Showing 1–50 of 350 results for author: Liu, R

A Cohomological Framework for Topological Phases from Momentum-Space Crystallographic Groups

Authors: T. R. Liu, Zheng Zhang, Y. X. Zhao

Abstract: Crystallographic groups are conventionally studied in real space to characterize crystal symmetries. Recent work has recognized that when these symmetries are realized projectively, momentum space inherently accommodates nonsymmorphic symmetries, thereby evoking the concept of \textit{momentum-space crystallographic groups} (MCGs). Here, we reveal that the cohomology of MCGs encodes fundamental da… ▽ More Crystallographic groups are conventionally studied in real space to characterize crystal symmetries. Recent work has recognized that when these symmetries are realized projectively, momentum space inherently accommodates nonsymmorphic symmetries, thereby evoking the concept of \textit{momentum-space crystallographic groups} (MCGs). Here, we reveal that the cohomology of MCGs encodes fundamental data of crystalline topological band structures. Specifically, the collection of second cohomology groups, $H^2(Γ_F,\mathbb{Z})$, for all MCGs $Γ_F$, provides an exhaustive classification of Abelian crystalline topological insulators, serving as an effective approximation to the full crystalline topological classification. Meanwhile, the third cohomology groups $H^3(Γ_F,\mathbb{Z})$ across all MCGs exhaustively classify all possible twistings of point-group actions on the Brillouin torus, essential data for twisted equivariant K-theory. Furthermore, we establish the isomorphism $H^{n+1}(Γ_F,\mathbb{Z})\cong H^n\big(Γ_F,\operatorname{\mathcal{F}}(\mathbb{R}^d_F,U(1))\big)$ for $ n\ge 1$, where $\operatorname{\mathcal{F}}(\mathbb{R}^d_F,U(1))$ denotes the space of continuous $U(1)$-valued functions on the $d$D momentum space $\mathbb{R}^d_F$. The case $n=1$ yields a complete set of topological invariants formulated in purely algebraic terms, which differs fundamentally from the conventional formulation in terms of differential forms. The case $n=2$, analogously, provides a fully algebraic description for all such twistings. Thus, the cohomological theory of MCGs serves as a key technical framework for analyzing crystalline topological phases within the general setting of projective symmetry. △ Less

Submitted 25 December, 2025; originally announced December 2025.

Comments: 16 pages, 2 figures and supplementary materials

Magnetic properties of molecular beam epitaxy-grown ultrathin Cr2Ge2Te6 films down to monolayer limit on Si substrates

Authors: Pengfei Ji, Ruixuan Liu, Tianchen Zhu, Jinxuan Liang, Yang Chen, Yitian Tong, Yunhe Bai, Zuhan Geng, Fangting Chen, Yunyi Zang, Xiyu Hong, Jiatong Zhang, Luyi Yang, Qi-Kun Xue, Ke He, Xiao Feng

Abstract: Cr2Ge2Te6, a prototypical van der Waals ferromagnetic semiconductor, have attracted significant interest for its potential applications in high-performance spintronics. However, the magnetic ground state of monolayer Cr2Ge2Te6 remains elusive due to fragile and irregular-shaped thin flake samples with weak magnetic signals. Here, we successfully grow uniform ferromagnetic Cr2Ge2Te6 films down to m… ▽ More Cr2Ge2Te6, a prototypical van der Waals ferromagnetic semiconductor, have attracted significant interest for its potential applications in high-performance spintronics. However, the magnetic ground state of monolayer Cr2Ge2Te6 remains elusive due to fragile and irregular-shaped thin flake samples with weak magnetic signals. Here, we successfully grow uniform ferromagnetic Cr2Ge2Te6 films down to monolayer by molecular beam epitaxy. By exploiting a self-limiting growth mode, we achieve synthesis of uniform monolayer Cr2Ge2Te6 films across entire millimeter-scale Si substrates. Through a combination of superconducting quantum interference device magnetometry and anomalous Hall effect measurements, we establish that monolayer Cr2Ge2Te6 exhibits intrinsic ferromagnetism with perpendicular magnetic anisotropy below ~10 K, albeit with strong magnetic fluctuations characteristic of its two-dimensional nature. Furthermore, a systematic thickness-dependent study reveals a crossover from this fluctuation-dominated two-dimensional magnetism turns into conventional long-range ferromagnetic order as the film thickness increases. Our work not only definitively establishes the intrinsic ferromagnetic ground state of monolayer Cr2Ge2Te6, but also provides a scalable, silicon-compatible route for preparing the two-dimensional magnet for future spintronic or quantum devices. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Spin-flop driven interfacial tunneling magnetoresistance in an antiferromagnetic tunnel junction

Authors: Xiaolin Ren, Ruizi Liu, Yiyang Zhang, Yuting Liu, Xuezhao Wu, Kun Qian, Kenji Watanabe, Takashi Taniguchi, Qiming Shao

Abstract: The utilization of two-dimensional (2D) materials in magnetic tunnel junctions (MTJs) has shown excellent performance and rich physics. As for 2D antiferromagnets, the magnetic moments in different layers respond asynchronously and can be configured at various states under different magnetic fields, showing the possibility of efficient magnetic and electrical tunability. In this report, A-type ant… ▽ More The utilization of two-dimensional (2D) materials in magnetic tunnel junctions (MTJs) has shown excellent performance and rich physics. As for 2D antiferromagnets, the magnetic moments in different layers respond asynchronously and can be configured at various states under different magnetic fields, showing the possibility of efficient magnetic and electrical tunability. In this report, A-type antiferromagnetic (AFM) material (Fe0.5Co0.5)5GeTe2 (FCGT) works as electrodes to realize full van der Waals magnetic tunnel junctions. Owing to the interfacial effect, the even-layer FCGT, although with zero net magnetization, exhibits spin selectivity in MTJ architecture contributing to a tunneling magnetoresistance (TMR) reaching about 25% at a low operating current 1 nA at 100 K and persists near room temperature. Due to the surface spin-flop (SSF) effect in antiferromagnetic FCGT, the alternation flexibility between the volatile and nonvolatile memory behavior is achieved. The interfacial TMR can be tuned efficiently in amplitude and even sign under different bias currents and temperatures. These findings show precise magnetoelectric manipulation in MTJs based on 2D antiferromagnets and highlight the promise of 2D antiferromagnets for spintronic devices. △ Less

Submitted 3 December, 2025; originally announced December 2025.

Chemical vapor deposition growth of continuous monolayer antiferromagnetic CrOCl films

Authors: Chao Chen, Yulu Liu, Hongyan Lu, Zihao Wang, Bowen Zheng, Qian Guo, Jingkuan Xiao, Ping Wang, Wanting Xu, Yulin Han, Mingxuan Chen, Xiaofan Cai, Jiabei Huang, Yaqing Han, Di Zhang, Renjun Du, Alexander S. Mayorov, Ziying Li, Shuai Zhang, Yi Huang, Tingting Cheng, Zhaolong Chen, Ronghua Liu, Nujiang Tang, Haibo Ni , et al. (7 additional authors not shown)

Abstract: The discovery of two-dimensional magnetic materials has provided an ideal platform for exploring physical phenomena in the two-dimensional limit. However, intrinsic two-dimensional antiferromagnetic materials have been rarely reported, limiting systematic studies of their electronic properties. The discovery of novel intrinsic two-dimensional antiferromagnets and the development of robust synthesi… ▽ More The discovery of two-dimensional magnetic materials has provided an ideal platform for exploring physical phenomena in the two-dimensional limit. However, intrinsic two-dimensional antiferromagnetic materials have been rarely reported, limiting systematic studies of their electronic properties. The discovery of novel intrinsic two-dimensional antiferromagnets and the development of robust synthesis strategies, therefore, remain significant challenges. Here, we report the chemical vapor deposition synthesis of CrOCl monolayer films and nanosheets that exhibit excellent air stability. The CrOCl morphology is tunable, ranging from two-dimensional nanosheets to three-dimensional flower-like structures, with lateral sizes ranging from several microns to continuous monolayer films. Structural characterization confirms the materials composition and high crystalline quality. Furthermore, magnetic measurements, supported by theoretical calculations, reveal a Néel temperature for CrOCl of ~14 K. This work provides a reliable route for preparing two-dimensional antiferromagnetic materials. △ Less

Submitted 18 November, 2025; originally announced November 2025.

A Fast, Accurate, and Reactive Equivariant Foundation Potential

Authors: Tsz Wai Ko, Runze Liu, Adesh Rohan Mishra, Zihan Yu, Ji Qi, Shyue Ping Ong

Abstract: Electrostatics govern charge transfer and reactivity in materials. Yet, most foundation potentials (FPs) either do not explicitly model such interactions or pay a prohibitive scaling penalty to do so. Here, we introduce charge-equilibrated TensorNet (QET), an equivariant, charge-aware architecture that attains linear scaling with system size via an analytically solvable charge-equilibration scheme… ▽ More Electrostatics govern charge transfer and reactivity in materials. Yet, most foundation potentials (FPs) either do not explicitly model such interactions or pay a prohibitive scaling penalty to do so. Here, we introduce charge-equilibrated TensorNet (QET), an equivariant, charge-aware architecture that attains linear scaling with system size via an analytically solvable charge-equilibration scheme. We demonstrate that a trained QET FP matches state-of-the-art FPs on standard materials property benchmarks but delivers qualitatively different predictions in systems dominated by charge transfer. The QET FP reproduces the correct structure and density of the NaCl-CaCl2 ionic liquid, which charge-agnostic FPs miss. We further show that a fine-tuned QET captures reactive processes at the Li/Li6PS5Cl solid-electrolyte interface and supports simulations under applied electrochemical potentials. These results remove a fundamental constraint in the atomistic simulation of accurate electrostatics at scale and establish a general, data-driven framework for charge-aware FPs with transformative applications in energy storage, catalysis, and beyond. △ Less

Submitted 10 November, 2025; v1 submitted 10 November, 2025; originally announced November 2025.

Comments: 39 pages (29 for Manuscript + 10 for SI), 13 figures (6 for Manuscript + 7 for SI)

Twisted locality-preserving automorphisms, anomaly index, and generalized Lieb-Schultz-Mattis theorems with anti-unitary symmetries

Authors: Ruizhi Liu, Jinmin Yi, Liujun Zou

Abstract: Symmetries and their anomalies are powerful tools to understand quantum matter. In this work, for quantum spin chains, we define twisted locality-preserving automorphisms and their Gross-Nesme-Vogts-Werner indices, which provide a unified framework to describe both unitary and anti-unitary symmetries, on-site and non-on-site symmetries, and internal and translation symmetries. For a symmetry $G$ w… ▽ More Symmetries and their anomalies are powerful tools to understand quantum matter. In this work, for quantum spin chains, we define twisted locality-preserving automorphisms and their Gross-Nesme-Vogts-Werner indices, which provide a unified framework to describe both unitary and anti-unitary symmetries, on-site and non-on-site symmetries, and internal and translation symmetries. For a symmetry $G$ with actions given by twisted locality-preserving automorphisms, we give a microscopic definition of its anomaly index, which is an element in $H^3_\varphi(G; U(1))$, where the subscript $\varphi$ means that anti-unitary elements of $G$ act on $U(1)$ by complex conjugation. We show that an anomalous symmetry leads to multiple Lieb-Schultz-Matttis-type theorems. In particular, any state with an anomalous symmetry must either have long-range correlation or violate the entanglement area law. Based on this theorem, we further deduce that any state with an anomalous symmetry must have long-range entanglement, and any Hamiltonian that has an anomalous symmetry cannot have a unique gapped symmetric ground state, as long as the interactions in the Hamiltonian decay fast enough as the range of the interaction increases. For Hamiltonians with only two-spin interactions, the last theorem holds if the interactions decay faster than $1/r^2$, where $r$ is the distance between the two interacting spins. We demonstrate these general theorems in various concrete examples. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Comments: 24 pages + appendices + references

Lovász Meets Lieb-Schultz-Mattis: Complexity in Approximate Quantum Error Correction

Authors: Jinmin Yi, Ruizhi Liu, Zhi Li

Abstract: Approximate quantum error correction (AQEC) provides a versatile framework for both quantum information processing and probing many-body entanglement. We reveal a fundamental tension between the error-correcting power of an AQEC and the hardness of code state preparation. More precisely, through a novel application of the Lovász local lemma, we establish a fundamental trade-off between local indis… ▽ More Approximate quantum error correction (AQEC) provides a versatile framework for both quantum information processing and probing many-body entanglement. We reveal a fundamental tension between the error-correcting power of an AQEC and the hardness of code state preparation. More precisely, through a novel application of the Lovász local lemma, we establish a fundamental trade-off between local indistinguishability and circuit complexity, showing that orthogonal short-range entangled states must be distinguishable via a local operator. These results offer a powerful tool for exploring quantum circuit complexity across diverse settings. As applications, we derive stronger constraints on the complexity of AQEC codes with transversal logical gates and establish strong complexity lower bounds for W state preparation. Our framework also provides a novel perspective for systems with Lieb-Schultz-Mattis type constraints. △ Less

Submitted 5 October, 2025; originally announced October 2025.

Patterning programmable spin arrays on DNA origami for quantum technologies

Authors: Zhiran Zhang, Taylor Morrison, Lillian Hughes, Weijie Wu, Ruiyao Liu, Dolev Bluvstein, Norman Yao, Deborah Fygenson, Ania C. Bleszynski Jayich

Abstract: The controlled assembly of solid-state spins with nanoscale spatial precision is an outstanding challenge for quantum technology. Here, we combine DNA-based patterning with nitrogen-vacancy (NV) ensemble quantum sensors in diamond to form and sense programmable 2D arrays of spins. We use DNA origami to control the spacing of chelated Gd$^{3+}$ spins, as verified by the observed linear relationship… ▽ More The controlled assembly of solid-state spins with nanoscale spatial precision is an outstanding challenge for quantum technology. Here, we combine DNA-based patterning with nitrogen-vacancy (NV) ensemble quantum sensors in diamond to form and sense programmable 2D arrays of spins. We use DNA origami to control the spacing of chelated Gd$^{3+}$ spins, as verified by the observed linear relationship between proximal NVs' relaxation rate, $1/T_1$, and the engineered number of Gd$^{3+}$ spins per origami unit. We further show that DNA origami provides a robust way of functionalizing the diamond surface with spins as it preserves the charge state and spin coherence of proximal, shallow NV centers. Our work enables the formation and interrogation of ordered, strongly interacting spin networks with applications in quantum sensing and quantum simulation. We quantitatively discuss the prospects of entanglement-enhanced metrology and high-throughput proteomics. △ Less

Submitted 12 September, 2025; originally announced September 2025.

Magnetic excitations in biaxial-strain detwinned $α$-RuCl$_{3}$

Authors: Yi Li, Yanyan Shangguan, Xinzhe Wang, Ruixian Liu, Chang Liu, Yongqi Han, Zhaosheng Wang, Christian Balz, Ross Stewart, Shun-Li Yu, Jinsheng Wen, Jian-Xin Li, Xingye Lu

Abstract: The honeycomb magnet $α$-RuCl$_{3}$ has been a leading candidate for realizing the Kitaev quantum spin liquid (QSL), but its intrinsic spin dynamics have remained obscured by crystal twinning. Here we apply biaxial anisotropic strain to detwin $α$-RuCl$_{3}$ single crystals and directly visualize the intrinsic magnetic excitations using inelastic neutron scattering. We discover that the low-energy… ▽ More The honeycomb magnet $α$-RuCl$_{3}$ has been a leading candidate for realizing the Kitaev quantum spin liquid (QSL), but its intrinsic spin dynamics have remained obscured by crystal twinning. Here we apply biaxial anisotropic strain to detwin $α$-RuCl$_{3}$ single crystals and directly visualize the intrinsic magnetic excitations using inelastic neutron scattering. We discover that the low-energy spin waves emerge from the $M$ points -- transverse to the magnetic Bragg peaks -- providing direct evidence of anisotropic magnetic interactions in $α$-RuCl$_{3}$. The intrinsic spin-wave spectrum imposes stringent constraints on the extended Kitaev Hamiltonian, yielding a refined, quantitatively consistent set of exchange couplings for the zigzag ground state and its low-energy dynamics. Above the magnon band, we uncover broad excitation continua: while a twofold-symmetric feature near 6 meV at $Γ$ is consistent with bimagnon scattering, the dominant spectral weight forms a sixfold-symmetric continuum extending up to $\sim 16$ meV that cannot be explained by conventional magnons. This strongly supports the presence of fractionalized excitations-a hallmark of Kitaev QSL physics. Our findings establish biaxial strain as a powerful symmetry-breaking probe to access the intrinsic spin dynamics of Kitaev materials and provide critical benchmarks for refining theoretical models of quantum magnetism in $α$-RuCl$_{3}$. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Comments: 8 pages, 4 figures

Signatures of three-state Potts nematicity in spin excitations of the van der Waals antiferromagnet FePSe$_3$

Authors: Weiliang Yao, Viviane Peçanha Antonio, Devashibhai Adroja, S. J. Gomez Alvarado, Bin Gao, Sijie Xu, Ruixian Liu, Xingye Lu, Pengcheng Dai

Abstract: In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with twofold rotational ($C_2$) symmetry that profoundly impacts their properties. For 2D materials with threefold rotational ($C_3$) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet… ▽ More In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with twofold rotational ($C_2$) symmetry that profoundly impacts their properties. For 2D materials with threefold rotational ($C_3$) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet (AFM) FePSe$_3$ via optical and thermodynamic methods under uniaxial strain. Here, we use neutron scattering to study the magnetic order and spin excitations of FePSe$_3$ under uniaxial strain. In the AFM ordered state, we find that $\sim$0.6% tensile strain significantly suppresses one zigzag domain and promotes the other two, lowering the AFM order and spin waves to $C_2$ symmetry. The broken $C_3$ symmetry in spin excitations persists slightly above $T_{\rm{N}}\approx 108.6$ K, where the zigzag AFM order is absent. Our results thus provide direct evidence of magnetoelastic coupling and suggest that the three-state Potts nematicity in paramagnetic spin excitations arises from the vestigial order associated with the low-temperature zigzag AFM order. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Correlation tuned Fermi-arc topology in a Weyl ferromagnet

Authors: Yiran Peng, Rui Liu, Pengyu Zheng, Zhiping Yin

Abstract: Electrons on Fermi arcs (FAs), a hallmark of Weyl semimetals, exhibit chiral transport harboring chiral anomaly, negative magnetoresistance, and Majorana zero modes. While FAs were observed in exemplary Weyl semimetal TaAs and Co3Sn2S2, the manipulation of FAs has been rarely explored. Here we take Co3Sn2S2 as an example and demonstrate that tuning the electronic correlation strength is an effecti… ▽ More Electrons on Fermi arcs (FAs), a hallmark of Weyl semimetals, exhibit chiral transport harboring chiral anomaly, negative magnetoresistance, and Majorana zero modes. While FAs were observed in exemplary Weyl semimetal TaAs and Co3Sn2S2, the manipulation of FAs has been rarely explored. Here we take Co3Sn2S2 as an example and demonstrate that tuning the electronic correlation strength is an effective way to control the topology and connectivity of FAs. After achieving a good agreement with experimentally measured band structure by employing combined density functional theory and dynamical mean field theory (DFT+DMFT) calculations, we show that the experimental charge dynamics are well reproduced by DFT+DMFT calculations but not DFT calculations. Electronic correlation renormalizes the bands around the Fermi level and modifies the energy and location of Weyl points, and the resulting FAs. In particular, on the Co-terminated surface, the FAs are formed by connecting Weyl points located in adjacent Brillouin zones in DFT+DMFT calculations and experiments, in strong contrast to the FAs connecting Weyl points within the same Brillouin zone in DFT calculations. We further show the evolution of FAs with correlation and reveal a topological change of the FAs on the Sn-terminated surface at stronger correlation strength. Our study sheds new light on experimental manipulation of FAs to improve the electronic properties of correlated Weyl semimetals. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Investigating CO Adsorption on Cu(111) and Rh(111) Surfaces Using Machine Learning Exchange-Correlation Functionals

Authors: Xinyuan Liang, Renxi Liu, Mohan Chen

Abstract: The "CO adsorption puzzle", a persistent failure of utilizing generalized gradient approximations (GGA) in density functional theory to replicate CO's experimental preference for top-site adsorption on transition-metal surfaces, remains a critical barrier in surface chemistry. While hybrid functionals such as HSE06 partially resolve this discrepancy, their prohibitive computational cost limits bro… ▽ More The "CO adsorption puzzle", a persistent failure of utilizing generalized gradient approximations (GGA) in density functional theory to replicate CO's experimental preference for top-site adsorption on transition-metal surfaces, remains a critical barrier in surface chemistry. While hybrid functionals such as HSE06 partially resolve this discrepancy, their prohibitive computational cost limits broader applications. We tackle this issue by adopting the Deep Kohn-Sham (DeePKS) method to train machine-learned exchange-correlation functionals. Principal component analysis reveals that the input descriptors for electronic structures separate distinctly across different chemical environments, enabling the DeePKS models to generalize to multi-element systems. We train system-specific DeePKS models for transition-metal surfaces Cu(111) and Rh(111). These models successfully recover experimental site preferences, yielding adsorption energy differences of about 10 meV compared to HSE06. Furthermore, a single model for the two surfaces is trained, and the model achieves comparable accuracy in predicting not only adsorption energies and site preference but also potential energy surfaces and relaxed surface adsorption structures. The above work demonstrates a promising path towards universal models, enabling catalyst exploration with hybrid functional accuracy at substantially reduced cost. △ Less

Submitted 29 July, 2025; originally announced July 2025.

The stability of long-range order in disordered systems: A generalized Ding-Zhuang argument

Authors: Yejia Chen, Jianwen Zhou, Ruifeng Liu, Hai-Jun Zhou

Abstract: The stability of long-range order against quenched disorder is a central problem in statistical mechanics. This paper develops a generalized framework extending the Ding-Zhuang method and integrated with the Pirogov-Sinai framework, establishing a systematic scheme for studying phase transitions of long-range order in disordered systems. We axiomatize the Ding-Zhuang approach into a theoretical fr… ▽ More The stability of long-range order against quenched disorder is a central problem in statistical mechanics. This paper develops a generalized framework extending the Ding-Zhuang method and integrated with the Pirogov-Sinai framework, establishing a systematic scheme for studying phase transitions of long-range order in disordered systems. We axiomatize the Ding-Zhuang approach into a theoretical framework consisting of the Peierls condition and a local symmetry condition. For systems in dimensions $d \geq 3$ satisfying these conditions, we prove the persistence of long-range order at low temperatures and under weak disorder, with multiple coexisting distinct Gibbs states. The framework's versatility is demonstrated for diverse models, providing a systematic extension of Peierls methods to disordered systems. △ Less

Submitted 15 July, 2025; originally announced July 2025.

Comments: 43 pages

MSC Class: 82B44; 60K35; 82B26; 82B20

Finite-size scaling of percolation on scale-free networks

Authors: Xuewei Zhao, Liwenying Yang, Dan Peng, Run-Ran Liu, Ming Li

Abstract: Critical phenomena on scale-free networks with a degree distribution $p_k \sim k^{-λ}$ exhibit rich finite-size effects due to its structural heterogeneity. We systematically study the finite-size scaling of percolation and identify two distinct crossover routes to mean-field behavior: one controlled by the degree exponent $λ$, the other by the degree cutoff $K \sim V^κ$, where $V$ is the system s… ▽ More Critical phenomena on scale-free networks with a degree distribution $p_k \sim k^{-λ}$ exhibit rich finite-size effects due to its structural heterogeneity. We systematically study the finite-size scaling of percolation and identify two distinct crossover routes to mean-field behavior: one controlled by the degree exponent $λ$, the other by the degree cutoff $K \sim V^κ$, where $V$ is the system size and $κ\in [0,1]$ is the cutoff exponent. Increasing $λ$ or decreasing $κ$ suppresses heterogeneity and drives the system toward mean-field behavior, with logarithmic corrections near the marginal case. These findings provide a unified picture of the crossover from heterogeneous to homogeneous criticality. In the crossover regime, we observe rich finite-size phenomena, including the transition from vanishing to divergent susceptibility, distinct exponents for the shift and fluctuation of pseudocritical points, and a numerical clarification of previous theoretical predictions. △ Less

Submitted 24 August, 2025; v1 submitted 8 July, 2025; originally announced July 2025.

Comments: 10 pages, 9 figures

Journal ref: Chaos, Solitons & Fractals 200, 117076 (2025)

Correlation-driven quantum geometry effects in a Kondo system

Authors: Ruizi Liu, Zehan Chen, Xingkai Cheng, Xiaolin Ren, Yiyang Zhang, Xuezhao Wu, Chengping Zhang, Kun Qian, Ching Ho Chan, Junwei Liu, Kam Tuen Law, Qiming Shao

Abstract: Quantum geometry, including quantum metric and Berry curvature, which describes the topology of electronic states, can induce fascinating physical properties. Symmetry-dependent nonlinear transport has emerged as a sensitive probe of these quantum geometric properties. However, its interplay with strong electronic correlations has rarely been explored in bulk materials, particularly in a Kondo lat… ▽ More Quantum geometry, including quantum metric and Berry curvature, which describes the topology of electronic states, can induce fascinating physical properties. Symmetry-dependent nonlinear transport has emerged as a sensitive probe of these quantum geometric properties. However, its interplay with strong electronic correlations has rarely been explored in bulk materials, particularly in a Kondo lattice system. Here, we uncover correlation-driven quantum geometry in centrosymmetric antiferromagnetic iron telluride (FeTe). We experimentally observe the quantum metric quadrupole-induced third-order nonlinear transport, whose angular dependence reflects magnetic structure in FeTe. The nonlinear transport signals follow Kondo lattice crossover and vanish at high temperatures. Our theory suggests that a Kondo lattice formed at low temperatures explains the emergence of quantum geometry, which is induced by the opening of a hybridization gap near the Fermi energy. This discovery establishes a paradigm where quantum geometry arises not from static symmetry breaking but from dynamic many-body effects and provides a zero-field probe for sensing antiferromagnetic order. △ Less

Submitted 2 July, 2025; originally announced July 2025.

Comments: 19 pages, 4 figures for the manuscript

Mic-hackathon 2024: Hackathon on Machine Learning for Electron and Scanning Probe Microscopy

Authors: Utkarsh Pratiush, Austin Houston, Kamyar Barakati, Aditya Raghavan, Dasol Yoon, Harikrishnan KP, Zhaslan Baraissov, Desheng Ma, Samuel S. Welborn, Mikolaj Jakowski, Shawn-Patrick Barhorst, Alexander J. Pattison, Panayotis Manganaris, Sita Sirisha Madugula, Sai Venkata Gayathri Ayyagari, Vishal Kennedy, Ralph Bulanadi, Michelle Wang, Kieran J. Pang, Ian Addison-Smith, Willy Menacho, Horacio V. Guzman, Alexander Kiefer, Nicholas Furth, Nikola L. Kolev , et al. (48 additional authors not shown)

Abstract: Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains d… ▽ More Microscopy is a primary source of information on materials structure and functionality at nanometer and atomic scales. The data generated is often well-structured, enriched with metadata and sample histories, though not always consistent in detail or format. The adoption of Data Management Plans (DMPs) by major funding agencies promotes preservation and access. However, deriving insights remains difficult due to the lack of standardized code ecosystems, benchmarks, and integration strategies. As a result, data usage is inefficient and analysis time is extensive. In addition to post-acquisition analysis, new APIs from major microscope manufacturers enable real-time, ML-based analytics for automated decision-making and ML-agent-controlled microscope operation. Yet, a gap remains between the ML and microscopy communities, limiting the impact of these methods on physics, materials discovery, and optimization. Hackathons help bridge this divide by fostering collaboration between ML researchers and microscopy experts. They encourage the development of novel solutions that apply ML to microscopy, while preparing a future workforce for instrumentation, materials science, and applied ML. This hackathon produced benchmark datasets and digital twins of microscopes to support community growth and standardized workflows. All related code is available at GitHub: https://github.com/KalininGroup/Mic-hackathon-2024-codes-publication/tree/1.0.0.1 △ Less

Submitted 27 June, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Superatomic hydrogen: achieving effective aggregation of hydrogen atoms at pressures lower than that of metallic hydrogen

Authors: Jia Fan, Chenxi Wan, Rui Liu, Zhen Gong, Hongbo Jing, Baiqiang Liu, Siyang Liu, Zhigang Wang

Abstract: Metal hydrogen exhibiting electron delocalization properties has been recognized as an important prospect for achieving controlled nuclear fusion, but the extreme pressure conditions required exceeding hundreds of GPa remain a daunting challenge. Here, we propose a model of superatomic hydrogen, aiming to reduce the pressure conditions required for the effective aggregation of elemental hydrogen a… ▽ More Metal hydrogen exhibiting electron delocalization properties has been recognized as an important prospect for achieving controlled nuclear fusion, but the extreme pressure conditions required exceeding hundreds of GPa remain a daunting challenge. Here, we propose a model of superatomic hydrogen, aiming to reduce the pressure conditions required for the effective aggregation of elemental hydrogen atoms. High-precision ab initio calculations indicate that the pressure required to compress the H13 system with one central atom and 12 surrounding atoms into a superatomic state is approximately two orders of magnitude lower than that of metallic hydrogen. Atomic-level analyses reveal that in the superatomic state of compressed H13, the central H atom donates its electron, and all electrons are delocalized on the superatomic molecular orbitals, which conforms to properties of metallic hydrogen. Our discovery in principle opens up the prospect of superatomic hydrogen in areas such as nuclear fusion. △ Less

Submitted 3 June, 2025; originally announced June 2025.

Comments: 7 pages, 3 figures

QR$^2$-code: An open-source program for double resonance Raman spectra

Authors: Jianqi Huang, Renhui Liu, Ye Zhang, Nguyen Tuan Hung, Huaihong Guo, Riichiro Saito, Teng Yang

Abstract: We present an open-source program, QR$^2$-code, that computes double-resonance Raman (DRR) spectra using first-principles calculations. QR$^2$-code can calculate not only two-phonon DRR spectra but also single-resonance Raman spectra and defect-induced DRR spectra. For defect-induced DDR spectra, we simply assume that the electron-defect matrix element of elastic scattering is a constant. Hands-on… ▽ More We present an open-source program, QR$^2$-code, that computes double-resonance Raman (DRR) spectra using first-principles calculations. QR$^2$-code can calculate not only two-phonon DRR spectra but also single-resonance Raman spectra and defect-induced DRR spectra. For defect-induced DDR spectra, we simply assume that the electron-defect matrix element of elastic scattering is a constant. Hands-on tutorials for graphene are given to show how to run QR$^2$-code for single-resonance, double-resonance, and defect-induced Raman spectra. We also compare the single-resonance Raman spectra by QR$^2$-code with that by QERaman code. In QR$^2$-code, the energy dispersions of electron and phonon are taken from Quantum ESPRESSO (QE) code, and the electron-phonon matrix element is obtained from the electron-phonon Wannier (EPW) code. All codes, examples, and scripts are available on the GitHub repository. △ Less

Submitted 15 May, 2025; originally announced May 2025.

Comments: 13 pages, 5 figures, 2 tables

Post-buckling of fiber-reinforced soft tissues

Authors: Yang Liu, Rui-Cheng Liu, Wanyu Ma, Alain Goriely

Abstract: Fiber-reinforcement is a universal feature of many biological tissues. It involves the interplay between fiber stiffness, fiber orientation, and the elastic properties of the matrix, influencing pattern formation and evolution in layered tissues. Here, we investigate the deformation of a compressed film bonded to a half-space, where either the film or the substrate exhibits anisotropy. Within the… ▽ More Fiber-reinforcement is a universal feature of many biological tissues. It involves the interplay between fiber stiffness, fiber orientation, and the elastic properties of the matrix, influencing pattern formation and evolution in layered tissues. Here, we investigate the deformation of a compressed film bonded to a half-space, where either the film or the substrate exhibits anisotropy. Within the framework of finite elasticity, we formulate nonlinear incremental equations, enabling linear and weakly nonlinear analyses. These analyses yield exact bifurcation conditions and an amplitude equation for surface wrinkling. In particular, for a simple fiber-reinforced model, we show that the bifurcation can be supercritical or subcritical depending on the ratio between the substrate and the film moduli. These findings underscore the pivotal role of fiber-reinforcement in shaping pattern formation in anisotropic tissues and provide insights into the morphological evolution of biological tissues. △ Less

Submitted 28 April, 2025; originally announced April 2025.

Comments: 37 pages, 17 figures

Orientation- and pressure-dependence of the vibrational response of a monolayer crystal on a vicinal diamond surface

Authors: Yi Zhao, Lingxiao Zhao, Chengjiang Du, Ruirui Liu, John A. McGuire, Yanpeng Qi

Abstract: We systematically investigate polarization-dependent Raman spectra of a monolayer crystal of WS2 on the (100) and (230) surfaces of diamond. At ambient pressure, identical polarization dependence of the Raman spectra is observed on the different surfaces, independent of the orientation of the monolayer crystal relative to the diamond crystal. However, when monolayer WS2 is compressed to about 4 GP… ▽ More We systematically investigate polarization-dependent Raman spectra of a monolayer crystal of WS2 on the (100) and (230) surfaces of diamond. At ambient pressure, identical polarization dependence of the Raman spectra is observed on the different surfaces, independent of the orientation of the monolayer crystal relative to the diamond crystal. However, when monolayer WS2 is compressed to about 4 GPa, an abrupt drop of the intensity of the 2LA mode relative to the A' mode occurs on the (100) surface and the (230) surface with the zigzag direction along the atomic step edges of the (230) surface. In contrast, no such drop is observed when the armchair direction is along or at 15° to the atomic steps of the (230) surface. We also observe a shift of the polarization angle of the intensity maxima of the 2LA and A' modes on (230) surface during compression. These results demonstrate that the atomic steps of a vicinal surface strongly modify the vibrational response under high pressure. △ Less

Submitted 25 April, 2025; originally announced April 2025.

Comments: 14 pages,5 figures

Journal ref: ACS Applied Electronic Materials 2025

Hyperuniform Mixing of Binary Active Spinners

Authors: Rui Liu, Mingcheng Yang, Ke Chen

Abstract: Spinner mixtures consisting of both clockwise and counterclockwise self-spinning particles are often expected to phase separate. However, we demonstrate that such a demixing is absent for dimer (or rod-like) spinners. These particles always mix, even in a globally-hyperuniform way, with the total structure factor $S(q\to 0)\sim q^α\,(α>0)$. This global hyperuniformity can be enhanced or weakened b… ▽ More Spinner mixtures consisting of both clockwise and counterclockwise self-spinning particles are often expected to phase separate. However, we demonstrate that such a demixing is absent for dimer (or rod-like) spinners. These particles always mix, even in a globally-hyperuniform way, with the total structure factor $S(q\to 0)\sim q^α\,(α>0)$. This global hyperuniformity can be enhanced or weakened by changes in the driving torques or the particle density in various ways. The corresponding microscopic mechanism is attributed to the competition between a dynamical heterocoordination effect and effective like-particle attractions. Critical scaling for the absorbing state transition of the system is also found to persist, with a significant shift in its critical point observed. The system can be further thermalized, by the driving torques or through thermostating, into an ideal solution with identical partial radial distribution functions, which denys the possibility of being multi-hyperuniform. A simply-extented coupled density-oscillator theory explains why the system can not be multi-hyperuniform, but can have a global hyperuniformity with the scaling exponent $α$ approaching $2$. Such a hyperuniform mixing provides a way to regulate the topological boundary flows of this chiral system, and this mixing regulation is found to barely affect the bulk density fluctuations and even preserve the localization of the flows and the bulk hyperuniformity. △ Less

Submitted 23 April, 2025; originally announced April 2025.

A Foundational Potential Energy Surface Dataset for Materials

Authors: Aaron D. Kaplan, Runze Liu, Ji Qi, Tsz Wai Ko, Bowen Deng, Janosh Riebesell, Gerbrand Ceder, Kristin A. Persson, Shyue Ping Ong

Abstract: Accurate potential energy surface (PES) descriptions are essential for atomistic simulations of materials. Universal machine learning interatomic potentials (UMLIPs)$^{1-3}$ offer a computationally efficient alternative to density functional theory (DFT)$^4$ for PES modeling across the periodic table. However, their accuracy today is fundamentally constrained due to a reliance on DFT relaxation da… ▽ More Accurate potential energy surface (PES) descriptions are essential for atomistic simulations of materials. Universal machine learning interatomic potentials (UMLIPs)$^{1-3}$ offer a computationally efficient alternative to density functional theory (DFT)$^4$ for PES modeling across the periodic table. However, their accuracy today is fundamentally constrained due to a reliance on DFT relaxation data.$^{5,6}$ Here, we introduce MatPES, a foundational PES dataset comprising $\sim 400,000$ structures carefully sampled from 281 million molecular dynamics snapshots that span 16 billion atomic environments. We demonstrate that UMLIPs trained on the modestly sized MatPES dataset can rival, or even outperform, prior models trained on much larger datasets across a broad range of equilibrium, near-equilibrium, and molecular dynamics property benchmarks. We also introduce the first high-fidelity PES dataset based on the revised regularized strongly constrained and appropriately normed (r$^2$SCAN) functional$^7$ with greatly improved descriptions of interatomic bonding. The open source MatPES initiative emphasizes the importance of data quality over quantity in materials science and enables broad community-driven advancements toward more reliable, generalizable, and efficient UMLIPs for large-scale materials discovery and design. △ Less

Submitted 5 March, 2025; originally announced March 2025.

Comments: The first three listed authors contributed equally to this work. For training data, see http://matpes.ai or https://materialsproject-contribs.s3.amazonaws.com/index.html#MatPES_2025_1/

Materials Graph Library (MatGL), an open-source graph deep learning library for materials science and chemistry

Authors: Tsz Wai Ko, Bowen Deng, Marcel Nassar, Luis Barroso-Luque, Runze Liu, Ji Qi, Elliott Liu, Gerbrand Ceder, Santiago Miret, Shyue Ping Ong

Abstract: Graph deep learning models, which incorporate a natural inductive bias for a collection of atoms, are of immense interest in materials science and chemistry. Here, we introduce the Materials Graph Library (MatGL), an open-source graph deep learning library for materials science and chemistry. Built on top of the popular Deep Graph Library (DGL) and Python Materials Genomics (Pymatgen) packages, ou… ▽ More Graph deep learning models, which incorporate a natural inductive bias for a collection of atoms, are of immense interest in materials science and chemistry. Here, we introduce the Materials Graph Library (MatGL), an open-source graph deep learning library for materials science and chemistry. Built on top of the popular Deep Graph Library (DGL) and Python Materials Genomics (Pymatgen) packages, our intention is for MatGL to be an extensible ``batteries-included'' library for the development of advanced graph deep learning models for materials property predictions and interatomic potentials. At present, MatGL has efficient implementations for both invariant and equivariant graph deep learning models, including the Materials 3-body Graph Network (M3GNet), MatErials Graph Network (MEGNet), Crystal Hamiltonian Graph Network (CHGNet), TensorNet and SO3Net architectures. MatGL also includes a variety of pre-trained universal interatomic potentials (aka ``foundational materials models (FMM)'') and property prediction models are also included for out-of-box usage, benchmarking and fine-tuning. Finally, MatGL includes support for Pytorch Lightning for rapid training of models. △ Less

Submitted 5 March, 2025; originally announced March 2025.

Comments: 50 pages, 13 figures including Manuscript and Supplementary Inoformation

Spontaneous rotational symmetry breaking induced by electronic instability in the normal state of La_{1-x} Sr_{x} NiO_{2}

Authors: Qiang Zhao, Rui Liu, Wen-Long Yang, Xue-Yan Wang, Jia-Kun Luo, Jing-Yuan Ma, Fang-Hui Zhu, Cheng-Xue Chen, Mei-Ling Yan, Rui-Fen Dou, Chang-Min Xiong, Chi Xu, Xing-Ye Lu, Hai-Wen Liu, Ji-Kun Chen, Zhi-Ping Yin, Jia-Cai Nie

Abstract: The spontaneous rotational symmetry breaking (RSB), a hallmark phenomenon in cuprate and iron-based high-temperature superconductors, is believed to intimately connected to superconductivity, both of which originate from interactions among different degrees of freedoms and competing quantum states. Understanding RSB is pivotal for unraveling the microscopic origin of unconventional superconductivi… ▽ More The spontaneous rotational symmetry breaking (RSB), a hallmark phenomenon in cuprate and iron-based high-temperature superconductors, is believed to intimately connected to superconductivity, both of which originate from interactions among different degrees of freedoms and competing quantum states. Understanding RSB is pivotal for unraveling the microscopic origin of unconventional superconductivity. Although infinite-layer nickelates (ILNs) share similar crystalline structure and the same nominal 3d-electron configurations with cuprates, they have significant differences in Fermi surface topology, electronic band characteristics, and charge order. These distinctions make ILNs an ideal platform for studying RSB in unconventional superconductors. Through angular-resolved resistivity measurements within a large temperature and doping range, we identify pronounced RSB signatures near doping concentrations x=0.05 and 0.25. Based on the strongly correlated electronic structures from combined density functional theory and dynamical mean field theory calculations, we find that the calculated electronic susceptibility has a peak structure at the corresponding doping concentration, indicating pronounced electronic instabilities which drive RSB. Detailed analysis of the electronic susceptibility demonstrates that the van Hove singularity at the Fermi level significantly contributes to the electronic instability at 0.05 Sr doping. Our findings reveal the important role of electronic correlation, Van Hove singularity, and Fermi surface nesting in the emergence of RSB. Our work not only deepens the understanding of electronic behavior in ILNs, but also provides new ideas and methods for exploring RSB in other unconventional superconductors. △ Less

Submitted 16 March, 2025; v1 submitted 5 March, 2025; originally announced March 2025.

Comments: 17pages,7figures

ACM Class: J.2

Unraveling the origin of Kondo-like behavior in the 3$d$-electron heavy-fermion compound YFe$_{2}$Ge$_{2}$

Authors: Bing Xu, Rui Liu, Hongliang Wo, Zhiyu Liao, Shaohui Yi, Chunhong Li, Jun Zhao, Xianggang Qiu, Zhiping Yin, Christian Bernhard

Abstract: The heavy fermion (HF) state of $d$-electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in $f$-electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the $3d$-electron HF compound YFe$_2$Ge$_2$. The infrared response exhibits several char… ▽ More The heavy fermion (HF) state of $d$-electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in $f$-electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the $3d$-electron HF compound YFe$_2$Ge$_2$. The infrared response exhibits several charge-dynamical hallmarks of HF and a corresponding scaling behavior that resemble those of the $f$-electron HF systems. In particular, the low-temperature spectra reveal a dramatic narrowing of the Drude response along with the appearance of a hybridization gap ($Δ\sim$ 50 meV) and a strongly enhanced quasiparticle effective mass. Moreover, the temperature dependence of the infrared response indicates a crossover around $T^{\ast} \sim$ 100 K from a coherent state at low temperature to a quasi-incoherent one at high temperature. Despite of these striking similarities, our band structure calculations suggest that the mechanism underlying the HF behavior in YFe$_2$Ge$_2$ is distinct from the Kondo scenario of the $f$-electron HF compounds and even from that of the $d$-electron iron-arsenide superconductor KFe$_2$As$_2$. For the latter, the HF state is driven by orbital-selective correlations due to a strong Hund's coupling. Instead, for YFe$_2$Ge$_2$ the HF behavior originates from the band flatness near the Fermi level induced by the combined effects of kinetic frustration from a destructive interference between the direct Fe-Fe and indirect Fe-Ge-Fe hoppings, band hybridization involving Fe $3d$ and Y $4d$ electrons, and electron correlations. This highlights that rather different mechanisms can be at the heart of the HF state in $d$-electron systems. △ Less

Submitted 28 February, 2025; originally announced February 2025.

Comments: 11 pages, 7 figures

Journal ref: PNAS 121, e2401430121 (2024)

Phases and critical transport of the SU(N) Hofstadter-Hubbard model on the triangular lattice

Authors: Lu Zhang, Rongning Liu, Xue-Yang Song

Abstract: We report the study of phases and transitions of SU(N) Hofstadter-Hubbard model subject to commensurate magnetic field on the triangular lattice. At filling one fermion per site, for the number of fermion flavors 2 <= N <= 8, we identify three distinct phases and calculate critical interaction strength from parton mean-field approximation. Integer quantum Hall, chiral spin liquid, and valence bond… ▽ More We report the study of phases and transitions of SU(N) Hofstadter-Hubbard model subject to commensurate magnetic field on the triangular lattice. At filling one fermion per site, for the number of fermion flavors 2 <= N <= 8, we identify three distinct phases and calculate critical interaction strength from parton mean-field approximation. Integer quantum Hall, chiral spin liquid, and valence bond solid states could be realized upon varying the Hubbard interaction U and the number of flavor N . We construct the critical theory for the putative continuous transition from quantum Hall states to chiral spin liquid and calculate the critical transport behavior using quantum Boltzmann equations for general N . These results could be validated in synthetic systems such as moir'e superlattices and cold atom platforms. △ Less

Submitted 15 August, 2025; v1 submitted 14 February, 2025; originally announced February 2025.

Comments: 13 pages, 6 figures

Unveiling resilient superconducting fluctuations in atomically thin NbSe$_2$ through Higgs mode spectroscopy

Authors: Yu Du, Gan Liu, Wei Ruan, Zhi Fang, Kenji Watanabe, Takashi Taniguchi, Ronghua Liu, Jian-Xin Li, Xiaoxiang Xi

Abstract: We report a combined electrical transport and optical study of the superconductivity in atomically thin NbSe$_2$. When subjected to an out-of-plane magnetic field, an anomalous metallic state emerges, characterized by a finite longitudinal resistance and a vanishing Hall resistance, suggesting the presence of particle-hole symmetry. We establish a superconducting Higgs mode in atomically thin samp… ▽ More We report a combined electrical transport and optical study of the superconductivity in atomically thin NbSe$_2$. When subjected to an out-of-plane magnetic field, an anomalous metallic state emerges, characterized by a finite longitudinal resistance and a vanishing Hall resistance, suggesting the presence of particle-hole symmetry. We establish a superconducting Higgs mode in atomically thin samples, which reveals enduring superconducting fluctuations that withstand unexpectedly high reduced magnetic fields. These findings provide evidence of robust locally paired electrons in the anomalous metallic state, affirming its bosonic nature. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. Lett. 134, 066002 (2025)

Spin correlations in La$_3$Ni$_2$O$_7$ superconducting thin films

Authors: Hengyang Zhong, Bo Hao, Zhijia Zhang, Anni Chen, Yuan Wei, Ruixian Liu, Xinru Huang, Chunyi Li, Wenting Zhang, Chang Liu, Xiao-Sheng Ni, Marli dos Reis Cantarino, Kurt Kummer, Nicholas Brookes, Kun Cao, Yuefeng Nie, Thorsten Schmitt, Xingye Lu

Abstract: The discovery of ambient-pressure superconductivity with $T_{c,\text{onset}} > 40$ K in {\LNO} (LNO) thin films grown on the SrLaAlO$_4$ (SLAO) substrate with compressive ($\varepsilon\approx-2\%$) epitaxial strain provides a unique platform for investigating the superconducting mechanisms in nickelate superconductors. Here, we use resonant inelastic X-ray scattering (RIXS) to unveil the dispersiv… ▽ More The discovery of ambient-pressure superconductivity with $T_{c,\text{onset}} > 40$ K in {\LNO} (LNO) thin films grown on the SrLaAlO$_4$ (SLAO) substrate with compressive ($\varepsilon\approx-2\%$) epitaxial strain provides a unique platform for investigating the superconducting mechanisms in nickelate superconductors. Here, we use resonant inelastic X-ray scattering (RIXS) to unveil the dispersive spin excitations in the LNO/SLAO superconducting thin film and establish the strain dependence of the electronic and spin excitations in LNO thin films with strain ranging from $\varepsilon\approx-2\%$ to $+1.9\%$. Compared with the bulk crystal, the LNO/SLAO thin film (with $\varepsilon\approx-2\%$) exhibits similar $dd$ excitations and spin dynamics with larger bandwidth. By contrast, tensile-strained LNO/SrTiO$_3$ ($\varepsilon \approx +1.9\%$) exhibits a marked suppression of both the spin excitations and the Ni 3{\dz}-derived $dd$ excitations. The strain dependence of the spin excitations reflects significant changes in the interlayer exchange coupling $J_z$, and the diminishing $dd$ excitations in tensile-strained samples indicate weaker Ni 3{\dz}-O 2$p_{z}$ hybridization. This strain evolution of the spin excitations and $J_z$ is attributed to the strain-tuned $c$-axis Ni-O-Ni bond angle $\varphi$, which controls the Ni 3{\dz}-O 2$p_{z}$ hybridization. Since superconductivity is observed only in films grown on SLAO, and spin correlations are enhanced along with the emergence of superconductivity, our results identify $\varphi$ as a key structural lever controlling $J_z$ and provide direct spectroscopic support for interlayer spin-fluctuation-mediated pairing scenarios in bilayer nickelates. △ Less

Submitted 16 November, 2025; v1 submitted 5 February, 2025; originally announced February 2025.

Comments: 7 pages, 4 figures. Supplementary is available upon reasonable request

ABACUS: An Electronic Structure Analysis Package for the AI Era

Authors: Weiqing Zhou, Daye Zheng, Qianrui Liu, Denghui Lu, Yu Liu, Peize Lin, Yike Huang, Xingliang Peng, Jie J. Bao, Chun Cai, Zuxin Jin, Jing Wu, Haochong Zhang, Gan Jin, Yuyang Ji, Zhenxiong Shen, Xiaohui Liu, Liang Sun, Yu Cao, Menglin Sun, Jianchuan Liu, Tao Chen, Renxi Liu, Yuanbo Li, Haozhi Han , et al. (33 additional authors not shown)

Abstract: ABACUS (Atomic-orbital Based Ab-initio Computation at USTC) is an open-source software for first-principles electronic structure calculations and molecular dynamics simulations. It mainly features density functional theory (DFT) and molecular dynamics functions and is compatible with both plane-wave basis sets and numerical atomic orbital basis sets. ABACUS serves as a platform that facilitates th… ▽ More ABACUS (Atomic-orbital Based Ab-initio Computation at USTC) is an open-source software for first-principles electronic structure calculations and molecular dynamics simulations. It mainly features density functional theory (DFT) and molecular dynamics functions and is compatible with both plane-wave basis sets and numerical atomic orbital basis sets. ABACUS serves as a platform that facilitates the integration of various electronic structure methods, such as Kohn-Sham DFT, stochastic DFT, orbital-free DFT, and real-time time-dependent DFT, etc. In addition, with the aid of high-performance computing, ABACUS is designed to perform efficiently and provide massive amounts of first-principles data for generating general-purpose machine learning potentials, such as DPA models. Furthermore, ABACUS serves as an electronic structure platform that interfaces with several AI-assisted algorithms and packages, such as DeePKS-kit, DeePMD, DP-GEN, DeepH, DeePTB, HamGNN, etc. △ Less

Submitted 22 October, 2025; v1 submitted 15 January, 2025; originally announced January 2025.

Large Scale Finite-Temperature Real-time Time Dependent Density Functional Theory Calculation with Hybrid Functional on ARM and GPU Systems

Authors: Rongrong Liu, Zhuoqiang Guo, Qiuchen Sha, Tong Zhao, Haibo Li, Wei Hu, Lijun Liu, Guangming Tan, Weile Jia

Abstract: Ultra-fast electronic phenomena originating from finite temperature, such as nonlinear optical excitation, can be simulated with high fidelity via real-time time dependent density functional theory (rt-TDDFT) calculations with hybrid functional. However, previous rt-TDDFT simulations of real materials using the optimal gauge--known as the parallel transport gauge--have been limited to low-temperat… ▽ More Ultra-fast electronic phenomena originating from finite temperature, such as nonlinear optical excitation, can be simulated with high fidelity via real-time time dependent density functional theory (rt-TDDFT) calculations with hybrid functional. However, previous rt-TDDFT simulations of real materials using the optimal gauge--known as the parallel transport gauge--have been limited to low-temperature systems with band gaps. In this paper, we introduce the parallel transport-implicit midpoint (PT-IM) method, which significantly accelerates finite-temperature rt-TDDFT calculations of real materials with hybrid function. We first implement PT-IM with hybrid functional in our plane wave code PWDFT, and optimized it on both GPU and ARM platforms to build a solid baseline code. Next, we propose a diagonalization method to reduce computation and communication complexity, and then, we employ adaptively compressed exchange (ACE) method to reduce the frequency of the most expensive Fock exchange operator. Finally, we adopt the ring\_based method and the shared memory mechanism to overlap computation and communication and alleviate memory consumption respectively. Numerical results show that our optimized code can reach 3072 atoms for rt-TDDFT simulation with hybrid functional at finite temperature on 192 computing nodes, the time-to-solution for one time step is 429.3s, which is 41.4 times faster compared to the baseline. △ Less

Submitted 6 January, 2025; originally announced January 2025.

Symmetry-Based Real-Space Framework for Realizing Flat Bands and Unveiling Nodal-Line Touchings

Authors: Rui-Heng Liu, Xin Liu

Abstract: Flat band (FB) systems provide ideal playgrounds for studying correlation physics, whereas multi-orbital characteristics in real materials are distinguished from most simple FB models. Here, we propose a systematic and versatile framework for FB constructions in tight-binding (TB) models based on symmetric compact localized states (CLSs), integrating lattice and orbital degrees of freedom. We firs… ▽ More Flat band (FB) systems provide ideal playgrounds for studying correlation physics, whereas multi-orbital characteristics in real materials are distinguished from most simple FB models. Here, we propose a systematic and versatile framework for FB constructions in tight-binding (TB) models based on symmetric compact localized states (CLSs), integrating lattice and orbital degrees of freedom. We first demonstrate that any CLS can be symmetrized into a representation of the point group, which remains valid for high orbitals with finite spin-orbit coupling (SOC). Second, we determine the candidate CLS sites according to lattice symmetry, and simplify the hopping as a linear mapping between two Hilbert spaces: one of CLS sites and another of their adjacent sites. The existence of FBs depends on a non-empty kernel of the mapping. Finally, we distinguish eigenstates in the kernel to qualify as a CLS. To illustrate the versatility of our framework, we construct three representative FB models: one in two dimensions (2D) and the rest in three dimensions (3D). All of them lack special lattice structures and incorporate high orbitals. Notably, the 3D FBs can exhibit not only band touchings at points but also along lines, a feature of significant physical interest. For a comprehensive understanding, we derive a concise criterion for determining band touchings, which provides a natural explanation for the occurrence of both gapped and gapless FBs. By unifying symmetry principles in real space, our work offers a systematic approach to constructing FBs across diverse lattice systems. This framework opens new avenues for understanding and engineering FB systems, with potential implications for correlated quantum phenomena and exotic phases of matter. △ Less

Submitted 25 March, 2025; v1 submitted 20 December, 2024; originally announced December 2024.

Comments: 25 pages, 11 figures

Exploring the energy landscape of aluminas through machine learning interatomic potential

Authors: Lei Zhang, Wenhao Luo, Renxi Liu, Mohan Chen, Zhongbo Yan, Kun Cao

Abstract: Aluminum oxide (alumina, Al$_2$O$_3$) exists in various structures and has broad industrial applications. While the crystal structure of $α$-Al$_2$O$_3$ is well-established, those of transitional aluminas remain highly debated. In this study, we propose a universal machine learning interatomic potential (MLIP) for aluminas, trained using the neuroevolution potential (NEP) approach. The dataset is… ▽ More Aluminum oxide (alumina, Al$_2$O$_3$) exists in various structures and has broad industrial applications. While the crystal structure of $α$-Al$_2$O$_3$ is well-established, those of transitional aluminas remain highly debated. In this study, we propose a universal machine learning interatomic potential (MLIP) for aluminas, trained using the neuroevolution potential (NEP) approach. The dataset is constructed through iterative training and farthest point sampling, ensuring the generation of the most representative configurations for an exhaustive sampling of the potential energy surface. The accuracy and generality of the potential are validated through simulations under a wide range of conditions, including high temperatures and pressures. A phase diagram is presented that includes both transitional aluminas and $α$-Al$_2$O$_3$ based on the NEP. We also successfully extrapolate the phase diagram of aluminas under extreme conditions ([0, 4000] K and [0, 200] GPa ranges of temperature and pressure, respectively), while maintaining high accuracy in describing their properties under more moderate conditions. Furthermore, combined with our developed structure search workflow, the NEP provides an evaluation of existing $γ$-Al$_2$O$_3$ structure models. The NEP developed in this work enables highly accurate dynamic simulations of various aluminas on larger scales and longer timescales, while also offering new insights into the study of transitional aluminas structures. △ Less

Submitted 4 December, 2024; v1 submitted 3 December, 2024; originally announced December 2024.

Journal ref: Phys. Rev. Materials 9, 023801(2025)

Higher obstructions to conformal boundary conditions and lattice realizations

Authors: Ruizhi Liu, Weicheng Ye

Abstract: Although it is long believed that vanishing of chiral central charges of a 2d conformal field theory (CFT) implies the existence of conformal boundary conditions, there are yet higher obstructions. In this paper, we focus on 2d rational CFTs, for which we identify a series of obstructions, known as higher central charges. We also discuss its implication for lattice realizations and the generalizat… ▽ More Although it is long believed that vanishing of chiral central charges of a 2d conformal field theory (CFT) implies the existence of conformal boundary conditions, there are yet higher obstructions. In this paper, we focus on 2d rational CFTs, for which we identify a series of obstructions, known as higher central charges. We also discuss its implication for lattice realizations and the generalization with global symmetries. △ Less

Submitted 24 May, 2025; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: The proof in this paper is incorrect and the original conjecture remains open

A facile route to synthesize cubic gauche polymeric nitrogen

Authors: Runteng Chen, Jun Zhang, Zelong Wang, Ke Lu, Yi Peng, Jianfa Zhao, Xiaodong Liu, Shaomin Feng, Ruibin Liu, Chuan Xiao, Changqing Jin

Abstract: In this work, the long-sought cg-N with N-N single bond has been synthesized for the first time by a thermal-driven-only chemical route at ambient conditions. The successful synthesis of cg-N was achieved by first creating a solution of azides, which was then pretreated under vacuum conditions. Following the pretreatment, the resultant concentrated azide was heated at temperatures ranging from 260… ▽ More In this work, the long-sought cg-N with N-N single bond has been synthesized for the first time by a thermal-driven-only chemical route at ambient conditions. The successful synthesis of cg-N was achieved by first creating a solution of azides, which was then pretreated under vacuum conditions. Following the pretreatment, the resultant concentrated azide was heated at temperatures ranging from 260°C to 330°C for a reaction time of 3 hours, ultimately leading to the formation of cg-N. The emergent intense Raman peak characterized of cg-N provides solid evidence that the double bonded nitrogen-nitrogen transforms into a single bond form, which agrees well with cg-N structure. To date, this is the only work achieving the quantity of cg-N synthesized at ambient conditions by a facile route that can be further developed for the scalable synthesis and applications of polymerized nitrogen-based materials as high energy density materials. △ Less

Submitted 28 November, 2024; v1 submitted 15 November, 2024; originally announced November 2024.

Comments: 8 pages, 2 figures, published in Science Bulletin

Journal ref: Science Bulletin, 69(24):3812 (2024)

Competing few-body correlations in ultracold Fermi polarons

Authors: Ruijin Liu, Xiaoling Cui

Abstract: Polaron, a typical quasi-particle that describes a single impurity dressed with surrounding environment, serves as an ideal platform for bridging few- and many-body physics. In particular, different few-body correlations can compete with each other and lead to many intriguing phenomena. In this work, we review the recent progresses made in understanding few-body correlation effects in attractive F… ▽ More Polaron, a typical quasi-particle that describes a single impurity dressed with surrounding environment, serves as an ideal platform for bridging few- and many-body physics. In particular, different few-body correlations can compete with each other and lead to many intriguing phenomena. In this work, we review the recent progresses made in understanding few-body correlation effects in attractive Fermi polarons of ultracold gases. By adopting a unified variational ansatz that incorporates different few-body correlations in a single framework, we will discuss their competing effects in Fermi polarons when the impurity and majority fermions have the same or different masses. For the equal-mass case, we review the nature of polaron-molecule transition that is driven by two-body correlations, and especially highlight the finite momentum character and huge degeneracy of molecule states. For the mass-imbalanced case, we focus on the smooth crossover between polaron and various dressed clusters that originate from high-order correlations. These competing few-body correlations reviewed in Fermi polarons suggest a variety of exotic new phases in the corresponding many-body system of Fermi-Fermi mixtures. △ Less

Submitted 26 October, 2024; originally announced October 2024.

Comments: 9 pages, 7 figures; invited review by AAPPS Bulletin

Journal ref: AAPPS Bulletin 34, 29 (2024)

High-temperature ferromagnetism and ferroelasticity in ultraflexible atomically thin square-shaped lattices

Authors: Xinyuan Huang, Yueqiao Qu, Yu Liao, Qian Zheng, Ran Liu, Yu Chen, Liang Liu, Junzhong Wang, Gang Yao

Abstract: The coexistence of high-temperature intrinsic ferromagnetic ordering, large magnetic anisotropy, along with novel mechanical properties such as ferroelasticity and flexibility, in experimental feasible two-dimensional (2D) crystals is greatly appealing for nanoscale spintronics. However, the progress in identifying such materials is limited. Here, by first-principles calculations, we report the fi… ▽ More The coexistence of high-temperature intrinsic ferromagnetic ordering, large magnetic anisotropy, along with novel mechanical properties such as ferroelasticity and flexibility, in experimental feasible two-dimensional (2D) crystals is greatly appealing for nanoscale spintronics. However, the progress in identifying such materials is limited. Here, by first-principles calculations, we report the findings of an extraordinary combination of the above qualities for the first time in a new 2D exfoliated FeSi nanosheet in the P4/nmm space group. Due to the strong anion-mediated superexchange interaction, the monolayer FeSi (ML-FeSi) exhibits a Curie temperature Tc as high as 830 K, surpassing the current experimental record (344 K for ML-Cr3Te4). Furthermore, including FeSi, such isostructural lattices all demonstrate exceptional softness, as evidenced by their ultra-low in-plane stiffness. Remarkably, the transition metal atom and square-shaped crystal form work together to give this family of ML materials unique properties that can transition from Ising-like 2D ferromagnets in FeSi, MnP, MnAs, CrP, FeI, and VAs to 2D-XY ones in CrAs, VP, and multiferroic MnGe and TiTe. Overall, our work highlights such 2D lattices as promising candidates in emerging multifunctional device applications and nontrivial topological spintronics. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Comments: 16 pages, 5 figures

Cryogenic Digital Image Correlation as a Probe of Strain in Iron-Based Superconductors

Authors: Ziye Mo, Chunyi Li, Wenting Zhang, Chang Liu, Yongxin Sun, Ruixian Liu, Xingye Lu

Abstract: Uniaxial strain is a powerful tuning parameter that can control symmetry and anisotropic electronic properties in iron-based superconductors. However, accurately characterizing anisotropic strain can be challenging and complex. Here, we utilize a cryogenic optical system equipped with a high-spatial-resolution microscope to characterize surface strains in iron-based superconductors using the digit… ▽ More Uniaxial strain is a powerful tuning parameter that can control symmetry and anisotropic electronic properties in iron-based superconductors. However, accurately characterizing anisotropic strain can be challenging and complex. Here, we utilize a cryogenic optical system equipped with a high-spatial-resolution microscope to characterize surface strains in iron-based superconductors using the digital image correlation method. Compared with other methods such as high-resolution X-ray diffraction, strain gauge, and capacitive sensor, digital image correlation offers a non-contact, full-field measurement approach, acting as an optical virtual strain gauge that provides high spatial resolution. The results measured on detwinned {\BFA} are quantitatively consistent with the distortion measured by X-ray diffraction and neutron Larmor diffraction. These findings highlight the potential of cryogenic digital image correlation as an effective and accessible tool for probing the isotropic and anisotropic strains, facilitating the application of uniaxial strain tuning in the study of quantum materials. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Comments: 6 pages, 4 figures. Published online in Chinese Physics Letters. DOI 10.1088/0256-307X/41/10/107102

Evolution of pairing symmetry in FeSe$_{1-x}$S$_x$ as probed by uniaxial-strain tuning of $T_c$

Authors: Ruixian Liu, Qi Tang, Chang Liu, Chunyi Li, Kaijuan Zhou, Qiaoyu Wang, Xingye Lu

Abstract: In iron-based superconductors (FeSCs), the interplay between electronic nematicity and superconductivity is essential for understanding the exotic superconducting ground state. In the nematic regime, uniaxial-strain ($\varepsilon$) tuning of the superconducting transition temperature $T_c$ [$ΔT_c(\varepsilon)=α\varepsilon+β\varepsilon^2$] offers a unique approach to investigating the evolution of… ▽ More In iron-based superconductors (FeSCs), the interplay between electronic nematicity and superconductivity is essential for understanding the exotic superconducting ground state. In the nematic regime, uniaxial-strain ($\varepsilon$) tuning of the superconducting transition temperature $T_c$ [$ΔT_c(\varepsilon)=α\varepsilon+β\varepsilon^2$] offers a unique approach to investigating the evolution of pairing symmetry if both $s$ and $d$ wave pairing instabilities are relevant. Here, we employ uniaxial strain to tune the $T_c$ of FeSe$_{1-x}$S$_x$, in which both nematicity and superconductivity undergo significant changes with doping. While $T_c$ is usually suppressed quadratically with $\varepsilon$ in optimally doped BaFe$_2$As$_2$, $ΔT_c(\varepsilon)$ in FeSe$_{1-x}$S$_x$ dominated by $ΔT_c(\varepsilon)=β\varepsilon^2$ changes its sign from $β$ < $0$ in FeSe to $β$ > $0$ in FeSe$_{1-x}$S$_x$ ($x\gtrsim0.10$), indicating an evolution of the pairing symmetry from an $s_{\pm}$ state towards an $s+d$ wave state. These findings highlight the $ΔT_c(\varepsilon)$ as a powerful probe for elucidating the superconducting pairing symmetry in the nematic regime of FeSCs and provide new insights into the evolution of pairing symmetry in FeSCs. △ Less

Submitted 18 October, 2024; v1 submitted 17 October, 2024; originally announced October 2024.

Comments: 6 pages, 4 figures. Supplementary is available upon reasonable request

Self-Assembly of a halogenated organic molecule on the Si(111) $\surd$3$\times$$\surd$3-Ag surface

Authors: R Liu, D. Marchese, R. C. Mawhinney, M. C. Gallagher

Abstract: We study the self-assembly of halogen-based organic molecules on a passivated silicon surface. The room temperature adsorption of 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on the Si(111)-$\surd$3$\times$$\surd$3-Ag surface is described. The adsorption is investigated primarily by room-temperature scanning tunneling microscopy (STM) and density-functional theoretical (DFT) calculations. The ex… ▽ More We study the self-assembly of halogen-based organic molecules on a passivated silicon surface. The room temperature adsorption of 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on the Si(111)-$\surd$3$\times$$\surd$3-Ag surface is described. The adsorption is investigated primarily by room-temperature scanning tunneling microscopy (STM) and density-functional theoretical (DFT) calculations. The experimental results is a dramatic example of how the substrate can influence the overall structure of the self-assembly. With increasing dose, the TIPT monomers form supramolecular structures defined by a two monomer, 2.07 $\pm$ 0.05 nm by 1.83 $\pm$ 0.05 nm rectangular cell. The unit cell is characterized by zig-zag rows of molecules aligned \pm13° from the high symmetry directions of the $\surd$3-Ag substrate. The 2.07 nm dimension along the zig-zag rows is very similar to self-assembled TIPT networks observed on HOPG, however the 1.83 nm dimension is extended considerably and commensurate with the $\surd$3-Ag substrate. The epitaxial relationship between the overlayer and the substrate, and the commensurate inter-row spacing indicate significant molecule-substrate interactions. In fact, DFT calculations of free standing TIPT hexamers reveal that increasing the inter row spacing comes at little energy cost. Experiments also indicate that the formation of supramolecular TIPT domains is extremely sensitive to the quality of the underlying $\surd3$-Ag reconstruction. Point defects in the $\surd3$-Ag reconstruction ultimately restricts the extent of the observed domains. △ Less

Submitted 3 October, 2024; originally announced October 2024.

Month-long-lifetime microwave spectral holes in an erbium-doped scheelite crystal at millikelvin temperature

Authors: Zhiren Wang, Sen Lin, Marianne Le Dantec, Miloš Rančić, Philippe Goldner, Sylvain Bertaina, Thierry Chanelière, Ren-Bao Liu, Daniel Esteve, Denis Vion, Emmanuel Flurin, Patrice Bertet

Abstract: Rare-earth-ion (REI) ensembles in crystals have remarkable optical and spin properties characterized by narrow homogeneous linewidths relative to the inhomogeneous ensemble broadening. This makes it possible to precisely tailor the ensemble spectral density and therefore the absorption profile by applying narrow-linewidth radiation to transfer population into auxiliary levels, a process broadly kn… ▽ More Rare-earth-ion (REI) ensembles in crystals have remarkable optical and spin properties characterized by narrow homogeneous linewidths relative to the inhomogeneous ensemble broadening. This makes it possible to precisely tailor the ensemble spectral density and therefore the absorption profile by applying narrow-linewidth radiation to transfer population into auxiliary levels, a process broadly known as spectral hole burning (SHB). REI-doped crystals find applications in information processing, both classical (pattern recognition, filtering, spectral analysis) and quantum (photon storage), all protocols requiring suitable ensemble preparation by SHB as a first step. In Er$^{3+}$-doped materials, the longest reported hole lifetime is one minute, and longer lifetimes are desirable. Here, we report SHB and accumulated echo measurements in a scheelite crystal of CaWO$_4$ by pumping the electron spin transition of Er$^{3+}$ ions at microwave frequencies and millikelvin temperatures, with nuclear spin states of neighboring $^{183}$W atoms serving as the auxiliary levels. The lifetime of the holes and accumulated echoes rises steeply as the sample temperature is decreased, exceeding a month at 10 mK. Our results demonstrate that millikelvin temperatures can be beneficial for signal processing applications requiring long spectral hole lifetimes. △ Less

Submitted 22 August, 2024; originally announced August 2024.

On-Demand Growth of Semiconductor Heterostructures Guided by Physics-Informed Machine Learning

Authors: Chao Shen, Yuan Li, Wenkang Zhan, Shujie Pan, Fuxin Lin, Kaiyao Xin, Hui Cong, Chi Xu, Xiaotian Cheng, Ruixiang Liu, Zhibo Ni, Chaoyuan Jin, Bo Xu, Siming Chen, Zhongming Wei, Chunlai Xue, Zhanguo Wang, Chao Zhao

Abstract: Developing tailored semiconductor heterostructures on demand represents a critical capability for addressing the escalating performance demands in electronic and optoelectronic devices. However, traditional fabrication methods remain constrained by simulation-based design and iterative trial-and-error optimization. Here, we introduce SemiEpi, a self-driving platform designed for molecular beam epi… ▽ More Developing tailored semiconductor heterostructures on demand represents a critical capability for addressing the escalating performance demands in electronic and optoelectronic devices. However, traditional fabrication methods remain constrained by simulation-based design and iterative trial-and-error optimization. Here, we introduce SemiEpi, a self-driving platform designed for molecular beam epitaxy (MBE) to perform multi-step semiconductor heterostructure growth through in-situ monitoring and on-the-fly feedback control. By integrating standard MBE reactors, physics-informed machine learning (ML) models, and parameter initialization, SemiEpi identifies optimal initial conditions and proposes experiments for heterostructure growth, eliminating the need for extensive expertise in MBE processes. As a proof of concept, we demonstrate the optimization of high-density InAs quantum dot (QD) growth with a target emission wavelength of 1240 nm, showcasing the power of SemiEpi. We achieve a QD density of 5 x 10^10 cm^-2, a 1.6-fold increase in photoluminescence (PL) intensity, and a reduced full width at half maximum (FWHM) of 29.13 meV, leveraging in-situ reflective high-energy electron diffraction monitoring with feedback control for adjusting growth temperatures. Taken together, our results highlight the potential of ML-guided systems to address challenges in multi-step heterostructure growth, facilitate the development of a hardware-independent framework, and enhance process repeatability and stability, even without exhaustive knowledge of growth parameters. △ Less

Submitted 5 October, 2025; v1 submitted 6 August, 2024; originally announced August 2024.

Comments: 5 figures

Invariant Discovery of Features Across Multiple Length Scales: Applications in Microscopy and Autonomous Materials Characterization

Authors: Aditya Raghavan, Utkarsh Pratiush, Mani Valleti, Richard Liu, Reece Emery, Hiroshi Funakubo, Yongtao Liu, Philip Rack, Sergei Kalinin

Abstract: Physical imaging is a foundational characterization method in areas from condensed matter physics and chemistry to astronomy and spans length scales from atomic to universe. Images encapsulate crucial data regarding atomic bonding, materials microstructures, and dynamic phenomena such as microstructural evolution and turbulence, among other phenomena. The challenge lies in effectively extracting a… ▽ More Physical imaging is a foundational characterization method in areas from condensed matter physics and chemistry to astronomy and spans length scales from atomic to universe. Images encapsulate crucial data regarding atomic bonding, materials microstructures, and dynamic phenomena such as microstructural evolution and turbulence, among other phenomena. The challenge lies in effectively extracting and interpreting this information. Variational Autoencoders (VAEs) have emerged as powerful tools for identifying underlying factors of variation in image data, providing a systematic approach to distilling meaningful patterns from complex datasets. However, a significant hurdle in their application is the definition and selection of appropriate descriptors reflecting local structure. Here we introduce the scale-invariant VAE approach (SI-VAE) based on the progressive training of the VAE with the descriptors sampled at different length scales. The SI-VAE allows the discovery of the length scale dependent factors of variation in the system. Here, we illustrate this approach using the ferroelectric domain images and generalize it to the movies of the electron-beam induced phenomena in graphene and topography evolution across combinatorial libraries. This approach can further be used to initialize the decision making in automated experiments including structure-property discovery and can be applied across a broad range of imaging methods. This approach is universal and can be applied to any spatially resolved data including both experimental imaging studies and simulations, and can be particularly useful for exploration of phenomena such as turbulence, scale-invariant transformation fronts, etc. △ Less

Submitted 31 July, 2024; originally announced August 2024.

Universal clusters in quasi-two-dimensional ultracold Fermi mixtures

Authors: Ruijin Liu, Tingting Shi, Matteo Zaccanti, Xiaoling Cui

Abstract: We study universal clusters in quasi-two dimensions (q2D) that consist of a light (L) atom interacting with two or three heavy (H) identical fermions, forming the trimer or tetramer bound state. The axial confinement in q2D is shown to lift the three-fold degeneracy of 3D trimer (tetramer) in $p$-wave channel and uniquely select the ground state with magnetic angular momentum $|m|=1$ ($m=0$). By v… ▽ More We study universal clusters in quasi-two dimensions (q2D) that consist of a light (L) atom interacting with two or three heavy (H) identical fermions, forming the trimer or tetramer bound state. The axial confinement in q2D is shown to lift the three-fold degeneracy of 3D trimer (tetramer) in $p$-wave channel and uniquely select the ground state with magnetic angular momentum $|m|=1$ ($m=0$). By varying the interaction or confinement strength, we explore the dimensional crossover of these clusters from 3D to 2D, characterized by a gradual change of critical H-L mass ratio for their emergence and momentum-space distribution. Importantly, we find that a finite effective range will {\it not} alter their critical mass ratios in the weak coupling regime. There, we establish an effective 2D model to quantitatively reproduce the properties of q2D clusters, and further identify the optimal interaction strengths for their detections in experiments. Our results suggest a promising prospect for observing universal clusters and associated high-order correlation effects in realistic q2D ultracold Fermi mixtures. △ Less

Submitted 5 October, 2024; v1 submitted 24 July, 2024; originally announced July 2024.

Comments: 6 pages, 4 figures, with supplementary material (8 pages, 4 figures)

Journal ref: Phys. Rev. Research 6, L042004 (2024)

Chern insulator phase realized in dual-gate-tuned MnBi2Te4 thin films grown by molecular beam epitaxy

Authors: Yunhe Bai, Yuanzhao Li, Ruixuan Liu, Jianli Luan, Yang Chen, Wenyu Song, Peng-Fei Ji, Cui Ding, Zongwei Gao, Qinghua Zhang, Fanqi Meng, Bingbing Tong, Lin Li, Tianchen Zhu, Lin Gu, Lili Wang, Jinsong Zhang, Yayu Wang, Qi-Kun Xue, Ke He, Yang Feng, Xiao Feng

Abstract: The intrinsic magnetic order, large topological-magnetic gap and rich topological phases make MnBi2Te4 a wonderful platform to study exotic topological quantum states such as axion insulator and Chern insulator. To realize and manipulate these topological phases in a MnBi2Te4 thin film, precise manipulation of the electric field across the film is essential, which requires a dual-gate structure. I… ▽ More The intrinsic magnetic order, large topological-magnetic gap and rich topological phases make MnBi2Te4 a wonderful platform to study exotic topological quantum states such as axion insulator and Chern insulator. To realize and manipulate these topological phases in a MnBi2Te4 thin film, precise manipulation of the electric field across the film is essential, which requires a dual-gate structure. In this work, we achieve dual-gate tuning of MnBi2Te4 thin films grown with molecular beam epitaxy on SrTiO3(111) substrates by applying the substrate and an AlOx layer as the gate dielectrics of bottom and top gates, respectively. Under magnetic field of 9T and temperature of 20 mK, the Hall and longitudinal resistivities of the films show inversed gate-voltage dependence, for both top- and bottom-gates, signifying the existence of the dissipationless edge state contributed by Chern insulator phase in the ferromagnetic configuration. The maximum of the Hall resistivity only reaches 0.8 h/e2, even with dual-gate tuning, probably due to the high density of bulk carriers introduced by secondary phases. In the antiferromagnetic state under zero magnetic field, the films show normal insulator behavior. The dual-gated MnBi2Te4 thin films lay the foundation for developing devices based on electrically tunable topological quantum states. △ Less

Submitted 9 June, 2024; originally announced June 2024.

Comments: 24 pages, 4 figures

Higher-Matter and Landau-Ginzburg Theory of Higher-Group Symmetries

Authors: Ruizhi Liu, Ran Luo, Yi-Nan Wang

Abstract: Higher-matter is defined by higher-representation of a symmetry algebra, such as the $p$-form symmetries, higher-group symmetries or higher-categorical symmetries. In this paper, we focus on the cases of higher-group symmetries, which are formulated in terms of the strictification of weak higher-groups. We systematically investigate higher-matter charged under 2-group symmetries, defined by automo… ▽ More Higher-matter is defined by higher-representation of a symmetry algebra, such as the $p$-form symmetries, higher-group symmetries or higher-categorical symmetries. In this paper, we focus on the cases of higher-group symmetries, which are formulated in terms of the strictification of weak higher-groups. We systematically investigate higher-matter charged under 2-group symmetries, defined by automorphism 2-representations. Furthermore, we construct a Lagrangian formulation of such higher-matter fields coupled to 2-group gauge fields in the path space of the spacetime manifold. We interpret such model as the Landau-Ginzburg theory for 2-group symmetries, and discuss the spontaneous symmetry breaking (SSB) of 2-group symmetries under this framework. Examples of discrete and continuous 2-groups are discussed. Interestingly, we find that a non-split 2-group symmetry can admit an SSB to a split 2-group symmetry, where the Postnikov class is trivialized. We also briefly discuss the strictification of weak 3-groups, weak 3-group gauge fields and 3-representations in special cases. △ Less

Submitted 12 July, 2024; v1 submitted 6 June, 2024; originally announced June 2024.

Comments: 75 pages

Journal ref: SciPost Phys. 18, 052 (2025)

Entanglement area law and Lieb-Schultz-Mattis theorem in long-range interacting systems, and symmetry-enforced long-range entanglement

Authors: Ruizhi Liu, Jinmin Yi, Shiyu Zhou, Liujun Zou

Abstract: We establish multiple interrelated, fundamental results in quantum many-body systems that can have long-range interactions. For a sufficiently long quantum spin chain, we first show that if the multi-spin interactions in the Hamiltonian decay fast enough as their ranges increase and the Hamiltonian is gapped, then the ground states satisfy the entanglement area law, even if there is a ground state… ▽ More We establish multiple interrelated, fundamental results in quantum many-body systems that can have long-range interactions. For a sufficiently long quantum spin chain, we first show that if the multi-spin interactions in the Hamiltonian decay fast enough as their ranges increase and the Hamiltonian is gapped, then the ground states satisfy the entanglement area law, even if there is a ground state degeneracy due to a spontaneously broken discrete symmetry. This area law also holds for certain excited states. Second, if such a long-range interacting Hamiltonian has an anomalous symmetry, then the Lieb-Schultz-Mattis theorem applies, i.e., the Hamiltonian cannot have a unique gapped symmetric ground state. If the Hamiltonian contains only 2-spin interactions, these results hold when the interactions decay faster than $1/r^2$, with $r$ the distance between the two interacting spins. Third, we show that pure states with an anomalous symmetry, which may not be a ground state of any natural Hamiltonian, must be long-range entangled. The symmetries we consider include on-site internal symmetries combined with lattice translation symmetries, and they can also extend to purely internal but non-on-site symmetries. Moreover, these internal symmetries can be discrete or continuous. We explore the applications of these results through various examples. △ Less

Submitted 4 December, 2025; v1 submitted 23 May, 2024; originally announced May 2024.

Comments: This is a much improved version with significantly more results and deeper understanding. The paper contains a 10-page main text and various appendices

Journal ref: Phys. Rev. B 112, 214408 (2025)

Robust field-free switching using large unconventional spin-orbit torque in an all-van der Waals heterostructure

Authors: Yiyang Zhang, Xiaolin Ren, Ruizi Liu, Zehan Chen, Xuezhao Wu, Jie Pang, Wei Wang, Guibin Lan, Kenji Watanabe, Takashi Taniguchi, Youguo Shi, Guoqiang Yu, Qiming Shao

Abstract: The emerging all-van der Waals (vdW) magnetic heterostructure provides a new platform to control the magnetization by the electric field beyond the traditional spintronics devices. One promising strategy is using unconventional spin-orbit torque (SOT) exerted by the out-of-plane polarized spin current to enable deterministic magnetization switching and enhance the switching efficiency. However, in… ▽ More The emerging all-van der Waals (vdW) magnetic heterostructure provides a new platform to control the magnetization by the electric field beyond the traditional spintronics devices. One promising strategy is using unconventional spin-orbit torque (SOT) exerted by the out-of-plane polarized spin current to enable deterministic magnetization switching and enhance the switching efficiency. However, in all-vdW heterostructures, large unconventional SOT remains elusive and the robustness of the field-free switching against external magnetic field hasn't been examined, which hinder further applications. Here we demonstrate the field-free switching in an all-vdW heterostructure combining a type-II Weyl semimetal TaIrTe4 and above-room-temperature ferromagnet Fe3GaTe2. The fully field-free switching can be achieved at 2.56 x 10^10 A per m2 at 300K and a large SOT effective field efficiency of the out-of-plane polarized spin current generated by TaIrTe4 is determined to be 0.37. Moreover, we find that the switching polarity cannot be changed until the external in-plane magnetic field reaches 252mT, indicating a robust switching against the magnetic field. The numerical simulation suggests the large unconventional SOT reduces the switching current density and enhances the robustness of the switching. Our work shows that all-vdW heterostructures are promising candidates for future highly efficient and stable SOT-based devices. △ Less

Submitted 8 August, 2024; v1 submitted 10 May, 2024; originally announced May 2024.

Comments: 15 pages, 4 figures, accepted by Advanced Materials

Non-Abelian line graph: A generalized approach to flat bands

Authors: Rui-Heng Liu, Xin Liu

Abstract: Flat bands (FBs) in materials can enhance the correlation effects, resulting in exotic phenomena. Line graph (LG) lattices are well known for hosting FBs with isotropic hoppings in $s$-orbital models. Despite their prevalent application in the Kagome metals, there has been a lack of a general approach for incorporating higher-angular-momentum orbitals with spin-orbit couplings (SOCs) into LGs to a… ▽ More Flat bands (FBs) in materials can enhance the correlation effects, resulting in exotic phenomena. Line graph (LG) lattices are well known for hosting FBs with isotropic hoppings in $s$-orbital models. Despite their prevalent application in the Kagome metals, there has been a lack of a general approach for incorporating higher-angular-momentum orbitals with spin-orbit couplings (SOCs) into LGs to achieve FBs. Here, we introduce a non-Abelian LG theory to construct FBs in realistic systems, which incorporates internal degrees of freedom and goes beyond $s$-orbital models. We modify the lattice edges and sites in the LG to be associated with arbitrary Hermitian matrices, referred to as the multiple LG. A fundamental aspect involves mapping the multiple LG Hamiltonian to a tight-binding (TB) model that respects the lattice symmetry through appropriate local non-Abelian transformations. We establish the general conditions to determine the local transformations. Based on this mechanism, we demonstrate the realization of $d$-orbital FBs in the Kagome lattice, which could serve as a minimal model for understanding the FBs in transition metal Kagome materials. Our approach bridges the gap between the known FBs in pure lattice models and their realization in multi-orbital systems. △ Less

Submitted 11 July, 2024; v1 submitted 1 May, 2024; originally announced May 2024.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. B 111, 035134 (2015)

Quantum theory of molecular orientations: topological classification, complete entanglement, and fault-tolerant encodings

Authors: Victor V. Albert, Eric Kubischta, Mikhail Lemeshko, Lee R. Liu

Abstract: We formulate a quantum phase space for molecular rotational and nuclear-spin states. Taking in molecular geometry and nuclear-spin data, we reproduce a molecule's admissible angular momentum states known from spectroscopy, introduce its angular position states using quantization theory, and develop a generalized Fourier transform converting between the two. We classify molecules into three types -… ▽ More We formulate a quantum phase space for molecular rotational and nuclear-spin states. Taking in molecular geometry and nuclear-spin data, we reproduce a molecule's admissible angular momentum states known from spectroscopy, introduce its angular position states using quantization theory, and develop a generalized Fourier transform converting between the two. We classify molecules into three types -- asymmetric, rotationally symmetric, and perrotationally symmetric -- with the last type having no macroscopic analogue due to nuclear-spin statistics constraints. We discuss two general features in perrotationally symmetric state spaces that are Hamiltonian-independent and induced solely by symmetry and spin statistics. First, we quantify when and how the state space of a molecular species is completely rotation-spin entangled, meaning that it does not admit any separable states. Second, we identify molecular species whose position states house an internal pseudo-spin or "fiber" degree of freedom, and the fiber's Berry phase or matrix after adiabatic changes in position yields naturally robust operations, akin to braiding anyonic quasiparticles or realizing fault-tolerant quantum gates. We outline how the fiber can be used as a quantum error-correcting code and discuss scenarios where these features can be experimentally probed. △ Less

Submitted 2 January, 2025; v1 submitted 7 March, 2024; originally announced March 2024.

Comments: 10 + 40 pages, 10 figures, 6 tables, 49 examples; v3 new QEC application

Polarity vs Chirality: Functionality from competing magneto-structural instabilities

Authors: M. Tardieux, E. Stylianidis, D. Behr, R. Liu, K. Yamaura, D. D. Khalyavin, D. R. Bowler, A. A. Belik, R. D. Johnson

Abstract: We report a phenomenological magneto-structural model based on competing free-energy terms that couple either polar or chiral distortions in cubic quadruple perovskites, depending on the global direction of magnetic moments. The model naturally explains why some compounds in this material system host magnetically-induced ferroelectricity at low temperature, while others such as CaMn$_3$(Cr$_3$Mn)O… ▽ More We report a phenomenological magneto-structural model based on competing free-energy terms that couple either polar or chiral distortions in cubic quadruple perovskites, depending on the global direction of magnetic moments. The model naturally explains why some compounds in this material system host magnetically-induced ferroelectricity at low temperature, while others such as CaMn$_3$(Cr$_3$Mn)O$_{12}$, which we characterise experimentally, do not. Importantly, our results suggest a new approach towards developing an applied multiferroic functionality, and can be generalised to other multi-sublattice systems where the magnetic interaction between sublattices is prohibited by spatial inversion. △ Less

Submitted 4 March, 2024; originally announced March 2024.

Comments: 6 pages, 3 figures