Showing 1–50 of 81 results for author: Fang, M

Evaluating Large Language Models in Scientific Discovery

Authors: Zhangde Song, Jieyu Lu, Yuanqi Du, Botao Yu, Thomas M. Pruyn, Yue Huang, Kehan Guo, Xiuzhe Luo, Yuanhao Qu, Yi Qu, Yinkai Wang, Haorui Wang, Jeff Guo, Jingru Gan, Parshin Shojaee, Di Luo, Andres M Bran, Gen Li, Qiyuan Zhao, Shao-Xiong Lennon Luo, Yuxuan Zhang, Xiang Zou, Wanru Zhao, Yifan F. Zhang, Wucheng Zhang , et al. (31 additional authors not shown)

Abstract: Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain exp… ▽ More Large language models (LLMs) are increasingly applied to scientific research, yet prevailing science benchmarks probe decontextualized knowledge and overlook the iterative reasoning, hypothesis generation, and observation interpretation that drive scientific discovery. We introduce a scenario-grounded benchmark that evaluates LLMs across biology, chemistry, materials, and physics, where domain experts define research projects of genuine interest and decompose them into modular research scenarios from which vetted questions are sampled. The framework assesses models at two levels: (i) question-level accuracy on scenario-tied items and (ii) project-level performance, where models must propose testable hypotheses, design simulations or experiments, and interpret results. Applying this two-phase scientific discovery evaluation (SDE) framework to state-of-the-art LLMs reveals a consistent performance gap relative to general science benchmarks, diminishing return of scaling up model sizes and reasoning, and systematic weaknesses shared across top-tier models from different providers. Large performance variation in research scenarios leads to changing choices of the best performing model on scientific discovery projects evaluated, suggesting all current LLMs are distant to general scientific "superintelligence". Nevertheless, LLMs already demonstrate promise in a great variety of scientific discovery projects, including cases where constituent scenario scores are low, highlighting the role of guided exploration and serendipity in discovery. This SDE framework offers a reproducible benchmark for discovery-relevant evaluation of LLMs and charts practical paths to advance their development toward scientific discovery. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Néel vector and Rashba SOC effects on RKKY interaction in 2D $d$-wave altermagnets

Authors: Hou-Jian Duan, Miao-Sheng Fang, Ming-Xun Deng, Ruiqiang Wang

Abstract: Altermagnets possess two key features: non-relativistic alternating spin splitting (i.e., altermagnetism) and a material-dependent Néel vector. The former naturally coexists with Rashba spin-orbit coupling (SOC) in real materials on substrates, prompting the question of how SOC affects the magnetic properties of altermagnets. The latter is crucial for information storage, making it essential to de… ▽ More Altermagnets possess two key features: non-relativistic alternating spin splitting (i.e., altermagnetism) and a material-dependent Néel vector. The former naturally coexists with Rashba spin-orbit coupling (SOC) in real materials on substrates, prompting the question of how SOC affects the magnetic properties of altermagnets. The latter is crucial for information storage, making it essential to determine its orientation. To address these issues, we study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in two-dimensional (2D) $d$-wave altermagnets by independently varying the Néel vector orientation and the SOC strength. Our results demonstrate that the Néel vector orientation can be accurately determined via the Ising term without SOC, or qualitatively inferred via the DM terms with SOC. Moreover, we observe a novel Dzyaloshinskii-Moriya (DM) component distinct from previous reports, whose emergence is attributed to the synergy between altermagnetism and SOC. Additionally, through tuning SOC strength, we reveal the evolution of the RKKY spin models governed by five distinct mechanisms: the spin model may be determined solely by altermagnetism, solely by SOC, or solely by the kinetic term; alternatively, altermagnetism may coincidentally yield the same moderately anisotropic spin model as SOC, or compete with SOC to produce a spin model with maximal anisotropy. Beyond SOC strength, which mechanism operates also relies on the Néel vector orientation and impurity configurations. All results are numerically verified. These findings -- which were inaccessible in prior studies due to the limitations of first-order SOC expansion and fixed Néel vector orientation -- provide important new insights into the magnetic properties of altermagnets. △ Less

Submitted 30 September, 2025; originally announced September 2025.

Topological nontrivial berry phase in altermagnet CrSb

Authors: Jianhua Du, Xin Peng, Yuzhi Wang, Shengnan Zhang, Yuran Sun, Chunxiang Wu, Tingyu Zhou, Le Liu, Hangdong Wang, Jinhu Yang, Bin Chen, Chuanying Xi, Zhiwei Jiao, Quansheng Wu, Minghu Fang

Abstract: The study of topological properties in magnetic materials has long been one of the forefront research areas in condensed matter physics. CrSb, as a prototypical candidate material for altermagnetism, has attracted significant attention due to its unique magnetic properties. This system provides a novel platform for exploring the intrinsic relationship between altermagnetic order and exotic topolog… ▽ More The study of topological properties in magnetic materials has long been one of the forefront research areas in condensed matter physics. CrSb, as a prototypical candidate material for altermagnetism, has attracted significant attention due to its unique magnetic properties. This system provides a novel platform for exploring the intrinsic relationship between altermagnetic order and exotic topological states. In this study, we combine systematic electrical transport experiments with first-principles calculations to investigate the possible realization mechanisms of topological semimetal states in CrSb and their manifestations in quantum transport phenomena. Our high field magneto-transport measurements reveal that the magnetoresistance of CrSb exhibits no sign of saturation up to 35 T, following a distinct power-law dependence with an exponent of 1.48. The nonlinear Hall resistivity further indicates a multiband charge transport mechanism. Under high magnetic fields, we observe pronounced Shubnikov-de Haas (SdH) quantum oscillations and discernible Zeeman-effect-induced band splitting at 1.6 K. Systematic Fermi surface and band calculations combined with Berry phase analysis confirm the nontrivial topological character of this material (with a Berry phase approaching π). These findings not only provide crucial experimental evidence for understanding the electronic structure of CrSb, but also establish an important foundation for investigating topological quantum states in altermagnets. △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 10 pages, 5 figures

Topological Dissipation as the Missing Link in Multiscale Polymer Dynamics

Authors: Xu-Ze Zhang, Rui Shi, Ming-Ji Fang, Zhong-Yuan Lu, Hu-Jun Qian

Abstract: We identify topological dissipation -- momentum transport along the polymer backbone with Ising-type exponential decay ($\sim \exp(-Δn/n_\text{d})$) -- as the missing link connecting atomistic and mesoscale dynamics. Simulations of four polymers reveal that dynamical correlation length $n_\text{d} \approx n_\text{k}/3$ (Kuhn length $n_\text{k}$), enabling a coarse-grained framework that \emph{expl… ▽ More We identify topological dissipation -- momentum transport along the polymer backbone with Ising-type exponential decay ($\sim \exp(-Δn/n_\text{d})$) -- as the missing link connecting atomistic and mesoscale dynamics. Simulations of four polymers reveal that dynamical correlation length $n_\text{d} \approx n_\text{k}/3$ (Kuhn length $n_\text{k}$), enabling a coarse-grained framework that \emph{explicitly separates} topological (intrachain) and spatial (interchain) dissipation channels without temporal memory kernels. The approach quantitatively reproduces dynamics from segmental relaxation to chain diffusion, solving the long-standing memory preservation challenge in Markovian coarse-graining. Our results establish topology-mediated dissipation as a key mechanism for polymer dynamics. △ Less

Submitted 17 August, 2025; originally announced August 2025.

Comments: 5 pages, 5 figures

Scaling Behavior of Magnetoresistance and Hall Resistivity in Altermagnet CrSb

Authors: Xin Peng, Yuzhi Wang, Shengnan Zhang, Yi Zhou, Yuran Sun, Yahui Su, Chunxiang Wu, Tingyu Zhou, Le Liu, Hangdong Wang, Jinhu Yang, Bin Chen, Zhong Fang, Jianhua Du, Zhiwei Jiao, Quansheng Wu, Minghu Fang

Abstract: The discovery of altermagnet (AM) marks a significant advancement in magnetic materials, combining characteristics of both ferromagnetism and antiferromagnetism. In this Letter, we focus on CrSb, which has been verified to be an AM and to exhibit substantial spin splitting near the Fermi level. After successfully growing high-quality CrSb single crystals, we performed comprehensive magnetization,… ▽ More The discovery of altermagnet (AM) marks a significant advancement in magnetic materials, combining characteristics of both ferromagnetism and antiferromagnetism. In this Letter, we focus on CrSb, which has been verified to be an AM and to exhibit substantial spin splitting near the Fermi level. After successfully growing high-quality CrSb single crystals, we performed comprehensive magnetization, magnetoresistance (MR), and Hall resistivity measurements, along with the electronic structure, and Fermi surface (FS) calculations, as well as the magneto-transport property numerical simulations. An antiferromagnetic transition occurring at $T_{N}$ = 712 K was reconfirmed. It was found that both experimental MR and Hall resistivity are consistent with the numerical simulation results, and exhibit obvious scaling behavior. The nonlinear Hall resistivity is due to its multi-band structure, rather than an anomalous Hall effect (AHE). Especially, the scaling behavior in Hall resistivity is first observed within an AM material. These findings demonstrate that the magneto-transport properties in CrSb originate from the intrinsic electronic structure and are dominated by the Lorentz force. △ Less

Submitted 16 December, 2024; originally announced December 2024.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 111, 144402(2025)

Universal Scaling Behavior of Transport Properties in Non-Magnetic RuO$_{2}$

Authors: Xin Peng, Zhihao Liu, Shengnan Zhang, Yi Zhou, Yuran Sun, Yahui Su, Chunxiang Wu, Tingyu Zhou, Le Liu, Yazhou Li, Hangdong Wang, Jinhu Yang, Bin Chen, Yuke Li, Chuanying Xi, Jianhua Du, Zhiwei Jiao, Quansheng Wu, Minghu Fang

Abstract: As a prototypical altermagnet, RuO$_{2}$ has been subject to many controversial reports regarding its magnetic ground state and the existence of crystal Hall effects. We obtained high-quality RuO$_{2}$ single crystal with a residual resistivity ratio (RRR = 152), and carefully measured its magnetization, longitudinal resistivity ($ρ_{xx}$) and Hall resistivity ($ρ_{yx}$) up to 35 T magnetic field.… ▽ More As a prototypical altermagnet, RuO$_{2}$ has been subject to many controversial reports regarding its magnetic ground state and the existence of crystal Hall effects. We obtained high-quality RuO$_{2}$ single crystal with a residual resistivity ratio (RRR = 152), and carefully measured its magnetization, longitudinal resistivity ($ρ_{xx}$) and Hall resistivity ($ρ_{yx}$) up to 35 T magnetic field. We also calculated its electronic band, Fermi surface, and conducted numerical simulations for its transport properties. It was found that no magnetic transition occurs below 400 K, and that all the transport properties are consistent with the numerical simulations results, indicating that the magnetotransport properties originate from the intrinsic electronic structures and are dominated by the Lorentz force. Particularly, no crystal Hall effects were observed in our RuO$_{2}$ samples and both magnetoresistance and Hall resistivity follow scaling behavior. These results demonstrate that RuO$_{2}$ is a typical semimetal, rather than an altermagnet. △ Less

Submitted 16 December, 2024; originally announced December 2024.

Comments: 5 pages, 4 figures

Journal ref: Commun Mater 6, 177 (2025)

Orthogonal Geometry of Magneto-Optical Kerr Effect Enabled by Magnetization Multipole of Berry Curvature

Authors: Haolin Pan, Han Li, Jixiang Huang, Zheng Liu, Mingyue Fang, Yanan Yuan, Daxiang Liu, Xintong Hu, Wenzhi Peng, Zhenguo Liang, Xiao Chang, Zhigao Sheng, Xianzhe Chen, Lingfei Wang, Qian Li, Peng Li, Qian Niu, Yang Gao, Qinghui Yang, Dazhi Hou

Abstract: The Magneto-Optical Kerr Effect (MOKE) is a fundamental tool in magnetometry, pivotal for advancing research in optics, magnetism, and spintronics as a direct probe of magnetization. Traditional MOKE measurements primarily detect the magnetization components parallel to the Poynting vector, which can only access the magnitude but not the direction of the orthogonal component. In this study, we int… ▽ More The Magneto-Optical Kerr Effect (MOKE) is a fundamental tool in magnetometry, pivotal for advancing research in optics, magnetism, and spintronics as a direct probe of magnetization. Traditional MOKE measurements primarily detect the magnetization components parallel to the Poynting vector, which can only access the magnitude but not the direction of the orthogonal component. In this study, we introduce an orthogonal MOKE geometry in which the Kerr signal detects both the magnitude and direction of the magnetization component perpendicular to the Poynting vector. We demonstrate the broad applicability of this orthogonal geometry through the MOKE measurements in cubic ferromagnets and van der Waals ferromagnet. We theoretically show that the orthogonal MOKE geometry is enabled by the multipolar structure of Berry curvature in the magnetization space, which generally induces a Voigt vector orthogonal to the magnetization, thereby accounting for the unique magnetization angle dependence distinct from conventional MOKE. The establishment of the orthogonal MOKE geometry not only introduces a new paradigm for magneto-optical measurements but also provides a framework for exploring the magnetization multipoles of Berry curvature across the electromagnetic spectrum. △ Less

Submitted 19 January, 2025; v1 submitted 12 December, 2024; originally announced December 2024.

Comments: 26 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

Layer Dependent Thermal Transport Properties of One- to Three-Layer Magnetic Fe:MoS2

Authors: Elham Easy, Mengqi Fang, Mingxing Li, Eui-Hyeok Yang, Xian Zhang

Abstract: Two-Dimensional (2D) transition metal dichalcogenides (TMDs) have been the subject of extensive attention thanks to their unique properties and atomically thin structure. Because of its unprecedented room-temperature magnetic properties, iron-doped MoS2 (Fe:MoS2) is considered the next-generation quantum and magnetic material. It is essential to understand Fe:MoS2's thermal behavior since temperat… ▽ More Two-Dimensional (2D) transition metal dichalcogenides (TMDs) have been the subject of extensive attention thanks to their unique properties and atomically thin structure. Because of its unprecedented room-temperature magnetic properties, iron-doped MoS2 (Fe:MoS2) is considered the next-generation quantum and magnetic material. It is essential to understand Fe:MoS2's thermal behavior since temperature and thermal load/activation are crucial for their magnetic properties and the current nano and quantum devices have been severely limited by thermal management. In this work, Fe:MoS2 is synthesized by doping Fe atoms into MoS2 using the chemical vapor deposition (CVD) synthesis and a refined version of opto-thermal Raman technique is used to study the thermal transport properties of Fe:MoS2 in the forms of single (1L), bilayer (2L), and tri-layer (3L). In the Opto-thermal Raman technique, a laser is focused on the center of a thin film and used to measure the peak position of a Raman-active mode. The lateral thermal conductivity of 1-3L of Fe:MoS2 and the interfacial thermal conductance between Fe:MoS2 and the substrate were obtained by analyzing the temperature-dependent and power-dependent Raman measurement, laser power absorption coefficient, and laser spot sizes. We also characterized Fe:MoS2's thermal transport at high temperature, and calculated Fe:MoS2's thermal transport by density theory function. These findings will shed light on the thermal management and thermoelectric designs for Fe:MoS2 based nano and quantum electronic devices. △ Less

Submitted 8 December, 2024; originally announced December 2024.

Reduction in Thermal Conductivity of Monolayer MoS2 by Large Mechanical Strains for Efficient Thermal Management

Authors: Jun Liu, Mengqi Fang, Eui-Hyeok Yang, Xian Zhang

Abstract: Two dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDC) have received extensive research interests and investigations in the past decade. In this research, we report the first experimental measurement of the in plane thermal conductivity of MoS2 monolayer under a large mechanical strain using optothermal Raman technique. This measurement technique is direct with… ▽ More Two dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDC) have received extensive research interests and investigations in the past decade. In this research, we report the first experimental measurement of the in plane thermal conductivity of MoS2 monolayer under a large mechanical strain using optothermal Raman technique. This measurement technique is direct without additional processing to the material, and MoS2's absorption coefficient is discovered during the measurement process to further increase this technique's precision. Tunable uniaxial tensile strains are applied on the MoS2 monolayer by stretching a flexible substrate it sits on. Experimental results demonstrate that, the thermal conductivity is substantially suppressed by tensile strains: under the tensile strain of 6.3%, the thermal conductivity of the MoS2 monolayer drops approximately by 62%. A serious of thermal transport properties at a group of mechanical strains are also reported, presenting a strain dependent trend. It is the first and original study of 2D materials' thermal transport properties under a large mechanical strain, and provides important information that the thermal transport of MoS2 will significantly decrease at a large mechanical strain. This finding provides the key information for flexible and wearable electronics thermal management and designs. △ Less

Submitted 8 December, 2024; originally announced December 2024.

Exciton-activated effective phonon magnetic moment in monolayer MoS2

Authors: Chunli Tang, Gaihua Ye, Cynthia Nnokwe, Mengqi Fang, Li Xiang, Masoud Mahjouri-Samani, Dmitry Smirnov, Eui-Hyeok Yang, Tingting Wang, Lifa Zhang, Rui He, Wencan Jin

Abstract: Optical excitation of chiral phonons plays a vital role in studying the phonon-driven magnetic phenomena in solids. Transition metal dichalcogenides host chiral phonons at high symmetry points of the Brillouin zone, providing an ideal platform to explore the interplay between chiral phonons and valley degree of freedom. Here, we investigate the helicity-resolved magneto-Raman response of monolayer… ▽ More Optical excitation of chiral phonons plays a vital role in studying the phonon-driven magnetic phenomena in solids. Transition metal dichalcogenides host chiral phonons at high symmetry points of the Brillouin zone, providing an ideal platform to explore the interplay between chiral phonons and valley degree of freedom. Here, we investigate the helicity-resolved magneto-Raman response of monolayer MoS2 and identify a doubly degenerate Brillouin-zone-center chiral phonon mode at ~270 cm-1. Our wavelength- and temperature-dependent measurements show that this chiral phonon is activated through the resonant excitation of A exciton. Under an out-of-plane magnetic field, the chiral phonon exhibits giant Zeeman splitting, which corresponds to an effective magnetic moment of ~2.5mu_B. Moreover, we carry out theoretical calculations based on the morphic effects in nonmagnetic crystals, which reproduce the linear Zeeman splitting and Raman cross-section of the chiral phonon. Our study provides important insights into lifting the chiral phonon degeneracy in an achiral covalent material, paving a new route to excite and control chiral phonons. △ Less

Submitted 7 April, 2024; v1 submitted 22 March, 2024; originally announced March 2024.

Journal ref: Phys. Rev. B 109, 155426 (2024)

Tensorial properties via the neuroevolution potential framework: Fast simulation of infrared and Raman spectra

Authors: Nan Xu, Petter Rosander, Christian Schäfer, Eric Lindgren, Nicklas Österbacka, Mandi Fang, Wei Chen, Yi He, Zheyong Fan, Paul Erhart

Abstract: Infrared and Raman spectroscopy are widely used for the characterization of gases, liquids, and solids, as the spectra contain a wealth of information concerning in particular the dynamics of these systems. Atomic scale simulations can be used to predict such spectra but are often severely limited due to high computational cost or the need for strong approximations that limit application range and… ▽ More Infrared and Raman spectroscopy are widely used for the characterization of gases, liquids, and solids, as the spectra contain a wealth of information concerning in particular the dynamics of these systems. Atomic scale simulations can be used to predict such spectra but are often severely limited due to high computational cost or the need for strong approximations that limit application range and reliability. Here, we introduce a machine learning (ML) accelerated approach that addresses these shortcomings and provides a significant performance boost in terms of data and computational efficiency compared to earlier ML schemes. To this end, we generalize the neuroevolution potential approach to enable the prediction of rank one and two tensors to obtain the tensorial neuroevolution potential (TNEP) scheme. We apply the resulting framework to construct models for the dipole moment, polarizability, and susceptibility of molecules, liquids, and solids, and show that our approach compares favorably with several ML models from the literature with respect to accuracy and computational efficiency. Finally, we demonstrate the application of the TNEP approach to the prediction of infrared and Raman spectra of liquid water, a molecule (PTAF-), and a prototypical perovskite with strong anharmonicity (BaZrO3). The TNEP approach is implemented in the free and open source software package GPUMD, which makes this methodology readily available to the scientific community. △ Less

Submitted 28 March, 2024; v1 submitted 8 December, 2023; originally announced December 2023.

Comments: 11 pages, 6 figures

Journal ref: Journal of Chemical Theory and Computation 20, 3273 (2024)

Observation of long-range ferromagnetism via anomalous supercurrents in a spin-orbit coupled superconductor

Authors: B. K. Xiang, Y. S. Lin, Q. S. He, J. J. Zhu, B. R. Chen, Y. F. Wang, K. Y. Liang, Z. J. Li, H. X. Yao, C. X. Wu, T. Y. Zhou, M. H. Fang, Y. Lu, I. V. Tokatly, F. S. Bergeret, Y. H. Wang

Abstract: Conventional superconductors naturally disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. In superconductors with strong Rashba spin-orbit coupling, impurity magnetic moments induce supercurrents through the spin-galvanic effect. As a result, long-range ferromagnetic interaction among the impurity moments may be mediated through such an… ▽ More Conventional superconductors naturally disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. In superconductors with strong Rashba spin-orbit coupling, impurity magnetic moments induce supercurrents through the spin-galvanic effect. As a result, long-range ferromagnetic interaction among the impurity moments may be mediated through such anomalous supercurrents in a similar fashion as in itinerant ferromagnets. Fe(Se,Te) is such a superconductor with topological surface bands, previously shown to exhibit quantum anomalous vortices around impurity spins. Here, we take advantage of the flux sensitivity of scanning superconducting quantum interference devices to investigate superconducting Fe(Se,Te) in the regime where supercurrents around impurities overlap. We find homogeneous remanent flux patterns after applying a supercurrent through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by in-plane magnetization, which occur above a current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Similar long-range magnetic orders can be generated by Meissner current under a small out-of-plane magnetic field. The magnetization weakens with increasing temperature and disappears after thermal cycling to above superconducting critical temperature; further suggesting superconductivity is central to establishing and maintaining the magnetic order. These observations demonstrate surface anomalous supercurrents as a mediator for ferromagnetism in a spin-orbit coupled superconductor, which may potentially be utilized for low-power cryogenic memory. △ Less

Submitted 20 July, 2023; v1 submitted 20 July, 2023; originally announced July 2023.

2D Magnetic Semiconductors via Substitutional Doping of Transition Metal Dichalcogenides

Authors: Mengqi Fang, Eui-Hyeok Yang

Abstract: Transition metal dichalcogenides (TMDs) are two-dimensional (2D) materials with remarkable electrical, optical and chemical properties. One promising strategy to tailor TMD properties of TMDs is to create alloys through dopant-induced modification. Dopants can introduce additional states within the bandgap of TMDs, leading to changes in their optical, electronic, and magnetic properties. This pape… ▽ More Transition metal dichalcogenides (TMDs) are two-dimensional (2D) materials with remarkable electrical, optical and chemical properties. One promising strategy to tailor TMD properties of TMDs is to create alloys through dopant-induced modification. Dopants can introduce additional states within the bandgap of TMDs, leading to changes in their optical, electronic, and magnetic properties. This paper overviews chemical vapor deposition (CVD) methods to introduce dopants into TMD monolayers. The advantages and limitations and their impacts on the doped TMDs' structural, electrical, optical, and magnetic properties are discussed. The dopants in TMDs modify the density and type of carriers in the material, thereby influencing the optical properties of the materials. The TMDs' magnetic moment and circular dichroism are also strongly affected by doping, which enhances the magnetic signal in the material. Finally, we highlight the different doping-induced magnetic properties of TMDs, including superexchange-induced ferromagnetism and valley Zeeman shift. Overall, this review paper provides a comprehensive summary of magnetic TMDs synthesized via CVD, which can guide future research on doped TMDs for various applications, such as spintronics, optoelectronics, and magnetic memory devices. △ Less

Submitted 20 March, 2023; originally announced March 2023.

A study of simulating Raman spectra for alkanes with a machine learning-based polarizability model

Authors: Mandi Fang, Shi Tang, Zheyong Fan, Yao Shi, Nan Xu, Yi He

Abstract: Polarizability is closely related to many fundamental characteristics of molecular systems and plays an indispensable role in simulating the Raman spectra. However, the calculations of polarizability for large systems still suffers from the limitations of processing ability of the quantum mechanical (QM) methods. This work assessed and compared the accuracy of the bond polarizability model (BPM) a… ▽ More Polarizability is closely related to many fundamental characteristics of molecular systems and plays an indispensable role in simulating the Raman spectra. However, the calculations of polarizability for large systems still suffers from the limitations of processing ability of the quantum mechanical (QM) methods. This work assessed and compared the accuracy of the bond polarizability model (BPM) and a ML-based atomic polarizability model (AlphaML) in predicting polarizability of alkanes and then also investigated the ability of simulating Raman spectra. We found that the AlphaML has appreciable advantages over the BPM in learning the polarizability in the training data set and predicting polarizability of molecules that configurational differently from training structures. In addition, the BPM has inherent disadvantages in predicting polarizability anisotropy due to many factors including large uncertainties of estimating bond anisotropy, omitting of off-diagonal parameters in the construction of the model. As a result, the BPM has larger errors than the AlphaML in the simulation of anisotropic Raman scattering. Finally, we demonstrated that both the BPM and AlphaML suffer from transference to alkanes larger than those used in the training data sets, but the problem for the AlphaML can be circumvented by exploring more proper training structures. △ Less

Submitted 31 January, 2023; originally announced January 2023.

Comments: 37 pages, 10 figures

Superconductivity in the nodal-line compound La$_3$Pt$_3$Bi$_4$

Authors: Liang Li, Guo-Xiang Zhi, Qinqing Zhu, Chunxiang Wu, Zhihua Yang, Jianhua Du, Jinhu Yang, Bin Chen, Hangdong Wang, Chao Cao, Minghu Fang

Abstract: Owing to the specific topological states in nodal-line semimetals, novel topological superconductivity is expected to emerge in these systems. In this letter, by combination of the first-principles calculations and resistivity, susceptibility and specific heat measurements, we demonstrate that La$_3$Pt$_3$Bi$_4$ is a topologically nontrivial nodal-ring semimetal protected by the gliding-mirror sym… ▽ More Owing to the specific topological states in nodal-line semimetals, novel topological superconductivity is expected to emerge in these systems. In this letter, by combination of the first-principles calculations and resistivity, susceptibility and specific heat measurements, we demonstrate that La$_3$Pt$_3$Bi$_4$ is a topologically nontrivial nodal-ring semimetal protected by the gliding-mirror symmetry even in the presence of spin-orbit coupling. Meanwhile, we discover bulk superconductivity with a transition temperature of $\sim$1.1 K, and an upper critical field of $\sim$0.41 T. These findings demonstrate that La$_3$Pt$_3$Bi$_4$ provides a material platform for studying novel superconductivity in the nodal-ring system. △ Less

Submitted 19 April, 2022; originally announced April 2022.

Journal ref: Phys. Rev. Research 4, L032004 (2022)

Superconductivity in TlBi$_2$ with a large Kadowaki-Woods ratio

Authors: Zhihua Yang, Zhen Yang, Qiping Su, Jianhua Du, Enda Fang, Chuxiang Wu, Jinhu Yang, Bin Chen, Hangdong Wang, Minghu Fang

Abstract: In this article, the superconducting and normal state properties of TlBi$_2$ with the AlB$_2$-type structure were studied by the resistivity, magnetization and specific heat measurements. It was found that bulk superconductivity with $T_{C}$ = 6.2 K emerges in TlBi$_2$, which is a phonon-mediated $s$-wave superconductor with a strong electron-phonon coupling ($λ$$_{ep}$ = 1.38) and a large superco… ▽ More In this article, the superconducting and normal state properties of TlBi$_2$ with the AlB$_2$-type structure were studied by the resistivity, magnetization and specific heat measurements. It was found that bulk superconductivity with $T_{C}$ = 6.2 K emerges in TlBi$_2$, which is a phonon-mediated $s$-wave superconductor with a strong electron-phonon coupling ($λ$$_{ep}$ = 1.38) and a large superconducting gap ($Δ_{0}$/$k_{B}T_{C}$ = 2.25). We found that the $ρ$($T$) exhibits an unusual $T$-linear dependence above 50 K, and can be well described by the Fermi-liquid theory below 20 K. Interestingly, its Kadowaki-Woods ratio $A/γ^{2}$ [9.2$\times$10$^{-5}$ $μΩ$ cm(mol K$^{2}$/mJ)$^{2}$] is unexpectedly one order of magnitude larger than that obtained in many heavy Fermi compounds, although the electron correlation is not so strong. △ Less

Submitted 11 April, 2022; originally announced April 2022.

Comments: 7 pages, 5 figures

Journal ref: Physical Review B 106, 224501 (2022)

Large Magnetoresistance and Nontrivial Berry Phase in Nb3Sb Crystals with A15 Structure

Authors: Qin Chen, Yuxing Zhou, Binjie Xu, Zhefeng Lou, Huancheng Chen, Shuijin Chen, Chunxiang Wu, Jianhua Du, Hangdong Wang, Jinhu Yang, Minghu Fang

Abstract: Compounds with the A15 structure have attracted extensive attention due to their superconductivity and nontrivial topological band structure. We have successfully grown Nb$_3$Sb single crystals with a A15 structure and systematically measured the longitudinal resistivity, Hall resistivity and quantum oscillations in magnetization. Similar to other topological trivial/nontrivial semimetals, Nb$_3$S… ▽ More Compounds with the A15 structure have attracted extensive attention due to their superconductivity and nontrivial topological band structure. We have successfully grown Nb$_3$Sb single crystals with a A15 structure and systematically measured the longitudinal resistivity, Hall resistivity and quantum oscillations in magnetization. Similar to other topological trivial/nontrivial semimetals, Nb$_3$Sb, exhibits large magnetoresistance (MR) at low temperatures (717$\%$, 2 K and 9 T), unsaturating quadratic field dependence of MR and up-turn behavior in $ρ_{xx}$(\emph{T}) curves under magnetic field, which is considered to result from a perfect hole-electron compensation, as evidenced by the Hall resistivity measurements. The nonzero Berry phase obtained from the de-Hass van Alphen (dHvA) oscillations demonstrates that Nb$_3$Sb is topologically nontrivial. These results indicate that Nb$_{3}$Sb superconductor is also a semimetal with large MR and nontrivial Berry phase, indicating that Nb$_{3}$Sb may be another platform to search for Majorana zero-energy mode. △ Less

Submitted 30 July, 2021; originally announced July 2021.

Comments: 5 pages, 5 figures. arXiv admin note: text overlap with arXiv:2007.04814, arXiv:2104.03799

Journal ref: CHIN. PHYS. LETT. Vol. 38, No. 8 (2021) 087501

Visualizing nematic transition and nanoscale suppression of superconductivity in Fe(Te,Se)

Authors: He Zhao, Hong Li, Lianyang Dong, Binjie Xu, John Schneeloch, Ruidan Zhong, Minghu Fang, Genda Gu, John Harter, Stephen D. Wilson, Ziqiang Wang, Ilija Zeljkovic

Abstract: The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-Tc superconductor FeTe1-xSex: it undergoes both a topological transition, linked to the electronic band inversion, and an electronic nematic phase transition, associated with rotation symmetry breaking, around the same critical composition xc where sup… ▽ More The interplay of different electronic phases underlies the physics of unconventional superconductors. One of the most intriguing examples is a high-Tc superconductor FeTe1-xSex: it undergoes both a topological transition, linked to the electronic band inversion, and an electronic nematic phase transition, associated with rotation symmetry breaking, around the same critical composition xc where superconducting Tc peaks. At this regime, nematic fluctuations and symmetry-breaking strain could have an enormous impact, but this is yet to be fully explored. Using spectroscopic-imaging scanning tunneling microscopy, we study the electronic nematic transition in FeTe1-xSex as a function of composition. Near xc, we reveal the emergence of electronic nematicity in nanoscale regions. Interestingly, we discover that superconductivity is drastically suppressed in areas where static nematic order is the strongest. By analyzing atomic displacement in STM topographs, we find that small anisotropic strain can give rise to these strongly nematic localized regions. Our experiments reveal a tendency of FeTe1-xSex near x~0.45 to form puddles hosting static nematic order, suggestive of nematic fluctuations pinned by structural inhomogeneity, and demonstrate a pronounced effect of anisotropic strain on superconductivity in this regime. △ Less

Submitted 14 June, 2021; originally announced June 2021.

Comments: This is the original submitted version. The final version will appear in Nature Physics

Journal ref: Nature Physics 17, 903 (2021)

2D Nb-doped MoS$_2$: Tuning the Exciton Transitions and Application to p-type FETs

Authors: Baokun Song, Honggang Gu, Mingsheng Fang, Zhengfeng Guo, Yen-Teng Ho, Xiuguo Chen, Hao Jiang, Shiyuan Liu

Abstract: Two-dimensional (2D) MoS$_2$ has been intensively investigated for its use in the fields of microelectronics, nanoelectronics, and optoelectronics. However, intrinsic 2D MoS$_2$ is usually used as the n-type semiconductor due to the unintentional sulphur vacancies and surface gas adsorption.The synthesis and characterization of 2D MoS$_2$ semiconductor of p-type are crucial for the development of… ▽ More Two-dimensional (2D) MoS$_2$ has been intensively investigated for its use in the fields of microelectronics, nanoelectronics, and optoelectronics. However, intrinsic 2D MoS$_2$ is usually used as the n-type semiconductor due to the unintentional sulphur vacancies and surface gas adsorption.The synthesis and characterization of 2D MoS$_2$ semiconductor of p-type are crucial for the development of relevant p-n junction devices, as well as the practical applications of 2D MoS$_2$ in the next-generation CMOS integrated circuit. Here, we synthesize high-quality, wafer-scale, 2D p-type MoS$_2$ (Mo$_{1-x}$Nb$_x$S$_2$) with various niobium (Nb) mole fractions from 0 to 7.6% by a creative two-step method. The dielectric functions of 2D Mo1-xNbxS2 are accurately determined by spectroscopic ellipsometry. We find that the increasing fraction of Nb dopant in 2D MoS$_2$ can modulate and promote the combination of A and B exciton peaks of 2D MoS$_2$. The direct causes of this impurity-tunable combination are interpreted as the joint influence of decreasing peak A and broadening peak B. We explain the broadening peak B as the multiple transitions from the impurity-induced valance bands to the conductive band minimum at K point of Brillouin zone by comparing and analyzing the simulated electronic structure of intrinsic and 2D Nb-doped MoS$_2$. A p-type FET based on the 2D Nb-doped MoS$_2$ was fabricated for characterization, and its working performance is expected to be adjustable as a function of concentration of Nb dopant according to our theoretical research. Our study is informative for comprehending optical and electronic properties of extrinsic 2D transitional metal dichalcogenides, which is important and imperative for the development and optimization of corresponding photonics and optoelectronics devices. △ Less

Submitted 7 April, 2021; originally announced April 2021.

Extremely large magnetoresistance in the "ordinary" metal ReO3

Authors: Qin Chen, Zhefeng Lou, ShengNan Zhang, Yuxing Zhou, Binjie Xu, Huancheng Chen, Shuijin Chen, Jianhua Du, Hangdong Wang, Jinhu Yang, QuanSheng Wu, Oleg V. Yazyev, Minghu Fang

Abstract: The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas-Van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance… ▽ More The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas-Van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance (MR) of ReO$_3$ with a simple cubic structure, an "ordinary" nonmagnetic metal considered previously. We found that ReO$_3$ exhibits almost all the characteristics of XMR semimetals: the nearly quadratic field dependence of MR, a field-induced upturn in resistivity followed by a plateau at low temperatures, high mobilities of charge carriers. It was found that for magnetic field \emph{H} applied along the \emph{c} axis, the MR exhibits an unsaturated \emph{H}$^{1.75}$ dependence, which was argued to arise from the complete carrier compensation supported by the Hall resistivity measurements. For \emph{H} applied along the direction of 15$^\circ$ relative to the \emph{c} axis, an unsaturated \emph{H}$^{1.90}$ dependence of MR up to 9.43~$\times$~$10^3$$\%$ at 10~K and 9~T was observed, which was explained by the existence of electron open orbits extending along the $k_{x}$ direction. Two mechanisms responsible for XMR observed usually in the semimetals occur also in the simple metal ReO$_3$ due to its peculiar FS (two closed electron pockets and one open electron pocket), once again indicating that the details of FS topology are a key factor for the observed XMR in materials. △ Less

Submitted 8 April, 2021; originally announced April 2021.

Comments: 9 pages, 8 figures. arXiv admin note: text overlap with arXiv:2007.04814

Journal ref: Phys. Rev. B 104, 115104 (2021)

Magnetoresistance and Kondo effect in the nodal-line semimetal VAs$_2$

Authors: Shuijin Chen, Zhefeng Lou, Yuxing Zhou, Qin Chen, Binjie Xu, Jianhua Du, Jinhu Yang, Haangdong Wang, Minghu Fang

Abstract: We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity $ρ_{xx}(T,H)$, Hall resistivity $ρ_{xy}(T,H)$, and magnetic susceptibility as a function of temperature and various magnetic fields for VAs$_2$ with a monoclinic crystal structure. The band structure calculations show that VAs$_2$ is a nodal-line semimetal when spin-orb… ▽ More We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity $ρ_{xx}(T,H)$, Hall resistivity $ρ_{xy}(T,H)$, and magnetic susceptibility as a function of temperature and various magnetic fields for VAs$_2$ with a monoclinic crystal structure. The band structure calculations show that VAs$_2$ is a nodal-line semimetal when spin-orbit coupling is ignored. The emergence of a minimum at around 11 K in $ρ_{xx}(T)$ measured at $H$ = 0 demonstrates that an additional magnetic impurity (V$^{4+}$, $S$ = 1/2) occurs in VAs$_2$ single crystals, evidenced by both the fitting of $ρ_{xx}(T)$ data and the susceptibility measurements. It was found that a large positive magnetoresistance (MR) reaching 649\% at 10 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of $ρ_{xx}(T)$ emerge also in VAs$_2$, although MR is not so large due to the existence of additional scattering compared with other topological nontrival/trival semimetals. The observed properties are attributed to a perfect charge-carrier compensation, which is evidenced by both calculations relying on the Fermi surface and the Hall resistivity measurements. These results indicate that the compounds containing V ($3d^3 4s^2$) element as a platform for studying the influence of magnetic impurities to the topological properties. △ Less

Submitted 22 December, 2020; originally announced December 2020.

Comments: 8 pages, 5 figures. arXiv admin note: text overlap with arXiv:2007.04814

Journal ref: CHIN. PHYS. LETT. 38, 017202 (2021)

Bulk Superconductivity in the Dirac Semimetal TlSb

Authors: Yuxing Zhou, Bin Li, Zhefeng Lou, Huancheng Chen, Qin Chen, Binjie Xu, Chunxiang Wu, Jianhua Du, Jinhu Yang, Hangdong Wang, Minghu Fang

Abstract: A feasible strategy to realize the Majorana fermions is searching for a simple compound with both bulk superconductivity and Dirac surface states. In this paper, we performed calculations of electronic band structure, the Fermi surface and surface states, as well as measured the resistivity, magnetization, specific heat for TlSb compound with a CsCl-type structure. The band structure calculations… ▽ More A feasible strategy to realize the Majorana fermions is searching for a simple compound with both bulk superconductivity and Dirac surface states. In this paper, we performed calculations of electronic band structure, the Fermi surface and surface states, as well as measured the resistivity, magnetization, specific heat for TlSb compound with a CsCl-type structure. The band structure calculations show that TlSb is a Dirac semimetal when spin-orbit coupling is taken into account. Meanwhile, we first found that TlSb is a type-II superconductor with $T_c$ = 4.38 K, $H_{c1}$(0) = 148 Oe, $H_{c2}$(0) = 1.12 T and $κ_{GL}$ = 10.6, and confirmed it to be a moderately coupled s-wave superconductor. Although we can not determine which bands near the Fermi level $E_F$ to be responsible for superconductivity, its coexistence with the topological surface states implies that TlSb compound may be a simple material platform to realize the fault-tolerant quantum computations. △ Less

Submitted 6 December, 2020; originally announced December 2020.

Comments: 7 pages, 8 figures

Journal ref: Sci. China-Phys. Mech. Astron. 64, 247411(2021)

Large magnetoresistance and non-zero Berry phase in the nodal-line semimetal MoO2

Authors: Qin Chen, Zhefeng Lou, ShengNan Zhang, Binjie Xu, Yuxing Zhou, Huancheng Chen, Shuijin Chen, Jianhua Du, Hangdong Wang, Jinhu Yang, QuanSheng Wu, Oleg V. Yazyev, Minghu Fang

Abstract: We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity rhoxx(T,H), Hall resistivity rhoxy(T,H) and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when spin-o… ▽ More We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity rhoxx(T,H), Hall resistivity rhoxy(T,H) and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching 5.03x10^4% at 2 K and 9 T, its nearly quadratic field dependence and a field-induced up-turn behavior of rhoxx(T), the characteristics common for many topologically non-trivial as well as trivial semimetals, emerge also in MoO2. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for magnetic field along the current direction and the non-zero Berry phase in de Haas-van Alphen measurements indicate that pairs of Weyl points appear in MoO2, which may be due to the crystal symmetry breaking. These results highlight MoO2 as a new platform materials for studying the topological properties of oxides. △ Less

Submitted 9 July, 2020; originally announced July 2020.

Comments: 8 papges, 7 figures

Report number: 2007.04814

Journal ref: Phys. Rev. B 102, 165133 (2020)

Metamagnetic transitions and anomalous magnetoresistance in EuAg$_4$As$_2$ single crystal

Authors: Qinqing Zhu, Liang Li, Zhihua Yang, Zhefeng Lou, Jianhua Du, Jinhu Yang, Bin Chen, Hangdong Wang, Minghu Fang

Abstract: In this paper, the magnetic and transport properties were systematically studied for EuAg$_4$As$_2$ single crystals, crystallizing in a centrosymmetric trigonal CaCu$_4$P$_2$ type structure. It was confirmed that two magnetic transitions occur at $\textit{T}$$_{N1}$ = 10 K and $\textit{T}$$_{N2}$ = 15 K, respectively. With the increasing field, the two transitions are noticeably driven to lower te… ▽ More In this paper, the magnetic and transport properties were systematically studied for EuAg$_4$As$_2$ single crystals, crystallizing in a centrosymmetric trigonal CaCu$_4$P$_2$ type structure. It was confirmed that two magnetic transitions occur at $\textit{T}$$_{N1}$ = 10 K and $\textit{T}$$_{N2}$ = 15 K, respectively. With the increasing field, the two transitions are noticeably driven to lower temperature. At low temperatures, applying a magnetic field in the $\textit{ab}$ plane induces two successive metamagnetic transitions. For both $\textit{H}$ $\parallel$ $\textit{ab}$ and $\textit{H}$ $\parallel$ $\textit{c}$, EuAg$_4$As$_2$ shows a positive, unexpected large magnetoresistance (up to 202\%) at low fields below 10 K, and a large negative magnetoresistance (up to -78\%) at high fields/intermediate temperatures. Such anomalous field dependence of magnetoresistance may have potential application in the future magnetic sensors. Finally, the magnetic phase diagrams of EuAg$_{4}$As$_{2}$ were constructed for both $\textit{H}$ $\parallel$ $\textit{ab}$ and $\textit{H}$ $\parallel$ $\textit{c}$. △ Less

Submitted 26 August, 2020; v1 submitted 7 May, 2020; originally announced May 2020.

Journal ref: SCIENCE CHINA Physics, Mechanics & Astronomy Vol. 64 No. 2: 227011 (2021)

Local Structure of Mott Insulating Iron Oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se)

Authors: B. Karki, A. Alfailakawi, Benjamin A. Frandsen, M. S. Everett, J. C. Neuefeind, Binjie Xu, Hangdong Wang, Minghu Fang, B. Freelon

Abstract: We describe the local structural properties of the iron oxychalcogenides, La$_2$O$_2$Fe$_2$O$M_2$ ($M$ = S, Se), by using pair distribution function (PDF) analysis applied to total scattering data. Our results of neutron powder diffraction show that $M$ = S and Se possess similar nuclear structure at low and room temperatures. The local crystal structures were studied by investigating deviations i… ▽ More We describe the local structural properties of the iron oxychalcogenides, La$_2$O$_2$Fe$_2$O$M_2$ ($M$ = S, Se), by using pair distribution function (PDF) analysis applied to total scattering data. Our results of neutron powder diffraction show that $M$ = S and Se possess similar nuclear structure at low and room temperatures. The local crystal structures were studied by investigating deviations in atomic positions and the extent of the formation of orthorhombicity. Analysis of the total scattering data suggests that buckling of the Fe$_2$O plane occurs below 100 K. The buckling may occur concomitantly with a change in octahedral height. Furthermore, within a typical range of 1-2 nm, we observed short-range orthorhombic-like structure suggestive of nematic fluctuations in both of these materials. △ Less

Submitted 2 July, 2021; v1 submitted 24 February, 2020; originally announced February 2020.

Journal ref: Phys. Rev. B 104, 064101 (2021)

Linear and quadratic magnetoresistance in the semimetal SiP2

Authors: Yuxing Zhou, Zhefeng Lou, ShengNan Zhang, Huancheng Chen, Qin Chen, Binjie Xu, Jianhua Du, Jinhu Yang, Hangdong Wang, QuanSheng Wu, Oleg V Yazyev, Minghu Fang

Abstract: Multiple mechanisms for extremely large magnetoresistance (XMR) found in many topologically nontrivial/trivial semimetals have been theoretically proposed, but experimentally it is unclear which mechanism is responsible in a particular sample. In this article, by the combination of band structure calculations, numerical simulations of magnetoresistance (MR), Hall resistivity and de Haas-van Alphen… ▽ More Multiple mechanisms for extremely large magnetoresistance (XMR) found in many topologically nontrivial/trivial semimetals have been theoretically proposed, but experimentally it is unclear which mechanism is responsible in a particular sample. In this article, by the combination of band structure calculations, numerical simulations of magnetoresistance (MR), Hall resistivity and de Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy of SiP$_{2}$ which is verified to be a topologically trivial, incomplete compensation semimetal. It was found that as magnetic field, $H$, is applied along the $a$ axis, the MR exhibits an unsaturated nearly linear $H$ dependence, which was argued to arise from incomplete carriers compensation. For the $H$ $\parallel$ [101] orientation, an unsaturated nearly quadratic $H$ dependence of MR up to 5.88 $\times$ 10$^{4}$$\%$ (at 1.8 K, 31.2 T) and field-induced up-turn behavior in resistivity were observed, which was suggested due to the existence of hole open orbits extending along the $k_{x}$ direction. Good agreement of the experimental results with the simulations based on the calculated Fermi surface (FS) indicates that the topology of FS plays an important role in its MR. △ Less

Submitted 14 August, 2020; v1 submitted 5 February, 2020; originally announced February 2020.

Comments: 9 pages, 8 figures

Report number: 2002.05258

Journal ref: Phys. Rev. B 102, 115145 (2020)

Nonvolatile Multilevel States in Multiferroic Tunnel Junctions

Authors: Mei Fang, Sangjian Zhang, Wenchao Zhang, Lu Jiang, Eric Vetter, Ho Nyung Lee, Xiaoshan Xu, Dali Sun, Jian Shen

Abstract: Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed Multiferroic Tunnel Junctions (MFTJs), can be achieved not only by the magnetic alignments of two ferromagnets but also by the electric polarization of the ferroelectric interlayer, providing great opportunities for next-generation multi-state memory devices. He… ▽ More Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed Multiferroic Tunnel Junctions (MFTJs), can be achieved not only by the magnetic alignments of two ferromagnets but also by the electric polarization of the ferroelectric interlayer, providing great opportunities for next-generation multi-state memory devices. Here we show that a La0.67Sr0.33MnO3 (LSMO)/PbZr0.2Ti0.8O3(PZT)/Co structured MFTJ device can exhibit multilevel resistance states in the presence of gradually reversed ferroelectric domains via tunneling electro-resistance and tunneling magnetoresistance, respectively. The nonvolatile ferroelectric control in the MFTJ can be attributed to separate contributions arising from two independent ferroelectric channels in the PZT interlayer with opposite polarization. Our study shows the dominant role of "mixed" ferroelectric states on achieving accumulative electrical modulation of multilevel resistance states in MFTJs, paving the way for multifunctional device applications. △ Less

Submitted 4 October, 2019; originally announced October 2019.

Journal ref: Physical Review APPLIED 12, 044049 (2019)

Projective Quasiparticle Interference of a Single Scatterer to Analyze the Electronic Band Structure of ZrSiS

Authors: Wenhao Zhang, Kunliang Bu, Fangzhou Ai, Zongxiu Wu, Ying Fei, Yuan Zheng, Jianhua Du, Minghu Fang, Yi Yin

Abstract: Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy (STM) to analyze the electronic band structure of materials. Single-defect induced QPI reveals defect-dependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise… ▽ More Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy (STM) to analyze the electronic band structure of materials. Single-defect induced QPI reveals defect-dependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise ratio (SNR) is relatively low in a standard two-dimensional QPI measurement. In this paper, we introduce a projective quasiparticle interference (PQPI) method, in which a one-dimensional measurement is taken along high-symmetry directions centered on a specified defect. We apply the PQPI method to a topological nodal-line semimetal ZrSiS. We focus on two special types of atomic defects that scatter the surface and bulk electronic bands. With enhanced SNR in PQPI, the energy dispersions are clearly resolved along high symmetry directions. We discuss the defect-dependent scattering of bulk bands with the non-symmorphic symmetry-enforced selection rules. Furthermore, an energy shift of the surface floating band is observed and a new branch of energy dispersion (q6) is resolved. This PQPI method can be applied to other complex materials to explore defect-dependent interactions in the future. △ Less

Submitted 13 June, 2020; v1 submitted 26 July, 2019; originally announced July 2019.

Comments: 21 pages, 5 figures, supplementary 3 pages, 2 figures

Journal ref: Phys. Rev. Research 2, 023419 (2020)

Nonlinearity in the Dark: Broadband Terahertz Generation with Extremely High Efficiency

Authors: Ming Fang, Nian-Hai Shen, Wei E. I. Sha, Zhixiang Huang, Thomas Koschny, Costas M. Soukouli

Abstract: Plasmonic metamaterials and metasurfaces offer new opportunities in developing high performance terahertz emitters and detectors beyond the limitations of conventional nonlinear materials. However, simple meta-atoms for second-order nonlinear applications encounter fundamental trade-offs in the necessary symmetry breaking and local-field enhancement due to radiation damping that is inherent to the… ▽ More Plasmonic metamaterials and metasurfaces offer new opportunities in developing high performance terahertz emitters and detectors beyond the limitations of conventional nonlinear materials. However, simple meta-atoms for second-order nonlinear applications encounter fundamental trade-offs in the necessary symmetry breaking and local-field enhancement due to radiation damping that is inherent to the operating resonant mode and cannot be controlled separately. Here we present a novel concept that eliminates this restriction obstructing the improvement of terahertz generation efficiency in nonlinear metasurfaces based on metallic nanoresonators. This is achieved by combining a resonant dark-state metasurface, which locally drives nonlinear nanoresonators in the near field, with a specific spatial symmetry that enables destructive interference of the radiating linear moments of the nanoresonators, and perfect absorption via simultaneous electric and magnetic critical coupling of the pump radiation to the dark mode. Our proposal allows eliminating linear radiation damping, while maintaining constructive interference and effective radiation of the nonlinear components. We numerically demonstrate a giant second-order nonlinear susceptibility around Hundred-Billionth m/V, a one order improvement compared with the previously reported split-ring-resonator metasurface, and correspondingly, a 2 orders of magnitude enhanced terahertz energy extraction should be expected with our configuration under the same conditions. Our study offers a paradigm of high efficiency tunable nonlinear metadevices and paves the way to revolutionary terahertz technologies and optoelectronic nanocircuitry. △ Less

Submitted 19 January, 2019; originally announced January 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 122, 027401, Jan. 2019

A Single-Crystal Neutron Diffraction Study on Magnetic Structure of CsCo2Se2

Authors: Juanjuan Liu, Jieming Sheng, Wei Luo, Jinchen Wang, Bao Wei, Jinhu Yang, Minghu Fang, S. A. Danilkin

Abstract: The magnetic structure of CsCo2Se2 was investigated using single-crystal neutron diffraction technique. An antiferromagnetic transition with the propagation vector (0,0,1) was observed at T_N = 78 K. The Co magnetic moment 0.772(6) μ_B at 10 K pointing in the basal plane couples ferromagnetically in the plane which stacks antiferromagnetically along the c direction. Tuning and suppressing the inte… ▽ More The magnetic structure of CsCo2Se2 was investigated using single-crystal neutron diffraction technique. An antiferromagnetic transition with the propagation vector (0,0,1) was observed at T_N = 78 K. The Co magnetic moment 0.772(6) μ_B at 10 K pointing in the basal plane couples ferromagnetically in the plane which stacks antiferromagnetically along the c direction. Tuning and suppressing the interplane antiferromagnetic interaction may be crucial to induce the material to a superconducting state. △ Less

Submitted 1 November, 2018; originally announced November 2018.

Comments: 5 pages, 2 figures, 2 tables

Journal ref: Chin. Phys. B 27, 117401 (2018)

Unconventional superconductivity in the nickel-chalcogenide superconductor, TlNi$_2$Se$_2$

Authors: Erik Jellyman, Philippa Jefferies, Stephen Pollard, Ted Forgan, Elizabeth Blackburn, Emma Campillo, Alex Holmes, Robert Cubitt, Jorge Gavilano, Hangdong Wang, Jianhua Du, Minghu Fang

Abstract: We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase dia… ▽ More We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature-dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of of multiple gaps. △ Less

Submitted 19 February, 2020; v1 submitted 9 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. B 101, 134523 (2020)

Extremely large magnetoresistance in topologically trivial semimetal $α$-WP$_2$

Authors: Jianhua Du, Zhefeng Lou, ShengNan Zhang, Yuxing Zhou, Binjie Xu, Qin Chen, Yanqing Tang, Shuijin Chen, Huancheng Chen, Qinqing Zhu, Hangdong Wang, Jinhu Yang, quanSheng Wu, Oleg V. Yazyev, Minghu Fang

Abstract: Extremely large magnetoresistance (XMR) was recently discovered in many non-magnetic materials, while its underlying mechanism remains poorly understood due to the complex electronic structure of these materials. Here, we report an investigation of the $α$-phase WP$_2$, a topologically trivial semimetal with monoclinic crystal structure (C2/m), which contrasts to the recently discovered robust typ… ▽ More Extremely large magnetoresistance (XMR) was recently discovered in many non-magnetic materials, while its underlying mechanism remains poorly understood due to the complex electronic structure of these materials. Here, we report an investigation of the $α$-phase WP$_2$, a topologically trivial semimetal with monoclinic crystal structure (C2/m), which contrasts to the recently discovered robust type-II Weyl semimetal phase in $β$-WP$_2$. We found that $α$-WP$_2$ exhibits almost all the characteristics of XMR materials: the near-quadratic field dependence of MR, a field-induced up-turn in resistivity following by a plateau at low temperature, which can be understood by the compensation effect, and high mobility of carriers confirmed by our Hall effect measurements. It was also found that the normalized MRs under different magnetic fields has the same temperature dependence in $α$-WP$_2$, the Kohler scaling law can describe the MR data in a wide temperature range, and there is no obvious change in the anisotropic parameter $γ$ value with temperature. The resistance polar diagram has a peanut shape when field is rotated in $\textit{ac}$ plane, which can be understood by the anisotropy of Fermi surface. These results indicate that both field-induced-gap and temperature-induced Lifshitz transition are not the origin of up-turn in resistivity in the $α$-WP$_2$ semimetal. Our findings establish $α$-WP$_2$ as a new reference material for exploring the XMR phenomena. △ Less

Submitted 11 April, 2018; originally announced April 2018.

Comments: 18 pages, 12 figures

Journal ref: Phys. Rev. B 97, 245101 (2018)

Magnetic and Structural Properties of the Iron Oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M_{2}$ ($M$= S, Se)

Authors: B. Freelon, Z. Yamani, Ian Swainson, R. Flauca, Yu Hao Liu, L. Craco, M. S. Laad, Meng Wang, Jiaqi Chen, R. J. Birgeneau, Minghu Fang

Abstract: We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se). Elastic neutron scattering reveals that $M$ = S and Se have similar nuclear structures at room and low temperatures. We find that both materials obtain antiferromagnetic ordering at a Neel temperature $T_{N}$ 90.1 $\pm$ 0.16 K and 107.2 $\pm$ 0.06 K for… ▽ More We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se). Elastic neutron scattering reveals that $M$ = S and Se have similar nuclear structures at room and low temperatures. We find that both materials obtain antiferromagnetic ordering at a Neel temperature $T_{N}$ 90.1 $\pm$ 0.16 K and 107.2 $\pm$ 0.06 K for $M$= Se and S, respectively. The magnetic arrangements of $M$ = S, Se are obtained through Rietveld refinement. We find the order parameter exponent $β$ to be 0.129 $\pm$ 0.006 for $M$ = Se and 0.133 $\pm$ 0.007 for $M$ = S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen exchange, the nature of the magnetic interactions is similar in these materials. △ Less

Submitted 16 September, 2018; v1 submitted 31 December, 2017; originally announced January 2018.

Journal ref: Phys. Rev. B 99, 024109 (2019)

Maxwell-Hydrodynamic Model for Simulating Nonlinear Terahertz Generation from Plasmonic Metasurfaces

Authors: Ming Fang, Zhixiang Huang, Wei E. I. Sha, Xianliang Wu

Abstract: The interaction between the electromagnetic field and plasmonic nanostructures leads to both the strong linear response and inherent nonlinear behavior. In this paper, a time-domain hydrodynamic model for describing the motion of electrons in plasmonic nanostructures is presented, in which both surface and bulk contributions of nonlinearity are considered. A coupled Maxwell-hydrodynamic system cap… ▽ More The interaction between the electromagnetic field and plasmonic nanostructures leads to both the strong linear response and inherent nonlinear behavior. In this paper, a time-domain hydrodynamic model for describing the motion of electrons in plasmonic nanostructures is presented, in which both surface and bulk contributions of nonlinearity are considered. A coupled Maxwell-hydrodynamic system capturing full-wave physics and free electron dynamics is numerically solved with the parallel finite-difference time-domain (FDTD) method. The validation of the proposed method is presented to simulate linear and nonlinear responses from a plasmonic metasurface. The linear response is compared with the Drude dispersion model and the nonlinear terahertz emission from a difference-frequency generation process is validated with theoretical analyses. The proposed scheme is fundamentally important to design nonlinear plasmonic nanodevices, especially for efficient and broadband THz emitters. △ Less

Submitted 17 September, 2017; originally announced September 2017.

Comments: 8 pages, 7 figures, IEEE Journal on Multiscale and Multiphysics Computational Techniques, 2017

Nematic Fluctuations in Iron-Oxychalcogenide Mott Insulators

Authors: B. Freelon, R. Sarkar, S. Kamusella, F. Brückner, V. Grinenko, Swagata Acharya, Mukul Laad, Luis Craco, Zahra Yamani, Roxana Flacau, Ian Swainson, Benjamin Frandsen, Robert Birgeneau, Yuhao Liu, Bhupendra Karki, Alaa Alfailakawi, Joerg C. Neuefeind, Michelle Everett, Hangdong Wang, Binjie Xu, Minghu Fang, H. -H. Klauss

Abstract: Nematic fluctuations occur in a wide range of physical systems from liquid crystals to biological molecules to solids such as exotic magnets, cuprates and iron-based high-$T_c$ superconductors. Nematic fluctuations are thought to be closely linked to the formation of Cooper-pairs in iron-based superconductors. It is unclear whether the anisotropy inherent in this nematicity arises from electronic… ▽ More Nematic fluctuations occur in a wide range of physical systems from liquid crystals to biological molecules to solids such as exotic magnets, cuprates and iron-based high-$T_c$ superconductors. Nematic fluctuations are thought to be closely linked to the formation of Cooper-pairs in iron-based superconductors. It is unclear whether the anisotropy inherent in this nematicity arises from electronic spin or orbital degrees of freedom. We have studied the iron-based Mott insulators La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ $M$ = (S, Se) which are structurally similar to the iron pnictide superconductors. They are also in close electronic phase diagram proximity to the iron pnictides. Nuclear magnetic resonance (NMR) revealed a critical slowing down of nematic fluctuations as observed by the spin-lattice relaxation rate ($1/T_1$). This is complemented by the observation of a change of electrical field gradient over a similar temperature range using Mössbauer spectroscopy. The neutron pair distribution function technique applied to the nuclear structure reveals the presence of local nematic $C_2$ fluctuations over a wide temperature range while neutron diffraction indicates that global $C_{4}$ symmetry is preserved. Theoretical modeling of a geometrically frustrated spin-$1$ Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of magnetic fluctuations in terms of hidden quadrupolar spin fluctuations. Nematicity is closely linked to geometrically frustrated magnetism, which emerges from orbital selectivity. The results highlight orbital order and spin fluctuations in the emergence of nematicity in Fe-based oxychalcogenides. The detection of nematic fluctuation within these Mott insulator expands the group of iron-based materials that show short-range symmetry-breaking. △ Less

Submitted 10 February, 2020; v1 submitted 4 August, 2017; originally announced August 2017.

Full Hydrodynamic Model of Nonlinear Electromagnetic Response in Metallic Metamaterials

Authors: Ming Fang, Zhixiang Huang, Wei E. I. Sha, Xiaoyan Y. Z. Xiong, Xianliang Wu

Abstract: Applications of metallic metamaterials have generated significant interest in recent years. Electromagnetic behavior of metamaterials in the optical range is usually characterized by a local-linear response. In this article, we develop a finite-difference time-domain (FDTD) solution of the hydrodynamic model that describes a free electron gas in metals. Extending beyond the local-linear response,… ▽ More Applications of metallic metamaterials have generated significant interest in recent years. Electromagnetic behavior of metamaterials in the optical range is usually characterized by a local-linear response. In this article, we develop a finite-difference time-domain (FDTD) solution of the hydrodynamic model that describes a free electron gas in metals. Extending beyond the local-linear response, the hydrodynamic model enables numerical investigation of nonlocal and nonlinear interactions between electromagnetic waves and metallic metamaterials. By explicitly imposing the current continuity constraint, the proposed model is solved in a self-consistent manner. Charge, energy and angular momentum conservation laws of high-order harmonic generation have been demonstrated for the first time by the Maxwell-hydrodynamic FDTD model. The model yields nonlinear optical responses for complex metallic metamaterials irradiated by a variety of waveforms. Consequently, the multiphysics model opens up unique opportunities for characterizing and designing nonlinear nanodevices. △ Less

Submitted 30 October, 2016; originally announced October 2016.

Comments: 11 pages, 14 figures

Journal ref: Progress In Electromagnetics Research, Vol. 157, 63-78, 2016

Superconductivity in a new layered nickel-selenide CsNi2Se2

Authors: Huimin Chen, Jinhu Yang, Chao Cao, Lin Li, Qiping Su, Bin Chen, Hangdong Wang, Qianhui Mao, Jianhua Du, Minghu Fang

Abstract: The physical properties of CsNi$_{2}$Se$_{2}$ were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi$_{2}$Se$_{2}$ compound is a superconductor with a transition temperature \textit{T$_{c}$}=2.7K. A large Sommerfeld coefficient $γ$$_{n}$ ($\sim$77.90 mJ/mol$\cdot$K$^{-2}$), was obtained from the normal state electronic spec… ▽ More The physical properties of CsNi$_{2}$Se$_{2}$ were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi$_{2}$Se$_{2}$ compound is a superconductor with a transition temperature \textit{T$_{c}$}=2.7K. A large Sommerfeld coefficient $γ$$_{n}$ ($\sim$77.90 mJ/mol$\cdot$K$^{-2}$), was obtained from the normal state electronic specific heat. However, the Kadowaki-Woods ratio of CsNi$_{2}$Se$_{2}$ was estimated to be about 0.041$\times$10$^{-5}$ $μΩ$$\cdot$cm(mol$\cdot$K/mJ)$^{2}$, indicating the absence of strong electron-electron correlations in this compound. In the superconducting state, we found that the zero-field electronic specific heat data, $C_{es}(T)$ (0.5K $\leq$ T $<$ 2.6K), can be well fitted with a two-gap BCS model. The comparison with the results of the density functional theory (DFT) calculations suggested that the large $γ$$_{n}$ in the nickel-selenide superconductors may be related to the large Density of States (DOS) at the fermi surface. △ Less

Submitted 20 August, 2015; originally announced August 2015.

Comments: 5 pagers, 4 figures

Journal ref: Supercond. Sci. Technol. 29 (2016) 045008

Unsaturated both large positive and negative magnetoresistance in Weyl Semimetal TaP

Authors: Jianhua Du, Hangdong Wang, Qianhui Mao, Rajwali Khan, Binjie Xu, Yuxing Zhou, Yannan Zhang, Jinhu Yang, Bin Chen, Chunmu Feng, Minghu Fang

Abstract: After growing successfully TaP single crystal, we measured its longitudinal resistivity (rhoxx) and Hall resistivity (rhoyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches to 328000 percent, at 300K to 176 percent at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-e… ▽ More After growing successfully TaP single crystal, we measured its longitudinal resistivity (rhoxx) and Hall resistivity (rhoyx) at magnetic fields up to 9T in the temperature range of 2-300K. It was found that at 2K its magnetoresistivity (MR) reaches to 328000 percent, at 300K to 176 percent at 8T, and both do not appear saturation. We confirmed that TaP is indeed a low carrier concentration, hole-electron compensated semimetal, with a high mobility of hole muh=371000 cm2V-1s-1, and found that a magnetic-field-induced metal-insulator transition occurs at room temperature. Remarkably, as a magnetic field (H) is applied in parallel to the electric field (E), the negative MR due to chiral anomaly is observed, and reaches to -3000 percent at 9T without any signature of saturation, too, which distinguishes with other Weyl semimetals (WSMs). The analysis on the Shubnikov-de Haas (SdH) oscillations superimposing on the MR reveals that a nontrivial Berry phase with strong offset of 0.3958 realizes in TaP, which is the characteristic feature of the charge carriers enclosing a Weyl nodes. These results indicate that TaP is a promising candidate not only for revealing fundamental physics of the WSM state but also for some novel applications. △ Less

Submitted 19 July, 2015; originally announced July 2015.

Comments: 5 papes, 4 figures

Journal ref: Sci. China-Phys. Mech. Astron. 59, 657406(2016)

Optical properties of TlNi2Se2: Observation of pseudogap formation

Authors: X. B. Wang, H. P. Wang, Hangdong Wang, Minghu Fang, N. L. Wang

Abstract: The quasi-two-dimensional nickel chalcogenides $TlNi_2Se_2$ is a newly discovered superconductor. We have performed optical spectroscopy study on $TlNi_2Se_2$ single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure $BaNi_2As_2$. Both the suppression in $R(ω)$ and the peaklike feature in… ▽ More The quasi-two-dimensional nickel chalcogenides $TlNi_2Se_2$ is a newly discovered superconductor. We have performed optical spectroscopy study on $TlNi_2Se_2$ single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure $BaNi_2As_2$. Both the suppression in $R(ω)$ and the peaklike feature in $σ_1(ω)$ suggest the progressive formation of a pseudogap feature in the midinfrared range with decreasing temperatures, which might be originated from the dynamic local fluctuation of charge-density-wave (CDW) instability. We propose that the CDW instability in $TlNi_2Se_2$ is driven by the saddle points mechanism, due to the existence of van Hove singularity very close to the Fermi energy. △ Less

Submitted 27 December, 2015; v1 submitted 7 July, 2015; originally announced July 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 92, 245129 (2015)

Identification of prototypical Brinkman-Rice Mott physics in a class of iron chalcogenides superconductors

Authors: X. H. Niu, S. D. Chen, J. Jiang, Z. R. Ye, T. L. Yu, D. F. Xu, M. Xu, Y. Feng, Y. J. Yan, B. P. Xie, J. Zhao, D. C. Gu, L. L. Sun, Qianhui Mao, Hangdong Wang, Minghu Fang, C. J. Zhang, J. P. Hu, Z. Sun, D. L. Feng

Abstract: The 122$^{*}$ series of iron-chalcogenide superconductors, for example K$_x$Fe$_{2-y}$Se$_{2}$, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their beha… ▽ More The 122$^{*}$ series of iron-chalcogenide superconductors, for example K$_x$Fe$_{2-y}$Se$_{2}$, only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the 122$^{*}$ series of iron-chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of 122$^{*}$ iron-chalcogenides, including the isovalently `doped' K$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$, Rb$_x$Fe$_{2-y}$Se$_{2-z}$Te$_z$ and (Tl,K)$_x$Fe$_{2-y}$Se$_{2-z}$S$_z$. We found that the bandwidths of the low energy Fe \textit{3d} bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known 122$^{*}$-series iron chalcogenides can be understood {\it via} one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity. △ Less

Submitted 23 February, 2016; v1 submitted 12 June, 2015; originally announced June 2015.

Comments: 19 pages, 5 figures

Journal ref: Phys. Rev. B 93, 054516 (2016)

The formation of the positive, fixed charge at c-Si(111)/a-Si$_3$N$_{3.5}$:H interfaces

Authors: L. E. Hintzsche, C. M. Fang, M. Marsman, M. W. P. E. Lamers, A. W. Weeber, G. Kresse

Abstract: Modern electronic devices are unthinkable without the well-controlled formation of interfaces at heterostructures. These often involve at least one amorphous material. Modeling such interfaces poses a significant challenge, since a meaningful result can only be expected by using huge models or by drawing from many statistically independent samples. Here we report on the results of high throughput… ▽ More Modern electronic devices are unthinkable without the well-controlled formation of interfaces at heterostructures. These often involve at least one amorphous material. Modeling such interfaces poses a significant challenge, since a meaningful result can only be expected by using huge models or by drawing from many statistically independent samples. Here we report on the results of high throughput calculations for interfaces between crystalline silicon (c-Si) and amorphous silicon nitride (a-Si$_3$N$_{3.5}$:H), which are omnipresent in commercially available solar cells. The findings reconcile only partly understood key features. At the interface, threefold coordinated Si atoms are present. These are caused by the structural mismatch between the amorphous and crystalline part. The local Fermi level of undoped c-Si lies well below that of a-SiN:H. To align the Fermi levels in the device, charge is transferred from the a-SiN:H part to the c-Si part resulting in an abundance of positively charged, threefold coordinated Si atoms at the interface. This explains the existence of a positive, fixed charge at the interface that repels holes. △ Less

Submitted 15 January, 2015; originally announced January 2015.

Journal ref: Phys. Rev. Applied 3, 064005 (2015)

Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides

Authors: B. Freelon, Yu Hao Liu, Jeng-Lung Chen, L. Craco, M. S. Laad, S. Leoni, Jiaqi Chen, Li Tao, Hangdong Wang, R. Flauca, Z. Yamani, Minghu Fang, Chinglin Chang, J. -H. Guo, Z. Hussain

Abstract: We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scatter… ▽ More We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality. △ Less

Submitted 1 January, 2015; originally announced January 2015.

MSC Class: revtex4

Journal ref: Phys. Rev. B 92, 155139 (2015)

Camelback-shaped band reconciles heavy electron behavior with weak electronic Coulomb correlations in superconducting TlNi2Se2

Authors: N. Xu, C. E. Matt, P. Richard, A. van Roekeghem, S. Biermann, X. Shi, S. -F. Wu, H. W. Liu, D. Chen, T. Qian, N. C. Plumb, M. Radovic, Hangdong Wang, Qianhui Mao, Jianhua Du, Minghu Fang, J. Mesot, H. Ding, M. Shi

Abstract: Using high-resolution photoemission spectroscopy and first-principles calculations, we characterize superconducting TlNi$_2$Se$_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides,… ▽ More Using high-resolution photoemission spectroscopy and first-principles calculations, we characterize superconducting TlNi$_2$Se$_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi$_2$Se$_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state specific heat coefficient (Phys. Rev. Lett. 112, 207001) in this weakly correlated compound. △ Less

Submitted 22 December, 2014; originally announced December 2014.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 92, 081116 (2015)

Anomalous Pressure Dependence of the Superconducting Transition Temperature in TlNi$_2$Se$_{2-x}$S$_x$

Authors: S. K. Goh, H. C. Chang, P. Reiss, P. L. Alireza, Y. W. Cheung, S. Y. Lau, Hangdong Wang, Qianhui Mao, Jinhu Yang, Minghu Fang, F. M. Grosche, M. L. Sutherland

Abstract: We report the pressure dependence of the superconducting transition temperature, $T_c$, in TlNi$_2$Se$_{2-x}$S$_x$ detected via the AC susceptibility method. The pressure-temperature phase diagram constructed for TlNi$_{2}$Se$_{2}$, TlNi$_{2}$S$_{2}$ and TlNi$_{2}$SeS exhibits two unexpected features: (a) a sudden collapse of the superconducting state at moderate pressure for all three composition… ▽ More We report the pressure dependence of the superconducting transition temperature, $T_c$, in TlNi$_2$Se$_{2-x}$S$_x$ detected via the AC susceptibility method. The pressure-temperature phase diagram constructed for TlNi$_{2}$Se$_{2}$, TlNi$_{2}$S$_{2}$ and TlNi$_{2}$SeS exhibits two unexpected features: (a) a sudden collapse of the superconducting state at moderate pressure for all three compositions and (b) a dome-shaped pressure dependence of $T_c$ for TlNi$_{2}$SeS. These results point to the nontrivial role of S substitution and its subtle interplay with applied pressure, as well as novel superconducting properties of the TlNi$_2$Se$_{2-x}$S$_x$ system. △ Less

Submitted 9 October, 2014; originally announced October 2014.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. B 90, 201105(R) (2014)

Multi-gap nodeless superconductivity in nickel chalcogenide TlNi2Se2

Authors: X. C. Hong, Z. Zhang, S. Y. Zhou, J. Pan, Y. Xu, Hangdong Wang, Qianhui Mao, Minghu Fang, J. K. Dong, S. Y. Li

Abstract: Low-temperature thermal conductivity measurements were performed on single crystals of TlNi$_2$Se$_2$, a nickel-chalcogenide heavy-electron superconductor with $T_c$ $\simeq$ 3.7 K. In zero field, the residual electronic contribution at $T$ $\rightarrow$ 0 K ($κ_0/T$) was well separated from the total thermal conductivity, which is less than 0.45\% of its normal-state value. Such a tiny residual… ▽ More Low-temperature thermal conductivity measurements were performed on single crystals of TlNi$_2$Se$_2$, a nickel-chalcogenide heavy-electron superconductor with $T_c$ $\simeq$ 3.7 K. In zero field, the residual electronic contribution at $T$ $\rightarrow$ 0 K ($κ_0/T$) was well separated from the total thermal conductivity, which is less than 0.45\% of its normal-state value. Such a tiny residual $κ_0/T$ is unlikely contributed by the nodal quasiparticles. Nodeless gap structure is supported by the very weak field dependence of $κ_0(H)/T$ in low magnetic fields. In the whole field range, $κ_0(H)/T$ exhibits an "$S$"-shape curve, as in the case of nickel pnictides BaNi$_2$As$_2$ and SrNi$_2$P$_2$. This common feature of nickel-based superconductors can be explained by multiple nodeless superconducting gaps. △ Less

Submitted 26 July, 2014; originally announced July 2014.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. B 90, 060504(R) (2014)

High-pressure single-crystal neutron scattering study of magnetic and Fe vacancy orders in (Tl,Rb)2Fe4Se5 superconductor

Authors: Feng Ye, Wei Bao, Songxue Chi, Antonio M. dos Santos, Jamie J. Molaison, Minghu Fang, Hangdong Wang, Qianhui Mao Jinchen Wang, Juanjuan Liu, Jieming Sheng

Abstract: The magnetic and iron vacancy orders in superconducting (Tl,Rb)2Fe4Se5 single-crystals are investigated by using a high-pressure neutron diffraction technique. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous disappearance at the critical pressure Pc = 8… ▽ More The magnetic and iron vacancy orders in superconducting (Tl,Rb)2Fe4Se5 single-crystals are investigated by using a high-pressure neutron diffraction technique. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous disappearance at the critical pressure Pc = 8.3 GPa. Combined with previously determined Pc for superconductivity, our phase diagram under pressure reveals the concurrence of the block AFM order, the iron vacancy order and superconductivity for the 245 superconductor. A synthesis of current experimental data in a coherent physical picture is attempted. △ Less

Submitted 12 December, 2014; v1 submitted 15 May, 2014; originally announced May 2014.

Comments: 4 pages, 4 figures

Journal ref: Chinese Phys. Lett. Vol. 31, 127401 (2014)

Qubit architecture with high coherence and fast tunable coupling

Authors: Yu Chen, C. Neill, P. Roushan, N. Leung, M. Fang, R. Barends, J. Kelly, B. Campbell, Z. Chen, B. Chiaro, A. Dunsworth, E. Jeffrey, A. Megrant, J. Y. Mutus, P. J. J. O'Malley, C. M. Quintana, D. Sank, A. Vainsencher, J. Wenner, T. C. White, Michael R. Geller, A. N. Cleland, John M. Martinis

Abstract: We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a ver… ▽ More We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamic coupling by implementing a novel adiabatic controlled-Z gate, at a speed approaching that of single-qubit gates. Integrating coherence and scalable control, our "gmon" architecture is a promising path towards large-scale quantum computation and simulation. △ Less

Submitted 28 February, 2014; originally announced February 2014.

Journal ref: Phys. Rev. Lett. 113, 220502(2014)

Exploration of iron-chalcogenide superconductors

Authors: ChiHeng Dong, Hangdong Wang, Minghu Fang

Abstract: Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-Tc superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as differ… ▽ More Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-Tc superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered. △ Less

Submitted 23 July, 2013; originally announced July 2013.

Comments: 12 pages, 14 figures; to be published on the "TOPICAL REVIEW: Iron-based high temperature superconductors" in Chinese Physics B

Journal ref: Chin. Phys. B, Vol. 22, No. 8 (2013) 087401

Superconductivity and Disorder Effect in TlNi$_2$Se$_{2-x}$S$_x$ crystals

Authors: Hangdong Wang, Chiheng Dong, Qianhui Mao, Rajwali Khan, Xi Zhou, Chenxia Li, Bin Chen, Jinhu Yang, Minghu Fang

Abstract: After our first discovery of superconductivity (SC) with $T_C$=3.7 K in TlNi$_2$Se$_2$, we grew successfully a series of TlNi$_2$Se$_{2-x}$S$_x$ (0.0 $\leq$ x $\leq$2.0) single crystals. The measurements of resistivity, susceptibility and specific heat were carried out. We found that SC with $T_C$=2.3 K also emerges in TlNi$_2$S$_2$ crystal, which appears to involve heavy electrons with an effecti… ▽ More After our first discovery of superconductivity (SC) with $T_C$=3.7 K in TlNi$_2$Se$_2$, we grew successfully a series of TlNi$_2$Se$_{2-x}$S$_x$ (0.0 $\leq$ x $\leq$2.0) single crystals. The measurements of resistivity, susceptibility and specific heat were carried out. We found that SC with $T_C$=2.3 K also emerges in TlNi$_2$S$_2$ crystal, which appears to involve heavy electrons with an effective mass $m^*$=13$\sim$25 $m_b$, as inferred from the normal state electronic specific heat and the upper critical field, $H_{C2}(T)$. It was found that the $T_C$ and superconducting volume fraction in TlNi$_2$Se$_{2-x}$S$_x$ crystals changes with the disorder degree induced by the partial substitution of S for Se, which is characterized by the residual resistivity ratio (\textit{RRR}). The effect of the disorder on SC may provide some information for understanding the mechanism of SC in this new Ni-chalcogenide system. △ Less

Submitted 28 May, 2015; v1 submitted 5 May, 2013; originally announced May 2013.

Comments: 5 papers, 6 figures

Journal ref: J. Phys.: Condens. Matter 27(2015)395701