Showing 1–50 of 191 results for author: Fang, C

Autonomous Optimization of Complex Oxides for Thermochemical Fuel Production

Authors: Shuiping Gong, Mingcheng Li, Han Hao, Zhenhao Zhou, Yi Li, Xiaobo Liao, Cheng Fang, Jian Deng, Jiangang He, Wenpei Gao, Yakun Yuan, Chris Wolverton, Tao Deng, Chaochao Dun, Runxia Cai, Zhenpeng Yao

Abstract: Two-step thermochemical fuel production, including H2O and CO2 splitting, offers a promising route to sustainable fuel manufacturing, with performance governed by redox-active oxides that enable cyclic reduction-oxidation reactions. Maximizing thermal-to-fuel conversion efficiency demands materials that simultaneously satisfy multiple stringent thermodynamic and kinetic targets. Addressing these r… ▽ More Two-step thermochemical fuel production, including H2O and CO2 splitting, offers a promising route to sustainable fuel manufacturing, with performance governed by redox-active oxides that enable cyclic reduction-oxidation reactions. Maximizing thermal-to-fuel conversion efficiency demands materials that simultaneously satisfy multiple stringent thermodynamic and kinetic targets. Addressing these requirements has increasingly driven materials design toward complex, multi-cation oxides, such as mixed-cation fluorites, perovskites, and high-entropy oxides, wherein composition, defect chemistry, phase stability, and morphology should be co-optimized. This creates a challenging materials optimization problem that is poorly suited to traditional trial-and-error approaches. In this review, we argue that thermochemical fuel production provides a compelling frontier for autonomous materials design and optimization. We first examine why redox-active complex oxides are difficult to develop, owing to multidimensional phase spaces, harsh operating conditions, and competing functional targets. We then discuss how high-throughput computation, automated synthesis, characterization and testing, and machine learning can be integrated into closed-loop workflows to address these challenges. Building on broader oxide materials research, we organize recent progress into a capability roadmap for complex-oxide optimization, spanning compositionally diverse synthesis, operando characterization, robotic testing, operation-condition computation, and multi-objective optimization. Finally, we outline key experimental, computational, and data challenges for building self-improving materials development platforms for materials development in thermochemical fuel production. △ Less

Submitted 7 August, 2026; originally announced August 2026.

Comments: 31 pages, 3 figures, 1 table

Dynamical Poles in Non-Hermitian Impurity Scattering

Authors: Ao Yang, Kai Zhang, Chen Fang

Abstract: In Hermitian impurity scattering, each isolated late-time exponential is the fingerprint of a bound state. We show that this correspondence breaks down in non-Hermitian bands. For a single impurity in a non-Hermitian lattice, the late-time signal is controlled by isolated complex frequencies selected by the analytic continuation of the Green's function relevant to real-time dynamics, which we term… ▽ More In Hermitian impurity scattering, each isolated late-time exponential is the fingerprint of a bound state. We show that this correspondence breaks down in non-Hermitian bands. For a single impurity in a non-Hermitian lattice, the late-time signal is controlled by isolated complex frequencies selected by the analytic continuation of the Green's function relevant to real-time dynamics, which we term dynamical poles (DPs). DPs need not coincide with static bound states: one may appear without any bound-state counterpart, while a static bound state may be dynamically invisible. The remainder of the signal is an incoherent background set by complex continuum edges. Our results establish that the real-time analytic structure of the Green's function, not the static eigenvalue problem alone, organizes non-Hermitian impurity scattering. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: 17 pages, 3 figures

Nonmonotonic Scaling of the Anomalous Hall Effect in a Bicollinear Antiferromagnet

Authors: Ruifeng Wang, Chi Fang, Ilya Kostanovski, Ke Xiao, Felix Küster, Jenny Davern, Naoto Nagaosa, Stuart S. P. Parkin

Abstract: An anomalous Hall effect (AHE) in antiferromagnetic (AF) systems with no net magnetization is of considerable interest for both fundamental physics and spintronic applications. Of particular interest is the two-dimensional van der Waals antiferromagnet FeTe that has an unusual fully magnetically compensated bicollinear AF structure and exhibits pronounced Kondo interaction leading to strong band r… ▽ More An anomalous Hall effect (AHE) in antiferromagnetic (AF) systems with no net magnetization is of considerable interest for both fundamental physics and spintronic applications. Of particular interest is the two-dimensional van der Waals antiferromagnet FeTe that has an unusual fully magnetically compensated bicollinear AF structure and exhibits pronounced Kondo interaction leading to strong band renormalization. Here, we investigate the AHE in epitaxial FeTe thin films grown by molecular beam epitaxy. A large anomalous Hall conductivity is exhibited below the Neel temperature (T_N ~ 60 K) and, strikingly, becomes nonlinear at high fields within a narrow temperature window around 49 K, deviating from conventional AHE scaling behavior versus its longitudinal conductivity. Linear fits reveal a pronounced negative peak in the intercept, accompanied by a field-induced canted magnetic moment. The AHE responses are related to the Berry curvature derived from FeTe's topological band structure, highlighting the intricate interplay between topology, magnetism, and electronic transport. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: 21 pages, 5 figures

The theory of topological-topological flat bands

Authors: Rui-Heng Liu, Jiangping Hu, Chen Fang

Abstract: Electronic flat bands have localized Wannier-like orbitals as zero modes. In the Lieb or the kagome models, the localized orbitals satisfy a topological condition that entails two non-contractible loop eigenstates along $x/y$-axis in real space, and one topological band touching point with other bands in momentum space. In these topological-flat bands, the Bloch state at the touching point is ill-… ▽ More Electronic flat bands have localized Wannier-like orbitals as zero modes. In the Lieb or the kagome models, the localized orbitals satisfy a topological condition that entails two non-contractible loop eigenstates along $x/y$-axis in real space, and one topological band touching point with other bands in momentum space. In these topological-flat bands, the Bloch state at the touching point is ill-defined, and so is any topological invariant for the entire band. We propose a new topological condition that the loop states in different directions be linearly dependent. Its satisfaction removes the singularity at the band touching point, and enforces nontrivial, well-defined topological invariants. Enforcing the new condition, we obtain topological-topological (top$^2$)-flat bands in 2D and 3D that have nontrivial invariants including the Chern numbers, the $\mathbb{Z}_2$ invariants, and the topological-crystalline invariants. Under small, generic interactions, top$^2$-flat bands flow to correlated topological insulators with a dynamically generated, symmetric mass term; and specially designed interacting models can have top$^2$-flat bands as exact zero modes. △ Less

Submitted 25 March, 2026; originally announced March 2026.

Comments: 8+21 pages, 3+4 figures; Comments are welcome

Resolution of Topology and Geometry from Momentum-Resolved Spectroscopies

Authors: Shaofeng Huang, Chen Fang

Abstract: Extracting the complete quantum geometric and topological character of Bloch wavefunctions from experiments remains a challenge in condensed matter physics. Here, we resolve this by introducing the "wavefunction form factor" (WFF) matrix, a quantity directly constructible from intensities in momentum- and energy-resolved spectroscopies like ARPES and INS. We demonstrate that band topology is encod… ▽ More Extracting the complete quantum geometric and topological character of Bloch wavefunctions from experiments remains a challenge in condensed matter physics. Here, we resolve this by introducing the "wavefunction form factor" (WFF) matrix, a quantity directly constructible from intensities in momentum- and energy-resolved spectroscopies like ARPES and INS. We demonstrate that band topology is encoded in "spectral nodes" -- momentum-space points where the WFF determinant vanishes, providing a direct readout of topological invariants via a topological selection rule. Furthermore, when the number of independent probes exceeds the number of the target bands, our framework yields an effective band projector. This enables the determination of Wilson loop spectra and the extraction of an effective quantum geometric tensor, providing a model-independent measurement of the non-Abelian Berry curvature and quantum metric as resolved by the experimental probes. △ Less

Submitted 14 January, 2026; originally announced January 2026.

A Unified Symmetry Classification of Magnetic Orders via Spin Space Groups: Prediction of Coplanar Even-Wave Phases

Authors: Ziyin Song, Ziyue Qi, Chen Fang, Zhong Fang, Hongming Weng

Abstract: Spin space groups (SSGs) impose fundamentally different constraints on magnetic configurations in real and reciprocal spaces. As a consequence, the correspondence between real-space and momentum-space spin arrangements is far richer than traditionally assumed. Building on the complete enumeration of SSGs, we develop a systematic, symmetry-based framework that classifies all possible spin arrangeme… ▽ More Spin space groups (SSGs) impose fundamentally different constraints on magnetic configurations in real and reciprocal spaces. As a consequence, the correspondence between real-space and momentum-space spin arrangements is far richer than traditionally assumed. Building on the complete enumeration of SSGs, we develop a systematic, symmetry-based framework that classifies all possible spin arrangements allowed by these groups. This unified approach naturally incorporates conventional magnetic orders, altermagnetism, and p-wave magnetism as distinct symmetry classes. Crucially, our classification predicts a variety of novel magnetic phases, highlighted by the discovery of the coplanar even-wave magnet: a state that is non-collinear in real space but hosts a collinear even-wave spin polarization in k-space. Analysis of a minimal model reveals that this phase is characterized by non-quantized spin polarization and exhibits a novel mechanism for symmetry-enforced zero polarization on non-degenerate bands. Extending the framework from bulk crystals to layer SSGs appropriate for two-dimensional systems, we further predict layered counterparts and provide symmetry guidelines for designing bilayer coplanar p-wave and even-wave magnets. We further validate this finding through first-principles calculations and propose CoCrO4 as a promising candidate for its experimental realization, thereby demonstrating the completeness and predictive power of the SSG-based classification of magnetic orders. △ Less

Submitted 14 December, 2025; v1 submitted 9 December, 2025; originally announced December 2025.

Journal ref: Phys. Rev. X 16, 031038 (2026)

Exceptional points and spectral cusps from density-wave fluctuation

Authors: Zixi Fang, Chen Fang

Abstract: We report two types of singularities that arise from fluctuations during the formation of charge- or spin-density waves. The first is the exceptional point (EP), corresponding to a higher-order pole of the retarded Green's function. Such EPs lead to algebraic corrections in the decay of quasiparticle occupations and are observable through time-resolved angle-resolved photoemission spectroscopy (Tr… ▽ More We report two types of singularities that arise from fluctuations during the formation of charge- or spin-density waves. The first is the exceptional point (EP), corresponding to a higher-order pole of the retarded Green's function. Such EPs lead to algebraic corrections in the decay of quasiparticle occupations and are observable through time-resolved angle-resolved photoemission spectroscopy (Tr-ARPES). The second is a spectral cusp, defined by the coalescence of three extrema in the real-frequency spectral function $A(\mathbf{k}, ω)$. This cusp enforces the formation of Fermi arcs and induces a "threading" structure in the nearby band structure, both of which are directly observable in ARPES. △ Less

Submitted 26 November, 2025; originally announced November 2025.

Comments: 11 pages, 6 figures

Superdiffusive transport protected by topology and symmetry in all dimensions

Authors: Shaofeng Huang, Yu-Peng Wang, Jie Ren, Chen Fang

Abstract: Superdiffusion is an anomalous transport behavior. Recently, a new mechanism, termed the ``nodal mechanism," has been proposed to induce superdiffusion in quantum models. However, existing realizations of the nodal mechanism have so far been proposed on fine-tuned, artificial Hamiltonians, posing a significant challenge for experimental observation. In this work, we propose a broad class of models… ▽ More Superdiffusion is an anomalous transport behavior. Recently, a new mechanism, termed the ``nodal mechanism," has been proposed to induce superdiffusion in quantum models. However, existing realizations of the nodal mechanism have so far been proposed on fine-tuned, artificial Hamiltonians, posing a significant challenge for experimental observation. In this work, we propose a broad class of models for generating superdiffusion potentially realizable in condensed matter systems across different spatial dimensions. A robust nodal structure emerges from the hybridization between the itinerant electrons and the local impurity orbitals, protected by the intrinsic symmetry and topology of the electronic band. We derive a universal scaling law for the conductance, $G \sim L^{-γ}$, revealing how the exponent is dictated by the dimensionality of the nodal structure ($D_{\text{node}}$) and its order $n$, and the dimensionality of the system $(D)$ at high temperatures or that of the Fermi surface ($D^F$) at low temperatures. Through numerical simulations, we validate these scaling relations at zero temperature for various models, including those based on graphene and multi-Weyl semimetals, finding excellent agreement between our theory and the computed exponents. Beyond the scaling of conductance, our framework predicts a suite of experimentally verifiable signatures, notably a new mechanism for linear-in-temperature resistivity ($ρ\sim T$) and a divergent low-frequency optical conductivity ($σ(ω) \sim ω^{γ-1}$), establishing a practical route to discovering and engineering anomalous transport in quantum materials. △ Less

Submitted 12 November, 2025; originally announced November 2025.

Comments: 25 pages, 7 figures

Excitation spectrum and low-temperature magnetism in disordered defect-fluorite Ho2Zr2O7

Authors: P. L. Oliveira Silva, J. G. A. Ramon, Viviane Peçanha-Antonio, Tatiana Guidi, J. S. Gardner, Chun Sheng Fang, R. S. Freitas

Abstract: In this work, we report on the thermomagnetic characterization and crystalline-electric field (CEF) energy scheme of the disordered defect-fluorite Ho2Zr2O7. This structural phase is distinguished by the coexistence of magnetic frustration and extensive disorder, with Ho3+ and Zr4+ sharing randomly the same 4a site with even 50% occupancy, and an average 1/8 oxygen vacancy per unit cell. AC magnet… ▽ More In this work, we report on the thermomagnetic characterization and crystalline-electric field (CEF) energy scheme of the disordered defect-fluorite Ho2Zr2O7. This structural phase is distinguished by the coexistence of magnetic frustration and extensive disorder, with Ho3+ and Zr4+ sharing randomly the same 4a site with even 50% occupancy, and an average 1/8 oxygen vacancy per unit cell. AC magnetic susceptibility measurements performed on powder samples down to 0.5 K revealed signs of slowing spin dynamics without glassy behavior, including a frequency dependent peak at 1 K. Yet, no evidence for long-range magnetic order is found down to 150 mK in specific heat. Inelastic neutron scattering measurements show a weak, low-lying CEF excitation around 2 meV, accompanied by a broad level centered at 60 meV. To fit our observations, we propose an approach to account for structural disorder in the crystal-field splitting of the non-Kramers Ho3+. Our model provides an explanation to the broadening of the high-energy, single-ion excitations and suggests that the zirconate ground-state wave function has zero magnetic moment. However, structural disorder acts as guarantor of the magnetism in Ho2Zr2O7, allowing the mixing of low lying states at finite temperatures. Finally, we show that this scenario is in good agreement with the bulk properties reported in this work. △ Less

Submitted 7 November, 2025; originally announced November 2025.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. B 113, 214423 (2026)

Tunable Multistage Refrigeration via Geometrically Frustrated Triangular Lattice Antiferromagnet for Space Cooling

Authors: Jianqiao Wang, Chushu Fang, Zhibin Qiu, Yang Zhao, Quan Xiao, Xiying Sun, Zhaoyi Li, Laifeng Li, Yuan Zhou, Changzhao Pan, Shu Guo

Abstract: Low-temperature refrigeration technology constitutes a crucial component in space exploration. The small-scale, low-vibration Stirling-type pulse tube refrigerators hold significant application potential for space cooling. However, the efficient operation of current Stirling-type pulse tube cryocoolers in space cooling applications remains challenging due to the rapid decay of the heat capacity of… ▽ More Low-temperature refrigeration technology constitutes a crucial component in space exploration. The small-scale, low-vibration Stirling-type pulse tube refrigerators hold significant application potential for space cooling. However, the efficient operation of current Stirling-type pulse tube cryocoolers in space cooling applications remains challenging due to the rapid decay of the heat capacity of regenerative materials below 10 K. This study adopts a novel material strategy: using a novel high-spin S = 7/2 magnetic regenerative material, Gd2O2Se, we construct a multistage tunable regenerative material structure to achieve an efficient cooling approach to the liquid helium temperature range. Under substantial geometric frustration from a double-layered triangular lattice, it exhibits two-step specific heat transition peaks at 6.22 K and 2.11 K, respectively. Its ultrahigh specific heat and broad two-step transition temperature range effectively bridge the gap between commercially used high-heat-capacity materials. Experimental verification shows that when Gd2O2Se is combined with Er3Ni and HoCu2 in the Stirling-type pulse tube cryocooler, the cooling efficiency of the pulse tube increases by 66.5 % at 7 K, and the minimum achievable temperature reaches 5.85 K. These results indicate that Gd2O2Se is an ideal magnetic regenerative material for space cooling △ Less

Submitted 5 November, 2025; originally announced November 2025.

Observation and Modulation of the Quantum Mpemba Effect on a Superconducting Quantum Processor

Authors: Yueshan Xu, Cai-Ping Fang, Bing-Jie Chen, Ming-Chuan Wang, Zi-Yong Ge, Yun-Hao Shi, Yu Liu, Cheng-Lin Deng, Kui Zhao, Zheng-He Liu, Tian-Ming Li, Hao Li, Ziting Wang, Gui-Han Liang, Da'er Feng, Xueyi Guo, Xu-Yang Gu, Yang He, Hao-Tian Liu, Zheng-Yang Mei, Yongxi Xiao, Yu Yan, Yi-Han Yu, Wei-Ping Yuan, Jia-Chi Zhang , et al. (11 additional authors not shown)

Abstract: In non-equilibrium quantum systems, the quantum Mpemba effect (QME) emerges as a counterintuitive phenomenon: systems exhibiting greater initial symmetry breaking restore symmetry faster. It has been attracting broad interest in studying QME dynamics and potential applications in quantum information science. While theoretical exploration of QME has surged, experimental studies, specifically on its… ▽ More In non-equilibrium quantum systems, the quantum Mpemba effect (QME) emerges as a counterintuitive phenomenon: systems exhibiting greater initial symmetry breaking restore symmetry faster. It has been attracting broad interest in studying QME dynamics and potential applications in quantum information science. While theoretical exploration of QME has surged, experimental studies, specifically on its flexible modulation, remain limited. Here, we report the observation and modulation of QME using a superconducting processor featuring an all-to-all connected, tunable-coupling architecture that enables precise control from short- to long-range interactions. This platform allows independent manipulation of coupling regimes, on-site potentials, and initial states, enabling us to elucidate their roles in QME. To quantify symmetry restoration, we employ entanglement asymmetry (EA), derived from the reconstructed density matrix via quantum state tomography, as a sensitive probe. In strong short-range coupling regimes, EA crossovers during quenches from tilted Néel states confirm the presence of QME. In contrast, in intermediate coupling regimes, synchronized EA and entanglement entropy dynamics reveal the suppression of QME. Remarkably, QME reemerges with the introduction of on-site linear potentials or quenches from tilted ferromagnetic states, the latter proving robust against on-site disorder. Our study demonstrates flexible QME modulation on a superconducting platform with multiple controllable parameters, shedding light on quantum many-body non-equilibrium dynamics and opening avenues for quantum information applications. △ Less

Submitted 29 May, 2026; v1 submitted 11 August, 2025; originally announced August 2025.

Comments: Figures modified and new discussions added. Final version published in PRL

Journal ref: Phys. Rev. Lett. 137, 010402 (2026)

Soliton Transitions Mediated by Skin-Mode Localization and Band Nonreciprocity

Authors: Shanyue Li, Mengying Hu, Jing Lin, Chen Fang, Zhensheng Tao, Kun Ding

Abstract: Solitons, typically resulting from a competition between band dispersion and nonlinearity, occur in lattices featuring the non-Hermitian skin effect as nonlinearity increases, accompanied by a transition in localization from linear skin modes to solitons. However, localization does not disentangle the role of skin modes in the soliton formation from that of band dispersion. Here, in such lattices,… ▽ More Solitons, typically resulting from a competition between band dispersion and nonlinearity, occur in lattices featuring the non-Hermitian skin effect as nonlinearity increases, accompanied by a transition in localization from linear skin modes to solitons. However, localization does not disentangle the role of skin modes in the soliton formation from that of band dispersion. Here, in such lattices, we uncover two distinct soliton phases, skin-mode-assisted solitons (SMASs) and nonreciprocity-dressed solitons (NRDSs). Rooted in fundamentally different mechanisms, SMASs originate from skin effect, while NRDSs stem from band nonreciprocity, each exhibiting unique spatial profiles. Using a stacked Su-Schrieffer-Heeger-like model as a prototype, we delineate the phase diagram of SMASs and NRDSs, each having clear phase boundaries. To interpret them, we formulate a Wannier-function-based nonlinear Hamiltonian, showing that soliton formation depends critically on how skin-mode localization and band nonreciprocity suppress or enhance wave dispersion. For SMASs, skin-mode localization reduces wave broadening at the localization sites, thereby lowering the formation threshold. This soliton phase is observable from edge dynamics and accompanied by a dynamical stability reentrance when transitioning from linear skin modes. In contrast, NRDSs, as well as their thresholds, originate from bulk band nonreciprocity and persist under periodic boundary conditions. Our framework offers predictive tools for characterizing and engineering solitons in experimentally realizable non-Hermitian systems, spanning optics to mechanics. △ Less

Submitted 4 August, 2025; originally announced August 2025.

Comments: 21 pages, 6 figures

Journal ref: National Science Review, nwag381 (2026)

Many-body delocalization with a two-dimensional 70-qubit superconducting quantum simulator

Authors: Tian-Ming Li, Zheng-Hang Sun, Yun-Hao Shi, Zhen-Ting Bao, Yong-Yi Wang, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Yi-Han Yu, Zheng-He Liu, Chi-Tong Chen, Li Li, Hao Li, Hao-Tian Liu, Si-Yun Zhou, Zhen-Yu Peng, Yan-Jun Liu, Ziting Wang, Yue-Shan Xu, Kui Zhao, Yang He, Da'er Feng, Jia-Cheng Song, Cai-Ping Fang, Junrui Deng , et al. (13 additional authors not shown)

Abstract: Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D… ▽ More Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D) superconducting quantum simulator, we experimentally explore the robustness of the MBL regime in controlled finite-size 2D systems. We observe that the decay of imbalance becomes more pronounced with increasing system sizes, scaling up from 21, 42 to 70 qubits, with a relatively large disorder strength, and for the first time, provide an evidence for the many-body delocalization in 2D disordered systems. Our experimental results are consistent with the avalanche theory that predicts the instability of MBL regime beyond one spatial dimension. This work establishes a scalable platform for probing high-dimensional non-equilibrium phases of matter and their finite-size effects using superconducting quantum circuits. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: main text: 7 pages, 3 figures; supplementary information: 19 pages, 17 figures

Algebraic States in Continuum in $ d\gt 1$ Dimensional Non-Hermitian Systems

Authors: Ao Yang, Kai Zhang, Chen Fang

Abstract: We report the existence of algebraically localized eigenstates embedded within the continuum spectrum of 2D non-Hermitian systems with a single impurity. These modes, which we term algebraic states in continuum (AICs), decay algebraically as $1/|r|$ from the impurity site, and their energies lie within the bulk continuum spectrum under periodic boundary conditions. We analytically derive the thres… ▽ More We report the existence of algebraically localized eigenstates embedded within the continuum spectrum of 2D non-Hermitian systems with a single impurity. These modes, which we term algebraic states in continuum (AICs), decay algebraically as $1/|r|$ from the impurity site, and their energies lie within the bulk continuum spectrum under periodic boundary conditions. We analytically derive the threshold condition for the impurity strength required to generate such states. Remarkably, AICs are forbidden in Hermitian systems and in 1D non-Hermitian systems, making them unique to non-Hermitian systems in two and higher dimensions. To detect AICs, we introduce a local density of states as an experimental observable, which is readily accessible in photonic/acoustic platforms. △ Less

Submitted 20 April, 2026; v1 submitted 8 July, 2025; originally announced July 2025.

Comments: 18 pages, 5 figures

Journal ref: Phys. Rev. Lett. 136, 156603 (2026)

Molecular Dynamics Simulations on Nuclear Recoils in Silicon Crystals towards Single Electron-Hole Pair Ionization Yields

Authors: Chang-Hao Fang, Shin-Ted Lin, Shu-Kui Liu, Henry Tsz-King Wong, Hao-Yang Xing, Li-Tao Yang, Qian Yue, Jing-Jun Zhu

Abstract: We have developed a novel methodology utilizing molecular dynamics simulations to evaluate the ionization yields of nuclear recoils in crystalline silicon. This approach enables analytical exploration of atomic-scale transport within the lattice without necessitating parameterization. The quenching factors across the nuclear recoil energy range from 20 eV to 10 keV have been thoroughly investigate… ▽ More We have developed a novel methodology utilizing molecular dynamics simulations to evaluate the ionization yields of nuclear recoils in crystalline silicon. This approach enables analytical exploration of atomic-scale transport within the lattice without necessitating parameterization. The quenching factors across the nuclear recoil energy range from 20 eV to 10 keV have been thoroughly investigated. A remarkable agreement with experimental data is achieved, particularly for the minimal energy regime conducted to date, reaching the level of a single electron-hole pair. This work presents evidence of a crucial and fundamental distribution of the quenching factor, which can be associated to the collisional interactions underlying the transport phenomena. The region below 4 keV of the quenching factor, where discrepancies have been observed with the Lindhard's model, is found to be significantly attributed to the lattice binding effect and the specific crystal structure. In contrast, a gradual functional relationship is identified below approximately 100 eV, indicating that the quenching factor is influenced by the crystallographic orientation of the target material. From a distributional perspective, our analysis allows for the determination of the minimum exclusion mass for the dark matter nucleon elastic scattering channel at 0.29 $\mathrm{GeV}/c^2$, thereby significantly enhancing sensitivity for the sub-$\mathrm{GeV}/c^2$ mass region. △ Less

Submitted 14 November, 2025; v1 submitted 4 June, 2025; originally announced June 2025.

Comments: 7 pages, 5 figures, revised manuscript

Journal ref: Phys. Rev. D 112, L101303 (2025)

Giant Tunneling Magnetoresistance in Graphene/$h$-BN Based van der Waals Magnetic Tunnel Junctions via 3$d$ Transition Metal Intercalation

Authors: Zhi Yan, Jianhua Xiao, Xujin Zhang, Cheng Fang, Xiaohong Xu

Abstract: Atomic intercalation offers a powerful route for engineering two-dimensional (2D) materials by precisely tuning interlayer electronic coupling and spin configurations. Here, we propose a generic strategy for the construction of fully 2D magnetic tunnel junctions (MTJs) based on transition metal-intercalated graphene electrodes with $h$-BN barrier layer. First-principles calculations reveal that in… ▽ More Atomic intercalation offers a powerful route for engineering two-dimensional (2D) materials by precisely tuning interlayer electronic coupling and spin configurations. Here, we propose a generic strategy for the construction of fully 2D magnetic tunnel junctions (MTJs) based on transition metal-intercalated graphene electrodes with $h$-BN barrier layer. First-principles calculations reveal that intercalation not only stabilizes uniform atomic dispersion via steric hindrance but also induces robust ferromagnetism in graphene. Manganese- and vanadium-intercalated systems (Mn-Gr and V-Gr) exhibit exceptional spintronic performance, with tunneling magnetoresistance (TMR) showing a pronounced odd-even oscillation as a function of barrier thickness. A giant TMR of $4.35 \times 10^8\,\%$ is achieved in the Mn-Gr system with a monolayer barrier $h$ -BN ($n=1$), while V-Gr reaches a maximum TMR of $1.86 \times 10^5\,\%$ for a trilayer barrier ($n=3$). Moreover, biaxial strain further enhances the TMR to $10^9\,\%$ and $10^7\,\%$ in Mn-Gr and V-Gr systems, respectively. The devices also exhibit perfect spin filtering and pronounced negative differential resistance, offering new opportunities for high-performance spintronic and memory applications based on 2D van der Waals heterostructures. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Comments: 13 pages, 8 figures

Prethermalization by Random Multipolar Driving on a 78-Qubit Superconducting Processor

Authors: Zheng-He Liu, Yu Liu, Gui-Han Liang, Cheng-Lin Deng, Keyang Chen, Yun-Hao Shi, Tian-Ming Li, Lv Zhang, Bing-Jie Chen, Cai-Ping Fang, Da'er Feng, Xu-Yang Gu, Yang He, Kaixuan Huang, Hao Li, Hao-Tian Liu, Li Li, Zheng-Yang Mei, Zhen-Yu Peng, Jia-Cheng Song, Ming-Chuan Wang, Shuai-Li Wang, Ziting Wang, Yongxi Xiao, Minke Xu , et al. (21 additional authors not shown)

Abstract: Time-dependent drives hold the promise of realizing non-equilibrium many-body phenomena that are absent in undriven systems. Yet, drive-induced heating normally destabilizes the systems, which can be parametrically suppressed in the high-frequency regime by using periodic (Floquet) drives. It remains largely unknown to what extent highly controllable quantum simulators can suppress heating in non-… ▽ More Time-dependent drives hold the promise of realizing non-equilibrium many-body phenomena that are absent in undriven systems. Yet, drive-induced heating normally destabilizes the systems, which can be parametrically suppressed in the high-frequency regime by using periodic (Floquet) drives. It remains largely unknown to what extent highly controllable quantum simulators can suppress heating in non-periodically driven systems. Using the 78-qubit superconducting quantum processor, Chuang-tzu 2.0, we report the experimental observation of long-lived prethermal phases in many-body systems with tunable heating rates, driven by structured random protocols, characterized by $n$-multipolar temporal correlations. By measuring both the particle imbalance and subsystem entanglement entropy, we monitor the entire heating process over 1,000 driving cycles and observe the existence of the prethermal plateau. The prethermal lifetime is `doubly tunable': one way by driving frequency, the other by multipolar order; it grows algebraically with the frequency with the universal scaling exponent $2n{+}1$. Using quantum state tomography on different subsystems, we demonstrate a non-uniform spatial entanglement distribution and observe a crossover from area-law to volume-law entanglement scaling. With 78 qubits and 137 couplers in a 2D configuration, the entire far-from-equilibrium heating dynamics are beyond the reach of simulation using tensor-network numerical techniques. Our work highlights superconducting quantum processors as a powerful platform for exploring universal scaling laws and non-equilibrium phases of matter in driven systems in regimes where classical simulation faces formidable challenges. △ Less

Submitted 1 April, 2025; v1 submitted 27 March, 2025; originally announced March 2025.

Journal ref: Nature 650, 79-85 (2026)

Real-time edge dynamics of non-Hermitian lattices

Authors: Tian-Hua Yang, Chen Fang

Abstract: We derive the asymptotic forms of the Green's function at the open edges of general non-Hermitian band systems in all dimensions in the long-time limit, using a modified saddle-point approximation and the analytic continuation of the momentum. The edge dynamics is determined by the "dominant saddle point", a complex momentum, which, contrary to previous conjectures, may lie outside the generalized… ▽ More We derive the asymptotic forms of the Green's function at the open edges of general non-Hermitian band systems in all dimensions in the long-time limit, using a modified saddle-point approximation and the analytic continuation of the momentum. The edge dynamics is determined by the "dominant saddle point", a complex momentum, which, contrary to previous conjectures, may lie outside the generalized Brillouin zone. From this result, we obtain the effective edge Hamiltonians that evidently, as demonstrated by extensive numerical simulations, characterize the dynamics on the edges, and can be probed in real-time experiments or spectroscopies. △ Less

Submitted 7 October, 2025; v1 submitted 14 March, 2025; originally announced March 2025.

Comments: Code available at https://github.com/ThomasYangth/NHWP-SP

Journal ref: Physical Review Letters 135, 186401 (2025)

Impact of on-site potentials on $q$-breathers in nonlinear chains

Authors: Lin Deng, Chengguan Fang, Hang Yu, Yisen Wang, Zhigang Zhu, Weicheng Fu, Sergej Flach, Liang Huang

Abstract: On-site potentials are ubiquitous in physical systems and strongly influence their heat transport and energy localization. These potentials will inevitably affect the dynamical properties of $q$-breathers (QBs), defined as periodic orbits exponentially localized in normal mode space. By integrating on-site terms into the Fermi-Pasta-Ulam-Tsingou-$β$ system, this work utilizes numerical simulations… ▽ More On-site potentials are ubiquitous in physical systems and strongly influence their heat transport and energy localization. These potentials will inevitably affect the dynamical properties of $q$-breathers (QBs), defined as periodic orbits exponentially localized in normal mode space. By integrating on-site terms into the Fermi-Pasta-Ulam-Tsingou-$β$ system, this work utilizes numerical simulations and Floquet analysis to systematically explore the influence of on-site potentials on QB stability. For most QBs, except those at the phonon band edges, the instability is primarily governed by parametric resonance, and effectively described by coupled Mathieu equations. This approach provides a theoretical expression for the instability thresholds, which aligns well with numerical results. We demonstrate that the instability thresholds can be controlled through the strength of on-site potentials, and for a strong enough quadratic on-site potential, the QBs are always stable. Furthermore, the instability threshold is highly sensitive to the seed mode, in stark contrast to systems without on-site potentials. In addition, the instability phase diagrams exhibit joint interplay between different terms in the Hamiltonian, such as the quadratic on-site and quartic inter-site interaction terms, in regulating the QB dynamics. These findings offer valuable insights into QB stability and the manipulation of localized excitations in diverse physical systems with on-site potentials. △ Less

Submitted 7 March, 2025; originally announced March 2025.

Giant Nonvolatile Multistate Resistance with Fully Magnetically Controlled van der Waals Multiferroic Tunnel Junctions

Authors: Zhi Yan, Xujin Zhang, Jianhua Xiao, Cheng Fang, Xiaohong Xu

Abstract: Ferroelectric polarization switching in electrically controlled van der Waals multiferroic tunnel junctions (vdW-MFTJs) causes atomic migration, compromising device stability and fatigue resistance. Here we propose a fully magnetically controlled vdW-MFTJ based on a \(\mathrm{CrBr_3/MnPSe_3/CrBr_3}\) vertical heterostructure, which achieves ferroelectric polarization reversal without relying on at… ▽ More Ferroelectric polarization switching in electrically controlled van der Waals multiferroic tunnel junctions (vdW-MFTJs) causes atomic migration, compromising device stability and fatigue resistance. Here we propose a fully magnetically controlled vdW-MFTJ based on a \(\mathrm{CrBr_3/MnPSe_3/CrBr_3}\) vertical heterostructure, which achieves ferroelectric polarization reversal without relying on atomic migration driven by inversion symmetry breaking. Using first-principles calculations, we investigate the spin-polarized quantum transport properties of the proposed structure. By integrating asymmetric PtTe$_2$/alkali-metal (Li/Na/K)-doped/intercalated CrBr$_3$ electrodes, the device demonstrates exceptional performance, with a maximum tunneling magnetoresistance (TMR) exceeding $8.1\times10^5$\% and tunneling electroresistance (TER) reaching 2499\%, while the spin-filtering channels can be flexibly controlled by the magnetization direction of the magnetic free layer, achieving perfect spin-filtering over a broad bias voltage range. Applying an external bias voltage further enhances these metrics, increasing TMR to $3.6\times 10^7$\% and TER to 9990\%. Notably, a pronounced negative differential resistance (NDR) effect is observed, yielding an unprecedented peak-to-valley ratio (PVR) of $9.55\times10^9$\%, representing the highest value reported for vertical tunnel junctions. These extraordinary characteristics highlight the potential of vdW-MFTJs for ultra-efficient electronic switching, a key feature for next-generation spintronic devices. Our findings provide a solid theoretical foundation for designing and developing high-performance magnetic storage and logic technologies. △ Less

Submitted 19 May, 2025; v1 submitted 1 January, 2025; originally announced January 2025.

Comments: 7 pages, 4 figures

Constructions and Applications of Irreducible Representations of Spin-Space Groups

Authors: Ziyin Song, A. Z. Yang, Yi Jiang, Zhong Fang, Jian Yang, Chen Fang, Hongming Weng, Zheng-Xin Liu

Abstract: Spin-space groups (SSGs), including the traditional space groups (SGs) and magnetic space groups (MSGs) as subsets, describe the complete symmetries of magnetic materials with weak spin-orbit coupling (SOC). In the present work, we systematically study the irreducible representations (irreps) of SSGs by focusing on the projective irreps of the little co-group $L(k)$ of any momentum point $\pmb k$.… ▽ More Spin-space groups (SSGs), including the traditional space groups (SGs) and magnetic space groups (MSGs) as subsets, describe the complete symmetries of magnetic materials with weak spin-orbit coupling (SOC). In the present work, we systematically study the irreducible representations (irreps) of SSGs by focusing on the projective irreps of the little co-group $L(k)$ of any momentum point $\pmb k$. We analysis the factor systems of $L(k)$, and then reduce the projective regular representation of $L(k)$ into direct sum of irreps using the Hamiltonian approach. Especially, for collinear SSGs which contain continuous spin rotation operations, we adopt discrete subgroups to effectively capture their characteristics. Furthermore, we apply the representation theory of SSGs to study the band structure of electrons and magnons in magnetic materials. After identifying the SSG symmetry group, we extract relevant irreps and determine the $k\cdot p$ models. As an example, we illustrate how our approach works for the material \ch{Mn3Sn}. Degeneracies facilitated by SSG symmetry are observed, underscoring the effectiveness of application in material analysis. The SSG recognition and representation code is uploaded to GitHub, the information of irreps of all SSGs is also available in the online Database. Our work provides a practical toolkit for exploring the intricate symmetries of magnetic materials and paves the way for future advances in materials science. △ Less

Submitted 20 September, 2024; originally announced September 2024.

Broadband light extraction from near-surface NV centers using crystalline-silicon antennas

Authors: Minjeong Kim, Maryam Zahedian, Wenxin Wu, Chengyu Fang, Zhaoning Yu, Raymond A. Wambold, Ricardo Vidrio, Yuhan Tong, Shenwei Yin, David A. Czaplewski, Jennifer T. Choy, Mikhail A. Kats

Abstract: We use crystalline silicon (Si) antennas to efficiently extract broadband single-photon fluorescence from shallow nitrogen-vacancy (NV) centers in diamond into free space. Our design features relatively easy-to-pattern high-index Si resonators on the diamond surface to boost photon extraction by overcoming total internal reflection and Fresnel reflection at the diamond-air interface, and providing… ▽ More We use crystalline silicon (Si) antennas to efficiently extract broadband single-photon fluorescence from shallow nitrogen-vacancy (NV) centers in diamond into free space. Our design features relatively easy-to-pattern high-index Si resonators on the diamond surface to boost photon extraction by overcoming total internal reflection and Fresnel reflection at the diamond-air interface, and providing modest Purcell enhancement, without etching or otherwise damaging the diamond surface. In simulations, ~17 times more single photons are collected from a single NV center compared to the case without the antenna; in experiments, we observe an enhancement of ~9 times, limited by spatial alignment between the NV and the antenna. Our approach can be readily applied to other color centers in diamond, and more generally to the extraction of light from quantum emitters in wide-bandgap materials. △ Less

Submitted 10 February, 2025; v1 submitted 1 September, 2024; originally announced September 2024.

Comments: Updated main text + supplementary

Fermi-surface diagnosis for topological superconductivity with $s$-wave-like pairing symmetries

Authors: Zhongyi Zhang, Ken Shiozaki, Chen Fang, Seishiro Ono

Abstract: Theoretical prediction of topological superconductivity is key to their discovery. Recently, it is proved that in 199 out of 230 space groups, topological superconductivity coexists with an $s$-wave-like pairing symmetry, raising the hope of finding more candidates for this exotic phase. However, a comprehensive and efficient method for diagnosing topological superconductivity in realistic materia… ▽ More Theoretical prediction of topological superconductivity is key to their discovery. Recently, it is proved that in 199 out of 230 space groups, topological superconductivity coexists with an $s$-wave-like pairing symmetry, raising the hope of finding more candidates for this exotic phase. However, a comprehensive and efficient method for diagnosing topological superconductivity in realistic materials remains elusive. Here, we derive Fermi-surface formulas for gapped and gapless topological phases of time-reversal symmetric superconductors with $s$-wave-like pairing symmetries in all layer and space groups, applicable to thin-film and bulk materials. Our diagnosis uses only the sign of the pairing and the Fermi velocity at several Fermi points, and yields complete (partial) diagnosis for gapped topological superconductivity in 159 (40) out of the 199 space groups. This provides a fundamental basis for the first-principles prediction of new topological superconductors. △ Less

Submitted 10 July, 2025; v1 submitted 29 July, 2024; originally announced July 2024.

Comments: See "Ancillary files" for Supplementary Materials II

Report number: RIKEN-iTHEMS-Report-24, YITP-24-67

Journal ref: Nature Communications 17, 4413 (2026)

Tailoring Bound State Geometry in High-Dimensional Non-Hermitian Systems

Authors: Ao Yang, Zixi Fang, Kai Zhang, Chen Fang

Abstract: It is generally believed that the non-Hermitian effect (NHSE), due to its non-reciprocal nature, creates barriers for the appearance of impurity bound states. In this paper, we find that in two and higher dimensions, the presence of geometry-dependent skin effect eliminates this barrier such that even an infinitesimal impurity potential can confine bound states in this type of non-Hermitian system… ▽ More It is generally believed that the non-Hermitian effect (NHSE), due to its non-reciprocal nature, creates barriers for the appearance of impurity bound states. In this paper, we find that in two and higher dimensions, the presence of geometry-dependent skin effect eliminates this barrier such that even an infinitesimal impurity potential can confine bound states in this type of non-Hermitian systems. By examining bound states around Bloch saddle points, we find that non-Hermiticity can disrupt the isotropy of bound states, resulting in concave dumbbell-shaped bound states. Our work reveals a geometry transition of bound state between concavity and convexity in high-dimensional non-Hermitian systems, offering theoretical insights for the experimental manipulation of bound states. △ Less

Submitted 6 February, 2026; v1 submitted 11 June, 2024; originally announced June 2024.

Comments: 14 pages, 4 figures

Journal ref: Commun Phys 8, 124 (2025)

Quasi-Nambu-Goldstone modes in many-body scar models

Authors: Jie Ren, Yu-Peng Wang, Chen Fang

Abstract: From the quasisymmetry-group perspective [Phys. Rev. Lett. 126, 120604 (2021)], we show the universal existence of collective, coherent modes of excitations in many-body scar models in the degenerate limit, where the energy spacing in the scar tower vanishes. The number of these modes, as well as the quantum numbers carried by them, are given, not by the symmetry of the Hamiltonian, but by the qua… ▽ More From the quasisymmetry-group perspective [Phys. Rev. Lett. 126, 120604 (2021)], we show the universal existence of collective, coherent modes of excitations in many-body scar models in the degenerate limit, where the energy spacing in the scar tower vanishes. The number of these modes, as well as the quantum numbers carried by them, are given, not by the symmetry of the Hamiltonian, but by the quasisymmetry of the scar tower: hence the name quasi-Nambu-Goldstone modes. Based on this, we draw a concrete analogy between the paradigm of spontaneous symmetry breaking and the many-body scar physics in the degenerate limit. △ Less

Submitted 9 July, 2024; v1 submitted 1 May, 2024; originally announced May 2024.

Comments: 27 pages, 14 figures

Journal ref: Phys. Rev. B 110, 245101 (2024)

Superdiffusive transport on lattices with nodal impurities

Authors: Yu-Peng Wang, Jie Ren, Chen Fang

Abstract: We show that 1D lattice models exhibit superdiffusive transport in the presence of random "nodal impurities" in the absence of interaction. Here a nodal impurity is defined as a localized state, the wave function of which has zeros (nodes) in momentum space. The dynamics exponent $z$, a defining quantity for transport behaviors, is computed to establish this result. To be specific, in a disordered… ▽ More We show that 1D lattice models exhibit superdiffusive transport in the presence of random "nodal impurities" in the absence of interaction. Here a nodal impurity is defined as a localized state, the wave function of which has zeros (nodes) in momentum space. The dynamics exponent $z$, a defining quantity for transport behaviors, is computed to establish this result. To be specific, in a disordered system having only nodal impurities, the dynamical exponent $z=4n/(4n-1)$ where $n$ is the order of the node. If the system has time reversal, the nodes appear in pairs and the dynamical exponent can be enhanced to $z=8n/(8n-1)$. As $1<z<2$, both cases indicate superdiffusive transport. △ Less

Submitted 27 September, 2024; v1 submitted 25 April, 2024; originally announced April 2024.

Journal ref: Phys. Rev. B 110, 144201(2024)

Giant electrode effect on tunneling magnetoresistance and electroresistance in van der Waals intrinsic multiferroic tunnel junctions using VS2

Authors: Zhi Yan, Ruixia Yang, Cheng Fang, Wentian Lu, Xiaohong Xu

Abstract: Van der Waals multiferroic tunnel junctions (vdW-MFTJs) with multiple nonvolatile resistive states are highly suitable for new physics and next-generation storage electronics. However, currently reported vdW-MFTJs are based on two types of materials, i.e., vdW ferromagnetic and ferroelectric materials, forming a multiferroic system. This undoubtedly introduces additional interfaces, increasing the… ▽ More Van der Waals multiferroic tunnel junctions (vdW-MFTJs) with multiple nonvolatile resistive states are highly suitable for new physics and next-generation storage electronics. However, currently reported vdW-MFTJs are based on two types of materials, i.e., vdW ferromagnetic and ferroelectric materials, forming a multiferroic system. This undoubtedly introduces additional interfaces, increasing the complexity of experimental preparation. Herein, we engineer vdW intrinsic MFTJs utilizing bilayer VS$_2$. By employing the nonequilibrium Green's function combined with density functional theory, we systematically investigate the influence of three types of electrodes (including non-vdW pure metal Ag/Au, vdW metallic 1T-MoS$_2$/2H-PtTe$_2$, and vdW ferromagnetic metallic Fe$_3$GaTe$_2$/Fe$_3$GeTe$_2$) on the electronic transport properties of VS$_2$-based intrinsic MFTJs. We demonstrate that these MFTJs manifest a giant electrode-dependent electronic transport characteristic effect. Comprehensively comparing these electrode pairs, the Fe$_3$GaTe$_2$/Fe$_3$GeTe$_2$ electrode combination exhibits optimal transport properties, the maximum TMR (TER) can reach 10949\% (69\%) and the minimum resistance-area product (RA) is 0.45 $Ω$$μ$m$^{2}$, as well as the perfect spin filtering and negative differential resistance effects. More intriguingly, TMR (TER) can be further enhanced to 34000\% (380\%) by applying an external bias voltage (0.1 V), while RA can be reduced to 0.16 $Ω$$μ$m$^{2}$ under the influence of biaxial stress (-3\%). Our proposed concept of designing vdW-MFTJs using intrinsic multiferroic materials points towards new avenues in experimental exploration. △ Less

Submitted 7 May, 2024; v1 submitted 19 March, 2024; originally announced March 2024.

Comments: 13 pages, 9 figures

Formation of highly stable interfacial nitrogen gas hydrate overlayers under ambient conditions

Authors: Chung-Kai Fang, Cheng-Hao Chuang, Chih-Wen Yang, Zheng-Rong Guo, Wei-Hao Hsu, Chia-Hsin Wang, Ing-Shouh Hwang

Abstract: Surfaces (interfaces) dictate many physical and chemical properties of solid materials and adsorbates considerably affect these properties. Nitrogen molecules, which are the most abundant constituent in ambient air, are considered to be inert. Our study combining atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), and thermal desorption spectroscopy (TDS) revealed that nitrogen… ▽ More Surfaces (interfaces) dictate many physical and chemical properties of solid materials and adsorbates considerably affect these properties. Nitrogen molecules, which are the most abundant constituent in ambient air, are considered to be inert. Our study combining atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), and thermal desorption spectroscopy (TDS) revealed that nitrogen and water molecules can self-assemble into two-dimensional domains, forming ordered stripe structures on graphitic surfaces in both water and ambient air. The stripe structures of this study were composed of approximately 90% and 10% water and nitrogen molecules, respectively, and survived in ultra-high vacuum (UHV) conditions at temperatures up to approximately 350 K. Because pure water molecules completely desorb from graphitic surfaces in a UHV at temperatures lower than 200 K, our results indicate that the incorporation of nitrogen molecules substantially enhanced the stability of the crystalline water hydrogen bonding network. Additional studies on interfacial gas hydrates can provide deeper insight into the mechanisms underlying formation of gas hydrates. △ Less

Submitted 29 January, 2024; originally announced January 2024.

Probing topological phase transition with non-Hermitian perturbations

Authors: Jingcheng Liang, Chen Fang, Jiangping Hu

Abstract: We demonstrate that non-Hermitian perturbations can probe topological phase transitions and unambiguously detect non-Abelian zero modes. We show that under carefully designed non-Hermitian perturbations, the Loschmidt echo(LE) decays into 1/N where N is the ground state degeneracy in the topological non-trivial phase, while it approaches 1 in the trivial phase. This distinction is robust against s… ▽ More We demonstrate that non-Hermitian perturbations can probe topological phase transitions and unambiguously detect non-Abelian zero modes. We show that under carefully designed non-Hermitian perturbations, the Loschmidt echo(LE) decays into 1/N where N is the ground state degeneracy in the topological non-trivial phase, while it approaches 1 in the trivial phase. This distinction is robust against small parameter deviations in the non-Hermitian perturbations. We further study four well-known models that support Majorana or parafermionic zero modes. By calculating their dynamical responses to specific non-Hermitian perturbations, we prove that the steady-state LE can indeed differentiate between different phases. This method avoids the ambiguity introduced by trivial zero-energy states and thus provides an alternative and promising way to demonstrate the emergence of topologically non-trivial phases. The experimental realizations of non-Hermitian perturbations are discussed. △ Less

Submitted 31 December, 2023; originally announced January 2024.

Comments: 11 pages, 4 figures

A cyclical route linking fundamental mechanism and AI algorithm: An example from tuning Poisson's ratio in amorphous networks

Authors: Changliang Zhu, Chenchao Fang, Zhipeng Jin, Baowen Li, Xiangying Shen, Lei Xu

Abstract: "AI for science" is widely recognized as a future trend in the development of scientific research. Currently, although machine learning algorithms have played a crucial role in scientific research with numerous successful cases, relatively few instances exist where AI assists researchers in uncovering the underlying physical mechanisms behind a certain phenomenon and subsequently using that mechan… ▽ More "AI for science" is widely recognized as a future trend in the development of scientific research. Currently, although machine learning algorithms have played a crucial role in scientific research with numerous successful cases, relatively few instances exist where AI assists researchers in uncovering the underlying physical mechanisms behind a certain phenomenon and subsequently using that mechanism to improve machine learning algorithms' efficiency. This article uses the investigation into the relationship between extreme Poisson's ratio values and the structure of amorphous networks as a case study to illustrate how machine learning methods can assist in revealing underlying physical mechanisms. Upon recognizing that the Poisson's ratio relies on the low-frequency vibrational modes of dynamical matrix, we can then employ a convolutional neural network, trained on the dynamical matrix instead of traditional image recognition, to predict the Poisson's ratio of amorphous networks with a much higher efficiency. Through this example, we aim to showcase the role that artificial intelligence can play in revealing fundamental physical mechanisms, which subsequently improves the machine learning algorithms significantly. △ Less

Submitted 9 July, 2024; v1 submitted 6 December, 2023; originally announced December 2023.

Comments: 15 pages, 5 figures

Superdiffusive Transport in Quasi-Particle Dephasing Models

Authors: Yu-Peng Wang, Chen Fang, Jie Ren

Abstract: Investigating the behavior of noninteracting fermions subjected to local dephasing, we reveal that quasi-particle dephasing can induce superdiffusive transport. This superdiffusion arises from nodal points within the momentum distribution of local dephasing quasi-particles, leading to asymptotic long-lived modes. By studying the dynamics of the Wigner function, we rigorously elucidate how the dyna… ▽ More Investigating the behavior of noninteracting fermions subjected to local dephasing, we reveal that quasi-particle dephasing can induce superdiffusive transport. This superdiffusion arises from nodal points within the momentum distribution of local dephasing quasi-particles, leading to asymptotic long-lived modes. By studying the dynamics of the Wigner function, we rigorously elucidate how the dynamics of these enduring modes give rise to Lévy walk processes, a renowned mechanism underlying superdiffusion phenomena. Our research demonstrates the controllability of dynamical scaling exponents by selecting quasi-particles and extends its applicability to higher dimensions, underlining the pervasive nature of superdiffusion in dephasing models. △ Less

Submitted 14 December, 2023; v1 submitted 4 October, 2023; originally announced October 2023.

Comments: 12 pages, 4 figures

Enumeration of spin-space groups: Towards a complete description of symmetries of magnetic orders

Authors: Yi Jiang, Ziyin Song, Tiannian Zhu, Zhong Fang, Hongming Weng, Zheng-Xin Liu, Jian Yang, Chen Fang

Abstract: Symmetries of three-dimensional periodic scalar fields are described by 230 space groups (SGs). Symmetries of three-dimensional periodic (pseudo-) vector fields, however, are described by the spin-space groups (SSGs), which were initially used to describe the symmetries of magnetic orders. In SSGs, the real-space and spin degrees of freedom are unlocked in the sense that an operation could have di… ▽ More Symmetries of three-dimensional periodic scalar fields are described by 230 space groups (SGs). Symmetries of three-dimensional periodic (pseudo-) vector fields, however, are described by the spin-space groups (SSGs), which were initially used to describe the symmetries of magnetic orders. In SSGs, the real-space and spin degrees of freedom are unlocked in the sense that an operation could have different spacial and spin rotations. SSGs gives a complete symmetry description of magnetic structures, and have natural applications in the band theory of itinerary electrons in magnetically ordered systems with weak spin-orbit coupling. Altermagnetism, a concept raised recently that belongs to the symmetry-compensated collinear magnetic orders but has non-relativistic spin splitting, is well described by SSGs. Due to the vast number and complicated group structures, SSGs have not yet been systematically enumerated. In this work, we exhaust SSGs based on the invariant subgroups of SGs, with spin operations constructed from three-dimensional (3D) real representations of the quotient groups for the invariant subgroups. For collinear and coplanar magnetic orders, the spin operations can be reduced into lower dimensional real representations. As the number of SSGs is infinite, we only consider SSGs that describe magnetic unit cells up to 12 times crystal unit cells. We obtain 157,289 non-coplanar, 24,788 coplanar-non-collinear, and 1,421 collinear SSGs. The enumerated SSGs are stored in an online database at \url{https://cmpdc.iphy.ac.cn/ssg} with a user-friendly interface. We also develop an algorithm to identify SSG for realistic materials and find SSGs for 1,626 magnetic materials. Our results serve as a solid starting point for further studies of symmetry and topology in magnetically ordered materials. △ Less

Submitted 30 August, 2024; v1 submitted 19 July, 2023; originally announced July 2023.

Comments: The manuscript is revised according to referees' reports

Journal ref: Phys. Rev. X 14, 031039 (2024)

Point-Gap Bound States in Non-Hermitian Systems

Authors: Zixi Fang, Chen Fang, Kai Zhang

Abstract: In this paper, we systematically investigate the impurity-induced bound states in 1D non-Hermitian systems. By establishing an exact relationship between impurity potential and bound-state energy, we determine the minimum impurity potential required to generate bound states within each point energy gap. We demonstrate that the absence of Bloch saddle points necessitates a finite threshold of impur… ▽ More In this paper, we systematically investigate the impurity-induced bound states in 1D non-Hermitian systems. By establishing an exact relationship between impurity potential and bound-state energy, we determine the minimum impurity potential required to generate bound states within each point energy gap. We demonstrate that the absence of Bloch saddle points necessitates a finite threshold of impurity potential; otherwise, infinitesimal impurity potential can create bound states. Furthermore, we show that the bound states residing in the point gaps with nonzero spectral winding exhibit sensitivity to boundary conditions and will be squeezed towards the edges when the boundaries are opened, indicating the bulk-boundary correspondence in terms of point-gap topology. △ Less

Submitted 23 June, 2023; v1 submitted 18 May, 2023; originally announced May 2023.

Comments: 16 pages, 10 figures

Observation of Fluctuation Spin Hall Effect in Antiferromagnet

Authors: Chi Fang, Caihua Wan, Xiaoyue Zhang, Satoshi Okamoto, Tianyi Ma, Jianying Qin, Xiao Wang, Chenyang Guo, Jing Dong, Guoqiang Yu, Zhenchao Wen, Ning Tang, Stuart S. P. Parkin, Naoto Nagaosa, Yuan Lu, Xiufeng Han

Abstract: The spin Hall effect (SHE) can generate a pure spin current by an electric current, which is promisingly used to electrically control magnetization. To reduce power consumption of this control, a giant spin Hall angle (SHA) in the SHE is desired in low-resistivity systems for practical applications. Here, critical spin fluctuation near the antiferromagnetic (AFM) phase-transition is proved as an e… ▽ More The spin Hall effect (SHE) can generate a pure spin current by an electric current, which is promisingly used to electrically control magnetization. To reduce power consumption of this control, a giant spin Hall angle (SHA) in the SHE is desired in low-resistivity systems for practical applications. Here, critical spin fluctuation near the antiferromagnetic (AFM) phase-transition is proved as an effective mechanism to create an additional part of SHE, named as fluctuation spin Hall effect (FSHE). This FSHE enhances the SHA due to the AFM spin fluctuation between conduction electrons and local spins. We detect the FSHE with the inverse and direct spin Hall effect (ISHE and DSHE) set-up and their temperature (T) dependences in the Cr/MgO/Fe magnetic tunnel junctions (MTJs). The SHA is significantly enhanced when temperature is approached to the Néel temperature (T_N) and has a peak value of -0.34 at 200 K near T_N. This value is higher than the room-temperature value by 240% and comparable to that of heavy metals Ta and W. Furthermore, the spin Hall resistivity of Cr well fits the modeled T-dependence when T approaches T_N from low temperatures, implying the AFM spin fluctuation nature of strong SHA enhancement. Thus, this study demonstrates the critical spin fluctuation as a prospective way of increasing SHA and enriches the AFM material candidates for spin-orbitronic devices. △ Less

Submitted 26 April, 2023; originally announced April 2023.

Comments: 27 pages, 9 figures

Dynamical Degeneracy Splitting and Directional Invisibility in Non-Hermitian Systems

Authors: Kai Zhang, Chen Fang, Zhesen Yang

Abstract: In this paper, we introduce the concept of dynamical degeneracy splitting to describe the anisotropic decay behaviors in non-Hermitian systems. We demonstrate that systems with dynamical degeneracy splitting exhibit two distinctive features: (i) the system shows frequency-resolved non-Hermitian skin effect; (ii) Green's function exhibits anomalous at given frequency, leading to uneven broadening i… ▽ More In this paper, we introduce the concept of dynamical degeneracy splitting to describe the anisotropic decay behaviors in non-Hermitian systems. We demonstrate that systems with dynamical degeneracy splitting exhibit two distinctive features: (i) the system shows frequency-resolved non-Hermitian skin effect; (ii) Green's function exhibits anomalous at given frequency, leading to uneven broadening in spectral function and anomalous scattering. As an application, we propose directional invisibility based on wave packet dynamics to investigate the geometry-dependent skin effect in higher dimensions. Our work elucidates a faithful correspondence between non-Hermitian skin effect and Green's function, offering a guiding principle for exploration of novel physical phenomena emerging from this effect. △ Less

Submitted 23 June, 2023; v1 submitted 14 November, 2022; originally announced November 2022.

Comments: 24 pages, 11 figures

Journal ref: Phys. Rev. Lett. 131, 036402 (2023)

Diversified Ruderman-Kittel-Kasuya-Yosida Interactions in a Nonsymmorphic Crystal

Authors: Zhongyi Zhang, Shengshan Qin, Chen Fang, Jiangping Hu, Fu-chun Zhang

Abstract: We show that there are diversified Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between magnetic impurities, mediated by itinerant electrons, in a centrosymmetric crystal respecting a nonsymmorphic space group. We take the $P4/nmm$ space group as an example. We demonstrate that the different type of interactions, including the Heisenberg-type, the Dzyaloshinskii-Moriya (DM)-type, the Ising-ty… ▽ More We show that there are diversified Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between magnetic impurities, mediated by itinerant electrons, in a centrosymmetric crystal respecting a nonsymmorphic space group. We take the $P4/nmm$ space group as an example. We demonstrate that the different type of interactions, including the Heisenberg-type, the Dzyaloshinskii-Moriya (DM)-type, the Ising-type and the anisotropic interactions, can appear in accordance with the positions of the impurities in the real space. Their strengths strongly depend on the location of the itinerant electrons in the reciprocal space. The diversity stems from the position-dependent site groups and the momentum-dependent electronic structures guaranteed by the nonsymmorphic symmetries. Our study unveils the role of the nonsymmorphic symmetries in affecting magnetism, and suggests that the nonsymmorphic crystals can be promising platforms to design magnetic interactions. △ Less

Submitted 16 October, 2022; originally announced October 2022.

Comments: 11pages, 5 figures

Journal ref: Science Bulletin 68, 1113 (2023)

Absence of measurement-induced entanglement transition due to feedback-induced skin effect

Authors: Yu-Peng Wang, Chen Fang, Jie Ren

Abstract: A quantum many-body system subject to unitary evolution and repeated local measurements with an increasing rate undergoes a measurement-induced entanglement transition from extensive (or subextensive) to area law entropy scaling. We find that certain open boundary systems under "generalized monitoring", consisting of "projective monitoring" and conditional feedback, display an anomalous late-time… ▽ More A quantum many-body system subject to unitary evolution and repeated local measurements with an increasing rate undergoes a measurement-induced entanglement transition from extensive (or subextensive) to area law entropy scaling. We find that certain open boundary systems under "generalized monitoring", consisting of "projective monitoring" and conditional feedback, display an anomalous late-time particle concentration on the edge, reminiscent of the "skin effect" in non-Hermitian systems. Such feedback-induced skin effect will suppress the entanglement generation, rendering the system short-range entangled without measurement-induced entanglement transition. While initially emerged in noninteracting models, such skin effect can also occur in chaotic interacting systems and Floquet quantum circuits subjected to random generalized measurements. Since the dynamics of the skin effect do not require post selection, and can be observed at the particle number level, the phenomenon is experimentally relevant and accessible in noisy intermediate-scale quantum platforms, such as trapped ions. △ Less

Submitted 3 July, 2024; v1 submitted 22 September, 2022; originally announced September 2022.

Comments: 18 pages, 8 figures; Accepted version

Journal ref: Phys. Rev. B 110, 035113 (2024)

Topological Superconductivity from Unconventional Band Degeneracy with Conventional Pairing

Authors: Zhongyi Zhang, Zhenfei Wu, Chen Fang, Fu-chun Zhang, Jiangping Hu, Yuxuan Wang, Shengshan Qin

Abstract: We present a new scheme for Majorana modes in systems with nonsymmporhic-symmetry-protected band degeneracy. We reveal that when the gapless fermionic excitations are encoded with conventional superconductivity and magnetism, which can be intrinsic or induced by proximity effect, topological superconductivity and Majorana modes can be obtained. We illustrate this outcome in a system which respects… ▽ More We present a new scheme for Majorana modes in systems with nonsymmporhic-symmetry-protected band degeneracy. We reveal that when the gapless fermionic excitations are encoded with conventional superconductivity and magnetism, which can be intrinsic or induced by proximity effect, topological superconductivity and Majorana modes can be obtained. We illustrate this outcome in a system which respects the space group P4/nmm and features a fourfold-degenerate fermionic mode at (π, π) in the Brillouin zone. We show that in the presence of conventional superconductivity, different types of topological superconductivity, i.e. first-order and second-order topological superconductivity, with coexisting fragile Wannier obstruction in the latter case, can be generated in accordance with the different types of magnetic orders; Majorana modes are shown to exist on the boundary, at the corner and in the vortices. To further demonstrate the effectiveness of our approach, another example related to the space group P4/ncc based on this scheme is also provided. Our study offers insights into constructing topological superconductors based on bulk energy bands and conventional superconductivity, and helps to find new material candidates and design new platforms for realizing Majorana modes. △ Less

Submitted 8 October, 2024; v1 submitted 22 August, 2022; originally announced August 2022.

Journal ref: Nat Commun 15, 7971 (2024)

Spin-triplet Superconductivity in Nonsymmorphic crystals

Authors: Shengshan Qin, Chen Fang, Fu-chun Zhang, Jiangping Hu

Abstract: Spin-triplet superconductivity is known to be a rare quantum phenomenon. Here we show that nonsymmorphic crystalline symmetries can dramatically assist spin-triplet superconductivity in the presence of spin-orbit coupling. Even with a weak spin-orbit coupling, the spin-triplet pairing can be the leading pairing instability in a lattice with a nonsymmorphic symmetry. The underlining mechanism is th… ▽ More Spin-triplet superconductivity is known to be a rare quantum phenomenon. Here we show that nonsymmorphic crystalline symmetries can dramatically assist spin-triplet superconductivity in the presence of spin-orbit coupling. Even with a weak spin-orbit coupling, the spin-triplet pairing can be the leading pairing instability in a lattice with a nonsymmorphic symmetry. The underlining mechanism is the spin-sublattice-momentum lock on electronic bands that are protected by the nonsymmorphic symmetry. We use the nonsymmorphic space group P4/nmm to demonstrate these results and discuss related experimental observables. Our work paves a new way in searching for spin-triplet superconductivity. △ Less

Submitted 19 August, 2022; originally announced August 2022.

The mechanism of Li deposition on the Cu substrates in the anode-free Li metal batteries

Authors: Genming Lai, Junyu Jiao, Chi Fang, Liyuan Sheng, Yao Jiang, Chuying Ouyang, Jiaxin Zheng

Abstract: Due to the rapid growth in the demand for high-energy-density Li batteries and insufficient global Li reserves, the anode-free Li metal batteries are receiving increasing attention. Various strategies, such as surface modification and structural design of Cu current collectors, have been proposed to stabilize the anode-free Li metal batteries. Unfortunately, the mechanism of Li deposition on the C… ▽ More Due to the rapid growth in the demand for high-energy-density Li batteries and insufficient global Li reserves, the anode-free Li metal batteries are receiving increasing attention. Various strategies, such as surface modification and structural design of Cu current collectors, have been proposed to stabilize the anode-free Li metal batteries. Unfortunately, the mechanism of Li deposition on the Cu surfaces with the different Miller indices is poorly understood, especially on the atomic scale. Here, a large-scale molecular dynamics simulation of Li deposition on the Cu substrates was performed in the anode-free Li metal batteries. The results show that the Li layers on the Cu (100), Cu (110), and Cu (111) surfaces are closer to the structures of Li (110), Li (100), and Li (110) surfaces, respectively. The mechanism was studied through the surface similarity analysis, potential energy surfaces, and lattice features. Finally, a proposal to reduce the fraction of the (110) facet in commercial Cu foils was made to improve the reversibility and stability of Li plating/stripping in the anode-free Li metal batteries. △ Less

Submitted 8 August, 2022; originally announced August 2022.

Chern numbers of topological phonon band crossing determined with inelastic neutron scattering

Authors: Zhendong Jin, Biaoyan Hu, Yiran Liu, Yangmu Li, Tiantian Zhang, Kazuki Iida, Kazuya Kamazawa, A. I. Kolesnikov, M. B. Stone, Xiangyu Zhang, Haiyang Chen, Yandong Wang, I. A. Zaliznyak, J. M. Tranquada, Chen Fang, Yuan Li

Abstract: Topological invariants in the band structure, such as Chern numbers, are crucial for the classification of topological matters and dictate the occurrence of exotic properties, yet their direct spectroscopic determination has been largely limited to electronic bands. Here, we use inelastic neutron scattering in conjunction with ab initio calculations to identify a variety of topological phonon band… ▽ More Topological invariants in the band structure, such as Chern numbers, are crucial for the classification of topological matters and dictate the occurrence of exotic properties, yet their direct spectroscopic determination has been largely limited to electronic bands. Here, we use inelastic neutron scattering in conjunction with ab initio calculations to identify a variety of topological phonon band crossings in MnSi and CoSi single crystals. We find a distinct relation between the Chern numbers of a band-crossing node and the scattering intensity modulation in momentum space around the node. Given sufficiently high resolution, our method can be used to determine arbitrarily large Chern numbers of topological phonon band-crossing nodes. △ Less

Submitted 25 July, 2022; originally announced July 2022.

Comments: 14 pages, 9 figures including supplemental material

Journal ref: Phys. Rev. B 106, 224304 (2022)

Magnetic molecular orbitals in MnSi

Authors: Zhendong Jin, Yangmu Li, Zhigang Hu, Biaoyan Hu, Yiran Liu, Kazuki Iida, Kazuya Kamazawa, M. B. Stone, A. I. Kolesnikov, D. L. Abernathy, Xiangyu Zhang, Haiyang Chen, Yandong Wang, Chen Fang, Biao Wu, I. A. Zaliznyak, J. M. Tranquada, Yuan Li

Abstract: A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units ar… ▽ More A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units are interconnected, extended molecular orbitals consisting of three Mn atoms each, rather than individual Mn atoms. This result is further corroborated by magnetic Wannier orbitals obtained by ab initio calculations. It contrasts the ionic picture with a concrete example, and presents a novel regime of the spin waves where the wavelength is comparable to the spatial extent of the molecular orbitals. Our discovery brings important insights into not only the magnetism of MnSi, but also a broad range of magnetic quantum materials where structural symmetry, electron itinerancy and correlations act in concert. △ Less

Submitted 27 June, 2022; originally announced June 2022.

Comments: 27 pages, 27 figures including supplemental; comments are welcome

Journal ref: Science Advances 9, eadd5239 (2023)

Wire Construction of Topological Crystalline Superconductors

Authors: Bingrui Peng, Hongming Weng, Chen Fang

Abstract: We present a unified framework to construct and classify topological crystalline superconductors (TCSCs). The building blocks are one-dimensional topological superconductors (TSCs) protected solely by onsite symmetries, which are arranged and glued by crystalline symmetries in real space. We call this real-space scheme "wire construction", and we show its procedure can be formulated mathematically… ▽ More We present a unified framework to construct and classify topological crystalline superconductors (TCSCs). The building blocks are one-dimensional topological superconductors (TSCs) protected solely by onsite symmetries, which are arranged and glued by crystalline symmetries in real space. We call this real-space scheme "wire construction", and we show its procedure can be formulated mathematically. For illustration, we treat TCSCs of Altland-Zirnbauer (AZ) class[1] DIII protected by wallpaper group as well as layer group symmetries, with the resulting states by wire construction being TCSCs in two-dimension. We also discuss how these real-space TCSCs by wire construction are characterized by anomalous boundary states. Our method provides a real-space picture for TCSCs versus momentum-space pictures. △ Less

Submitted 26 May, 2022; originally announced May 2022.

Methods pressure control apparatus for lithium metal battery

Authors: Bingyu Lu, Wurigumula Bao, Weiliang Yao, Jean-Marie Doux, Chengcheng Fang, Ying Shirley Meng

Abstract: Lithium (Li) metal anodes are essential for developing next-generation high-energy-density batteries. However, Li dendrite/whisker formation caused short-circuiting issue and short cycle life have prevented lithium metal from being viably used in rechargeable batteries. Numerous works have been done to study how to regulate the Li growth in electrochemical cycling by using external stacking forces… ▽ More Lithium (Li) metal anodes are essential for developing next-generation high-energy-density batteries. However, Li dendrite/whisker formation caused short-circuiting issue and short cycle life have prevented lithium metal from being viably used in rechargeable batteries. Numerous works have been done to study how to regulate the Li growth in electrochemical cycling by using external stacking forces. While it is widely agreed that stack pressure positively affects the lithium plating/stripping process, the optimized pressure range provided by different works varies greatly because of the difference in the pressure control setup. In this work, a pressure control apparatus is designed for Li metal batteries with liquid and solid-state electrolytes (SSE). With considerations of minimizing cell to cell variation, a reusable split cell and pressure load cell are made for testing electrochemical cells with high precision pressure control. The capability of the designed setup is demonstrated by studying the pressure effect on the Li plating/stripping process. △ Less

Submitted 19 April, 2022; originally announced April 2022.

Suppressing chemical corrosions of lithium metal anodes

Authors: Bingyu Lu, Weikang Li, Diyi Cheng, Miguel Ceja, Wurigumula Bao, Chengcheng Fang, Ying Shirley Meng

Abstract: The lithium (Li) metal anode is essential for next generation high energy density rechargeable Li metal batteries. Although extensive studies have been performed to prolong the cycle life of Li metal batteries, the calendar life, which associates with chemical corrosion of Li metal in liquid electrolytes, has not been quantitatively understood. Here, by combing the Titration Gas Chromatography (TG… ▽ More The lithium (Li) metal anode is essential for next generation high energy density rechargeable Li metal batteries. Although extensive studies have been performed to prolong the cycle life of Li metal batteries, the calendar life, which associates with chemical corrosion of Li metal in liquid electrolytes, has not been quantitatively understood. Here, by combing the Titration Gas Chromatography (TGC) method and Cryogenic Focused Ion Beam (Cryo-FIB), we established a quantitative relationship between the chemical corrosion rate and electrochemically deposited Li morphology in various liquid electrolyte systems. We identified that the corrosion rate is dominated by the porosity of the deposited Li. The larger the porosity of deposited Li has, the faster the corrosion rate will be. We further proposed strategies to mitigate the chemical corrosion on Li thus to extend the calendar life of Li metal batteries. By strictly controlling the stacking pressure during Li plating, Li deposits with ultra-low porosity can be achieved, suppressing the corrosion rate to 0.08% per day compared with 1.71% per day of the high-porosity Li. △ Less

Submitted 19 April, 2022; originally announced April 2022.

Observation of Mesoscopic Clathrate Structures in Ethanol-Water Mixtures

Authors: Wei-Hao Hsu, Tzu-Chieh Yen, Chien-Chun Chen, Chih-Wen Yang, Chung-Kai Fang, Ing-Shouh Hwang

Abstract: Water-alcohol mixtures exhibit many abnormal physicochemical properties, the origins of which remain controversial. Here we use transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA), and atomic force microscopy (AFM) to study ethanol-water mixtures. TEM reveals mesoscopic clathrate structures with water molecules forming a crystalline matrix hosting a high density of tiny ce… ▽ More Water-alcohol mixtures exhibit many abnormal physicochemical properties, the origins of which remain controversial. Here we use transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA), and atomic force microscopy (AFM) to study ethanol-water mixtures. TEM reveals mesoscopic clathrate structures with water molecules forming a crystalline matrix hosting a high density of tiny cells. The presence of these mesoscopic clathrate structures is further supported by a refractive index of 1.27+-0.02 at 405 nm measured via NTA and the hydrophilic nature of the mesoscopic structures implied by AFM observations, explaining many long-standing puzzles related to water-alcohol mixtures. △ Less

Submitted 7 April, 2022; originally announced April 2022.

Topological states and topological phase transition in Cu2SnS3 and Cu2SnSe3

Authors: Liqin Zhou, Yuting Qian, Changming Yue, Zhong Fang, Wei Zhang, Chen Fang, Hongming Weng

Abstract: Based on the first-principles calculations within local density approximation and model analysis, we propose that the iso-structural compounds Cu2SnS3 and Cu2SnSe3 are both the simplest nodal line semimetals with only one nodal line in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The including of SOC drives Cu2SnS3 into a Weyl semimetal (WSM) state with only two pairs of… ▽ More Based on the first-principles calculations within local density approximation and model analysis, we propose that the iso-structural compounds Cu2SnS3 and Cu2SnSe3 are both the simplest nodal line semimetals with only one nodal line in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The including of SOC drives Cu2SnS3 into a Weyl semimetal (WSM) state with only two pairs of Weyl nodes, the minimum number required for WSM with time reversal symmetry. In contrast, SOC leads Cu2SnSe3 to strong topological insulator (TI) state. This difference can be well understood as there is a topological phase transition (TPT). In it, the Weyl nodes are driven by tunable SOC and annihilate in a mirror plane, resulting in a TI. This TPT, together with the evolution of Weyl nodes, the changing of mirror Chern numbers of mirror plane and the Z2 indices protected by time-reversal symmetry has been demonstrated by the calculation of Cu2SnS3-xSex within virtual crystal approximation and an effective k $\cdot$ p model analysis. Though our first-principles calculations have overestimated the topological states in both compounds, we believe that the theoretical demonstration of controlling TPT and evolution of Weyl nodes will stimulate further efforts in exploring them. △ Less

Submitted 9 January, 2022; originally announced January 2022.

Exactly solvable lattice models for interacting electronic insulators in two dimensions

Authors: Qing-Rui Wang, Yang Qi, Chen Fang, Meng Cheng, Zheng-Cheng Gu

Abstract: In the past decade, tremendous efforts have been made towards understanding fermionic symmetry protected topological (FSPT) phases in interacting systems. Nevertheless, for systems with continuum symmetry, e.g., electronic insulators, it is still unclear how to construct an exactly solvable model with a finite dimensional Hilbert space in general. In this paper, we give a lattice model constructio… ▽ More In the past decade, tremendous efforts have been made towards understanding fermionic symmetry protected topological (FSPT) phases in interacting systems. Nevertheless, for systems with continuum symmetry, e.g., electronic insulators, it is still unclear how to construct an exactly solvable model with a finite dimensional Hilbert space in general. In this paper, we give a lattice model construction and classification for 2D interacting electronic insulators. Based on the physical picture of $\mathrm{U(1)}_f$-charge decorations, we illustrate the key idea by considering the well known 2D interacting topological insulator. Then we generalize our construction to an arbitrary 2D interacting electronic insulator with symmetry $G_f=\mathrm{U(1)}_f \rtimes_{ρ_1,ω_2} G$, where $\mathrm{U(1)}_f$ is the charge conservation symmetry and $ρ_1, ω_2$ are additional data which fully characterize the group structure of $G_f$. Finally we study more examples, including the full interacting classification of 2D crystalline topological insulators. △ Less

Submitted 31 December, 2021; originally announced December 2021.

Comments: 6+11 pages, 2 figures, 3 tables

Journal ref: Phys. Rev. B 108, L121104 (2023)

The evolution of Weyl nodes in Ni doped thallium niobate pyrochlore Tl$_{2-x}$Ni$_x$Nb$_2$O$_7$

Authors: Yuefang Hu, Changming Yue, Danwen Yuan, Jiacheng Gao, Zhigao Huang, Zhong Fang, Chen Fang, Hongming Weng, Wei Zhang

Abstract: Magnetic Weyl semimetal (WSM) is of great importance both for fundamental physics and potential applications due to its spontaneous magnetism, robust band topology, and enhanced Berry curvature. It possesses many unique quantum effects, including large intrinsic anomalous Hall effect, Fermi arcs, and chiral anomaly. In this work, using ab initio calculations, we propose that Ni doped pyrochlore Tl… ▽ More Magnetic Weyl semimetal (WSM) is of great importance both for fundamental physics and potential applications due to its spontaneous magnetism, robust band topology, and enhanced Berry curvature. It possesses many unique quantum effects, including large intrinsic anomalous Hall effect, Fermi arcs, and chiral anomaly. In this work, using ab initio calculations, we propose that Ni doped pyrochlore Tl$_2$Nb$_2$O$_7$ is a magnetic WSM caused by exchange field splitting on bands around its quadratic band crossing point. The exchange field tuned by Ni 3d on-site Coulomb interaction parameter $U$ drives the evolution of Weyl nodes and the resulting topological phase transition. Since Weyl nodes can exist at generic point in Brillouin zone and are hard to be exactly identified, the creation and annihilation of them, i.e., the change in their number, chirality and distribution, have been consistently confirmed with a combined theoretical approach, which employs parity criterion, symmetry indicator analysis and the Wilson loop of Wannier center. We find that Weyl nodes remain in a quite large range of $U$ and are close to Fermi level, which makes the experimental observation very possible. We think this method and our proposal of magnetic WSM will be useful in finding more WSMs and adding our understanding on the topological phase transition. △ Less

Submitted 8 December, 2021; originally announced December 2021.

Journal ref: Sci. China-Phys. Mech. Astron. 65, 297211, 2022

Topological classification of intrinsic 3D superconductors using anomalous surface construction

Authors: Zhongyi Zhang, Jie Ren, Yang Qi, Chen Fang

Abstract: Intrinsic topological superconductors have protected gapless Majorana modes, bound and/or propagating, at the natural boundaries of the sample, without requiring field, defect, or heterostructure. We establish the complete classification/construction of intrinsic topological superconductors jointly protected by point-group and time-reversal symmetries in three dimensions. This is obtained from enu… ▽ More Intrinsic topological superconductors have protected gapless Majorana modes, bound and/or propagating, at the natural boundaries of the sample, without requiring field, defect, or heterostructure. We establish the complete classification/construction of intrinsic topological superconductors jointly protected by point-group and time-reversal symmetries in three dimensions. This is obtained from enumerating distinct ways for stacking $n$th-order irreducible building blocks, minimal anomalous surface states of $n$th-order topological superconductors. Particularly, our method provides a unified description of possible surface anomalies away from high-symmetry points/lines in terms of the homotopy group of the surface mass field. △ Less

Submitted 21 September, 2022; v1 submitted 29 September, 2021; originally announced September 2021.

Comments: 43 pages, 9 figures, 6 tables

Journal ref: Phys. Rev. B 106.L121108 (2022)