The STM system is equipped with a 1K pot cryogenic insert and a 9 Tesla superconducting magnet, capable of continuous SI-STM measurements for 7 days. A field ion microscopy system is installed for in situ STM tip treatment. We present the detailed vibrational noise analysis of the hybrid vibration isolation system and demonstrate the performance of our STM system by taking high resolution spectroscopic maps and topographic images on several quantum materials. Our results establish a new strategy to achieve an effective vibration isolation system for high-resolution STM and other scanning probe microscopy to investigate the nanoscale quantum phenomena. △ Less

Submitted 27 November, 2023; v1 submitted 17 November, 2023; originally announced November 2023.

Uncertainty Quantification in Multivariable Regression for Material Property Prediction with Bayesian Neural Networks

Authors: Longze Li, Jiang Chang, Aleksandar Vakanski, Yachun Wang, Tiankai Yao, Min Xian

Abstract: With the increased use of data-driven approaches and machine learning-based methods in material science, the importance of reliable uncertainty quantification (UQ) of the predicted variables for informed decision-making cannot be overstated. UQ in material property prediction poses unique challenges, including the multi-scale and multi-physics nature of advanced materials, intricate interactions b… ▽ More With the increased use of data-driven approaches and machine learning-based methods in material science, the importance of reliable uncertainty quantification (UQ) of the predicted variables for informed decision-making cannot be overstated. UQ in material property prediction poses unique challenges, including the multi-scale and multi-physics nature of advanced materials, intricate interactions between numerous factors, limited availability of large curated datasets for model training, etc. Recently, Bayesian Neural Networks (BNNs) have emerged as a promising approach for UQ, offering a probabilistic framework for capturing uncertainties within neural networks. In this work, we introduce an approach for UQ within physics-informed BNNs, which integrates knowledge from governing laws in material modeling to guide the models toward physically consistent predictions. To evaluate the effectiveness of this approach, we present case studies for predicting the creep rupture life of steel alloys. Experimental validation with three datasets of collected measurements from creep tests demonstrates the ability of BNNs to produce accurate point and uncertainty estimates that are competitive or exceed the performance of the conventional method of Gaussian Process Regression. Similarly, we evaluated the suitability of BNNs for UQ in an active learning application and reported competitive performance. The most promising framework for creep life prediction is BNNs based on Markov Chain Monte Carlo approximation of the posterior distribution of network parameters, as it provided more reliable results in comparison to BNNs based on variational inference approximation or related NNs with probabilistic outputs. The codes are available at: https://github.com/avakanski/Creep-uncertainty-quantification. △ Less

Submitted 14 May, 2024; v1 submitted 4 November, 2023; originally announced November 2023.

Comments: 24 pages, 4 figures

ACM Class: I.2.6

Journal ref: Scientific Reports, 14(1):10543, 2024

Searching for the signature of a pair density wave in YBa$_2$Cu$_3$O$_{6.67}$ using high energy X-ray diffraction

Authors: Elizabeth Blackburn, Oleh Ivashko, Emma Campillo, Martin von Zimmermann, Ruixing Liang, Douglas A. Bonn, Walter N. Hardy, Johan Chang, Edward M. Forgan, Stephen M. Hayden

Abstract: We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked. We have carried out a search for a pair density wave signature using high-energy X-ray diffraction in fields up to 16 T. We do not see evidence for a signal at the predicted wavevector. This is a report on the details of our experiment, with information on where in reciprocal space we looked. △ Less

Submitted 27 October, 2023; originally announced October 2023.

Comments: 5 pages, report on experimental results

Investigation of the mechanism of the anomalous Hall effects in Cr2Te3/(BiSb)2(TeSe)3 heterostructure

Authors: Seong Won Cho, In Hak Lee, Youngwoong Lee, Sangheon Kim, Yeong Gwang Khim, Seung-Young Park, Younghun Jo, Junwoo Choi, Seungwu Han, Young Jun Chang, Suyoun Lee

Abstract: The interplay between ferromagnetism and the non-trivial topology has unveiled intriguing phases in the transport of charges and spins. For example, it is consistently observed the so-called topological Hall effect (THE) featuring a hump structure in the curve of the Hall resistance (Rxy) vs. a magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a topological insulator (TI… ▽ More The interplay between ferromagnetism and the non-trivial topology has unveiled intriguing phases in the transport of charges and spins. For example, it is consistently observed the so-called topological Hall effect (THE) featuring a hump structure in the curve of the Hall resistance (Rxy) vs. a magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a topological insulator (TI). The origin of the hump structure is still controversial between the topological Hall effect model and the multi-component anomalous Hall effect (AHE) model. In this work, we have investigated a heterostructure consisting of BixSb2-xTeySe3-y (BSTS) and Cr2Te3 (CT), which are well-known TI and two-dimensional FM, respectively. By using the so-called minor-loop measurement, we have found that the hump structure observed in the CT/BSTS is more likely to originate from two AHE channels. Moreover, by analyzing the scaling behavior of each amplitude of two AHE with the longitudinal resistivities of CT and BSTS, we have found that one AHE is attributed to the extrinsic contribution of CT while the other is due to the intrinsic contribution of BSTS. It implies that the proximity-induced ferromagnetic layer inside BSTS serves as a source of the intrinsic AHE, resulting in the hump structure explained by the two AHE model. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Journal ref: Nano Convergence (2023) 10:11

Contact holes in vertical electrode structures analyzed by voltage contrast-SEM and conducting AFM

Authors: Minsun Gu, Moon Seop Hyun, Moonsup Han, Gyungtae Kim, Young Jun Chang

Abstract: Soaring demands of multi-stacked memory devices request urgent development of backside contact electrode technologies, such as high aspect ratio etching, metallization, and inspection methods. Especially the complex metal contact process should be monitored for each manufacturing step to filter the defective samples and to maintain the high yield of production. Among the inspection methods for det… ▽ More Soaring demands of multi-stacked memory devices request urgent development of backside contact electrode technologies, such as high aspect ratio etching, metallization, and inspection methods. Especially the complex metal contact process should be monitored for each manufacturing step to filter the defective samples and to maintain the high yield of production. Among the inspection methods for detecting the electrical connections, there is voltage contrast (VC)-SEM and conducting AFM (C-AFM). In this report, we investigated the two inspection methods for testing designed samples with different contact hole states. The VC-SEM data shows the contrast variation at the contact holes, from which one may discern the contact status with an optimum voltage. The C-AFM results clearly demonstrate a finite electrical current in the connected contact, while a negligible current in the disconnected one. Finally, we discuss insights of using the two methods for analyzing the contact hole technologies with high aspect ratios. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 17 pages, 4 figures

Journal ref: Current Applied Physics 53, 46 (2023)

Electrical conductivity enhancement of epitaxially grown TiN thin films

Authors: Yeong Gwang Khim, Beomjin Park, Jin Eun Heo, Young Hun Khim, Young Rok Khim, Minsun Gu, Tae Gyu Rhee, Seo Hyoung Chang, Moonsup Han, Young Jun Chang

Abstract: Titanium nitride (TiN) presents superior electrical conductivity with mechanical and chemical stability and compatibility with the semiconductor fabrication process. Here, we fabricated epitaxial and polycrystalline TiN (111) thin films on MgO (111), sapphire (001), and mica substrates at 640oC and room temperature by using a DC sputtering, respectively. The epitaxial films show less amount of sur… ▽ More Titanium nitride (TiN) presents superior electrical conductivity with mechanical and chemical stability and compatibility with the semiconductor fabrication process. Here, we fabricated epitaxial and polycrystalline TiN (111) thin films on MgO (111), sapphire (001), and mica substrates at 640oC and room temperature by using a DC sputtering, respectively. The epitaxial films show less amount of surface oxidation than the polycrystalline ones grown at room temperature. The epitaxial films show drastically reduced resistivity (~30 micro-ohm-cm), much smaller than the polycrystalline films. Temperature-dependent resistivity measurements show a nearly monotonic temperature slope down to low temperature. These results demonstrate that high temperature growth of TiN thin films leads to significant enhancement of electrical conductivity, promising for durable and scalable electrode applications. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 14 pages, 3 figures

Journal ref: Journal of the Korean Physical Society 82, 486 (2023)

Atomic arrangement of van der Waals heterostructures using X-ray scattering and crystal truncation rod analysis

Authors: Ryung Kim, Byoung Ki Choi, Kyeong Jun Lee, Hyuk Jin Kim, Hyun Hwi Lee, Tae Gyu Rhee, Yeong Gwang Khim, Young Jun Chang, Seo Hyoung Chang

Abstract: Vanadium diselenide (VSe2) has intriguing physical properties such as unexpected ferromagnetism at the two-dimensional limit. However, the experimental results for room temperature ferromagnetism are still controversial and depend on the detailed crystal structure and stoichiometry. Here we introduce crystal truncation rod (CTR) analysis to investigate the atomic arrangement of bilayer VSe2 and bi… ▽ More Vanadium diselenide (VSe2) has intriguing physical properties such as unexpected ferromagnetism at the two-dimensional limit. However, the experimental results for room temperature ferromagnetism are still controversial and depend on the detailed crystal structure and stoichiometry. Here we introduce crystal truncation rod (CTR) analysis to investigate the atomic arrangement of bilayer VSe2 and bilayer graphene (BLG) hetero-structures grown on a 6H-SiC(0001) substrate. Using non-destructive CTR analysis, we were able to obtain electron density profiles and detailed crystal structure of the VSe2/BLG heterostructures. Specifically, the out-of-plane lattice parameters of each VSe2 layer were modulated by the interface compared to that of the bulk VSe2 1T phase. The atomic arrangement of the VSe2/BLG heterostructure provides deeper understanding and insight for elucidating the magnetic properties of the van der Waals heterostructure. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 17 pages, 4 figures

Journal ref: Current Applied Physics 46, 70 (2023)

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth

Authors: Hyuk Jin Kim, Minsu Chong, Tae Gyu Rhee, Yeong Gwang Khim, Min-Hyoung Jung, Young-Min Kim, Hu Young Jeong, Byoung Ki Choi, Young Jun Chang

Abstract: In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data al… ▽ More In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 21 pages, 4 figures

Journal ref: Nano Convergence 10, 10 (2023)

Controlling spin-orbit coupling to tailor type-II Dirac bands

Authors: Nguyen Huu Lam, Phuong Lien Nguyen, Byoung Ki Choi, Trinh Thi Ly, Ganbat Duvjir, Tae Gyu Rhee, Yong Jin Jo, Tae Heon Kim, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Younghun Hwang, Young Jun Chang, Jaekwang Lee, Jungdae Kim

Abstract: NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density-functional theory (DFT) calculations, we report that the strength of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dir… ▽ More NiTe2, a type-II Dirac semimetal with strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density-functional theory (DFT) calculations, we report that the strength of spin-orbit coupling (SOC) in NiTe2 can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dirac band. Indeed, combined studies using scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) confirm that the BDP in the NiTe2-xSex alloy moves from +0.1 eV (NiTe2) to -0.3 eV (NiTeSe) depending on the Se concentrations, indicating the effective tunability of type-II Dirac fermions. Our results demonstrate an approach to tailor the type-II Dirac band in NiTe2 by controlling the SOC strength via chalcogen substitution. This approach can be applicable to different types of topological materials. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Comments: 25 pages, 4 figures

Journal ref: ACS Nano 16, 11227 (2022)

Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca$_2$RuO$_4$

Authors: M. Horio, F. Forte, D. Sutter, M. Kim, C. G. Fatuzzo, C. E. Matt, S. Moser, T. Wada, V. Granata, R. Fittipaldi, Y. Sassa, G. Gatti, H. M. Rønnow, M. Hoesch, T. K. Kim, C. Jozwiak, A. Bostwick, Eli Rotenberg, I. Matsuda, A. Georges, G. Sangiovanni, A. Vecchione, M. Cuoco, J. Chang

Abstract: Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca$_2$RuO$_4$, upon a… ▽ More Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca$_2$RuO$_4$, upon alkali-metal surface doping, develops a single-band metal skin. Our dynamical mean field theory calculations reveal that homogeneous electron doping of Ca$_2$RuO$_4$ results in a multi-band metal. All together, our results provide compelling evidence for an orbital-selective Mott insulator breakdown, which is unachievable via simple electron doping. Supported by a cluster model and cluster perturbation theory calculations, we demonstrate a novel type of skin metal-insulator transition induced by surface dopants that orbital-selectively hybridize with the bulk Mott state and in turn produce coherent in-gap states. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: A revised version of this manuscript will appear in Communications Physics

Journal ref: Commun. Phys. 6, 323 (2023)

Designing superhard magnetic material in clathrate \b{eta}-C3N2 through atom embeddedness

Authors: Liping Sun, Botao Fu, Jing Chang

Abstract: Designing new compounds with the coexistence of diverse physical properties is of great significance for broad applications in multifunctional electronic devices. In this work, based on density functional theory, we predict the coexistence of mechanical superhardness and the controllable magnetism in the clathrate material \b{eta}-C3N2 through the implant of the external atom into the intrinsic ca… ▽ More Designing new compounds with the coexistence of diverse physical properties is of great significance for broad applications in multifunctional electronic devices. In this work, based on density functional theory, we predict the coexistence of mechanical superhardness and the controllable magnetism in the clathrate material \b{eta}-C3N2 through the implant of the external atom into the intrinsic cage structure. Taking hydrogen-doping (H@\b{eta}-C3N2) and fluorine-doping (F@\b{eta}-C3N2) as examples, our calculations indicate these two doped configurations are stable and discovered that they belong to antiferromagnetic semiconductor and ferromagnetic semi-metal, respectively. These intriguing magnetic phase transitions originate from their distinctive band structure around the Fermi level and can be well understood by the 3D Hubbard model with half-filling occupation and the Stoner model. Moreover, the high Vickers hardness of 49.0 GPa for H@\b{eta}-C3N2 and 48.2 GPa for F@\b{eta}-C3N2 are obtained, suggesting they are clathrate superhard materials as its host. Therefore, the incorporation of H and F in \b{eta}-C3N2 gives rise to a new type of superhard antiferromagnetic semiconductor and superhard ferromagnetic semimetal, respectively, which could have potential applications in harsh conditions. Our work provides an effective strategy to design a new class of highly desirable multifunctional materials with excellent mechanical properties and magnetic properties, which may arouse spintronic applications in superhard materials in the future. △ Less

Submitted 26 September, 2023; originally announced September 2023.

Comments: 14 pages, 5 figures