Showing 1–50 of 116 results for author: Xie, M

Wavelength-dependent photo-creep in halide perovskite single crystals

Authors: Ruitian Chen, Jincong Pang, Lizhong Lang, Jiaze Wu, Mingyu Xie, Shuo Yang, Kaiqi Qiu, Tobin Filleter, Kai Huang, Guangda Niu, Jiang Tang, Yu Zou

Abstract: Halide perovskites are promising optoelectronic materials, but their time-dependent permanent deformation under illumination (i.e., photo-creep) is poorly understood, limiting their mechanical stability. Here we report wavelength-dependent photo-creep phenomena in CsPbBr3 and FAPbBr3 single crystals, studied by constant-load nanoindentation under controlled light with various wavelengths. Compared… ▽ More Halide perovskites are promising optoelectronic materials, but their time-dependent permanent deformation under illumination (i.e., photo-creep) is poorly understood, limiting their mechanical stability. Here we report wavelength-dependent photo-creep phenomena in CsPbBr3 and FAPbBr3 single crystals, studied by constant-load nanoindentation under controlled light with various wavelengths. Compared with creep in dark, continuous green light (near-bandgap) suppresses creep by 19% in CsPbBr3 and 10% in FAPbBr3, whereas violet (far above-bandgap) light enhances creep by 16% in CsPbBr3 and 8% in FAPbBr3. In contrast, when light is onset during creep, blue light enhances creep most prominently, whereas green light exhibits minimal influence. Such photo-creep behavior in halide perovskites are distinct with photo-plasticity phenomenon in conventional semiconductors. By combining the photoluminescence and photocurrent measurements, we unveil that ion migration promotes dislocation climb and creep, while carrier trapping suppresses dislocation glide and related creep in halide perovskites. Such competition between carrier trapping and ion migration tuned by wavelength governs the photo-creep response. Our findings uncover a photomechanical effect in halide perovskites and highlight how coupled carrier and ion dynamics under illumination affect their device reliability. △ Less

Submitted 12 April, 2026; originally announced April 2026.

Dissecting superconductivity in the Ruddlesden-Popper nickelates: The role of electron correlation and interlayer magnetic exchange

Authors: Xiaoyang Chen, Zezhong Li, Mei Xie, Deyuan Hu, Yiu-Fung Chiu, Stefano Agrestini, Wenliang Zhang, Yi Lu, Meng Wang, Mirian Garcia-Fernandez, Donglai Feng, Ke-Jin Zhou

Abstract: The discovery of superconductivity in the Ruddlesden-Popper (RP) nickelates has opened a new chapter in the search for high superconducting transition temperatures ($T_\mathrm{c}$) materials. A central and puzzling feature of this family is the wide variation in $T_\mathrm{c}$ despite their common NiO$_2$ building blocks, as highlighted by the recent observation of superconductivity at $\sim$ 30 K… ▽ More The discovery of superconductivity in the Ruddlesden-Popper (RP) nickelates has opened a new chapter in the search for high superconducting transition temperatures ($T_\mathrm{c}$) materials. A central and puzzling feature of this family is the wide variation in $T_\mathrm{c}$ despite their common NiO$_2$ building blocks, as highlighted by the recent observation of superconductivity at $\sim$ 30 K in trilayer $\mathrm{La_4Ni_3O_{10}}$, significantly lower than 80 K reported in bilayer $\mathrm{La_3Ni_2O_7}$. Understanding the factors that control $T_\mathrm{c}$ in this family is therefore of paramount importance. Here, we use resonant inelastic x-ray scattering (RIXS) to investigate the electronic and magnetic excitations of $\mathrm{La_4Ni_3O_{10}}$ in direct comparison with its bilayer counterpart. Our results reveal a markedly different landscape. $\mathrm{La_4Ni_3O_{10}}$ exhibits a more itinerant character, evidenced by broader Ni $dd$ orbital excitations and a strong Ni 3$d$ fluorescence continuum, suggesting weaker electronic correlations than in the bilayer. Despite this, well-defined collective spin excitations persist, including dispersive acoustic and optical magnon branches alongside an incommensurate spin density wave. Using linear spin wave theory, we extract the interlayer superexchange interaction ($J_z$) to be $\sim$ 22 meV, much smaller than that in $\mathrm{La_3Ni_2O_7}$. The weaker correlation and reduced interlayer exchange together provide a consistent explanation for the substantially lower $T_\mathrm{c}$ in the trilayer compound. Our findings establish interlayer magnetic coupling and electronic correlation as key parameters governing superconductivity in layered nickelates and offer critical constraints for understanding the pairing mechanism in this emerging family. △ Less

Submitted 2 April, 2026; originally announced April 2026.

Comments: 11 pages, 4 figures

Collective Spin Excitations in Correlated Moiré Chern Ferromagnets

Authors: Ming Xie, Sankar Das Sarma

Abstract: Moiré-induced narrow electronic bands in transition metal dichalcogenide superlattices support many correlated quantum phases characterized by novel charge, flavor, and topological orders. Among these, magnetic ordering emerges as the most ubiquitous, often serving as the parent state for other correlated phases, including quantum anomalous Hall states, as well as chiral superconducting state. Bec… ▽ More Moiré-induced narrow electronic bands in transition metal dichalcogenide superlattices support many correlated quantum phases characterized by novel charge, flavor, and topological orders. Among these, magnetic ordering emerges as the most ubiquitous, often serving as the parent state for other correlated phases, including quantum anomalous Hall states, as well as chiral superconducting state. Because of electron-electron correlation, the stability of magnetic order is critically influenced by low-energy collective spin fluctuations, or magnon excitations. We investigate the nature of magnon excitations and their impact on the stability and transition temperature of the magnetic state at integer filling factor $ν= -1$. We find that the magnon spectrum exhibits isolated low-energy bands whose topological character undergoes a transition upon tuning the interlayer displacement field. The magnon gap is found to depend sensitively on the topology of the magnetic ground state, resulting in an order-of-magnitude enhancement of the transition temperature $T_c$ in the quantum anomalous Hall phase compared to the topologically trivial correlated insulator. Our findings provide insight into the interplay between electron and magnon topology and suggest new routes for controlling magnetism and topology via moiré engineering. △ Less

Submitted 20 March, 2026; originally announced March 2026.

Spatiotemporal Magnonic Vortex Beams with Alternating Transverse Orbital Angular Momentum

Authors: Muyang Xie, Chenchen Liu, Jian Huang, Zhenyu Wang, Xinwei Dong, Ruifang Wang

Abstract: Recent theoretical and experimental studies revealed spatiotemporal photonic, and acoustic, vortex beams in open space. The spatiotemporal vortex beams carry orbital angular momentum perpendicular to the wave propagation direction. Here, we report spatiotemporal magnonic vortex beams in a confined geometry of a ferromagnetic nanostrip. The spatiotemporal magnonic vortex beam contains immobile phas… ▽ More Recent theoretical and experimental studies revealed spatiotemporal photonic, and acoustic, vortex beams in open space. The spatiotemporal vortex beams carry orbital angular momentum perpendicular to the wave propagation direction. Here, we report spatiotemporal magnonic vortex beams in a confined geometry of a ferromagnetic nanostrip. The spatiotemporal magnonic vortex beam contains immobile phase dislocations and the wave propagates in a zigzag-like route. It is remarkable that the transverse orbital angular momentums, carried by the phase dislocations, are spatially alternating. Our findings are in sharp contrast to the photonic and acoustic counterparts, and open a new area in the study of spatiotemporal vortex beams. △ Less

Submitted 15 March, 2026; originally announced March 2026.

Comments: 12 pages, 9 figures

Infinite Magnetoresistance and Vortex Coupling in the Pb/BSCCO Heterostructure

Authors: Weifan Zhu, Jiamin Yao, Shuntianjiao Ling, Shanyin Fu, Yifu Xu, Pengyue Xiong, Jiawen Zhang, Mengwei Xie, Yanan Zhang, Ye Chen, Huiqiu Yuan, Xin Lu, Qing-Hu Chen, Yang Liu

Abstract: Combining superconductivity with spintronics provides exciting opportunities to realize low-dissipation quantum devices. Here we report the synthesis, characterization and magnetotransport measurements of the Pb/Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO) superconducting heterostructures, where an insulating PbO$_{x}$ layer spontaneously forms at the interface. Non-volatile switching between superconduc… ▽ More Combining superconductivity with spintronics provides exciting opportunities to realize low-dissipation quantum devices. Here we report the synthesis, characterization and magnetotransport measurements of the Pb/Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO) superconducting heterostructures, where an insulating PbO$_{x}$ layer spontaneously forms at the interface. Non-volatile switching between superconducting (logical "0") and normal ("1") states in Pb films by an external field, i.e., infinite magnetoresistance (IMR), can be realized and are attributed to the strong trapping and pinning of vortices in BSCCO. Furthermore, butterfly-shaped hysteresis loops in magnetoresistance, pronounced resistance dips/jumps and thermal reset to superconducting states can be observed and are direct manifestations of the peculiar vortex dynamics in BSCCO and vortex coupling across the Pb/BSCCO interface. Our work demonstrates a simple and effective way to realize IMR through superconducting vortices and opens up new opportunities to study the vortex interactions across the superconducting interfaces. △ Less

Submitted 30 January, 2026; originally announced January 2026.

Comments: 9 pages, 4 figures, accepted by PRB

Multigap nodeless superconductivity in Dirac semimetal PdTe

Authors: Fengrui Shi, Weilong Qiu, Chufan Chen, Chunqiang Xu, Yan Zhang, Hao Zheng, Yuwei Zhou, Dongting Zhang, Mengwei Xie, Huiqiu Yuan, Shiyan Li, Yang Liu, Chao Cao, Xiaofeng Xu, Xin Lu

Abstract: PdTe has recently been reported to be a type-II Dirac semimetal while a bulk nodal and surface nodeless superconductivity (SC) has been claimed to coexist. In this work, we applied point-contact spectroscopy (PCS) method to systematically study the superconducting gap in PdTe single crystals with a SC transition temperature $T_{c}=4.3$ K. The obtained differential conductance curves show a common… ▽ More PdTe has recently been reported to be a type-II Dirac semimetal while a bulk nodal and surface nodeless superconductivity (SC) has been claimed to coexist. In this work, we applied point-contact spectroscopy (PCS) method to systematically study the superconducting gap in PdTe single crystals with a SC transition temperature $T_{c}=4.3$ K. The obtained differential conductance curves show a common deviation from a single-gap superconducting behavior and can be better fitted by a two-gap Blonder-Tinkham-Klapwijk model, suggesting the larger gap $Δ_{L}$ with $2Δ_{L}$=3.7 $k_{B}T_{c}$ and the smaller gap $Δ_S$ yielding $2Δ_{S}$=1.1-2.2 $k_{B}T_{c}$ with a weak interband scattering. The variations of conductance spectra among different contacts are proposed to be caused by the anisotropy of Fermi surface topology associated with different gaps. △ Less

Submitted 8 January, 2026; originally announced January 2026.

Comments: 6 pages,5 figures

Journal ref: Phys. Rev. B 112, 224518 Published 23 December, 2025

Anomalous enhancement of magnetism by nonmagnetic doping in the honeycomb-lattice antiferromagnet ErOCl

Authors: Yanzhen Cai, Mingtai Xie, Jing Kang, Weizhen Zhuo, Wei Ren, Xijing Dai, Anmin Zhang, Jianting Ji, Feng Jin, Zheng Zhang, Qingming Zhang

Abstract: Tuning magnetic anisotropy through chemical doping is a powerful strategy for designing functional materials with enhanced magnetic properties. Here, we report an enhanced Er^3+ magnetic moment resulting from nonmagnetic Lu^3+ substitution in the honeycomb-lattice antiferromagnet ErOCl. Unlike the Curie-Weiss type divergence typically observed in diluted magnetic systems, our findings reveal a dis… ▽ More Tuning magnetic anisotropy through chemical doping is a powerful strategy for designing functional materials with enhanced magnetic properties. Here, we report an enhanced Er^3+ magnetic moment resulting from nonmagnetic Lu^3+ substitution in the honeycomb-lattice antiferromagnet ErOCl. Unlike the Curie-Weiss type divergence typically observed in diluted magnetic systems, our findings reveal a distinct enhancement of magnetization per Er^3+ ion under high magnetic fields, suggesting an unconventional mechanism. Structural analysis reveals that Lu^3+ doping leads to a pronounced contraction of the c axis, which is attributed to chemical pressure effects, while preserving the layered SmSI-type crystal structure with space group R-3m. High-resolution Raman spectroscopy reveals a systematic blueshift of the first and seventh crystalline electric field (CEF) excitations, indicating an increase in the axial CEF parameter B_2^0. This modification enhances the magnetic anisotropy along the c axis, leading to a significant increase in magnetization at low temperatures and under high magnetic fields, contrary to conventional expectations for magnetic dilution. Our work not only clarifies the intimate connection between magnetism and CEF in rare-earth compounds, but more importantly, it reveals a physical pathway to effectively tune magnetic anisotropy via anisotropic lattice distortion induced by chemical pressure. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Comments: 12 pages, 4 figures

Journal ref: Physical Review B 112, 134448 (2025)

Hidden integer quantum ferroelectricity in chiral Tellurium

Authors: Wei Luo, Sihan Deng, Muting Xie, Junyi Ji, Hongjun Xiang, Laurent Bellaiche

Abstract: Ferroelectricity is a cornerstone of functional materials research, enabling diverse technologies from non-volatile memory to optoelectronics. Recently, type-I integer quantum ferroelectricity (IQFE), unconstrained by symmetry, has been proposed and experimentally demonstrated; however, as it arises from ionic displacements of an integer lattice vector, the initial and final states are macroscopic… ▽ More Ferroelectricity is a cornerstone of functional materials research, enabling diverse technologies from non-volatile memory to optoelectronics. Recently, type-I integer quantum ferroelectricity (IQFE), unconstrained by symmetry, has been proposed and experimentally demonstrated; however, as it arises from ionic displacements of an integer lattice vector, the initial and final states are macroscopically indistinguishable, rendering the physical properties unchanged. Here, we propose for the first time the nontrivial counterpart (i.e., type-II IQFE) where the polarization difference between the initial and final states is quantized but the macroscopical properties differ. We further demonstrate the existence of type-II IQFE in bulk chiral tellurium. In few-layer tellurium, the total polarization remains nearly quantized, composed of a bulk-inherited quantum component and a small surface-induced contribution. Molecular dynamics simulations reveal surface-initiated, layer-by-layer switching driven by reduced energy barriers, explaining why ferroelectricity was observed experimentally in few-layer tellurium, but not in bulk tellurium yet. Interestingly, the chirality of the initial and final states in bulk tellurium is opposite, suggesting a novel way to control structural chirality with electric field in chiral photonics and nonvolatile ferroelectric memory devices. △ Less

Submitted 9 October, 2025; originally announced October 2025.

Comments: 16 pages, 4 figures

Defect-Charge-Driven 90° Switching in HfO2

Authors: Muting Xie, Hongyu Yu, Zhihao Dai, Yingfen Wei, Changsong Xu, Hongjun Xiang

Abstract: Hafnium dioxide (HfO2) is a CMOS-compatible ferroelectric showing both 180° and 90° switching, yet the microscopic nature of the 90° pathway remains unresolved. We show that the 90° rotation pathway, negligible in pristine HfO2, becomes dominant under E// [111] when induced by charged oxygen vacancies. This pathway is more fatigue-resistant than the 180° reversal pathway, while delivering the same… ▽ More Hafnium dioxide (HfO2) is a CMOS-compatible ferroelectric showing both 180° and 90° switching, yet the microscopic nature of the 90° pathway remains unresolved. We show that the 90° rotation pathway, negligible in pristine HfO2, becomes dominant under E// [111] when induced by charged oxygen vacancies. This pathway is more fatigue-resistant than the 180° reversal pathway, while delivering the same polarization change along [111] (2Pr=60 μC/cm^2 ). This charge-driven switching arises from two factors: the crystal geometry of HfO2 and the intrinsic nature of rotational pathways, the latter suggesting a possible general tendency for defect charge to bias rotation over reversal in ferroelectrics. Together these findings reveal a pathway-level origin of fatigue resistance and establish defect charge as a general control parameter for polarization dynamics. △ Less

Submitted 25 September, 2025; originally announced September 2025.

DiffSyn: A Generative Diffusion Approach to Materials Synthesis Planning

Authors: Elton Pan, Soonhyoung Kwon, Sulin Liu, Mingrou Xie, Alexander J. Hoffman, Yifei Duan, Thorben Prein, Killian Sheriff, Yuriy Roman-Leshkov, Manuel Moliner, Rafael Gomez-Bombarelli, Elsa Olivetti

Abstract: The synthesis of crystalline materials, such as zeolites, remains a significant challenge due to a high-dimensional synthesis space, intricate structure-synthesis relationships and time-consuming experiments. Considering the one-to-many relationship between structure and synthesis, we propose DiffSyn, a generative diffusion model trained on over 23,000 synthesis recipes spanning 50 years of litera… ▽ More The synthesis of crystalline materials, such as zeolites, remains a significant challenge due to a high-dimensional synthesis space, intricate structure-synthesis relationships and time-consuming experiments. Considering the one-to-many relationship between structure and synthesis, we propose DiffSyn, a generative diffusion model trained on over 23,000 synthesis recipes spanning 50 years of literature. DiffSyn generates probable synthesis routes conditioned on a desired zeolite structure and an organic template. DiffSyn achieves state-of-the-art performance by capturing the multi-modal nature of structure-synthesis relationships. We apply DiffSyn to differentiate among competing phases and generate optimal synthesis routes. As a proof of concept, we synthesize a UFI material using DiffSyn-generated synthesis routes. These routes, rationalized by density functional theory binding energies, resulted in the successful synthesis of a UFI material with a high Si/Al$_{\text{ICP}}$ of 19.0, which is expected to improve thermal stability and is higher than that of any previously recorded. △ Less

Submitted 24 September, 2025; v1 submitted 21 September, 2025; originally announced September 2025.

Dominant Kitaev interaction and field-induced quantum phase transitions in triangular-lattice KCeSe2

Authors: Mingtai Xie, Zheng Zhang, Weizhen Zhuo, Wei Xu, Jinfeng Zhu, Jan Embs, Lei Wang, Zikang Li, Huanpeng Bu, Anmin Zhang, Feng Jin, Jianting Ji, Zhongwen Ouyang, Liusuo Wu, Jie Ma, Qingming Zhang

Abstract: Realizing Kitaev interactions on triangular lattices offers a compelling platform for exploring quantum-spin-liquid physics beyond the conventional honeycomb lattice framework. Here, we investigate the triangular-lattice antiferromagnet KCeSe2, where multiple probes reveal strong magnetic anisotropy suggesting significant Kitaev physics. Through detailed and combined analysis of magnetization, neu… ▽ More Realizing Kitaev interactions on triangular lattices offers a compelling platform for exploring quantum-spin-liquid physics beyond the conventional honeycomb lattice framework. Here, we investigate the triangular-lattice antiferromagnet KCeSe2, where multiple probes reveal strong magnetic anisotropy suggesting significant Kitaev physics. Through detailed and combined analysis of magnetization, neutron scattering, and thermodynamic experiments, we identify dominant ferromagnetic Kitaev ($K = -1.82$ K) and antiferromagnetic Heisenberg ($J = 1.34$ K) interactions that stabilize a stripe-$yz$ ordered ground state via an order-by-disorder mechanism. Magnetic fields applied along the Kitaev bond direction induce two phase transitions at 1.67 T and 3.8 T, consistent with density matrix renormalization group (DMRG) calculations predictions of a progression from stripe-$yz$ to stripe-canted and spin-polarized phases. Near the 1.67 T quantum critical point, enhanced quantum fluctuations suggest conditions favorable for exotic excitations. These results establish KCeSe2 as a platform for exploring Kitaev physics on triangular lattices. △ Less

Submitted 29 May, 2025; originally announced May 2025.

Comments: 8 Pages, 4 Figures

Journal ref: Phys. Rev. Research 7, 023198 (2025)

Gapless spinon excitations emerging from a multipolar transverse field in the triangular-lattice Ising antiferromagnet NaTmSe2

Authors: Zheng Zhang, Jinlong Jiao, Weizhen Zhuo, Mingtai Xie, D. T. Adroja, Toni Shiroka, Guochu Deng, Anmin Zhang, Feng Jin, Jianting Ji, Jie Ma, Qingming Zhang

Abstract: The triangular-lattice quantum Ising antiferromagnet is a promising platform for realizing Anderson's quantum spin liquid, though finding suitable materials to realize it remains a challenge. Here, we present a comprehensive study of NaTmSe2 using magnetization, specific heat, neutron scattering, and muon spin relaxation, combined with theoretical calculations. We demonstrate that NaTmSe2 realizes… ▽ More The triangular-lattice quantum Ising antiferromagnet is a promising platform for realizing Anderson's quantum spin liquid, though finding suitable materials to realize it remains a challenge. Here, we present a comprehensive study of NaTmSe2 using magnetization, specific heat, neutron scattering, and muon spin relaxation, combined with theoretical calculations. We demonstrate that NaTmSe2 realizes the transverse field Ising model and quantitatively determine its exchange parameters. Our results reveal a multipolar spin-polarized state coexisting with a dipolar spin-disordered state. These states feature gapless spinon excitations mediated by the multipolar moments. The study shows how multiple types of magnetism can emerge in distinct magnetic channels (dipolar and multipolar) within a single magnet, advancing our understanding of spin-frustrated Ising physics and opening pathways for different quantum computing applications. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 111, L180405 (2025) (letter)

Unveiling competitions between carrier recombination pathways in semiconductors via mechanical damping

Authors: Mingyu Xie, Ruitian Chen, Jiaze Wu, Kaiqi Qiu, Mingqiang Li, Huicong Chen, Kai Huang, Yu Zou

Abstract: The total rate of carrier recombination in semiconductors has conventionally been expressed using an additive model, r_total = Σr_i , which rules out the interactions between carrier recombination pathways. Here we challenge this paradigm by demonstrating pathway competitions using our newly developed light-induced mechanical absorption spectroscopy (LIMAS), which allows us to probe genuine recomb… ▽ More The total rate of carrier recombination in semiconductors has conventionally been expressed using an additive model, r_total = Σr_i , which rules out the interactions between carrier recombination pathways. Here we challenge this paradigm by demonstrating pathway competitions using our newly developed light-induced mechanical absorption spectroscopy (LIMAS), which allows us to probe genuine recombination dynamics in semiconductors via mechanical damping. We show that the total recombination rate in zinc sulfide (ZnS), a model semiconductor material, follows a multiplicative weighting model, r_total \propto Πr_i ^(w_i) with Σw_i=1. Under both steady-state and switch-on illuminations, the weighting factors w_i for each recombination pathway-direct, trap-assisted, and sublinear-are dictated by the carrier generation mechanism: (i) interband transition favors direct recombination; (ii) single-defect level-mediated generation promotes trap-assisted recombination; (iii) generation involving multiple saturated defect levels gives rise to sublinear recombination. Upon light switch-off, localized state changes drive a dynamic evolution of w_i, altering pathway competitions. These findings reshape our fundamental understanding of carrier dynamics and provide a new strategy to optimize next-generation optoelectronic devices. △ Less

Submitted 1 May, 2025; originally announced May 2025.

Probing Quantum Anomalous Hall States in Twisted Bilayer WSe2 via Attractive Polaron Spectroscopy

Authors: Beini Gao, Mahdi Ghafariasl, Mahmoud Jalali Mehrabad, Tsung-Sheng Huang, Lifu Zhang, Deric Session, Pranshoo Upadhyay, Rundong Ma, Ghadah Alshalan, Daniel Gustavo Suárez Forero, Supratik Sarkar, Suji Park, Houk Jang, Kenji Watanabe, Takashi Taniguchi, Ming Xie, You Zhou, Mohammad Hafezi

Abstract: Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects. A recent study hinted that twisted WSe2 homobilayer (tWSe2) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase. Here, we report the first direct evidence of QAH states in tWSe2 with spontaneous ferroma… ▽ More Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects. A recent study hinted that twisted WSe2 homobilayer (tWSe2) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase. Here, we report the first direct evidence of QAH states in tWSe2 with spontaneous ferromagnetism. Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2 degree tWSe2 and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling ν= 1. Together with a Chern number measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by C = 1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe2 as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes. △ Less

Submitted 10 March, 2026; v1 submitted 15 April, 2025; originally announced April 2025.

Engineering diamond interfaces free of dark spins

Authors: Xiaofei Yu, Evan J. Villafranca, Stella Wang, Jessica C. Jones, Mouzhe Xie, Jonah Nagura, Ignacio Chi-Durán, Nazar Delegan, Alex B. F. Martinson, Michael E. Flatté, Denis R. Candido, Giulia Galli, Peter C. Maurer

Abstract: Nitrogen-vacancy (NV) centers in diamond are extensively utilized as quantum sensors for imaging fields at the nanoscale. The ultra-high sensitivity of NV magnetometers has enabled the detection and spectroscopy of individual electron spins, with potentially far-reaching applications in condensed matter physics, spintronics, and molecular biology. However, the surfaces of these diamond sensors nat… ▽ More Nitrogen-vacancy (NV) centers in diamond are extensively utilized as quantum sensors for imaging fields at the nanoscale. The ultra-high sensitivity of NV magnetometers has enabled the detection and spectroscopy of individual electron spins, with potentially far-reaching applications in condensed matter physics, spintronics, and molecular biology. However, the surfaces of these diamond sensors naturally contain electron spins, which create a background signal that can be hard to differentiate from the signal of the target spins. In this study, we develop a surface modification approach that eliminates the unwanted signal of these so-called dark electron spins. Our surface passivation technique, based on coating diamond surfaces with a thin titanium oxide (TiO$_2$) layer, reduces the dark spin density. The observed reduction in dark spin density aligns with our findings on the electronic structure of the diamond-TiO$_2$ interface. The reduction, from a typical value of $2,000$~$μ$m$^{-2}$ to a value below that set by the detection limit of our NV sensors ($200$~$μ$m$^{-2}$), results in a two-fold increase in Hahn-echo coherence time of near surface NV centers. Furthermore, we derive a comprehensive spin model that connects dark spin relaxation with NV coherence, providing additional insights into the mechanisms behind the observed spin dynamics. Our findings are directly transferable to other quantum platforms, including nanoscale solid state qubits and superconducting qubits. △ Less

Submitted 19 December, 2025; v1 submitted 11 April, 2025; originally announced April 2025.

Comments: Main text: 7 pages, 5 figures

Radiation-induced Instability of Organic-Inorganic Halide Perovskite Single Crystals

Authors: Ruitian Chen, Mingyu Xie, Tianyi Lyu, Jincong Pang, Lewei Zeng, Jiahui Zhang, Changjun Cheng, Renfei Feng, Guangda Niu, Jiang Tang, Yu Zou

Abstract: Organic-inorganic halide perovskites (OIHPs) are promising optoelectronic materials, but their instability under radiation environments restricts their durability and practical applications. Here we employ electron and synchrotron X-ray beams, individually, to investigate the radiation-induced instability of two types of OIHP single crystals (FAPbBr3 and MAPbBr3). Under the electron beam, we obser… ▽ More Organic-inorganic halide perovskites (OIHPs) are promising optoelectronic materials, but their instability under radiation environments restricts their durability and practical applications. Here we employ electron and synchrotron X-ray beams, individually, to investigate the radiation-induced instability of two types of OIHP single crystals (FAPbBr3 and MAPbBr3). Under the electron beam, we observe that 3-point star-style cracks grow on the surface of FAPbBr3, and bricklayer-style cracks are formed on the surface of MAPbBr3. Under the X-ray beam, a new composition without organic components appears in both FAPbBr3 and MAPbBr3. Such cracking and composition changes are attributed to the volatilization of organic components. We propose a volume-strain-based mechanism, in which the energy conversion results from the organic cation loss. Using nanoindentation, we reveal that beam radiations reduce the Youngs modulus and increase the hardness of both OIHPs. This study provides valuable insights into the structural and mechanical stabilities of OIHP single crystals in radiation environments. △ Less

Submitted 10 January, 2026; v1 submitted 8 April, 2025; originally announced April 2025.

Thermodynamics and heat transport of quantum spin liquid candidates NaYbS$_2$ and NaYbSe$_2$

Authors: N. Li, M. T. Xie, Q. Huang, Z. W. Zhuo, Z. Zhang, E. S. Choi, Y. Y. Wang, H. Liang, Y. Sun, D. D. Wu, Q. J. Li, H. D. Zhou, G. Chen, X. Zhao, Q. M. Zhang, X. F. Sun

Abstract: We study the ultralow-temperature thermodynamics and thermal conductivity ($κ$) of the single-crystal rare-earth chalcogenides NaYbS$_2$ and NaYbSe$_2$, which have an ideal triangular lattice of the Yb$^{3+}$ ions and have been proposed to be quantum spin liquid candidates. The magnetic specific heat divided by temperature $C_{\rm{mag}}/T$ is nearly constant at $T <$ 200 mK, which is indeed the in… ▽ More We study the ultralow-temperature thermodynamics and thermal conductivity ($κ$) of the single-crystal rare-earth chalcogenides NaYbS$_2$ and NaYbSe$_2$, which have an ideal triangular lattice of the Yb$^{3+}$ ions and have been proposed to be quantum spin liquid candidates. The magnetic specific heat divided by temperature $C_{\rm{mag}}/T$ is nearly constant at $T <$ 200 mK, which is indeed the indication of the gapless magnetic excitations with a constant density of states. However, we observe a vanishingly small residual term $κ_0/T$, which points to the absence of mobile fermionic excitations in these materials. Both the weak temperature dependence of $κ$ and the strong magnetic-field dependence of $κ$ suggest the significant scattering between the spinons and phonons, which actually supports the existence of gapless or tiny-gapped quantum spin liquid. Moreover, the $κ(B)/κ(0)$ isotherms show a series of field-induced magnetic transitions for $B \parallel a$, confirming the easy-plane anisotropy, which is consistent with the results of ac magnetic susceptibility. We expect our results to inspire further interests in the understanding of the spinon-phonon coupling in the spin liquid systems. △ Less

Submitted 15 December, 2024; originally announced December 2024.

Comments: 10 pages, 10 figures

Report number: Phys. Rev. B 110, 224414 (2024)

Crystalline electric field excitations and their nonlinear splitting under magnetic fields in YbOCl

Authors: Yanzhen Cai, Wei Ren, Xijing Dai, Jing Kang, Weizhen Zhuo, Mingtai Xie, Anmin Zhang, Jianting Ji, Feng Jin, Zheng Zhang, Qingming Zhang

Abstract: Recently reported van der Waals layered honeycomb rare-earth chalcohalides REChX (RE = rare earth, Ch = chalcogen, and X = halogen) are considered to be promising Kitaev spin liquid (KSL) candidates. The high-quality single crystals of YbOCl, a representative member of the family with an effective spin of 1/2, are available now. The crystalline electric field (CEF) excitations in a rare-earth spin… ▽ More Recently reported van der Waals layered honeycomb rare-earth chalcohalides REChX (RE = rare earth, Ch = chalcogen, and X = halogen) are considered to be promising Kitaev spin liquid (KSL) candidates. The high-quality single crystals of YbOCl, a representative member of the family with an effective spin of 1/2, are available now. The crystalline electric field (CEF) excitations in a rare-earth spin system are fundamentally important for understanding both finite-temperature and ground-state magnetism, but remain unexplored in YbOCl so far. In this paper, we conduct a comprehensive Raman scattering study to unambiguously identify the CEF excitations in YbOCl and determine the CEF parameters and wave functions. Our Raman experiments further reveal the anomalous nonlinear CEF splitting under magnetic fields. We have grown single crystals of YbOCl, the nonmagnetic LuOCl, and the diluted magnetic Lu_{0.86}Yb_{0.14}OCl to make a completely comparative investigation. Polarized Raman spectra on the samples at 1.8 K allow us to clearly assign all the Raman-active phonon modes and explicitly identify the CEF excitations in YbOCl. The CEF excitations are further examined using temperature-dependent Raman measurements and careful symmetry analysis based on Raman tensors related to CEF excitations. By applying the CEF Hamiltonian to the experimentally determined CEF excitations, we extract the CEF parameters and eventually determine the CEF wave functions. The study experimentally pins down the CEF excitations in the Kitaev compound YbOCl and sets a foundation for understanding its finite-temperature magnetism and exploring the possible nontrivial spin ground state. △ Less

Submitted 24 October, 2024; v1 submitted 24 October, 2024; originally announced October 2024.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. Research 6, 043061 (2024)

Rare-Earth Chalcogenides: An Inspiring Playground For Exploring Frustrated Magnetism

Authors: Mingtai Xie, Wenzhen Zhuo, Yanzhen Cai, Zheng Zhang, Qingming Zhang

Abstract: The rare-earth chalcogenide $ARECh_{2}$ family ($A =$ alkali metal or monovalent ions, $RE =$ rare earth, $Ch =$ chalcogen) has emerged as a paradigmatic platform for studying frustrated magnetism on a triangular lattice. The family members exhibit a variety of ground states, from quantum spin liquid to exotic ordered phases, providing fascinating insight into quantum magnetism. Their simple cryst… ▽ More The rare-earth chalcogenide $ARECh_{2}$ family ($A =$ alkali metal or monovalent ions, $RE =$ rare earth, $Ch =$ chalcogen) has emerged as a paradigmatic platform for studying frustrated magnetism on a triangular lattice. The family members exhibit a variety of ground states, from quantum spin liquid to exotic ordered phases, providing fascinating insight into quantum magnetism. Their simple crystal structure and chemical tunability enable systematic exploration of competing interactions in quantum magnets. Recent neutron scattering and thermodynamic studies have revealed rich phase diagrams and unusual excitations, refining theoretical models of frustrated systems. This review provides a succinct introduction to $ARECh_{2}$ research. It summarizes key findings on crystal structures, single-ion physics, magnetic Hamiltonians, ground states, and low-energy excitations. By highlighting current developments and open questions, we aim to catalyze further exploration and deeper physical understanding on this frontier of quantum magnetism. △ Less

Submitted 2 December, 2024; v1 submitted 23 October, 2024; originally announced October 2024.

Comments: 15 pages, 5 figures

Journal ref: Chin. Phys. Lett. 41,117505 (2024)

Broadband measurement of Feibelman's quantum surface response functions

Authors: Zeling Chen, Shu Yang, Zetao Xie, Jinbing Hu, Xudong Zhang, Yipu Xia, Yonggen Shen, Huirong Su, Maohai Xie, Thomas Christensen, Yi Yang

Abstract: The Feibelman $d$-parameter, a mesoscopic complement to the local bulk permittivity, describes quantum optical surface responses for interfaces, including nonlocality, spill-in and-out, and surface-enabled Landau damping. It has been incorporated into the macroscopic Maxwellian framework for convenient modeling and understanding of nanoscale electromagnetic phenomena, calling for the compilation o… ▽ More The Feibelman $d$-parameter, a mesoscopic complement to the local bulk permittivity, describes quantum optical surface responses for interfaces, including nonlocality, spill-in and-out, and surface-enabled Landau damping. It has been incorporated into the macroscopic Maxwellian framework for convenient modeling and understanding of nanoscale electromagnetic phenomena, calling for the compilation of a $d$-parameter database for interfaces of interest in nano-optics. However, accurate first-principles calculations of $d$-parameters face computational challenges, whereas existing measurements of $d$-parameters are scarce and restricted to narrow spectral windows. We demonstrate a general broadband ellipsometric approach to measure $d$-parameters at a gold--air interface across the visible--ultraviolet regimes. Gold is found to spill in and spill out at different frequencies. We also observe gold's Bennett mode, a surface-dipole resonance associated with a pole of the $d$-parameter, around 2.5 eV. Our measurements give rise to and are further validated by the passivity and Kramers--Kronig causality analysis of $d$-parameters. Our work advances the understanding of quantum surface response and may enable applications like enhanced electron field emission. △ Less

Submitted 28 November, 2024; v1 submitted 25 September, 2024; originally announced September 2024.

Thermal crossover from a Chern insulator to a fractional Chern insulator in pentalayer graphene

Authors: Sankar Das Sarma, Ming Xie

Abstract: By theoretically analyzing the recent temperature dependent transport data in pentalayer graphene [Lu et al., arXiv:2408.10203], we establish that the experimentally observed transition from low-temperature quantum anomalous Hall effect to higher-temperature fractional quantum anomalous Hall effect is a crossover phenomenon arising from the competition between interaction and disorder energy scale… ▽ More By theoretically analyzing the recent temperature dependent transport data in pentalayer graphene [Lu et al., arXiv:2408.10203], we establish that the experimentally observed transition from low-temperature quantum anomalous Hall effect to higher-temperature fractional quantum anomalous Hall effect is a crossover phenomenon arising from the competition between interaction and disorder energy scales, with the likely zero temperature ground state of the system being either a localized insulator or a Chern insulator with a quantized anomalous Hall effect. In particular, the intriguing suppression of FQAHE in favor of QAHE with decreasing temperature is explained as arising from the low-temperature localization of the carriers where disorder overcomes the interaction effects. We provide a detailed analysis of the data in support of the crossover scenario. △ Less

Submitted 30 October, 2024; v1 submitted 20 August, 2024; originally announced August 2024.

Comments: Published version

Journal ref: Phys. Rev. B 110, 155148 (2024) - Editors' Suggestion

Role of Domain Walls on Imprint and Fatigue in HfO2-Based Ferroelectrics

Authors: Muting Xie, Hongyu Yu, Binhua Zhang, Changsong Xu, Hongjun Xiang

Abstract: HfO2-based ferroelectric materials are promising for the next generation of memory devices, attracting significant attention. However, their potential applications are significantly limited by fatigue and imprint phenomena, which affect device lifetime and memory capabilities. Here, to accurately describe the dynamics and field effects of HfO2, we adopt our newly developed DREAM-Allegro network sc… ▽ More HfO2-based ferroelectric materials are promising for the next generation of memory devices, attracting significant attention. However, their potential applications are significantly limited by fatigue and imprint phenomena, which affect device lifetime and memory capabilities. Here, to accurately describe the dynamics and field effects of HfO2, we adopt our newly developed DREAM-Allegro network scheme and develop a comprehensive machine-learning model for HfO2. Such model can not only predict the interatomic potential, but also predict Born effective charges. Applying such model, we explore the role of domain dynamics in HfO2 and find that the fatigue and imprint phenomena are closely related to the so-called E-path and T-path switching pathways. Based on the different atomic motions in the two paths, we propose that an inclined electric field can sufficiently suppress fatigue and enhancing the performance of HfO2-based ferroelectric devices. △ Less

Submitted 27 July, 2024; originally announced July 2024.

Switchable Ferroelectricity in Subnano Silicon Thin Films

Authors: Hongyu Yu, Shihan deng, Muting Xie, Yuwen Zhang, Xizhi Shi, Jianxin Zhong, Chaoyu He, Hongjun Xiang

Abstract: Recent advancements underscore the critical need to develop ferroelectric materials compatible with silicon. We systematically explore possible ferroelectric silicon quantum films and discover a low-energy variant (hex-OR-2*2-P) with energy just 1 meV/atom above the ground state (hex-OR-2*2). Both hex-OR-2*2 and hex-OR-2*2-P are confirmed to be dynamically and mechanically stable semiconductors wi… ▽ More Recent advancements underscore the critical need to develop ferroelectric materials compatible with silicon. We systematically explore possible ferroelectric silicon quantum films and discover a low-energy variant (hex-OR-2*2-P) with energy just 1 meV/atom above the ground state (hex-OR-2*2). Both hex-OR-2*2 and hex-OR-2*2-P are confirmed to be dynamically and mechanically stable semiconductors with indirect gaps of 1.323 eV and 1.311 eV, respectively. The ferroelectric hex-OR-2*2-P exhibits remarkable in-plane spontaneous polarization up to 120 Pc/m and is protected by a potential barrier (13.33 meV/atom) from spontaneously transitioning to hex-OR-22. To simulate the switching ferroelectricity in electric fields of the single-element silicon bilayer, we develop a method that simultaneously learns interatomic potentials and Born effective charges (BEC) in a single equivariant model with a physically informed loss. Our method demonstrates good performance on several ferroelectrics. Simulations of hex-OR-2*2-P silicon suggest a depolarization temperature of approximately 300 K and a coercive field of about 0.05 V/Å. These results indicate that silicon-based ferroelectric devices are feasible, and the ground state phase of the silicon bilayer (hex-OR-2*2) is an ideal system. Our findings highlight the promise of pure silicon ferroelectric materials for future experimental synthesis and applications in memory devices, sensors, and energy converters. △ Less

Submitted 1 July, 2024; originally announced July 2024.

Comments: 18 pages, 3 figures

Superconductivity in twisted transition metal dichalcogenide homobilayers

Authors: Jihang Zhu, Yang-Zhi Chou, Ming Xie, Sankar Das Sarma

Abstract: For the first time, robust superconductivity has been independently observed in twisted WSe$_2$ bilayers by two separate groups [Y. Xia et al., arXiv:2405.14784; Y. Guo et al., arXiv:2406.03418.]. In light of this, we explore the possibility of a universal superconducting pairing mechanism in twisted WSe$_2$ bilayers. Using a continuum band structure model and a phenomenological boson-mediated eff… ▽ More For the first time, robust superconductivity has been independently observed in twisted WSe$_2$ bilayers by two separate groups [Y. Xia et al., arXiv:2405.14784; Y. Guo et al., arXiv:2406.03418.]. In light of this, we explore the possibility of a universal superconducting pairing mechanism in twisted WSe$_2$ bilayers. Using a continuum band structure model and a phenomenological boson-mediated effective electron-electron attraction, we find that intervalley intralayer pairing predominates over interlayer pairing. Notably, despite different experimental conditions, both twisted WSe$_2$ samples exhibit a comparable effective attraction strength. This consistency suggests that the dominant pairing glue is likely independent of the twist angle and layer polarization, pointing to a universal underlying boson-induced pairing mechanism. △ Less

Submitted 6 February, 2025; v1 submitted 27 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. B 111, L060501 (2025)

Magnetism of $\mathrm{NaYbS_2}$: From finite temperatures to ground state

Authors: Weizhen Zhuo, Zheng Zhang, Mingtai Xie, Anmin Zhang, Jianting Ji, Feng Jin, Qingming Zhang

Abstract: Rare-earth chalcogenide compounds $\mathrm{ARECh_2}$ (A = alkali or monovalent metal, RE = rare earth, Ch = O, S, Se, Te) are a large family of quantum spin liquid (QSL) candidate materials. $\mathrm{NaYbS_2}$ is a representative member of the family. Several key issues on $\mathrm{NaYbS_2}$, particularly how to determine the highly anisotropic spin Hamiltonian and describe the magnetism at finite… ▽ More Rare-earth chalcogenide compounds $\mathrm{ARECh_2}$ (A = alkali or monovalent metal, RE = rare earth, Ch = O, S, Se, Te) are a large family of quantum spin liquid (QSL) candidate materials. $\mathrm{NaYbS_2}$ is a representative member of the family. Several key issues on $\mathrm{NaYbS_2}$, particularly how to determine the highly anisotropic spin Hamiltonian and describe the magnetism at finite temperatures and the ground state, remain to be addressed. In this paper, we conducted an in-depth and comprehensive study on the magnetism of $\mathrm{NaYbS_2}$ from finite temperatures to the ground state. Firstly, we successfully detected three crystalline electric field (CEF) excitation energy levels using low-temperature Raman scattering technique. Combining them with the CEF theory and magnetization data, we worked out the CEF parameters, CEF energy levels, and CEF wavefunctions. We further determined a characteristic temperature of $\sim$40 K, above which the magnetism is dominated by CEF excitations while below which the spin-exchange interactions play a main role. The characteristic temperature has been confirmed by the temperature-dependent electron spin resonance (ESR) linewidth. Low-temperature ESR experiments on the dilute magnetic doped crystal of $\mathrm{NaYb_{0.1}Lu_{0.9}S_2}$ further helped us to determine the accurate $g$-factor. Next, we quantitatively obtained the spin-exchange interactions in the spin Hamiltonian by consistently simulating the magnetization and specific heat data. Finally, the above studies allow us to explore the ground state magnetism of $\mathrm{NaYbS_2}$ by using the density matrix renormalization group. We combined numerical calculations and experimental results to demonstrate that the ground state of $\mathrm{NaYbS_2}$ is a Dirac-like QSL. △ Less

Submitted 7 June, 2024; originally announced June 2024.

Comments: 13 pages, 6 figures

Journal ref: Sci. China Phys. Mech. Astron. 67, 107411 (2024)

Analytical photoresponses of Schottky contact MoS2 phototransistors

Authors: Jianyong Wei, Yumeng Liu, Yizhuo Wang, Kai Li, Zhentao Lian, Maosong Xie, Xinhan Yang, Seyed Saleh Mousavi Khaleghi, Fuxing Dai, Weida Hu, Xuejiao Gao, Rui Yang, Yaping Dan

Abstract: High-gain photodetectors based on two-dimensional (2D) semiconductors, in particular those in photoconductive mode, have been extensively investigated in the past decade. However, the classical photoconductive theory was derived on two misplaced assumptions. In this work, we established an explicit analytical device model for Schottky contact MoS2 phototransistors that fits well with experimental… ▽ More High-gain photodetectors based on two-dimensional (2D) semiconductors, in particular those in photoconductive mode, have been extensively investigated in the past decade. However, the classical photoconductive theory was derived on two misplaced assumptions. In this work, we established an explicit analytical device model for Schottky contact MoS2 phototransistors that fits well with experimental data. From the fitting results, we found that the Richardson constant of the MoS2 Schottky contact is temperature dependent, indicating that the Schottky contacts for the 2D material is best described by the mixed thermionic emission and diffusion model. Based on this device model, we further established an analytical photoresponse for the few-layer MoS2 phototransistors, from which we found the voltage distribution on the two Schottky contacts and the channel, and extracted the minority carrier recombination lifetimes. The lifetimes are comparable with the values found from transient photoluminescence measurements, which therefore validates our analytical photoresponses for Schottky contact 2D semiconducting phototransistors. △ Less

Submitted 25 May, 2024; originally announced May 2024.

Comments: 15 pages, 6 figures

Fermi-Bose Machine achieves both generalization and adversarial robustness

Authors: Mingshan Xie, Yuchen Wang, Haiping Huang

Abstract: Distinct from human cognitive processing, deep neural networks trained by backpropagation can be easily fooled by adversarial examples. To design a semantically meaningful representation learning, we discard backpropagation, and instead, propose a local contrastive learning, where the representation for the inputs bearing the same label shrink (akin to boson) in hidden layers, while those of diffe… ▽ More Distinct from human cognitive processing, deep neural networks trained by backpropagation can be easily fooled by adversarial examples. To design a semantically meaningful representation learning, we discard backpropagation, and instead, propose a local contrastive learning, where the representation for the inputs bearing the same label shrink (akin to boson) in hidden layers, while those of different labels repel (akin to fermion). This layer-wise learning is local in nature, being biological plausible. A statistical mechanics analysis shows that the target fermion-pair-distance is a key parameter. Moreover, the application of this local contrastive learning to MNIST benchmark dataset demonstrates that the adversarial vulnerability of standard perceptron can be greatly mitigated by tuning the target distance, i.e., controlling the geometric separation of prototype manifolds. △ Less

Submitted 18 July, 2024; v1 submitted 21 April, 2024; originally announced April 2024.

Comments: 32 pages, 6 figures, a physics inspired machine without backpropagation yet with enhanced adversarial robustness

Journal ref: Sci. China-Phys. Mech. Astron. 68, 210511 (2025)

Prediction of topotactic transition from black to blue phosphorus induced by surface Br adsorption

Authors: Hao Tian, Wenjun Xie, Maohai Xie, Chuanhui Zhu, Hu Xu, Shuk-Yin Tong

Abstract: Based on first-principles calculations, we propose a potential access to the yet unrealized freestanding blue phosphorus (blueP) through transformation of black phosphorus (blackP) induced by surface bromine (Br) adsorption. Formation of the Br-P bonds disrupts the original sp3 configurations in blackP, generates unpaired pz electrons and induces a structural transformation that results in blueP f… ▽ More Based on first-principles calculations, we propose a potential access to the yet unrealized freestanding blue phosphorus (blueP) through transformation of black phosphorus (blackP) induced by surface bromine (Br) adsorption. Formation of the Br-P bonds disrupts the original sp3 configurations in blackP, generates unpaired pz electrons and induces a structural transformation that results in blueP formation by re-pairing the pz orbitals. Ab initio molecular dynamics simulations confirm that randomly adsorbed Br adatoms on bilayer blackP spontaneously diffuse into specific patterns to render the emergence of the blueP phase. The expected obtainment Br-passivated blueP nanoribbons exhibit tunable band gaps in a wide range and high carrier mobilities of the order of 1000 cm2V-1s-1. This study provides an opportunity to fabricate blueP through the conversion from blackP by tuning its surface chemistry. △ Less

Submitted 8 April, 2024; originally announced April 2024.

Integer and fractional quantum anomalous Hall effects in pentalayer graphene

Authors: Ming Xie, Sankar Das Sarma

Abstract: We critically analyze the recently reported observation of integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects at zero applied magnetic field in pentalayer graphene. Our quantitative activation and variable range hopping transport analysis of the experimental data reveals that the observed IQAHE and FQAHE at different fillings all have similar excitation gaps of the order of… ▽ More We critically analyze the recently reported observation of integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects at zero applied magnetic field in pentalayer graphene. Our quantitative activation and variable range hopping transport analysis of the experimental data reveals that the observed IQAHE and FQAHE at different fillings all have similar excitation gaps of the order of $5-10$ K. In addition, we also find that the observed FQAHE manifests a large hidden background contact series resistance >10 k$Ω$ of unknown origin whereas this contact resistance is much smaller ~500 $Ω$ in the observed IQAHE. Both of these findings are surprising as well as inconsistent with the well-established phenomenology of the corresponding high-field integer and fractional quantum Hall effects in 2D semiconductor systems. △ Less

Submitted 10 July, 2024; v1 submitted 2 April, 2024; originally announced April 2024.

Comments: Final published version

Journal ref: Phys. Rev. B 109, L241115 (2024) (Editor's Suggestion)

$π$ Phase Interlayer Shift and Stacking Fault in the Kagome Superconductor CsV$_3$Sb$_5$

Authors: Feng Jin, Wei Ren, Mingshu Tan, Mingtai Xie, Bingru Lu, Zheng Zhang, Jianting Ji, Qingming Zhang

Abstract: The stacking degree of freedom is a crucial factor in tuning material properties and has been extensively investigated in layered materials. The kagome superconductor CsV$_3$Sb$_5$ was recently discovered to exhibit a three-dimensional CDW phase below TCDW ~94 K. Despite the thorough investigation of in-plane modulation, the out-of-plane modulation has remained ambiguous. Here, our polarization- a… ▽ More The stacking degree of freedom is a crucial factor in tuning material properties and has been extensively investigated in layered materials. The kagome superconductor CsV$_3$Sb$_5$ was recently discovered to exhibit a three-dimensional CDW phase below TCDW ~94 K. Despite the thorough investigation of in-plane modulation, the out-of-plane modulation has remained ambiguous. Here, our polarization- and temperature-dependent Raman measurements reveal the breaking of C$_6$ rotational symmetry and the presence of three distinct domains oriented at approximately 120°to each other. The observations demonstrate that the CDW phase can be naturally explained as a 2c staggered order phase with adjacent layers exhibiting a relative $π$ phase shift. Further, we discover a first-order structural phase transition at approximately 65 K and suggest that it is a stacking order-disorder phase transition due to stacking fault, supported by the thermal hysteresis behavior of a Cs-related phonon mode. Our findings highlight the significance of the stacking degree of freedom in CsV$_3$Sb$_5$ and offer structural insights to comprehend the entanglement between superconductivity and CDW. △ Less

Submitted 7 March, 2024; originally announced March 2024.

Comments: This manuscript was published in Phys. Rev. Lett

Journal ref: Physical Review Letters 132, 066501 (2024)

Long-lived Topological Flatband Excitons in Semiconductor Moiré Heterostructures: a Bosonic Kane-Mele Model Platform

Authors: Ming Xie, Mohammad Hafezi, Sankar Das Sarma

Abstract: Moiré superlattices based on two-dimensional transition metal dichalcogenides (TMDs) have emerged as a highly versatile and fruitful platform for exploring correlated topological electronic phases. One of the most remarkable examples is the recently discovered fractional quantum anomalous Hall effect (FQAHE) under zero magnetic field. Here we propose a minimal structure that hosts long-lived excit… ▽ More Moiré superlattices based on two-dimensional transition metal dichalcogenides (TMDs) have emerged as a highly versatile and fruitful platform for exploring correlated topological electronic phases. One of the most remarkable examples is the recently discovered fractional quantum anomalous Hall effect (FQAHE) under zero magnetic field. Here we propose a minimal structure that hosts long-lived excitons -- a ubiquitous bosonic excitation in TMD semiconductors -- with narrow topological bosonic bands. The nontrivial exciton topology originates from hybridization of moiré interlayer excitons, and is tunable by controlling twist angle and electric field. At small twist angle, the lowest exciton bands are isolated from higher energy bands and provide a solid-state realization of the bosonic Kane-Mele model with topological flatbands, which could potentially support the bosonic version of FQAHE. △ Less

Submitted 30 September, 2024; v1 submitted 29 February, 2024; originally announced March 2024.

Comments: Published version

Journal ref: Phys. Rev. Lett. 133, 136403 (2024)

Distinct pressure evolution of superconductivity and charge-density-wave in kagome superconductor CsV$_3$Sb$_5$ thin flakes

Authors: Ge Ye, Mengwei Xie, Chufan Chen, Yanan Zhang, Dongting Zhang, Xin Ma, Xiangyu Zeng, Fanghang Yu, Yi Liu, Xiaozhi Wang, Guanghan Cao, Xiaofeng Xu, Xianhui Chen, Huiqiu Yuan, Chao Cao, Xin Lu

Abstract: It is intriguing to explore the coexistence and (or) competition between charge-density-wave (CDW) and superconductivity (SC) in many correlated electron systems, such as cuprates, organic superconductors and dichacolgenides. Among them, the recently discovered $\mathbb{Z} _2$ topological kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs) serve as an ideal platform to study the intricate relation between th… ▽ More It is intriguing to explore the coexistence and (or) competition between charge-density-wave (CDW) and superconductivity (SC) in many correlated electron systems, such as cuprates, organic superconductors and dichacolgenides. Among them, the recently discovered $\mathbb{Z} _2$ topological kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs) serve as an ideal platform to study the intricate relation between them. Here, we report the electrical resistance measurements on CsV$_3$Sb$_5$ thin flakes ($\approx$ 60 nm) under hydrostatic pressure up to 2.12 GPa to compare its pressure phase diagram of CDW and SC with its bulk form. Even though the CDW transition temperature (T$_{CDW}$) in CsV$_3$Sb$_5$ thin flakes is still monotonically suppressed under pressure and totally vanishes at P$_2$=1.83 GPa similar to the bulk, the superconducting transition temperature (T$_c$) shows an initial decrease and consequent increase up to its maximum $\sim$ 8.03 K at P$_2$, in sharp contrast with the M-shaped double domes in the bulk CsV$_3$Sb$_5$. Our results suggest the important role of reduced dimensionality on the CDW state and its interplay with the SC, offering a new perspective to explore the exotic nature of CsV$_3$Sb$_5$. △ Less

Submitted 9 February, 2024; originally announced February 2024.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 109, 054501(2024)

On the zero-field quantization of the anomalous quantum Hall effect in moiré 2D layers

Authors: Sankar Das Sarma, Ming Xie

Abstract: In recent breakthrough experiments, twisted moiré layers of transition metal dichalcogenides are found to manifest both integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects in zero applied magnetic field because of the underlying flat band topology and spontaneous breaking of the time reversal invariance. In the current work, we critically analyze the experimental values of the qu… ▽ More In recent breakthrough experiments, twisted moiré layers of transition metal dichalcogenides are found to manifest both integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects in zero applied magnetic field because of the underlying flat band topology and spontaneous breaking of the time reversal invariance. In the current work, we critically analyze the experimental values of the quantized conductance in each case to emphasize the role of disorder in the problem, pointing out that obtaining accurate quantized conductance in future experiments would necessitate better contacts and lower disorder. △ Less

Submitted 22 February, 2024; v1 submitted 10 January, 2024; originally announced January 2024.

Comments: Updated version (including added Fig. 8) to appear in Phys. Rev. B (Letter)

Journal ref: Phys. Rev. B 109, L121104 (2024)

Electrical control and transport of tightly bound interlayer excitons in a MoSe2/hBN/MoSe2 heterostructure

Authors: Lifu Zhang, Ruihao Ni, Liuxin Gu, Ming Xie, Suji Park, Houk Jang, Takashi Taniguchi, Kenji Watanabe, You Zhou

Abstract: Controlling interlayer excitons in van der Waals heterostructures holds promise for exploring Bose-Einstein condensates and developing novel optoelectronic applications, such as excitonic integrated circuits. Despite intensive studies, several key fundamental properties of interlayer excitons, such as their binding energies and interactions with charges, remain not well understood. Here we report… ▽ More Controlling interlayer excitons in van der Waals heterostructures holds promise for exploring Bose-Einstein condensates and developing novel optoelectronic applications, such as excitonic integrated circuits. Despite intensive studies, several key fundamental properties of interlayer excitons, such as their binding energies and interactions with charges, remain not well understood. Here we report the formation of momentum-direct interlayer excitons in a high-quality MoSe2/hBN/MoSe2 heterostructure under an electric field, characterized by bright photoluminescence (PL) emission with high quantum yield and a narrow linewidth of less than 4 meV. These interlayer excitons show electrically tunable emission energy spanning ~180 meV through the Stark effect, and exhibit a sizable binding energy of ~81 meV in the intrinsic regime, along with trion binding energies of a few millielectronvolts. Remarkably, we demonstrate the long-range transport of interlayer excitons with a characteristic diffusion length exceeding ten micrometers, which can be attributed, in part, to their dipolar repulsive interactions. Spatially and polarization-resolved spectroscopic studies reveal rich exciton physics in the system, such as valley polarization, local trapping, and the possible existence of dark interlayer excitons. The formation and transport of tightly bound interlayer excitons with narrow linewidth, coupled with the ability to electrically manipulate their properties, open exciting new avenues for exploring quantum many-body physics, including excitonic condensate and superfluidity, and for developing novel optoelectronic devices, such as exciton and photon routers. △ Less

Submitted 5 April, 2024; v1 submitted 4 December, 2023; originally announced December 2023.

Comments: 4 figures

Topological interfacial states in ferroelectric domain walls of two-dimensional bismuth

Authors: Wei Luo, Yang Zhong, Hongyu Yu, Muting Xie, Yingwei Chen, Hongjun Xiang, Laurent Bellaiche

Abstract: Using machine learning methods, we explore different types of domain walls in the recently unveiled single-element ferroelectric, the bismuth monolayer [Nature 617, 67 (2023)]. Remarkably, our investigation reveals that the charged domain wall configuration exhibits lower energy compared to the uncharged domain wall structure. We also demonstrate that the experimentally discovered tail-to-tail dom… ▽ More Using machine learning methods, we explore different types of domain walls in the recently unveiled single-element ferroelectric, the bismuth monolayer [Nature 617, 67 (2023)]. Remarkably, our investigation reveals that the charged domain wall configuration exhibits lower energy compared to the uncharged domain wall structure. We also demonstrate that the experimentally discovered tail-to-tail domain wall maintains topological interfacial states caused by the change in the Z_2 number between ferroelectric and paraelectric states. Interestingly, due to the intrinsic built-in electric fields in asymmetry DW configurations, we find that the energy of topological interfacial states splits, resulting in an accidental band crossing at the Fermi level. Our study suggests that domain walls in two-dimensional bismuth hold potential as a promising platform for the development of ferroelectric domain wall devices. △ Less

Submitted 23 May, 2024; v1 submitted 8 August, 2023; originally announced August 2023.

Comments: 15 pages, 4 figs

Journal ref: Letter in PRB (2026)

Giant optical nonlinearity of Fermi polarons in atomically thin semiconductors

Authors: Liuxin Gu, Lifu Zhang, Ruihao Ni, Ming Xie, Dominik S. Wild, Suji Park, Houk Jang, Takashi Taniguchi, Kenji Watanabe, Mohammad Hafezi, You Zhou

Abstract: Realizing strong nonlinear optical responses is a long-standing goal of both fundamental and technological importance. Recently significant efforts have focused on exploring excitons in solids as a pathway to achieving nonlinearities even down to few-photon levels. However, a crucial tradeoff arises as strong light-matter interactions require large oscillator strength and short radiative lifetime… ▽ More Realizing strong nonlinear optical responses is a long-standing goal of both fundamental and technological importance. Recently significant efforts have focused on exploring excitons in solids as a pathway to achieving nonlinearities even down to few-photon levels. However, a crucial tradeoff arises as strong light-matter interactions require large oscillator strength and short radiative lifetime of the excitons, which limits their interaction strength and nonlinearity. Here we experimentally demonstrate strong nonlinear optical responses by exploiting the coupling between excitons and carriers in an atomically thin semiconductor of trilayer tungsten diselenide. By controlling the electric field and electrostatic doping of the trilayer, we observe the hybridization between intralayer and interlayer excitons along with the formation of Fermi polarons due to the interactions between excitons and free carriers. We find substantial optical nonlinearity can be achieved under both continuous wave and pulsed laser excitation, where the resonance of the hole-doped Fermi polaron blueshifts by as much as ~10 meV. Intriguingly, we observe a remarkable asymmetry in the optical nonlinearity between electron and hole doping, which is tunable by the applied electric field. We attribute these features to the strong interactions between excitons and free charges with optically induced valley polarization. Our results establish that atomically thin heterostructures are a highly versatile platform for engineering nonlinear optical response with applications to classical and quantum optoelectronics, and open avenues for exploring many-body physics in hybrid Fermionic-Bosonic systems. △ Less

Submitted 19 June, 2023; originally announced June 2023.

Comments: 4 figures with SI

Excitonic Mott insulator in a Bose-Fermi-Hubbard system of moiré $\rm{WS}_2$/$\rm{WSe}_2$ heterobilayer

Authors: Beini Gao, Daniel G. Suárez-Forero, Supratik Sarkar, Tsung-Sheng Huang, Deric Session, Mahmoud Jalali Mehrabad, Ruihao Ni, Ming Xie, Pranshoo Upadhyay, Jonathan Vannucci, Sunil Mittal, Kenji Watanabe, Takashi Taniguchi, Atac Imamoglu, You Zhou, Mohammad Hafezi

Abstract: Understanding the Hubbard model is crucial for investigating various quantum many-body states and its fermionic and bosonic versions have been largely realized separately. Recently, transition metal dichalcogenides heterobilayers have emerged as a promising platform for simulating the rich physics of the Hubbard model. In this work, we explore the interplay between fermionic and bosonic population… ▽ More Understanding the Hubbard model is crucial for investigating various quantum many-body states and its fermionic and bosonic versions have been largely realized separately. Recently, transition metal dichalcogenides heterobilayers have emerged as a promising platform for simulating the rich physics of the Hubbard model. In this work, we explore the interplay between fermionic and bosonic populations, using a $\rm{WS}_2$/$\rm{WSe}_2$ heterobilayer device that hosts this hybrid particle density. We independently tune the fermionic and bosonic populations by electronic doping and optical injection of electron-hole pairs, respectively. This enables us to form strongly interacting excitons that are manifested in a large energy gap in the photoluminescence spectrum. The incompressibility of excitons is further corroborated by measuring exciton diffusion, which remains constant upon increasing pumping intensity, as opposed to the expected behavior of a weakly interacting gas of bosons, suggesting the formation of a bosonic Mott insulator. We explain our observations using a two-band model including phase space filling. Our system provides a controllable approach to the exploration of quantum many-body effects in the generalized Bose-Fermi-Hubbard model. △ Less

Submitted 28 March, 2024; v1 submitted 19 April, 2023; originally announced April 2023.

Comments: Main text: 7 pages, 5 figures. Supplementary Material: 10 pages, 7 figures

Journal ref: Nature Communications, 15, 2305 (2024)

Flavor symmetry breaking in spin-orbit coupled bilayer graphene

Authors: Ming Xie, Sankar Das Sarma

Abstract: Recent experimental discovery of flavor symmetry breaking metallic phases in Bernal-stacked bilayer graphene points to the strongly interacting nature of electrons near the top (bottom) of its valence (conduction) band. Superconductivity was also observed in between these symmetry breaking phases when the graphene bilayer is placed under a small in-plane magnetic field or in close proximity to a m… ▽ More Recent experimental discovery of flavor symmetry breaking metallic phases in Bernal-stacked bilayer graphene points to the strongly interacting nature of electrons near the top (bottom) of its valence (conduction) band. Superconductivity was also observed in between these symmetry breaking phases when the graphene bilayer is placed under a small in-plane magnetic field or in close proximity to a monolayer WSe$_2$ substrate. Here we address the correlated nature of the band edge electrons and obtain the quantum phase diagram of their many-body ground states incorporating the effect of proximity induced spin-orbit coupling. We find that in addition to the spin/valley flavor polarized half and quarter metallic states, two types of intervalley coherent phases emerge near the phase boundaries between the flavor polarized metals. Both spin-orbit coupling and in-plane magnetic field disfavor the spin-unpolarized valley coherent phase. Our findings suggest possible competition between intervalley coherence and superconducting orders, arising from the intriguing correlation effects in bilayer graphene in the presence of spin-orbit coupling. △ Less

Submitted 21 June, 2023; v1 submitted 23 February, 2023; originally announced February 2023.

Journal ref: Phys. Rev. B 107, L201119 (2023)

Representations of Materials for Machine Learning

Authors: James Damewood, Jessica Karaguesian, Jaclyn R. Lunger, Aik Rui Tan, Mingrou Xie, Jiayu Peng, Rafael Gómez-Bombarelli

Abstract: High-throughput data generation methods and machine learning (ML) algorithms have given rise to a new era of computational materials science by learning relationships among composition, structure, and properties and by exploiting such relations for design. However, to build these connections, materials data must be translated into a numerical form, called a representation, that can be processed by… ▽ More High-throughput data generation methods and machine learning (ML) algorithms have given rise to a new era of computational materials science by learning relationships among composition, structure, and properties and by exploiting such relations for design. However, to build these connections, materials data must be translated into a numerical form, called a representation, that can be processed by a machine learning model. Datasets in materials science vary in format (ranging from images to spectra), size, and fidelity. Predictive models vary in scope and property of interests. Here, we review context-dependent strategies for constructing representations that enable the use of materials as inputs or outputs of machine learning models. Furthermore, we discuss how modern ML techniques can learn representations from data and transfer chemical and physical information between tasks. Finally, we outline high-impact questions that have not been fully resolved and thus, require further investigation. △ Less

Submitted 20 January, 2023; originally announced January 2023.

Comments: 20 pages, 5 figures, To Appear in Annual Review of Materials Research 53

Magic-Angle Twisted Symmetric Trilayer Graphene as Topological Heavy Fermion Problem

Authors: Jiabin Yu, Ming Xie, B. Andrei Bernevig, Sankar Das Sarma

Abstract: Recently, Ref. [1] reformulated magic-angle twisted bilayer graphene (MATBG) as a topological heavy fermion problem, and used this reformulation to provide a deeper understanding for the correlated phases at integer fillings. In this work, we generalize this heavy-fermion paradigm to magic-angle twisted symmetric trilayer graphene (MATSTG), and propose a low-energy $f-c-d$ model that reformulates… ▽ More Recently, Ref. [1] reformulated magic-angle twisted bilayer graphene (MATBG) as a topological heavy fermion problem, and used this reformulation to provide a deeper understanding for the correlated phases at integer fillings. In this work, we generalize this heavy-fermion paradigm to magic-angle twisted symmetric trilayer graphene (MATSTG), and propose a low-energy $f-c-d$ model that reformulates MATSTG as heavy localized $f$ modes coupled to itinerant topological semimetalic $c$ modes and itinerant Dirac $d$ modes. Our $f-c-d$ model well reproduces the single-particle band structure of MATSTG at low energies for displacement field $\mathcal{E}\in[0,300]$meV. By performing Hartree-Fock calculations with the $f-c-d$ model for $ν=0,-1,-2$ electrons per Moiré unit cell, we reproduce all the correlated ground states obtain from the previous numerical Hartree-Fock calculations with the Bistritzer-MacDonald-type (BM-type) model, and we find additional new correlated ground states at high displacement field. Based on the numerical results, we propose a simple rule for the ground states at high displacement fields by using the $f-c-d$ model, and provide analytical derivation for the rule at charge neutrality. We also provide analytical symmetry arguments for the (nearly-)degenerate energies of the high-$\mathcal{E}$ ground states at all the integer fillings of interest, and make experimental predictions of which charge-neutral states are stabilized in magnetic fields. Our $f-c-d$ model provides a new perspective for understanding the correlated phenomena in MATSTG, suggesting that the heavy fermion paradigm of Ref. [1] should be the generic underpinning of correlated physics in multilayer moire graphene structures. △ Less

Submitted 18 August, 2023; v1 submitted 10 January, 2023; originally announced January 2023.

Comments: Close to published version

Journal ref: Phys. Rev. B 108, 035129 (2023)

Nematic excitonic insulator in transition metal dichalcogenide moiré heterobilayers

Authors: Ming Xie, Haining Pan, Fengcheng Wu, Sankar Das Sarma

Abstract: We study the effect of inter-electron Coulomb interactions on the displacement field induced topological phase transition in transition metal dichalcogenide (TMD) moiré heterobilayers. We find a nematic excitonic insulator (NEI) phase that breaks the moiré superlattice's three-fold rotational symmetry and preempts the topological phase transition in both AA and AB stacked heterobilayers when the i… ▽ More We study the effect of inter-electron Coulomb interactions on the displacement field induced topological phase transition in transition metal dichalcogenide (TMD) moiré heterobilayers. We find a nematic excitonic insulator (NEI) phase that breaks the moiré superlattice's three-fold rotational symmetry and preempts the topological phase transition in both AA and AB stacked heterobilayers when the interlayer tunneling is weak, or when the Coulomb interaction is not strongly screened. The nematicity originates from the frustration between the nontrivial spatial structure of the interlayer tunneling, which is crucial to the existence of the topological Chern band, and the interlayer coherence induced by the Coulomb interaction that favors uniformity in layer pseudo-spin orientations. We construct a unified effective two-band model that captures the physics near the band inversion and applies to both AA and AB stacked heterobilayers. Within the two-band model, the competition between the NEI phase and the Chern insulator phase can be understood as the switching of the energetic order between the $s$-wave and the $p$-wave excitons upon increasing the interlayer tunneling. △ Less

Submitted 30 October, 2023; v1 submitted 24 June, 2022; originally announced June 2022.

Comments: 6 pages and 4 figures

Journal ref: Phys. Rev. Lett. 131. 046402 (2023)

Spontaneous emission modulation in biaxial hyperbolic van der Waals material

Authors: Haotuo Liu, Yang Hu, Qing Ai, Ming Xie, Xiaohu Wu

Abstract: As a natural van der Waals crystal, α-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its actively tunable properties are generally neglected. In this work, we achieved active modulation of spontaneous emission from a single-layer flat plate using the rotation method for the first time. Numerical results and theoretical analysis show that α-MoO3… ▽ More As a natural van der Waals crystal, α-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its actively tunable properties are generally neglected. In this work, we achieved active modulation of spontaneous emission from a single-layer flat plate using the rotation method for the first time. Numerical results and theoretical analysis show that α-MoO3 exhibits good tunability when rotated in the y-z or x-y plane. A modulation factor of more than three orders of magnitude can be obtained at 634 cm-1. However, when the rotation is in the x-z plane, the spontaneous emission of the material exhibits strong angle independence. The theoretical formulation and the physical mechanism analysis explain the above phenomenon well. In addition, for the semi-infinite α-MoO3 flat structure, we give the distribution of the modulation factor of spontaneous emission with wavenumber and rotation angle. Finally, we extended the calculation results from semi-infinite media to finite thickness films. We obtained the general evolution law of the peak angle of the modulation factor with thickness, increasing the modulation factor to about 2000. We believe that the results of this paper can guide the active modulation of spontaneous emission based on anisotropic materials. △ Less

Submitted 23 May, 2022; originally announced May 2022.

Euler Obstructed Cooper Pairing in Twisted Bilayer Graphene: Nematic Nodal Superconductivity and Bounded Superfluid Weight

Authors: Jiabin Yu, Ming Xie, Fengcheng Wu, Sankar Das Sarma

Abstract: Magic-angle twisted bilayer graphene (MATBG) hosts normal-state nearly-flat bands with nonzero Euler numbers and shows superconductivity. In this work, we study the effects of the nontrivial normal-state band topology on the intervalley $C_{2z}\mathcal{T}$-invariant mean-field Cooper pairing order parameter in MATBG. We show that the pairing order parameter can always be split into a trivial chann… ▽ More Magic-angle twisted bilayer graphene (MATBG) hosts normal-state nearly-flat bands with nonzero Euler numbers and shows superconductivity. In this work, we study the effects of the nontrivial normal-state band topology on the intervalley $C_{2z}\mathcal{T}$-invariant mean-field Cooper pairing order parameter in MATBG. We show that the pairing order parameter can always be split into a trivial channel and an Euler obstructed channel in all gauges for the normal-state basis, generalizing the previously-studied channel splitting in the Chern gauge. The nonzero normal-state Euler numbers require the pairing gap function of the Euler obstructed channel to have zeros, while the trivial channel can have a nonvanishing pairing gap function. When the pairing is spontaneously nematic, we find that a sufficiently-dominant Euler obstructed channel with two zeros typically leads to nodal superconductivity. Under the approximation of exactly-flat bands, we find that the mean-field zero-temperature superfluid weight is generally bounded from below, no matter whether the Euler obstructed channel is dominant or not, generalizing the previously-derived bound for the uniform s-wave pairing. We numerically verify these statements for pairings derived from a local attractive interaction. Our work suggests that Euler obstructed Cooper pairing may play an essential role in the superconducting MATBG. △ Less

Submitted 21 February, 2022; v1 submitted 4 February, 2022; originally announced February 2022.

Comments: Revised wording and updated references

Journal ref: Phys. Rev. B 107, L201106 (2023)

Low-energy Spin Dynamics of Quantum Spin Liquid Candidate $NaYbSe_{2}$

Authors: Zheng Zhang, Jianshu Li, Mingtai Xie, Weizhen Zhuo, D. T. Adroja, Peter J. Baker, T. G. Perring, Anmin Zhang, Feng Jin, Jianting Ji, Xiaoqun Wang, Jie Ma, Qingming Zhang

Abstract: The family of rare earth chalcogenides $ARECh_{2}$ (A = alkali or monovalent ions, RE = rare earth, and Ch = O, S, Se, and Te) appears as an inspiring playground for studying quantum spin liquids (QSL). The crucial low-energy spin dynamics remain to be uncovered. By employing muon spin relaxation ($μ$SR) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL i… ▽ More The family of rare earth chalcogenides $ARECh_{2}$ (A = alkali or monovalent ions, RE = rare earth, and Ch = O, S, Se, and Te) appears as an inspiring playground for studying quantum spin liquids (QSL). The crucial low-energy spin dynamics remain to be uncovered. By employing muon spin relaxation ($μ$SR) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL in $NaYbSe_{2}$, a representative member with an effective spin-1/2, and explore its unusual spin dynamics. The ZF $μ$SR experiments unambiguously rule out spin ordering or freezing in $NaYbSe_{2}$ down to 50 mK, two orders of magnitude smaller than the exchange coupling energies. The spin relaxation rate, $λ$, approaches a constant below 0.3 K, indicating finite spin excitations featured by a gapless QSL ground state. This is consistently supported by our AC susceptibility measurements. The careful analysis of the longitudinal field (LF) $μ$SR spectra reveals a strong spatial correlation and a temporal correlation in the spin-disordered ground state, highlighting the unique feature of spin entanglement in the QSL state. The observations allow us to establish an experimental H-T phase diagram. The study offers insight into the rich and exotic magnetism of the rare earth family. △ Less

Submitted 10 August, 2022; v1 submitted 14 December, 2021; originally announced December 2021.

Comments: 21 pages, 18 figures

Journal ref: Phys. Rev. B 106, 085115 (2022)

Equivalence between algorithmic instability and transition to replica symmetry breaking in perceptron learning systems

Authors: Yang Zhao, Junbin Qiu, Mingshan Xie, Haiping Huang

Abstract: Binary perceptron is a fundamental model of supervised learning for the non-convex optimization, which is a root of the popular deep learning. Binary perceptron is able to achieve a classification of random high-dimensional data by computing the marginal probabilities of binary synapses. The relationship between the algorithmic instability and the equilibrium analysis of the model remains elusive.… ▽ More Binary perceptron is a fundamental model of supervised learning for the non-convex optimization, which is a root of the popular deep learning. Binary perceptron is able to achieve a classification of random high-dimensional data by computing the marginal probabilities of binary synapses. The relationship between the algorithmic instability and the equilibrium analysis of the model remains elusive. Here, we establish the relationship by showing that the instability condition around the algorithmic fixed point is identical to the instability for breaking the replica symmetric saddle point solution of the free energy function. Therefore, our analysis would hopefully provide insights towards other learning systems in bridging the gap between non-convex learning dynamics and statistical mechanics properties of more complex neural networks. △ Less

Submitted 7 March, 2022; v1 submitted 25 November, 2021; originally announced November 2021.

Comments: 24 pages, 2 figures, revision to journal

Journal ref: Phys. Rev. Research 4, 023023 (2022)

Topological Phases in AB-Stacked MoTe$_2$/WSe$_2$: $\mathbb{Z}_2$ Topological Insulators, Chern Insulators, and Topological Charge Density Waves

Authors: Haining Pan, Ming Xie, Fengcheng Wu, Sankar Das Sarma

Abstract: We present a theory on the quantum phase diagram of AB-stacked MoTe$_2$/WSe$_2$ using a self-consistent Hartree-Fock calculation performed in the plane-wave basis, motivated by the observation of topological states in this system. At filling factor $ν=2$ (two holes per moiré unit cell), Coulomb interaction can stabilize a $\mathbb{Z}_2$ topological insulator by opening a charge gap. At $ν=1$, the… ▽ More We present a theory on the quantum phase diagram of AB-stacked MoTe$_2$/WSe$_2$ using a self-consistent Hartree-Fock calculation performed in the plane-wave basis, motivated by the observation of topological states in this system. At filling factor $ν=2$ (two holes per moiré unit cell), Coulomb interaction can stabilize a $\mathbb{Z}_2$ topological insulator by opening a charge gap. At $ν=1$, the interaction induces three classes of competing states, spin density wave states, an in-plane ferromagnetic state, and a valley polarized state, which undergo first-order phase transitions tuned by an out-of-plane displacement field. The valley polarized state becomes a Chern insulator for certain displacement fields. Moreover, we predict a topological charge density wave forming a honeycomb lattice with ferromagnetism at $ν=2/3$. Future directions on this versatile system hosting a rich set of quantum phases are discussed. △ Less

Submitted 9 August, 2022; v1 submitted 1 November, 2021; originally announced November 2021.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. Lett. 129, 056804 (2022)

Finite-size effects in one-dimensional Bose-Einstein condensation of photons

Authors: Zhi-Jie Liu, Mi Xie

Abstract: The Bose-Einstein condensation (BEC) of photons has been realized in one- and two-dimensional systems. When considering the influence of finite-size effect, the condensation in the one-dimensional fibre is of special interest since such a condensation cannot occur in the thermodynamic limit due to the linear dispersion relation of photons. The finite-size effect must play a key role in this system… ▽ More The Bose-Einstein condensation (BEC) of photons has been realized in one- and two-dimensional systems. When considering the influence of finite-size effect, the condensation in the one-dimensional fibre is of special interest since such a condensation cannot occur in the thermodynamic limit due to the linear dispersion relation of photons. The finite-size effect must play a key role in this system and needs a detailed description. However, the previous theoretical analysis of finite-size effect is often not accurate enough and only gives the leading-order contribution due to a divergence difficulty. In this paper, by using an analytical continuation method to overcome the divergence difficulty, we give an analytical treatment for the finite-size effect in BEC. The analytical expressions of the critical temperature or critical particle number with higher order correction and the chemical potential below the transition point are presented. Our result shows that in a recent experiment, the deviation between experiment and theory is overestimated, most of which is caused by the inaccurate theoretical treatment of the finite-size effect. By taking into account the next-to-leading correction, we find that the actual deviation is much smaller. △ Less

Submitted 31 October, 2021; originally announced November 2021.

Comments: 10 pages, 1 figure

Materials challenges for quantum technologies based on color centers in diamond

Authors: Lila V. H. Rodgers, Lillian B. Hughes, Mouzhe Xie, Peter C. Maurer, Shimon Kolkowitz, Ania C. Bleszynski Jayich, Nathalie P. de Leon

Abstract: Emerging quantum technologies require precise control over quantum systems of increasing complexity. Defects in diamond, particularly the negatively charged nitrogen-vacancy (NV) center, are a promising platform with the potential to enable technologies ranging from ultra-sensitive nanoscale quantum sensors, to quantum repeaters for long distance quantum networks, to simulators of complex dynamica… ▽ More Emerging quantum technologies require precise control over quantum systems of increasing complexity. Defects in diamond, particularly the negatively charged nitrogen-vacancy (NV) center, are a promising platform with the potential to enable technologies ranging from ultra-sensitive nanoscale quantum sensors, to quantum repeaters for long distance quantum networks, to simulators of complex dynamical processes in many-body quantum systems, to scalable quantum computers. While these advances are due in large part to the distinct material properties of diamond, the uniqueness of this material also presents difficulties, and there is a growing need for novel materials science techniques for characterization, growth, defect control, and fabrication dedicated to realizing quantum applications with diamond. In this review we identify and discuss the major materials science challenges and opportunities associated with diamond quantum technologies. △ Less

Submitted 22 June, 2021; originally announced June 2021.

A group method solving many-body systems in intermediate statistical representation

Authors: Yao Shen, Chi-Chun Zhou, Wu-sheng Dai, Mi Xie

Abstract: The exact solution of the interacting many-body system is important and is difficult to solve. In this paper, we introduce a group method to solve the interacting many-body problem using the relation between the permutation group and the unitary group. We prove a group theorem first, then using the theorem, we represent the Hamiltonian of the interacting many-body system by the Casimir operators o… ▽ More The exact solution of the interacting many-body system is important and is difficult to solve. In this paper, we introduce a group method to solve the interacting many-body problem using the relation between the permutation group and the unitary group. We prove a group theorem first, then using the theorem, we represent the Hamiltonian of the interacting many-body system by the Casimir operators of unitary group. The eigenvalues of Casimir operators could give the exact values of energy and thus solve those problems exactly. This method maps the interacting many-body system onto an intermediate statistical representation. We give the relation between the conjugacy-class operator of permutation group and the Casimir operator of unitary group in the intermediate statistical representation, called the Gentile representation. Bose and Fermi cases are two limitations of the Gentile representation. We also discuss the representation space of symmetric and unitary group in the Gentile representation and give an example of the Heisenberg model to demonstrate this method. It is shown that this method is effective to solve interacting many-body problems. △ Less

Submitted 26 May, 2021; originally announced May 2021.

Comments: 11 pages, 0 figures

A Two-Kind-Boson Mixture Honeycomb Hamiltonian of Bloch Exciton-Polaritons

Authors: Haining Pan, K. Winkler, Mats Powlowski, Ming Xie, A. Schade, M. Emmerling, M. Kamp, S. Klemt, C. Schneider, Tim Byrnes, S. Hoefling, Na Young Kim

Abstract: The electronic bandstructure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the bandstructure with band parameters explains intrinsic physical, chemical and mechanical properties of the solid. Here we report the artificial polaritonic bandstructures of two-dimensional honeycomb lattice… ▽ More The electronic bandstructure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the bandstructure with band parameters explains intrinsic physical, chemical and mechanical properties of the solid. Here we report the artificial polaritonic bandstructures of two-dimensional honeycomb lattices for microcavity exciton-polaritons using GaAs semiconductors in the wide-range detuning values, from cavity-photon-like (red-detuned) to exciton-like (blue-detuned) regimes. In order to understand the experimental bandstructures and their band parameters, such as gap energies, bandwidths, hopping integrals and density of states, we originally establish a polariton band theory within an augmented plane wave method with two-kind-bosons, cavity photons trapped at the lattice sites and freely moving excitons. In particular, this two-kind-band theory is absolutely essential to elucidate the exciton effect in the bandstructures of blue-detuned exciton-polaritons, where the flattened exciton-like dispersion appears at larger in-plane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values. △ Less

Submitted 1 May, 2021; originally announced May 2021.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. B 99, 045302 (2019)