Showing 1–50 of 113 results for author: Liu, N

Imaging short- and long-range magnetic order in a quantum anomalous Hall insulator

Authors: Andriani Vervelaki, Boris Gross, Daniel Jetter, Katharina Kress, Timur Weber, Dieter Koelle, Kajetan M. Fijalkowski, Martin Klement, Nan Liu, Karl Brunner, Charles Gould, Laurens W. Molenkamp, Martino Poggio, Floris Braakman

Abstract: The quantum anomalous Hall effect has been observed in several magnetically doped topological insulators, where its robustness and macroscopic magnetization properties have been taken to suggest the presence of long-range ferromagnetic order. However, experiments in such systems have found evidence for both long- and short-range order, leaving the precise nature of the magnetism in these systems u… ▽ More The quantum anomalous Hall effect has been observed in several magnetically doped topological insulators, where its robustness and macroscopic magnetization properties have been taken to suggest the presence of long-range ferromagnetic order. However, experiments in such systems have found evidence for both long- and short-range order, leaving the precise nature of the magnetism in these systems unclear. Here, we use scanning superconducting quantum interference device microscopy to study magnetic domains in V-doped (Bi,Sb)$_2$Te$_3$ exhibiting a quantum anomalous Hall effect with precise quantization. By imaging stray magnetic fields as a function of applied field, we map the formation and evolution of domains through magnetic reversal. We reconstruct the magnetization configuration underlying the measured stray field and find that magnetic domains and crystallographic grains are of similar size. Moreover, magnetic reversal is found to occur through domain expansion, typical of ferromagnets, rather than through nucleation at random sites. Our measurements thus reveal a coexistence of both local magnetic interactions within crystallographic grains and long-range ferromagnetic coupling between grains. This behavior in V-doped (Bi,Sb)$_2$Te$_3$ is markedly distinct from that previously reported for Cr-doped (Bi,Sb)$_2$Te$_3$. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Fermi-Dirac thermal measurements: A framework for quantum hypothesis testing and semidefinite optimization

Authors: Nana Liu, Mark M. Wilde

Abstract: Quantum measurements are the means by which we recover messages encoded into quantum states. They are at the forefront of quantum hypothesis testing, wherein the goal is to perform an optimal measurement for arriving at a correct conclusion. Mathematically, a measurement operator is Hermitian with eigenvalues in [0,1]. By noticing that this constraint on each eigenvalue is the same as that imposed… ▽ More Quantum measurements are the means by which we recover messages encoded into quantum states. They are at the forefront of quantum hypothesis testing, wherein the goal is to perform an optimal measurement for arriving at a correct conclusion. Mathematically, a measurement operator is Hermitian with eigenvalues in [0,1]. By noticing that this constraint on each eigenvalue is the same as that imposed on fermions by the Pauli exclusion principle, we interpret every eigenmode of a measurement operator as an independent effective fermionic mode. Under this perspective, various objective functions in quantum hypothesis testing can be viewed as the total expected energy associated with these fermionic occupation numbers. By instead fixing a temperature and minimizing the total expected fermionic free energy, we find that optimal measurements for these modified objective functions are Fermi-Dirac thermal measurements, wherein their eigenvalues are specified by Fermi-Dirac distributions. In the low-temperature limit, their performance closely approximates that of optimal measurements for quantum hypothesis testing, and we show that their parameters can be learned by classical or hybrid quantum-classical optimization algorithms. This leads to a new quantum machine-learning model, termed Fermi-Dirac machines, consisting of parameterized Fermi-Dirac thermal measurements-an alternative to quantum Boltzmann machines based on thermal states. Beyond hypothesis testing, we show how general semidefinite optimization problems can be solved using this approach, leading to a novel paradigm for semidefinite optimization on quantum computers, in which the goal is to implement thermal measurements rather than prepare thermal states. Finally, we propose quantum algorithms for implementing Fermi-Dirac thermal measurements, and we also propose second-order hybrid quantum-classical optimization algorithms. △ Less

Submitted 4 March, 2026; originally announced March 2026.

Comments: 35 pages, 3 figures

Spin-orbit-driven quarter semimetals in rhombohedral graphene

Authors: Jing Ding, Hanxiao Xiang, Naitian Liu, Wenqiang Zhou, Xinjie Fang, Zhangyuan Chen, Le Zhang, Kenji Watanabe, Takashi Taniguchi, Shuigang Xu

Abstract: Semimetals exhibit intriguing characteristics attributed to the coexistence of both electrons and holes. In rhombohedral multilayer graphene, a strong trigonal warping effect gives rise to a semi-metallic state near the Fermi surface, offering unique opportunities to explore the interplay of semi-metallic properties with strong correlations and topologies. Here, the observation of quarter semimeta… ▽ More Semimetals exhibit intriguing characteristics attributed to the coexistence of both electrons and holes. In rhombohedral multilayer graphene, a strong trigonal warping effect gives rise to a semi-metallic state near the Fermi surface, offering unique opportunities to explore the interplay of semi-metallic properties with strong correlations and topologies. Here, the observation of quarter semimetals in rhombohedral multilayer graphene by introducing spin-orbit coupling (SOC) is reported. The semi-metallic characteristics of rhombohedral graphene manifest as nearly vanished Hall resistance and parabolic longitudinal resistance. The strong correlations arising from the surface flat band lead to spontaneous symmetry breaking. SOC proximitized by WSe2 further lifts the valley degeneracy, resulting in the spontaneous time-reversal symmetry breaking, as evidenced by the hysteretic anomalous Hall effect. The coexistence of fully polarized electrons and holes allows for the observation of a non-monotonic temperature dependence of the anomalous Hall resistance. Furthermore, the application of moderate magnetic fields induces a phase transition from quarter semimetals to Chern insulators. These findings establish rhombohedral multilayer graphene as an ideal platform for studying strong correlations and topologies in semimetals. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Comments: Published in Advanced Materials

Journal ref: Adv. Mater., 38, e12713, (2025)

Layer-engineered quantum anomalous Hall effect in twisted rhombohedral graphene family

Authors: Zhangyuan Chen, Naitian Liu, Jiannan Hua, Hanxiao Xiang, Wenqiang Zhou, Jing Ding, Xinjie Fang, Linfeng Wu, Le Zhang, Qianmei Chen, Xuanyu Chen, Kenji Watanabe, Takashi Taniguchi, Na Xin, Wei Zhu, Shuigang Xu

Abstract: The quantum anomalous Hall (QAH) insulator is uniquely characterized by the topological Chern number C. Controlling the Chern number is a key step toward functional topological electronics and enables access to exotic quantum phases beyond the traditional quantum Hall physics. Here, we report a series of QAH insulators in twisted rhombohedral graphene family, in which the Chern number can be tuned… ▽ More The quantum anomalous Hall (QAH) insulator is uniquely characterized by the topological Chern number C. Controlling the Chern number is a key step toward functional topological electronics and enables access to exotic quantum phases beyond the traditional quantum Hall physics. Here, we report a series of QAH insulators in twisted rhombohedral graphene family, in which the Chern number can be tuned through layer configuration, in-situ electrostatic doping, and displacement field. Specifically, in twisted monolayer-rhombohedral N-layer graphene, denoted as (1+N) L, we observe QAH states with C=N at moire filling v=1, where N=3,4,5 represents the layer number of rhombohedral graphene. These results are experimentally confirmed by quantized Hall resistance and the Streda formula. In twisted monolayer-trilayer graphene, we also observe states with |C|=3 at v=3, whose sign can be switched by either electrostatic doping or displacement field. Furthermore, in twisted Bernal bilayer-rhombohedral tetralayer graphene denoted as (2+4) L, we demonstrate a displacement-field-driven topological phase transition between two distinct QAH states with C=3 and C=4 at v=1. Our work establishes twisted rhombohedral graphene as a highly versatile, layer-engineered platform for designing and dynamically controlling high-Chern-number topological matters. △ Less

Submitted 20 January, 2026; originally announced January 2026.

Anisotropic collective excitations of Bose gases in modified Newtonian dynamics

Authors: Ning Liu

Abstract: Collective excitations are fundamental in quantum many-body physics, yet their spectra have traditionally been studied within Newtonian dynamics. In this paper, we investigate collective excitations in Bose gases under Modified Newtonian Dynamics (MOND). We derive an anisotropic excitation spectrum in the MOND regime. This anisotropy arises directly from the intrinsic nonlinear structure of the MO… ▽ More Collective excitations are fundamental in quantum many-body physics, yet their spectra have traditionally been studied within Newtonian dynamics. In this paper, we investigate collective excitations in Bose gases under Modified Newtonian Dynamics (MOND). We derive an anisotropic excitation spectrum in the MOND regime. This anisotropy arises directly from the intrinsic nonlinear structure of the MOND Poisson equation, forming a distinctive signature of the modified gravitational response. We then analyze the Jeans instability, obtaining analytic expressions for the direction-dependent critical wavelength and mass. These results advance our understanding of collective behavior in quantum systems under modified dynamics and establish clear theoretical signatures for testing MOND-like effects in quantum simulators. △ Less

Submitted 31 January, 2026; v1 submitted 19 January, 2026; originally announced January 2026.

Comments: 7 pages,3 figures

Symmetry-engineered and electrically tunable in-plane anomalous Hall effect in oxide heterostructures

Authors: Kunjie Dai, Zhen Wang, Wenfeng Wu, Feng Jin, Enda Hua, Nan Liu, Jingdi Lu, Jinfeng Zhang, Yuyue Zhao, Linda Yang, Kai Liu, Huan Ye, Qiming Lv, Zhengguo Liang, Ao Wang, Dazhi Hou, Yang Gao, Shengchun Shen, Jing Tao, Liang Si, Wenbin Wu, Lingfei Wang

Abstract: The family of Hall effects has long served as a premier probe of how symmetry, magnetic order, and topology intertwine in solids. Recently, the in-plane anomalous Hall effect (IP-AHE), a transverse Hall response driven by in-plane magnetization, has emerged as a distinct member of this family, offering innovative spintronic functionalities and illuminating intricate interplay between mirror-symmet… ▽ More The family of Hall effects has long served as a premier probe of how symmetry, magnetic order, and topology intertwine in solids. Recently, the in-plane anomalous Hall effect (IP-AHE), a transverse Hall response driven by in-plane magnetization, has emerged as a distinct member of this family, offering innovative spintronic functionalities and illuminating intricate interplay between mirror-symmetry breaking and in-plane magnetic order. However, practical routes to deterministically and reversibly control IP-AHE remain limited. Here, we establish a symmetry-engineered IP-AHE platform, CaRuO3/La2/3Ca1/3MnO3/CaRuO3 heterostructure on NdGaO3(110), that turns strict mirror-symmetry breaking constraints into effective tuning knobs. IP-AHE in these epitaxial trilayers unambiguously couples to the CaRuO3-buffer-induced mirror-symmetry breaking and faithfully reproduces the ferromagnetic hysteresis. Ionic liquid gating further enables reversible reconfigurations of the symmetry breaking, thereby achieving electrical modulation and ON/OFF switching of IP-AHE. This highly tunable IP-AHE platform opens pathways for exploring nontrivial magnetic order and developing programmable Hall functionalities in planar geometries. △ Less

Submitted 8 January, 2026; originally announced January 2026.

Ground State and Collective Modes of Bose-Einstein Condensates in Newtonian and MOND-inspired gravitational potentials

Authors: Ning Liu

Abstract: We analytically and numerically study the ground state and collective dynamics of Bose-Einstein condensates in two traps: a Newtonian potential and a logarithmic potential inspired by Modified Newtonian Dynamics (MOND). In the ground state, the MOND potential supports bound states only in the deep-MOND regime, where the condensate becomes significantly larger than its Newtonian counterpart. The si… ▽ More We analytically and numerically study the ground state and collective dynamics of Bose-Einstein condensates in two traps: a Newtonian potential and a logarithmic potential inspired by Modified Newtonian Dynamics (MOND). In the ground state, the MOND potential supports bound states only in the deep-MOND regime, where the condensate becomes significantly larger than its Newtonian counterpart. The size increases with repulsive coupling parameter $β$ in both potentials. A clear scaling law of the size with $β^{1/3}$ emerges in the MOND case and is confirmed numerically over a wide parameter range, while for the Newtonian potential no simple scaling law exists as the Thomas-Fermi approximation ceases to be valid. For the dynamics, we derive and solve equations for the monopole collective mode. The larger MOND-bound condensate oscillates at a lower frequency, which scales as $β^{-1/3}$ in the strong-interaction limit. These scaling laws provide insights for quantum-simulation experiments aiming to probe modified-gravity scenarios with cold atoms. △ Less

Submitted 2 January, 2026; originally announced January 2026.

Comments: 9 pages, 3 figures

Constrained free energy minimization for the design of thermal states and stabilizer thermodynamic systems

Authors: Michele Minervini, Madison Chin, Jacob Kupperman, Nana Liu, Ivy Luo, Meghan Ly, Soorya Rethinasamy, Kathie Wang, Mark M. Wilde

Abstract: A quantum thermodynamic system is described by a Hamiltonian and a list of conserved, non-commuting charges, and a fundamental goal is to determine the minimum energy of the system subject to constraints on the charges. Recently, [Liu et al., arXiv:2505.04514] proposed first- and second-order classical and hybrid quantum-classical algorithms for solving a dual chemical potential maximization probl… ▽ More A quantum thermodynamic system is described by a Hamiltonian and a list of conserved, non-commuting charges, and a fundamental goal is to determine the minimum energy of the system subject to constraints on the charges. Recently, [Liu et al., arXiv:2505.04514] proposed first- and second-order classical and hybrid quantum-classical algorithms for solving a dual chemical potential maximization problem, and they proved that these algorithms converge to global optima by means of gradient-ascent approaches. In this paper, we benchmark these algorithms on several problems of interest in thermodynamics, including one- and two-dimensional quantum Heisenberg models with nearest- and next-nearest neighbor interactions and with the charges set to the total x, y, and z magnetizations. We also offer an alternative compelling interpretation of these algorithms as methods for designing ground and thermal states of controllable Hamiltonians, with potential applications in molecular and material design. Furthermore, we introduce stabilizer thermodynamic systems as thermodynamic systems based on stabilizer codes, with the Hamiltonian constructed from a given code's stabilizer operators and the charges constructed from the code's logical operators. We benchmark the aforementioned algorithms on several examples of stabilizer thermodynamic systems, including those constructed from the one-to-three-qubit repetition code, the perfect one-to-five-qubit code, and the two-to-four-qubit error-detecting code. Finally, we observe that the aforementioned hybrid quantum-classical algorithms, when applied to stabilizer thermodynamic systems, can serve as alternative methods for encoding quantum information into stabilizer codes at a fixed temperature, and we provide an effective method for warm-starting these encoding algorithms whenever a single qubit is encoded into multiple physical qubits. △ Less

Submitted 3 April, 2026; v1 submitted 12 August, 2025; originally announced August 2025.

Comments: v3: 21 pages of main text, 15 pages of appendices, 12 figures

Journal ref: Physical Review A, vol. 113, no. 4, page 042407, April 2026

Diverse high-Chern-number quantum anomalous Hall insulators in twisted rhombohedral graphene

Authors: Naitian Liu, Zhangyuan Chen, Jing Ding, Wenqiang Zhou, Hanxiao Xiang, Xinjie Fang, Linfeng Wu, Xiaowan Zhan, Le Zhang, Qianmei Chen, Kenji Watanabe, Takashi Taniguchi, Na Xin, Shuigang Xu

Abstract: Quantum anomalous Hall (QAH) insulators with high Chern number (C) enables multiple dissipationless edge channels for low-power-consumption electronics. We report the realization of multiple high-C QAH insulators including C=3,5,6, and 7 in twisted monolayer-rhombohedral pentalayer graphene. In twist angles of approximately 1.40°, we observe QAH effect with C=5 at a filling of one electron per moi… ▽ More Quantum anomalous Hall (QAH) insulators with high Chern number (C) enables multiple dissipationless edge channels for low-power-consumption electronics. We report the realization of multiple high-C QAH insulators including C=3,5,6, and 7 in twisted monolayer-rhombohedral pentalayer graphene. In twist angles of approximately 1.40°, we observe QAH effect with C=5 at a filling of one electron per moiré unit cell, persisting up to 2 Kelvin. Furthermore, incommensurate QAH insulators with C=5,6, and 7 emerge at partial fillings. In twist angles of 0.89°, Chern insulators with C=3 and C=6 appear at fillings of two and three electrons, respectively. Our findings establish twisted rhombohedral multilayer graphene as a highly tunable platform for multichannel, dissipationless electronics and for the exploration of exotic quantum Hall states beyond traditional Landau level paradigm. △ Less

Submitted 15 July, 2025; originally announced July 2025.

An eco-friendly universal strategy via ribavirin to achieve highly efficient and stable perovskite solar cells

Authors: Xianhu Wu, Gaojie Xia, Guanglei Cui, Jieyu Bi, Nian Liu, Jiaxin Jiang, Jilong Sun, Luyang Liu, Ping Li, Ning Lu, Zewen Zuo, Min Gu

Abstract: The grain boundaries of perovskite films prepared by the solution method are highly disordered, with a large number of defects existing at the grain boundaries. These defect sites promote the decomposition of perovskite. Here, we use ribavirin obtained through bacillus subtilis fermentation to regulate the crystal growth of perovskite, inducing changes in the work function and energy level structu… ▽ More The grain boundaries of perovskite films prepared by the solution method are highly disordered, with a large number of defects existing at the grain boundaries. These defect sites promote the decomposition of perovskite. Here, we use ribavirin obtained through bacillus subtilis fermentation to regulate the crystal growth of perovskite, inducing changes in the work function and energy level structure of perovskite, which significantly reduces the defect density. Based on density functional theory calculations, the defect formation energies of VI, VMA, VPb, and PbI in perovskite are improved. This increases the open-circuit voltage of perovskite solar cells (PSCs) (ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag) from 1.077 to 1.151 V, and the PCE increases significantly from 17.05% to 19.86%. Unencapsulated PSCs were stored in the environment (humidity approximately 35+-5%) for long-term stability testing. After approximately 900 hours of storage, the PCE of the ribavirin-based device retains 84.33% of its initial PCE, while the control-based device retains only 13.44% of its initial PCE. The PCE of PSCs (ITO/SnO2/perovskite/Spiro-OMETAD/Ag) is increased from 20.16% to 22.14%, demonstrating the universality of this doping method. This universal doping strategy provides a new approach for improving the efficiency and stability of PSCs using green molecular doping strategies. △ Less

Submitted 2 July, 2025; originally announced July 2025.

Detecting Collective Excitations in Self-Gravitating Bose-Einstein Condensates via Faraday Waves

Authors: Ning Liu, Guodong Cheng

Abstract: We propose Faraday waves as a probe for collective excitations in self-gravitating Bose-Einstein condensates (SGBECs). Using a semi-classical approach based on linear stability analysis of the Gross-Pitaevskii-Newton equations, we derive a damped Mathieu equation governing parametric instabilities. Our analysis reveals well-separated regions of parametric resonance and Jeans instability in paramet… ▽ More We propose Faraday waves as a probe for collective excitations in self-gravitating Bose-Einstein condensates (SGBECs). Using a semi-classical approach based on linear stability analysis of the Gross-Pitaevskii-Newton equations, we derive a damped Mathieu equation governing parametric instabilities. Our analysis reveals well-separated regions of parametric resonance and Jeans instability in parameter space, with distinct growth rate characteristics: Jeans instability decreases monotonically to zero at the critical wavenumber $k_J$, while parametric resonance exhibits non-monotonic behavior with a clear maximum. These findings provide explicit experimental guidelines for accessing the parametric resonance regime. Numerical simulations demonstrate the transition from Faraday wave formation to Jeans collapse as gravitational strength increases, validating our theoretical framework. △ Less

Submitted 19 November, 2025; v1 submitted 23 June, 2025; originally announced June 2025.

Comments: 26pages, 7 figures

Angle-dependent resonant tunneling and thermoelectric energy management in a hybrid 1D-2D-1D semiconductor nanostructure

Authors: Xiaoguang Luo, Jiaming Wang, Jiawen Dai, Junqiang Zhang, Nian Liu

Abstract: Low-dimensional semiconductors have been widely exploited in thermoelectric energy conversion for high efficiencies due to their suppressed lattice thermal conduction, sharply defined electronic density of states, and tunable energy-selective electron transmission. However, the widespread challenge of Fermi-level pinning or doping constraints limit precise control over thermoelectric energy manage… ▽ More Low-dimensional semiconductors have been widely exploited in thermoelectric energy conversion for high efficiencies due to their suppressed lattice thermal conduction, sharply defined electronic density of states, and tunable energy-selective electron transmission. However, the widespread challenge of Fermi-level pinning or doping constraints limit precise control over thermoelectric energy management via chemical potential modulation. Here, we proposed an alternative strategy: leveraging angle-dependent electron incidence to dynamically manipulate electron transmission and heat transport, which was implemented theoretically in a two-dimensional InP/InAs/InP double-barrier heterostructure integrated with laterally one-dimensional electrodes. By combining the transfer matrix method and Landauer formalism, we demonstrated the angle-dependent resonant tunneling dynamics, tunable negative differential resistance effect, and near-Carnot limits in thermoelectric energy conversions. Angular modulation enables precise control over transmission resonances, facilitating dynamic transitions among thermoelectric regimes (power generation, cooling, and hybrid heating) without requiring extreme chemical potential shifts. This work establishes angularly resolved electron transmission as a versatile mechanism for on-chip thermal management and cryogenic applications, offering a pathway to circumvent material limitations in next-generation nanoelectronics and quantum devices. △ Less

Submitted 16 May, 2025; originally announced May 2025.

Comments: 19 pages, 7 figures

Journal ref: Chinese Journal of Physics, Volume 98, December 2025, Pages 659-669

Quantum thermodynamics and semi-definite optimization

Authors: Nana Liu, Michele Minervini, Dhrumil Patel, Mark M. Wilde

Abstract: In quantum thermodynamics, a system is described by a Hamiltonian and a list of non-commuting charges representing conserved quantities like particle number or electric charge, and an important goal is to determine the system's minimum energy in the presence of these conserved charges. In optimization theory, a semi-definite program (SDP) involves a linear objective function optimized over the con… ▽ More In quantum thermodynamics, a system is described by a Hamiltonian and a list of non-commuting charges representing conserved quantities like particle number or electric charge, and an important goal is to determine the system's minimum energy in the presence of these conserved charges. In optimization theory, a semi-definite program (SDP) involves a linear objective function optimized over the cone of positive semi-definite operators intersected with an affine space. These problems arise from differing motivations in the physics and optimization communities and are phrased using very different terminology, yet they are essentially identical mathematically. By adopting Jaynes' mindset motivated by quantum thermodynamics, we observe that minimizing free energy in the aforementioned thermodynamics problem, instead of energy, leads to an elegant solution in terms of a dual chemical potential maximization problem that is concave in the chemical potential parameters. As such, one can employ standard (stochastic) gradient ascent methods to find the optimal values of these parameters, and these methods are guaranteed to converge quickly. At low temperature, the minimum free energy provides an excellent approximation for the minimum energy. We then show how this Jaynes-inspired gradient-ascent approach can be used in both first- and second-order classical and hybrid quantum-classical algorithms for minimizing energy, and equivalently, how it can be used for solving SDPs, with guarantees on the runtimes of the algorithms. The approach discussed here is well grounded in quantum thermodynamics and, as such, provides physical motivation underpinning why algorithms published fifty years after Jaynes' seminal work, including the matrix multiplicative weights update method, the matrix exponentiated gradient update method, and their quantum algorithmic generalizations, perform well at solving SDPs. △ Less

Submitted 12 May, 2025; v1 submitted 7 May, 2025; originally announced May 2025.

Comments: v2: 16 pages of main text, 15 pages of appendices, 3 figures, corrections introduced

Continuously tunable anomalous Hall crystals in rhombohedral heptalayer graphene

Authors: Hanxiao Xiang, Jing Ding, Jiannan Hua, Naitian Liu, Wenqiang Zhou, Qianmei Chen, Kenji Watanabe, Takashi Taniguchi, Na Xin, Wei Zhu, Shuigang Xu

Abstract: The interplay of electronic interactions and nontrivial topology can give rise to a wealth of exotic quantum states. A notable example is the formation of Wigner crystals driven by strong electron-electron interactions. When these electronic crystals emerge in a parent band carrying a large Berry curvature, they can exhibit topologically nontrivial properties as anomalous Hall crystals, spontaneou… ▽ More The interplay of electronic interactions and nontrivial topology can give rise to a wealth of exotic quantum states. A notable example is the formation of Wigner crystals driven by strong electron-electron interactions. When these electronic crystals emerge in a parent band carrying a large Berry curvature, they can exhibit topologically nontrivial properties as anomalous Hall crystals, spontaneously breaking both continuous translational symmetry and time-reversal symmetry. Here, we report the experimental observation of tunable anomalous Hall crystals in rhombohedral heptalayer graphene moiré superlattices. At filling factors near one electron per moiré unit cell (v=1), we identify a series of incommensurate Chern insulators with a Chern number of C=1. Furthermore, we observe spontaneous time-reversal symmetry breaking spanning the entire filling range from v=1 to v=2, manifesting as anomalous Hall effects with pronounced magnetic hysteresis. Notably, anomalous Hall crystals with a high Chern number C=3 are observed over generic fillings ranging from v=1.5 to v=2. These anomalous Hall crystals are incommensurate with the moiré superlattice and exhibit dispersive fan diagrams consistent with the Streda formula, with their positions continuously tunable through displacement fields. Remarkably, these partially filled Chern insulators display Chern numbers distinct from their parent bands. Our findings demonstrate the rich variety of electronic crystalline states in rhombohedral graphene moiré superlattices, offering valuable insights into the strongly correlated topological phases. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Comments: 4 figures

Localization transitions of correlated particles in nonreciprocal quasicrystals

Authors: Lei Wang, Juan Kang, Ni Liu, Chaohua Wu, Gang Chen

Abstract: The interplay among interaction, non-Hermiticity, and disorder opens a new avenue for engineering novel phase transitions. We here study the spectral and localization features of two interacting bosons in one-dimensional nonreciprocal quasicrystals. Specifically, by considering a quasiperiodic Hubbard lattice with nonreciprocal hoppings, we show that the interaction can lead to a mobility edge, wh… ▽ More The interplay among interaction, non-Hermiticity, and disorder opens a new avenue for engineering novel phase transitions. We here study the spectral and localization features of two interacting bosons in one-dimensional nonreciprocal quasicrystals. Specifically, by considering a quasiperiodic Hubbard lattice with nonreciprocal hoppings, we show that the interaction can lead to a mobility edge, which arises from the fact that the bound states display a much lower threshold for spectral and extended-localized transitions than scattering states. The localization transition of bound or scattering states is accompanied by a complex-real spectrum transition. Moreover, while the two-particle localized states are robust to the boundary conditions, the two-particle extended states turn into skin modes under open boundary condition. We also show the correlated dynamics to characterize these localization transitions. Finally, we reveal that the bound states can form mobility edge on their own by introducing a dimerized nonreciprocal quasicrystal. Our paper may pave the way for the study of non-Hermitian few-body physics. △ Less

Submitted 24 December, 2024; originally announced December 2024.

Comments: 12 pages, 10 figures

Magnetic Switching in Monolayer 2D Diluted Magnetic Semiconductors via Spin-to- Spin Conversion

Authors: Siwei Chen, Zitao Tang, Mengqi Fang, Rui Sun, Xiaotong Zhang, Licheng Xiao, Seyed Sepehr Mohajerani, Na Liu, Yuze Zhang, Abdus Salam Sarkar, Dali Sun, Stefan Strauf, Eui- Hyeok Yang

Abstract: The integration of two-dimensional (2D) van der Waals (vdW) magnets with topological insulators or heavy metals holds great potential for realizing next-generation spintronic memory devices. However, achieving high-efficiency SOT switching of monolayer vdW magnets at room temperature poses a significant challenge, particularly without an external magnetic field. Here, we show field-free, determini… ▽ More The integration of two-dimensional (2D) van der Waals (vdW) magnets with topological insulators or heavy metals holds great potential for realizing next-generation spintronic memory devices. However, achieving high-efficiency SOT switching of monolayer vdW magnets at room temperature poses a significant challenge, particularly without an external magnetic field. Here, we show field-free, deterministic, and nonvolatile SOT switching of perpendicular magnetization in the monolayer, diluted magnetic semiconductor (DMS), Fe-doped MoS2(Fe:MoS2) at up to 380 K with a current density of $7\times10^4 A cm^{-2}$. The in situ doping of Fe into monolayer MoS2 via chemical vapor deposition and the geometry-induced strain in the crystal break the rotational switching symmetry in Fe:MoS2, promoting field-free SOT switching by generating out-of-plane spins via spin-to-spin conversion. An apparent anomalous Hall effect (AHE) loop shift at a zero in-plane magnetic field verifies the existence of z spins in Fe:MoS2, inducing an antidamping-like torque that facilitates field-free SOT switching. A strong topological Hall effect (THE) was also observed, attributed to the interfacial Dzyaloshinskii-Moriya interaction (DMI), reducing the energy barrier for SOT switching. This field-free SOT application using a 2D ferromagnetic monolayer provides a new pathway for developing highly power-efficient spintronic memory devices. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Comments: 26 pages, including SOI

Report number: https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202418647

Next generation Co-Packaged Optics Technology to Train & Run Generative AI Models in Data Centers and Other Computing Applications

Authors: John Knickerbocker, Jean Benoit Heroux, Griselda Bonilla, Hsiang Hsu, Neng Liu, Adrian Paz Ramos, Francois Arguin, Yan Tribodeau, Badr Terjani, Mark Schultz, Raghu Kiran Ganti, Linsong Chu, Chinami Marushima, Yoichi Taira, Sayuri Kohara, Akihiro Horibe, Hiroyuki Mori, Hidetoshi Numata

Abstract: We report on the successful design and fabrication of optical modules using a 50 micron pitch polymer waveguide interface, integrated for low loss, high density optical data transfer with very low space requirements on a Si photonics die. This prototype module meets JEDEC reliability standards and promises to increase the number of optical fibers that can be connected at the edge of a chip, a meas… ▽ More We report on the successful design and fabrication of optical modules using a 50 micron pitch polymer waveguide interface, integrated for low loss, high density optical data transfer with very low space requirements on a Si photonics die. This prototype module meets JEDEC reliability standards and promises to increase the number of optical fibers that can be connected at the edge of a chip, a measure known as beachfront density, by six times compared to state of the art technology. Scalability of the polymer waveguide to less than 20 micron pitch stands to improve the bandwidth density upwards of 10 Tbps/mm. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Ultrahigh-temperature ferromagnetism in ultrathin insulating films with ripple-infinite-layer structure

Authors: Yazhuo Yi, Haoliang Huang, Ruiwen Shao, Yukuai Liu, Guangzheng Chen, Jiahui Ou, Xi Zhang, Ze Hua, Lang Chen, Chi Wah Leung, Xie-Rong Zeng, Feng Rao, Nan Liu, Heng Wang, Liang Si, Hongyu An, Zhuoyu Chen, Chuanwei Huang

Abstract: Ferromagnetism and electrical insulation are often at odds, signifying an inherent trade off. The simultaneous optimization of both in one material, essential for advancing spintronics and topological electronics, necessitates the individual manipulation over various degrees of freedom of strongly correlated electrons. Here, by selective control of the spin exchange and Coulomb interactions, we re… ▽ More Ferromagnetism and electrical insulation are often at odds, signifying an inherent trade off. The simultaneous optimization of both in one material, essential for advancing spintronics and topological electronics, necessitates the individual manipulation over various degrees of freedom of strongly correlated electrons. Here, by selective control of the spin exchange and Coulomb interactions, we report the achievement of SrFeO2 thin films with resistivity above 106 Ohm.cm and strong magnetization with Curie temperature extrapolated to be 1200 K. Robust ferromagnetism is obtained down to 1.0 nm thickness on substrate and 2.0 nm for freestanding films. Featuring an out of plane oriented ripple infinite layer structure, this ferromagnetic insulating phase is obtained through extensive reduction of as grown brownmillerite SrFeO2.5 films at high compressive strains. Pronounced spin Hall magnetoresistance signals up to 0.0026 is further demonstrated with a Pt Hall bar device. Our findings promise emerging spintronic and topological electronic functionalities harnessing spin dynamics with minimized power dissipations. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Zero external magnetic field quantum standard of resistance at the 10-9 level

Authors: D. K. Patel, K. M. Fijalkowski, M. Kruskopf, N. Liu, M. Götz, E. Pesel, M. Jaime, M. Klement, S. Schreyeck, K. Brunner, C. Gould, L. W. Molenkamp, H. Scherer

Abstract: The quantum anomalous Hall effect holds promise as a disruptive innovation in condensed matter physics and metrology, as it gives access to Hall resistance quantization in terms of the von-Klitzing constant RK = h/e2 at zero external magnetic field. In this work, we study the accuracy of Hall resistance quantization in a device based on the magnetic topological insulator material (V,Bi,Sb)2Te3. We… ▽ More The quantum anomalous Hall effect holds promise as a disruptive innovation in condensed matter physics and metrology, as it gives access to Hall resistance quantization in terms of the von-Klitzing constant RK = h/e2 at zero external magnetic field. In this work, we study the accuracy of Hall resistance quantization in a device based on the magnetic topological insulator material (V,Bi,Sb)2Te3. We show that the relative deviation of the Hall resistance from RK at zero external magnetic field is (4.4 +/- 8.7) nohm/ohm when extrapolated to zero measurement current, and (8.6 +/- 6.7) nohm/ohm when extrapolated to zero longitudinal resistivity (each with combined standard uncertainty, k = 1), which sets a new benchmark for the quantization accuracy in topological matter. This precision and accuracy at the nohm/ohm level (or 10-9 of relative uncertainty) achieve the thresholds for relevant metrological applications and establish a zero external magnetic field quantum standard of resistance - an important step towards the integration of quantum-based voltage and resistance standards into a single universal quantum electrical reference. △ Less

Submitted 22 January, 2025; v1 submitted 17 October, 2024; originally announced October 2024.

Comments: 12 pages (incl. 8 pages main text and 4 pages supplementary information), with 6 figures and 2 tables

Journal ref: Nature Electronics 7, 1111-1116 (2024)

Phase behaviors and dynamics of active particle systems in double-well potential

Authors: Lu Chen, Baopi Liu, Ning Liu

Abstract: In this study, we investigate the behaviors and dynamics of self-propelled particles with active reorientation (AR) in a double-well potential. We explore the competition between AR and external potentials, revealing that self-propelled particles exhibit flocking and clustering behaviors in an asymmetric potential trap. Through molecular dynamics simulations, we obtain a phase diagram that illustr… ▽ More In this study, we investigate the behaviors and dynamics of self-propelled particles with active reorientation (AR) in a double-well potential. We explore the competition between AR and external potentials, revealing that self-propelled particles exhibit flocking and clustering behaviors in an asymmetric potential trap. Through molecular dynamics simulations, we obtain a phase diagram that illustrates flocking behavior as a function of active reorientation and potential asymmetry. We compare the responses of inactive and active particles to the potential, finding that active reorientation significantly increases aggregation on one side of the asymmetric potential well. Additionally, by calculating the mean squared displacement and scaling exponent, we identify distinct diffusion regimes. Our findings demonstrate that active particles with active reorientation are more sensitive to variations in double-well potentials. △ Less

Submitted 13 March, 2025; v1 submitted 31 August, 2024; originally announced September 2024.

Comments: 7 pages, 9 figures

Journal ref: Physica A: Statistical Mechanics and its Applications Volume 667, 1 June 2025, 130538

Highly Efficient and Stable Perovskite Solar Cells via MultiFunctional Curcumin Modified Buried Interface

Authors: Xianhu Wu, Jieyu Bi, Guanglei Cu, Nian Liu, Gaojie Xia, Jilong Sun, Jiaxin Jiang, Ning Lu, Ping Li, Chunyi Zhao, Zewen Zuo, Min Gu

Abstract: The buried interface between the electron transport layer and the perovskite layer suffers from severe interface defects and imperfect energy level alignment. To address this issue, this study employs a multifunctional organic molecule, curcumin, to modify the interface between SnO2 and the perovskite layer. The functional groups on curcumin effectively passivate the defects on both sides of the i… ▽ More The buried interface between the electron transport layer and the perovskite layer suffers from severe interface defects and imperfect energy level alignment. To address this issue, this study employs a multifunctional organic molecule, curcumin, to modify the interface between SnO2 and the perovskite layer. The functional groups on curcumin effectively passivate the defects on both sides of the interface, reducing -OH and oxygen vacancy defects on the SnO2 surface and passivating uncoordinated Pb2+ in the perovskite layer. This results in a more compatible energy level alignment and lower defect density at the interface, enhancing carrier transport across it. Consequently, the devices based on curcumin achieve an impressive champion power conversion efficiency (PCE) of 24.46%, compared to 22.03% for control devices. This work demonstrates a simple, green, hydrophobic, and efficient molecular modification method for the buried interface, laying the foundation for the development of high-performance and stable perovskite solar cells. △ Less

Submitted 30 August, 2024; originally announced August 2024.

Electric-field switchable chirality in rhombohedral graphene Chern insulators stabilized by tungsten diselenide

Authors: Jing Ding, Hanxiao Xiang, Jiannan Hua, Wenqiang Zhou, Naitian Liu, Le Zhang, Na Xin, Bing Wu, Kenji Watanabe, Takashi Taniguchi, Zdenek Sofer, Wei Zhu, Shuigang Xu

Abstract: Chern insulators host topologically protected chiral edge currents with quantized conductance characterized by their Chern number. Switching the chirality of a Chern insulator, namely, the direction of the edge current, is highly challenging due to topologically forbidden backscattering but is of considerable importance for the design of topological devices. Nevertheless, this can be achieved by r… ▽ More Chern insulators host topologically protected chiral edge currents with quantized conductance characterized by their Chern number. Switching the chirality of a Chern insulator, namely, the direction of the edge current, is highly challenging due to topologically forbidden backscattering but is of considerable importance for the design of topological devices. Nevertheless, this can be achieved by reversing the sign of the Chern number through a topological phase transition. Here, we report electrically switchable chirality in rhombohedral multilayer graphene-based Chern insulators. By introducing moire superlattices in rhombohedral heptalayer graphene, we observed a cascade of topological phase transitions at quarter electron filling of a moire band with the Chern number tunable from -1, 1 to 2. Furthermore, integrating monolayer tungsten diselenide at the moireless interface of rhombohedral decalayer graphene/h-BN superlattices stabilizes the Chern insulators, enabling quantized anomalous Hall resistance of h/2e^2. Remarkably, the Chern number can be switched from -1 to 2 using displacement fields. Our work establishes rhombohedral multilayer graphene moire superlattices as a versatile platform for topological engineering, with switchable chirality offering significant promise for integrating chiral edge currents into topological electronic circuits. △ Less

Submitted 23 January, 2025; v1 submitted 20 June, 2024; originally announced June 2024.

Comments: Include the data from 10L graphene devices. To appear in Physical Review X

Journal ref: Phys. Rev. X 15, 011052 (2025)

Engineering band structures of two-dimensional materials with remote moire ferroelectricity

Authors: Jing Ding, Hanxiao Xiang, Wenqiang Zhou, Naitian Liu, Xinjie Fang, Kangyu Wang, Linfeng Wu, Kenji Watanabe, Takashi Taniguchi, Shuigang Xu

Abstract: The stacking order and twist angle provide abundant opportunities for engineering band structures of two-dimensional materials, including the formation of moire bands, flat bands, and topologically nontrivial bands. The inversion symmetry breaking in rhombohedral-stacked transitional metal dichalcogenides (TMDCs) endows them with an interfacial ferroelectricity associated with an out-of-plane elec… ▽ More The stacking order and twist angle provide abundant opportunities for engineering band structures of two-dimensional materials, including the formation of moire bands, flat bands, and topologically nontrivial bands. The inversion symmetry breaking in rhombohedral-stacked transitional metal dichalcogenides (TMDCs) endows them with an interfacial ferroelectricity associated with an out-of-plane electric polarization. By utilizing twist angle as a knob to construct rhombohedral-stacked TMDCs, antiferroelectric domain networks with alternating out-of-plane polarization can be generated. Here, we demonstrate that such spatially periodic ferroelectric polarizations in parallel-stacked twisted WSe2 can imprint their moire potential onto a remote bilayer graphene. This remote moire potential gives rise to pronounced satellite resistance peaks besides the charge-neutrality point in graphene, which are tunable by the twist angle of WSe2. Our observations of ferroelectric hysteresis at finite displacement fields suggest the moire is delivered by a long-range electrostatic potential. The constructed superlattices by moire ferroelectricity represent a highly flexible approach, as they involve the separation of the moire construction layer from the electronic transport layer. This remote moire is identified as a weak potential and can coexist with conventional moire. Our results offer a comprehensive strategy for engineering band structures and properties of two-dimensional materials by utilizing moire ferroelectricity. △ Less

Submitted 21 May, 2024; originally announced May 2024.

Journal ref: Nat. Commun. 15, 9087 (2024)

An eco-friendly passivation strategy of resveratrol for highly efficient and antioxidative perovskite solar cells

Authors: Xianhu Wu, Jieyu Bi, Guanglei Cui, Nian Liu, Gaojie Xia, Ping Li, Chunyi Zhao, Zewen Zuo, Min Gu

Abstract: The stability of perovskite solar cells is closely related to the defects in perovskite crystals, and there are a large number of crystal defects in the perovskite thin films prepared by the solution method, which is not conducive to the commercial production of PSCs. In this study, resveratrol(RES), a green natural antioxidant abundant in knotweed and grape leaves, was introduced into perovskite… ▽ More The stability of perovskite solar cells is closely related to the defects in perovskite crystals, and there are a large number of crystal defects in the perovskite thin films prepared by the solution method, which is not conducive to the commercial production of PSCs. In this study, resveratrol(RES), a green natural antioxidant abundant in knotweed and grape leaves, was introduced into perovskite films to passivate the defect. RES achieves defect passivation by interacting with uncoordinated Pb2+ in perovskite films. The results show that the quality of the perovskite film is significantly improved, and the energy level structure of the device is optimized, and the power conversion efficiency of the device is increased from 21.62% to 23.44%. In addition, RES can hinder the degradation of perovskite structures by O2- and CO2- free radicals, and the device retained 88% of its initial PCE after over 1000 hours in pure oxygen environment. The device retains 91% of the initial PCE after more than 1000 hours at 25°C and 50+5% relative humidity. This work provides a strategy for the use of natural and environmentally friendly additives to improve the efficiency and stability of devices, and provides an idea for the development of efficient, stable and environmentally friendly PSCs. △ Less

Submitted 2 May, 2024; originally announced May 2024.

Electronic ferroelectricity in monolayer graphene for multifunctional neuromorphic electronics

Authors: Le Zhang, Jing Ding, Hanxiao Xiang, Naitian Liu, Wenqiang Zhou, Linfeng Wu, Na Xin, Kenji Watanabe, Takashi Taniguchi, Shuigang Xu

Abstract: Ferroelectricity is intriguing for its spontaneous electric polarization, which is switchable by an external electric field. Expanding ferroelectric materials to two-dimensional limit will provide versatile applications for the development of next-generation nonvolatile devices. Conventional ferroelectricity requires the materials consisting of at least two constituent elements associated with pol… ▽ More Ferroelectricity is intriguing for its spontaneous electric polarization, which is switchable by an external electric field. Expanding ferroelectric materials to two-dimensional limit will provide versatile applications for the development of next-generation nonvolatile devices. Conventional ferroelectricity requires the materials consisting of at least two constituent elements associated with polar crystalline structures. Monolayer graphene as an elementary two-dimensional material unlikely exhibits ferroelectric order due to its highly centrosymmetric hexagonal lattices. Nevertheless, two-dimensional moire superlattices offer a powerful way to engineer diverse electronic orders in non-polar materials. Here, we report the observations of electronic ferroelectricity in monolayer graphene by introducing asymmetric moire superlattice at the graphene/h-BN interface. Utilizing Hall measurements, the electric polarization is identified to stem from electron-hole dipoles, suggesting the electronic dynamics of the observed ferroelectricity. Standard polarization-electric field hysteresis loops, as well as unconventional multiple switchable polarization states, have been achieved. By in-situ comparing with control devices, we found that the electronic ferroelectricity in graphene moire systems is independent of layer number of graphene and the corresponding fine band structures. Furthermore, we demonstrate the applications of this ferroelectric moire structures in multi-state non-volatile data storage and the emulation of versatile synaptic behaviors, including short-term plasticity, long-term potentiation and long-term depression. This work not only enriches the fundamental understanding of ferroelectricity, but also demonstrates the promising applications of graphene in multi-state memories and neuromorphic computing. △ Less

Submitted 4 April, 2024; originally announced April 2024.

Chlorine and zinc co-doping effects on the electronic structure and optical properties of γ-CuI

Authors: Chao Li, Meicong Li, Zhuli Zhang, Qiang Zhao, Naixin Liu, Kailei Wang, Fan Zhang, Xiaoping Ouyang

Abstract: The effects of chlorine (Cl) and zinc (Zn) co-doping on the electronic structure and optical properties of the zinc blende (γ) phase of copper iodide (γ-CuI) scintillator material are investigated by using first-principles density functional theory calculations. The band structure, density of states, dielectric function, absorption coefficients, and reflectivity were analyzed before and after dopi… ▽ More The effects of chlorine (Cl) and zinc (Zn) co-doping on the electronic structure and optical properties of the zinc blende (γ) phase of copper iodide (γ-CuI) scintillator material are investigated by using first-principles density functional theory calculations. The band structure, density of states, dielectric function, absorption coefficients, and reflectivity were analyzed before and after doping. Results show co-doping significantly modifies the band structure, reduces the band gap, and generates impurity energy levels. Cl doping enhances absorption in the high energy region while reducing visible light absorption. Zn doping induces a redshift in absorption and n-type conductivity at high concentrations. With suitable co-doping ratios, the absorption coefficient and reflectivity of γ-CuI can be optimized in the visible range to improve scintillation light yield. The calculations provide guidance for co-doping γ-CuI scintillators to achieve superior detection performance. The n-type conductivity also makes doped γ-CuI promising for optoelectronic applications. △ Less

Submitted 8 March, 2024; originally announced March 2024.

Balanced Quantum Hall Resistor

Authors: Kajetan M. Fijalkowski, Nan Liu, Martin Klement, Steffen Schreyeck, Karl Brunner, Charles Gould, Laurens W. Molenkamp

Abstract: The quantum anomalous Hall effect in magnetic topological insulators has been recognized as a promising platform for applications in quantum metrology. The primary reason for this is the electronic conductance quantization at zero external magnetic field, which allows to combine it with the quantum standard of voltage. Here we demonstrate a measurement scheme that increases the robustness of the z… ▽ More The quantum anomalous Hall effect in magnetic topological insulators has been recognized as a promising platform for applications in quantum metrology. The primary reason for this is the electronic conductance quantization at zero external magnetic field, which allows to combine it with the quantum standard of voltage. Here we demonstrate a measurement scheme that increases the robustness of the zero magnetic field quantum anomalous Hall resistor, allowing for higher operational currents. This is achieved by simultaneous current injection into the two disconnected perimeters of a multi-terminal Corbino device to balance the electrochemical potential between the edges, screening the electric field that drives back-scattering through the bulk, and thus improving the stability of the quantization at increased currents. This approach is not only applicable to devices based on the quantum anomalous Hall effect, but more generally can also be applied to existing quantum resistance standards that rely on the integer quantum Hall effect. △ Less

Submitted 6 February, 2024; originally announced February 2024.

Journal ref: Nature Electronics (2024)

Weak and strong coupling polarons in binary Bose-Einstein condensates

Authors: Ning Liu

Abstract: The Bose polaron is a quasiparticle that arises from the interaction between impurities and Bogoliubov excitation in Bose-Einstein condensates, analogous to the polaron formed by electrons and phonons in solid-state physics. In this paper, we investigate the effect of phase separation on weakly coupled and strongly coupled Bose polarons. Our findings reveal that phase separation induces a remarkab… ▽ More The Bose polaron is a quasiparticle that arises from the interaction between impurities and Bogoliubov excitation in Bose-Einstein condensates, analogous to the polaron formed by electrons and phonons in solid-state physics. In this paper, we investigate the effect of phase separation on weakly coupled and strongly coupled Bose polarons. Our findings reveal that phase separation induces a remarkable alteration in the properties of weakly coupled Bose polarons. However, in the case of strong coupling, phase separation cannot destroy the highly self-trapping state of Bose polarons. △ Less

Submitted 27 July, 2024; v1 submitted 22 January, 2024; originally announced January 2024.

Comments: 10 pages, 1 figures

Journal ref: Phys. Scr. 99 095412 (2024)

Peridynamic Neural Operators: A Data-Driven Nonlocal Constitutive Model for Complex Material Responses

Authors: Siavash Jafarzadeh, Stewart Silling, Ning Liu, Zhongqiang Zhang, Yue Yu

Abstract: Neural operators, which can act as implicit solution operators of hidden governing equations, have recently become popular tools for learning the responses of complex real-world physical systems. Nevertheless, most neural operator applications have thus far been data-driven and neglect the intrinsic preservation of fundamental physical laws in data. In this work, we introduce a novel integral neur… ▽ More Neural operators, which can act as implicit solution operators of hidden governing equations, have recently become popular tools for learning the responses of complex real-world physical systems. Nevertheless, most neural operator applications have thus far been data-driven and neglect the intrinsic preservation of fundamental physical laws in data. In this work, we introduce a novel integral neural operator architecture called the Peridynamic Neural Operator (PNO) that learns a nonlocal constitutive law from data. This neural operator provides a forward model in the form of state-based peridynamics, with objectivity and momentum balance laws automatically guaranteed. As applications, we demonstrate the expressivity and efficacy of our model in learning complex material behaviors from both synthetic and experimental data sets. We show that, owing to its ability to capture complex responses, our learned neural operator achieves improved accuracy and efficiency compared to baseline models that use predefined constitutive laws. Moreover, by preserving the essential physical laws within the neural network architecture, the PNO is robust in treating noisy data. The method shows generalizability to different domain configurations, external loadings, and discretizations. △ Less

Submitted 11 January, 2024; originally announced January 2024.

Inversion symmetry-broken tetralayer graphene probed by second harmonic generation

Authors: Wenqiang Zhou, Jiannan Hua, Naitian Liu, Jing Ding, Hanxiao Xiang, Wei Zhu, Shuigang Xu

Abstract: Symmetry breaking governs most fascinating phenomena in crystals, such as ferroelectricity, nonlinear optics, piezoelectricity, ferromagnetism, and superconductivity. In two-dimensional materials, a wide variety of tuning knobs presents extraordinary opportunities for engineering symmetry breaking, leading to the emergence and manipulation of novel physical properties. Recently, tetralayer graphen… ▽ More Symmetry breaking governs most fascinating phenomena in crystals, such as ferroelectricity, nonlinear optics, piezoelectricity, ferromagnetism, and superconductivity. In two-dimensional materials, a wide variety of tuning knobs presents extraordinary opportunities for engineering symmetry breaking, leading to the emergence and manipulation of novel physical properties. Recently, tetralayer graphene with ABCB stacking order is predicted to possess atypical elemental ferroelectricity arising from the symmetry breaking induced by its specific stacking configuration. Experimentally unveiling the stacking-order dependent symmetry in tetralayer graphene is crucial to understand the intricate properties in the emergent graphene allotropes. Here, we observe pronounced nonlinear optical second harmonic generation (SHG) in ABCB-stacked tetralayer graphene, but absent in both ABAB- and ABCA-stacked allotropes. Our results provide direct evidence of symmetry breaking in ABCB-stacked tetralayer graphene. The remarkable contrast in the SHG spectra of tetralayer graphene allows straightforward identification of ABCB domains from the other two kinds of stacking order and facilitates the characterization of their crystalline orientation. The employed SHG technique serves as a convenient tool for exploring the intriguing physics and novel nonlinear optics in ABCB-stacked graphene, where spontaneous polarization and intrinsic gapped flat bands coexist. Our results establish ABCB-stacked graphene as a unique platform for studying the rare ferroelectricity in non-centrosymmetric elemental structures. △ Less

Submitted 28 November, 2023; originally announced November 2023.

Journal ref: Nano Letters 24, 27, 8378-8385 (2024)

Observation of unconventional van der Waals multiferroics near room temperature

Authors: Yangliu Wu, Haipeng Lu, Xiaocang Han, Chendi Yang, Nanshu Liu, Xiaoxu Zhao, Liang Qiao, Wei Ji, Renchao Che, Longjiang Deng, Bo Peng

Abstract: The search for two-dimensional (2D) van der Waals (vdW) multiferroics is an exciting yet challenging endeavor. Room-temperature 2D vdW few-layer multiferroic is a much bigger insurmountable obstacle. Here we report the discovery of an unconventional 2D vdW multiferroic with out-of-plane ferroelectric polarization and long-range magnetic orders in trilayer NiI2 device from 10 K to 295 K. The evolut… ▽ More The search for two-dimensional (2D) van der Waals (vdW) multiferroics is an exciting yet challenging endeavor. Room-temperature 2D vdW few-layer multiferroic is a much bigger insurmountable obstacle. Here we report the discovery of an unconventional 2D vdW multiferroic with out-of-plane ferroelectric polarization and long-range magnetic orders in trilayer NiI2 device from 10 K to 295 K. The evolutions of magnetic domains with magnetic field, and the evolutions between ferroelectric and antiferroelectric phase have been unambiguously observed. More significantly, we realize a robust mutual control of magnetism and ferroelectricity at room temperature. The magnetic domains are manipulated by a small voltage ranging from 1 V to 6 V at 0 T and 295 K. This work opens opportunities for exploring multiferroic physics at the limit of few atomic layers. △ Less

Submitted 23 February, 2024; v1 submitted 24 November, 2023; originally announced November 2023.

Comments: 4 figures

MSC Class: 14J60 ACM Class: F.2.2; I.2.7

Spin-resolved imaging of atomic-scale helimagnetism in monolayer NiI2

Authors: Mao-Peng Miao, Nanshu Liu, Wen-Hao Zhang, Dao-Bo Wang, Wei Ji, Ying-Shuang Fu

Abstract: Identifying intrinsic noncollinear magnetic order in monolayer van der Waals (vdW) crystals is highly desirable for understanding the delicate magnetic interactions at reduced spatial constraints and miniaturized spintronic applications, but remains elusive in experiments. Here, we achieved spin-resolved imaging of helimagnetism at atomic scale in monolayer NiI2 crystals, that were grown on graphe… ▽ More Identifying intrinsic noncollinear magnetic order in monolayer van der Waals (vdW) crystals is highly desirable for understanding the delicate magnetic interactions at reduced spatial constraints and miniaturized spintronic applications, but remains elusive in experiments. Here, we achieved spin-resolved imaging of helimagnetism at atomic scale in monolayer NiI2 crystals, that were grown on graphene-covered SiC(0001) substrate, using spin-polarized scanning tunneling microscopy. Our experiments identify the existence of a spin spiral state with canted plane in monolayer NiI2. The spin modulation Q vector of the spin spiral is determined as (0.2203, 0, 0), which is different from its bulk value or its in-plane projection, but agrees well with our first principles calculations. The spin spiral surprisingly indicates collective spin switching behavior under magnetic field, whose origin is ascribed to the incommensurability between the spin spiral and the crystal lattice. Our work unambiguously identifies the helimagnetic state in monolayer NiI2, paving the way for illuminating its expected type-II multiferroic order and developing spintronic devices based on vdW magnets. △ Less

Submitted 28 September, 2023; originally announced September 2023.

Comments: 22 pages, 4 figures

Amorphous shear bands in crystalline materials as drivers of plasticity

Authors: Xuanxin Hu, Nuohao Liu, Vrishank Jambur, Siamak Attarian, Ranran Su, Hongliang Zhang, Jianqi Xi, Hubin Luo, John Perepezko, Izabela Szlufarska

Abstract: Traditionally, the formation of amorphous shear bands (SBs) in crystalline materials has been undesirable, because SBs can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently SBs were found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here, we have discovered trends… ▽ More Traditionally, the formation of amorphous shear bands (SBs) in crystalline materials has been undesirable, because SBs can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently SBs were found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here, we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit SB deformation, and by varying the composition, we were able to switch from ductile to brittle behavior. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing toughness of nominally brittle materials. △ Less

Submitted 7 August, 2023; originally announced August 2023.

Journal ref: Nature Materials (2023): 1-7

Manipulate Quantum Emission by Interface States between Multi-component Moiré Lattice and Metasurface

Authors: Z. N. Liu, X. Q. Zhao, Y. L. Zhao, S. N. Zhu, H. Liu

Abstract: In recent years, moiré lattice has become a hot topic and inspired the research upsurge of moiré lattice. In this work, we propose a method of constructing a multi-composite moiré lattice, which is composed of over three periodic component structures. Moreover, we propose the moiré lattice-metasurface structure, which can realize the multi-wavelength interface states between these kinds of moiré l… ▽ More In recent years, moiré lattice has become a hot topic and inspired the research upsurge of moiré lattice. In this work, we propose a method of constructing a multi-composite moiré lattice, which is composed of over three periodic component structures. Moreover, we propose the moiré lattice-metasurface structure, which can realize the multi-wavelength interface states between these kinds of moiré lattices and metasurfaces. The wavelength, polarization, and number of moiré interface states can be manipulated flexibly, with anisotropic metasurfaces. These multi-wavelength interface states are employed to enhance quantum emission (QE) and over 20 times QE efficiency can be obtained. △ Less

Submitted 17 July, 2023; originally announced July 2023.

Comments: 26 pages, 5 figures

Journal ref: Physical Review A 108, 013502 (2023)

Designing a Transition Photonic Band with a Synthetic Moire Sphere

Authors: Z. N. Liu, X. Q. Zhao, J. Yao, C. Zhang, J. L. Xu, S. N. Zhu, H. Liu

Abstract: In recent years, twisted bilayer graphene has become a hot topic and inspired the research upsurge of photonic moiré lattice. Here, we designed a photonic moiré superlattice with two synthetic twist angles and constructed a synthetic moiré sphere based on these two angles. Thus, we have more degrees of freedom to design the band structure flexibly. A type of transition photonic bands (TPBs) is obt… ▽ More In recent years, twisted bilayer graphene has become a hot topic and inspired the research upsurge of photonic moiré lattice. Here, we designed a photonic moiré superlattice with two synthetic twist angles and constructed a synthetic moiré sphere based on these two angles. Thus, we have more degrees of freedom to design the band structure flexibly. A type of transition photonic bands (TPBs) is obtained in such a moiré superlattice. We investigate the influence of two twist angles on TPBs and find a series of magic angle pairs with optimal band compression of TPB. The interesting optical properties of TPBs are experimentally demonstrated, including pulse delay, nonlinear optical enhancement, and pulse width compression. Our work introduces a new path to explore multi-twist angles moiré superlattices and reveals that the designed photonic moiré superlattice based on moiré spheres has broad application prospects including optical signal processing, nonlinear optics processes ,and other light-matter interactions. △ Less

Submitted 15 May, 2023; originally announced May 2023.

Journal ref: Phys. Rev. Applied 19, 044054 (2023)

Domain Agnostic Fourier Neural Operators

Authors: Ning Liu, Siavash Jafarzadeh, Yue Yu

Abstract: Fourier neural operators (FNOs) can learn highly nonlinear mappings between function spaces, and have recently become a popular tool for learning responses of complex physical systems. However, to achieve good accuracy and efficiency, FNOs rely on the Fast Fourier transform (FFT), which is restricted to modeling problems on rectangular domains. To lift such a restriction and permit FFT on irregula… ▽ More Fourier neural operators (FNOs) can learn highly nonlinear mappings between function spaces, and have recently become a popular tool for learning responses of complex physical systems. However, to achieve good accuracy and efficiency, FNOs rely on the Fast Fourier transform (FFT), which is restricted to modeling problems on rectangular domains. To lift such a restriction and permit FFT on irregular geometries as well as topology changes, we introduce domain agnostic Fourier neural operator (DAFNO), a novel neural operator architecture for learning surrogates with irregular geometries and evolving domains. The key idea is to incorporate a smoothed characteristic function in the integral layer architecture of FNOs, and leverage FFT to achieve rapid computations, in such a way that the geometric information is explicitly encoded in the architecture. In our empirical evaluation, DAFNO has achieved state-of-the-art accuracy as compared to baseline neural operator models on two benchmark datasets of material modeling and airfoil simulation. To further demonstrate the capability and generalizability of DAFNO in handling complex domains with topology changes, we consider a brittle material fracture evolution problem. With only one training crack simulation sample, DAFNO has achieved generalizability to unseen loading scenarios and substantially different crack patterns from the trained scenario. Our code and data accompanying this paper are available at https://github.com/ningliu-iga/DAFNO. △ Less

Submitted 28 October, 2023; v1 submitted 30 April, 2023; originally announced May 2023.

A Framework for Ductility in Metallic Glasses

Authors: Sungwoo Sohn, Naijia Liu, Geun Hee Yoo, Aya Ochiai, Jade Chen, Callie Levitt, Guannan Liu, Samuel Charles Schroers, Ethen Lund, Eun Soo Park, Jan Schroers

Abstract: The understanding and quantification of ductility in crystalline metals, which has led to their widespread and effective usage as a structural material, is lacking in metallic glasses (MGs). Here, we introduce such a framework for ductility. This very practical framework is based on a MGs ability to support stable shear band growth, quantified in a stress gradient, gradSDB, which we measure and ca… ▽ More The understanding and quantification of ductility in crystalline metals, which has led to their widespread and effective usage as a structural material, is lacking in metallic glasses (MGs). Here, we introduce such a framework for ductility. This very practical framework is based on a MGs ability to support stable shear band growth, quantified in a stress gradient, gradSDB, which we measure and calculate for a range of MGs. Whether a MG behaves ductile or brittle in an application is determined by the comparison between gradsDB the applied stress field gradient, gradsapp. If gradsDB > gradsapp, the MG will behave brittle, if gradsDB < gradsapp, the MG will behave ductile, and gradsapp - gradsDB indicates how ductile. This framework can explain observed plastic properties of MGs and their apparent contradicting brittle and ductile characteristics. Looking forward, proposed framework provides the constitutive relation to quantitatively model their plastic behavior in any application, a requirement to use MGs as structural materials. △ Less

Submitted 15 April, 2023; originally announced April 2023.

Layer-number-dependent spin Hall effects in transition metal monocarbides $M_{2}\rm{C}$ ($M=\rm{V}, \rm{Nb}, \rm{Ta}$)

Authors: Xi Zuo, Yulin Feng, Na Liu, Bing Huang, Desheng Liu, Bin Cui

Abstract: The recent discovery of strong spin Hall effects (SHE) in 2D layered topological semimetals has attracted intensive attention due to its exotic electronic properties and potential applications in spintronic devices. In this paper, we systematically study the topological properties and intrinsic SHE of layered transition metal carbides $M_{2}\rm{C}$ ($M=\rm{V}, \rm{Nb}, \rm{Ta}$). The results show… ▽ More The recent discovery of strong spin Hall effects (SHE) in 2D layered topological semimetals has attracted intensive attention due to its exotic electronic properties and potential applications in spintronic devices. In this paper, we systematically study the topological properties and intrinsic SHE of layered transition metal carbides $M_{2}\rm{C}$ ($M=\rm{V}, \rm{Nb}, \rm{Ta}$). The results show that both bulk and monolayer $M_{2}\rm{C}$ have symmetry-protected nodal points (NPs) and lines (NLs) originating from the $d$ band crossing near the Fermi level ($E_F$). The inclusion of SOC breaks the degeneracy of NLs and NPs, contributing to large spin Hall conductivity (SHC) up to $\sim$1100 and $\sim$200 $(\hbar / e)(Ω\mathrm{cm})^{-1}$ for bulk and monolayer Ta$_{2}$C, respectively. Remarkably, we find that magnitude of SHC exhibits a significant enhancement by increasing the layer number. For eight-layer Ta$_{2}$C, the maximum value of SHC can reach up to $\sim$600 $(\hbar / e)(Ω\mathrm{cm})^{-1}$, comparable to many reported 3D topological materials. Analysis of spin Berry curvature reveals that the large SHC originates from layer-number-dependent nodal line structure near the $E_F$, in which the repeated crossover between valence and conduction bands creates large amounts of NPs along the $Γ\rm{-K}$ route. Our findings not only provide a new platform for experimental research of low-dimensional SHE, but also suggest an effective way of realizing giant SHE by controlling layer thickness. △ Less

Submitted 3 January, 2023; originally announced January 2023.

Comments: 13 pages, 5 figures; references added

Competing multiferroic phases in monolayer and few-layer NiI$_{2}$

Authors: Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, Wei Ji

Abstract: A recent experiment reported type-II multiferroicity in monolayer (ML) NiI$_{2}$ based on a presumed spiral magnetic configuration (Spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI$_{2}$ breaks its C$_{3}$ symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinemen… ▽ More A recent experiment reported type-II multiferroicity in monolayer (ML) NiI$_{2}$ based on a presumed spiral magnetic configuration (Spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI$_{2}$ breaks its C$_{3}$ symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the C$_{3}$ symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order (Spiral-$IV^X$), while 2L shows a different order (Spiral-$V^Y$) and Spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, Spiral-B+FE, Spiral-$IV^X$ +FE, Spiral-$V^Y$+FE, and an anti-multiferroic AABB-AFM+AFE one, show layer-thickness-dependent and geometry-dependent dominance, ascribed to competitions among thickness-dependent Kitaev, biquadratic, and Heisenberg spin-exchange interactions and single-ion magnetic anisotropy. Our theoretical results clarify the debate on the multiferroicity of ML NiI$_{2}$ and shed light on the role of layer-stacking-induced changes in noncollinear spin-exchange interactions and magnetic anisotropy in thickness-dependent magnetism. △ Less

Submitted 3 June, 2024; v1 submitted 25 November, 2022; originally announced November 2022.

Comments: 48 pages, 21 figures

Journal ref: Physical Review B 109, 195422 (2024)

Polarons in Binary Bose-Einstein Condensates

Authors: Ning Liu, Z. C. Tu

Abstract: Bose polarons are quasiparticles formed through the interaction between impurities and Bose-Einstein condensates. In this paper, we derive an effective Fröhlich Hamiltonian using the generalized Bogoliubov transformation. The effective Fröhlich Hamiltonian encompasses two types of effective interactions: impurity-density (ID) coupling and impurity-spin (IS) coupling. Furthermore, we employ the Lee… ▽ More Bose polarons are quasiparticles formed through the interaction between impurities and Bose-Einstein condensates. In this paper, we derive an effective Fröhlich Hamiltonian using the generalized Bogoliubov transformation. The effective Fröhlich Hamiltonian encompasses two types of effective interactions: impurity-density (ID) coupling and impurity-spin (IS) coupling. Furthermore, we employ the Lee-Low-Pines variational approach to investigate the relevant properties of Bose polarons induced by the ID and IS coupling. These properties include the ground state energy, effective mass, and average number of virtual phonons. Our findings reveal that the contribution resulting from IS couplings to the ground energy decreases to zero near the miscible-immiscible boundary. Additionally, the increase of the IS coupling induces a greater number of virtual phonons, impeding the movement of impurities and leading to a significant increase in the effective mass of Bose polarons. △ Less

Submitted 24 August, 2023; v1 submitted 27 June, 2022; originally announced June 2022.

Comments: 24 pages, 8 figures

Report number: 093101

Journal ref: J. Stat. Mech. (2023) 093101

Exchange field enhanced upper critical field of the superconductivity in compressed antiferromagnetic EuTe2

Authors: Hualei Sun, Liang Qiu, Yifeng Han, Yunwei Zhang, Weiliang Wang, Chaoxin Huang, Naitian Liu, Mengwu Huo, Lisi Li, Hui Liu, Zengjia Liu, Peng Cheng, Hongxia Zhang, Hongliang Wang, Lijie Hao, Man-Rong Li, Dao-Xin Yao, Yusheng Hou, Pengcheng Dai, Meng Wang

Abstract: We report high pressure studies on the C-type antiferromagnetic semiconductor EuTe2 up to 36.0 GPa. A structural transition from the I4/mcm to C2/m space group is identified at ~16 GPa. Superconductivity is discovered above ~5 GPa in both the I4/mcm and C2/m space groups. In the low-pressure phase (< 16 GPa), the antiferromagnetic transition temperature is enhanced with increasing pressure due to… ▽ More We report high pressure studies on the C-type antiferromagnetic semiconductor EuTe2 up to 36.0 GPa. A structural transition from the I4/mcm to C2/m space group is identified at ~16 GPa. Superconductivity is discovered above ~5 GPa in both the I4/mcm and C2/m space groups. In the low-pressure phase (< 16 GPa), the antiferromagnetic transition temperature is enhanced with increasing pressure due to the enhanced magnetic exchange interactions. Magnetoresistance measurements indicate an interplay between the local moments of Eu2+ and the conduction electrons of Te 5p orbits. The upper critical field of the superconductivity is well above the Pauli limit. Across the structural transition to the high-pressure phase (> 16 GPa), EuTe2 becomes nonmagnetic and the superconducting transition temperature evolves smoothly with the upper critical field below the Pauli limit. Therefore, the high upper critical field of EuTe2 in the low-pressure phase is due to the exchange field compensation effect of the Eu magnetic order and the superconductivity in both structures may arise in the framework of the BCS theory. △ Less

Submitted 6 July, 2022; v1 submitted 14 June, 2022; originally announced June 2022.

Comments: 11 pages,5 figures with 8 pages supplementary

Macroscopic Quantum Tunneling of a Topological Ferromagnet

Authors: Kajetan M. Fijalkowski, Nan Liu, Pankaj Mandal, Steffen Schreyeck, Karl Brunner, Charles Gould, Laurens W. Molenkamp

Abstract: The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall effect[1-3] is a prime example due to its potential for applications in quantum metrology[4, 5] as well as its influence on fundamental research into the underlying topological and magnetic states[6-11] and axion electrodynamics[2, 12-14]. Here, we perform electronic transport studies… ▽ More The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall effect[1-3] is a prime example due to its potential for applications in quantum metrology[4, 5] as well as its influence on fundamental research into the underlying topological and magnetic states[6-11] and axion electrodynamics[2, 12-14]. Here, we perform electronic transport studies on a (V,Bi,Sb)2Te3 ferromagnetic topological insulator nanostructure in the quantum anomalous Hall regime. This allows us access to the dynamics of an individual ferromagnetic domain. The volume of the domain is estimated to be about 85 000 nm3, containing some 50 000 vanadium atoms, spread over a macroscopic distance of 115 nm. Telegraph noise resulting from the magnetization fluctuations of this domain is observed in the Hall signal. Careful analysis of the influence of temperature and external magnetic field on the domain switching statistics provides evidence for quantum tunneling of magnetization[15-22] in a macrospin state. This ferromagnetic macrospin is not only the largest magnetic object in which quantum tunneling has been observed, but also the first observation of the effect in a topological state of matter. △ Less

Submitted 8 June, 2022; originally announced June 2022.

Journal ref: Advanced Science 10, 2303165 (2023)

Strain Effect on Air-Stability of Monolayer CrSe2

Authors: Jun Chen, Linwei Zhou, Nanshu Liu, Jingsi Qiao, Xieyu Zhou, Cong Wang, Wei Ji

Abstract: The discovery of two dimensional (2D) magnetic materials has brought great research value for spintronics and data storage devices. However, their air-stability as well as the oxidation mechanism has not been unveiled, which limits their further applications. Here, by first-principles calculations, we carried out a detailed study on the oxidation process of monolayer CrSe2 and biaxial tensile stra… ▽ More The discovery of two dimensional (2D) magnetic materials has brought great research value for spintronics and data storage devices. However, their air-stability as well as the oxidation mechanism has not been unveiled, which limits their further applications. Here, by first-principles calculations, we carried out a detailed study on the oxidation process of monolayer CrSe2 and biaxial tensile strain effect. We found dissociation process of O2 on pristine CrSe2 sheet is an endothermic reaction with a reaction energy barrier of 0.53 eV, indicating its thermodynamics stability. However, such a process becomes exothermic under a biaxial tensile strain reaching 1%, accompanying with a decreased reaction barrier, leading to reduced stability. These results manifest that in-plane strain plays a significant role in modifying air-stability in CrSe2 and shed considerable light on searching appropriate substrate to stabilize 2D magnetic materials. △ Less

Submitted 4 April, 2022; v1 submitted 4 April, 2022; originally announced April 2022.

Antiferromagnetic order in MnBi2Te4 films grown on Si(111) by molecular beam epitaxy

Authors: N. Liu, S. Schreyeck, K. M. Fijalkowski, M. Kamp, K. Brunner, C. Gould, L. W. Molenkamp

Abstract: MnBi2Te4 has recently been predicted and shown to be a magnetic topological insulator with intrinsic antiferromagnetic order. However, it remains a challenge to grow stoichiometric MnBi2Te4 films by molecular beam epitaxy (MBE) and to observe pure antiferromagnetic order by magnetometry. We report on a detailed study of MnBi2Te4 films grown on Si(111) by MBE with elemental sources. Films of about… ▽ More MnBi2Te4 has recently been predicted and shown to be a magnetic topological insulator with intrinsic antiferromagnetic order. However, it remains a challenge to grow stoichiometric MnBi2Te4 films by molecular beam epitaxy (MBE) and to observe pure antiferromagnetic order by magnetometry. We report on a detailed study of MnBi2Te4 films grown on Si(111) by MBE with elemental sources. Films of about 100 nm thickness are analyzed in stoichiometric, structural, magnetic and magnetotransport properties with high accuracy. High-quality MnBi2Te4 films with nearly perfect septuple-layer structure are realized and structural defects typical for epitaxial van-der-Waals layers are analyzed. The films reveal antiferromagnetic order with a Neel temperature of 19 K, a spin-flop transition at a magnetic field of 2.5 T and a resistivity of 1.6 mOhm cm. These values are comparable to that of bulk MnBi2Te4 crystals. Our results provide an important basis for realizing and identifying single-phase MnBi2Te4 films with antiferromagnetic order grown by MBE. △ Less

Submitted 27 November, 2021; originally announced November 2021.

Two-dimensional ZIF-L nanosheets as high performance non-enzymatic glucose sensor

Authors: Nana Liu, Ningyan Cheng, Chengwu Yang, Weichang Hao, Yi Du

Abstract: An effective biosensor based on two-dimensional (2D) Co-ZIF-L nanosheets for sensitive electrochemical non-enzymatic glucose detection is developed, which exhibits high electrocalalytic activities towards glucose due to the ordered porous structure as well as ultrahigh specific surface area. The fabricated Co-ZIF-L nanosheets electrodes present an outstanding performance with higher sensitivity of… ▽ More An effective biosensor based on two-dimensional (2D) Co-ZIF-L nanosheets for sensitive electrochemical non-enzymatic glucose detection is developed, which exhibits high electrocalalytic activities towards glucose due to the ordered porous structure as well as ultrahigh specific surface area. The fabricated Co-ZIF-L nanosheets electrodes present an outstanding performance with higher sensitivity of 769.5 *10$^{-6}$ A mM$^{-1}$ cm$^{-2}$ and lower detect limit of 90.4 nM, while the constructed 3D ZIF-67 nanoparticles electrodes show a weaker sensitivity of 697.4 *10$^{-6}$ A mM$^{-1}$ cm$^{-2}$ and a limited detection range from 2 *10$^{-6}$ M to 414 *10$^{-6}$ M. Furthermore, the Co-ZIF-L based non-enzymatic glucose biosensors possess an acceptable selectivity, long-term stability as well as reproducibility. This work may offer a new approach to develop 2D ZIF nanosheets as a potential candidate in electrochemical biosensors. △ Less

Submitted 14 February, 2022; v1 submitted 9 October, 2021; originally announced October 2021.

Comments: do not want to publish

Quantum anomalous Hall edge channels survive up to the Curie temperature

Authors: Kajetan M. Fijalkowski, Nan Liu, Pankaj Mandal, Steffen Schreyeck, Karl Brunner, Charles Gould, Laurens W. Molenkamp

Abstract: Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a car… ▽ More Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials. △ Less

Submitted 3 September, 2021; originally announced September 2021.

Journal ref: Nature Communications 12, 5599 (2021)

Any Axion Insulator Must be a Bulk Three-Dimensional Topological Insulator

Authors: K. M. Fijalkowski, N. Liu, M. Hartl, M. Winnerlein, P. Mandal, A. Coschizza, A. Fothergill, S. Grauer, S. Schreyeck, K. Brunner, M. Greiter, R. Thomale, C. Gould, L. W. Molenkamp

Abstract: In recent attempts to observe axion electrodynamics, much effort has focused on trilayer heterostructures of magnetic topological insulators, and in particular on the examination of a so-called zero Hall plateau, which has misguidedly been overstated as direct evidence of an axion insulator state. We investigate the general notion of axion insulators, which by definition must contain a nontrivial… ▽ More In recent attempts to observe axion electrodynamics, much effort has focused on trilayer heterostructures of magnetic topological insulators, and in particular on the examination of a so-called zero Hall plateau, which has misguidedly been overstated as direct evidence of an axion insulator state. We investigate the general notion of axion insulators, which by definition must contain a nontrivial volume to host the axion term. We conduct a detailed magneto-transport analysis of Chern insulators comprised of a single magnetic topological insulator layer of varying thickness as well as trilayer structures, for samples optimized to yield a perfectly quantized anomalous Hall effect. Our analysis gives evidence for a topological magneto-electric effect quantized in units of e$^2$/2h, allowing us to identify signatures of axion electrodynamics. Our observations may provide direct experimental access to electrodynamic properties of the universe beyond the traditional Maxwell equations, and challenge the hitherto proclaimed exclusive link between the observation of a zero Hall plateau and an axion insulator. △ Less

Submitted 20 May, 2021; originally announced May 2021.

Journal ref: Phys. Rev. B 103, 235111 (2021)

Superconductivity in Layered van der Waals Hydrogenated Germanene at High Pressure

Authors: Yilian Xi, Xiaoling Jing, Zhongfei Xu, Nana Liu, Yani Liu, Miao-Ling Lin, Ming Yang, Ying Sun, Jincheng Zhuang, Xun Xu, Weichang Hao, Yanchun Li, Xiaodong Li, Ping-Heng Tan, Quanjun Li, Bingbing Liu, Shi Xue Dou, Yi Du

Abstract: Structural and superconducting transitions of layered van der Waals (vdW) hydrogenated germanene (GeH) were observed under high-pressure compression and decompression processes. GeH possesses a superconducting transition at critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa while superconductivity remains. An abnormally increased Tc up to 6.1… ▽ More Structural and superconducting transitions of layered van der Waals (vdW) hydrogenated germanene (GeH) were observed under high-pressure compression and decompression processes. GeH possesses a superconducting transition at critical temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition occurs at 16.80 GPa while superconductivity remains. An abnormally increased Tc up to 6.1 K has been observed in the decompression process while the GeH remained amorphous. Thorough in-situ high-pressure synchrotron X-ray diffraction and in-situ high-pressure Raman spectroscopy with the density functional theory simulations suggest that the superconductivity of GeH should be attributed to the increased density of states at the Fermi level as well as the enhanced electron-phonon coupling effect under high pressure. The decompression-driven superconductivity enhancement arises from pressure-induced phonon softening related to an in-plane Ge-Ge phonon mode. As an amorphous metal hydride superconductor, GeH provides a platform to study amorphous hydride superconductivity in layered vdW materials. △ Less

Submitted 3 June, 2021; v1 submitted 9 May, 2021; originally announced May 2021.

Comments: 22 Pages, 4 Figures

DNA origami

Authors: Swarup Dey, Chunhai Fan, Kurt V. Gothelf, Jiang Li, Chenxiang Lin, Longfei Liu, Na Liu, Minke A. D. Nijenhuis, Barbara Sacca, Friedrich C. Simmel, Hao Yan, Pengfei Zhan

Abstract: Biological materials are self-assembled with near-atomic precision in living cells, whereas synthetic 3D structures generally lack such precision and controllability. Recently, DNA nanotechnology, especially DNA origami technology, has been useful in the bottom-up fabrication of well-defined nanostructures ranging from tens of nanometres to sub-micrometres. In this Primer, we summarize the methodo… ▽ More Biological materials are self-assembled with near-atomic precision in living cells, whereas synthetic 3D structures generally lack such precision and controllability. Recently, DNA nanotechnology, especially DNA origami technology, has been useful in the bottom-up fabrication of well-defined nanostructures ranging from tens of nanometres to sub-micrometres. In this Primer, we summarize the methodologies of DNA origami technology, including origami design, synthesis, functionalization and characterization. We highlight applications of origami structures in nanofabrication, nanophotonics and nanoelectronics, catalysis, computation, molecular machines, bioimaging, drug delivery and biophysics. We identify challenges for the field, including size limits, stability issues and the scale of production, and discuss their possible solutions. We further provide an outlook on next-generation DNA origami techniques that will allow in vivo synthesis and multiscale manufacturing. △ Less

Submitted 30 April, 2021; originally announced April 2021.

Journal ref: Nature Reviews Methods Primers 1, Article number: 13 (2021)

Stabilizing $γ$-MgH$_2$ at Nanotwins in Mechanically Constrained Nanoparticles

Authors: Jochen A. Kammerer, Xiaoyang Duan, Frank Neubrech, Rasmus R. Schröder, Na Liu, Martin Pfannmöller

Abstract: Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH$_2$ system a prime candidate for solid state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable $γ$-MgH$_2$. A nanoparticle (NP) design, where $γ$-MgH… ▽ More Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH$_2$ system a prime candidate for solid state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable $γ$-MgH$_2$. A nanoparticle (NP) design, where $γ$-MgH$_2$ forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed.Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of $β$-MgH$_2$ via (301) $β$ twinning. It is proposed that these twins nucleate $γ$-MgH$_2$ nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle-stable, Mg-based dynamic plasmonic and hydrogen-storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform $γ$-MgH$_2$ during each rehydrogenation △ Less

Submitted 30 April, 2021; originally announced April 2021.

Journal ref: Adv. Mater. 33, 2008259 (2021)