Two ultra-stable novel allotropes of tellurium few-layers

At least four two-or quasi-one-dimensional allotropes and a mixture of them were theoretically predicted or experimentally observed for low-dimensional Te,namely theα,β,γ,δ,and chiral-α+δphases.Among them theγandαphases were found to be the most stable phases for monolayer and thicker layers,respectively.Here,we found two novel low-dimensional phases,namely theεandζphases.Theζphase is over 29 meV/Te more stable than the most stable monolayerγphase,and theεphase shows comparable stability with the most stable monolayerγphase.The energetic difference between theζandαphases reduces with respect to the increased layer thickness and vanishes at the four-layer(12-sublayer)thickness,while this thickness increases under change doping.Bothεandζphases are metallic chains and layers,respectively.Theζphase,with very strong interlayer coupling,shows quantum well states in its layer-dependent bandstructures.These results provide significantly insight into the understanding of polytypism in Te few-layers and may boost tremendous studies on properties of various few-layer phases.

Framework for Contrastive Learning Phases of Matter Based on Visual Representations

A main task in condensed-matter physics is to recognize,classify,and characterize phases of matter and the corresponding phase transitions,for which machine learning provides a new class of research tools due to the remarkable development in computing power and algorithms.Despite much exploration in this new field,usually different methods and techniques are needed for different scenarios.Here,we present SimCLP:a simple framework for contrastive learning phases of matter,which is inspired by the recent development in contrastive learning of visual representations.We demonstrate the success of this framework on several representative systems,including non-interacting and quantum many-body,conventional and topological.SimCLP is flexible and free of usual burdens such as manual feature engineering and prior knowledge.The only prerequisite is to prepare enough state configurations.Furthermore,it can generate representation vectors and labels and hence help tackle other problems.SimCLP therefore paves an alternative way to the development of a generic tool for identifying unexplored phase transitions.

Tuning the Mottness in Sr_(3)Ir_(2)O_(7)via Bridging Oxygen Vacancies

The electronic evolution of Mott insulators into exotic correlated phases remains puzzling,because of electron interaction and inhomogeneity.Introduction of individual imperfections in Mott insulators could help capture the main mechanism and serve as a basis to understand the evolution.Here we utilize scanning tunneling microscopy to probe the atomic scale electronic structure of the spin-orbit-coupling assisted Mott insulator Sr_(3)Ir_(2)O_(7).It is found that the tunneling spectra exhibit a homogeneous Mott gap in defect-free regions,but near the oxygen vacancy in the rotated Ir O_(2)plane the local Mott gap size is significantly enhanced.We attribute the enhanced gap to the locally reduced hopping integral between the 5d electrons of neighboring Ir sites via the bridging planar oxygen p orbitals.Such bridging defects have a dramatic influence on local bandwidth,thus provide a new way to manipulate the strength of Mottness in a Mott insulator.

Superconductivity in electron-doped arsenene

Based on the first-principles density functional theory electronic structure calculation,we investigate the possible phonon-mediated superconductivity in arsenene,a two-dimensional buckled arsenic atomic sheet,under electron doping.We find that the strong superconducting pairing interaction results mainly from the pz-like electrons of arsenic atoms and the A1 phonon mode around the K point,and the superconducting transition temperature can be as high as 30.8 K in the arsenene with 0.2 doped electrons per unit cell and 12%-applied biaxial tensile strain.This transition temperature is about ten times higher than that in the bulk arsenic under high pressure.It is also the highest transition temperature that is predicted for electron-doped two-dimensional elemental superconductors,including graphene,silicene,phosphorene,and borophene.

Quasi-One-Dimensional Free-Electron-Like States Selected by Intermolecular Hydrogen Bonds at the Glycine/Cu(100) Interface

We carry out ab initio density functional theory calculations to study manipulation of electronic structures of selfassembled molecular nanostructures on metal surfaces by investigating the geometric and electronic properties of glycine molecules on Cu(100).It is shown that a glycine monolayer on Cu(100)forms a two-dimensional hydrogen-bonding network between the carboxyl and amino groups of glycine using a first principles atomistic calculation on the basis of a recently found structure.This network includes at least two hydrogen-bonding chains oriented roughly perpendicular to each other.Through molecule–metal electronic hybridization,these two chains selectively hybridized with the two isotropic degenerate Cu(100)surface states,leading to two anisotropic quasi-one-dimensional surface states.Electrons occupying these two states can near-freely move from a molecule to its adjacent molecules directly through the intermolecular hydrogen bonds,rather than mediated by the substrate.This results in the experimentally observed anisotropic free-electron-like behavior.Our results suggest that hydrogen-bonding chains are likely candidates for charge conductors.

Symmetry-enforced two-dimensional Dirac node-line semimetals

Based on symmetry analysis and lattice model calculations,we demonstrate that Dirac nodal line(DNL)can stably exist in two-dimensional(2D)nonmagnetic as well as antiferromagnetic systems.We focus on the situations where the DNLs are enforced by certain symmetries and the degeneracies on the DNLs are inevitable even if spin–orbit coupling is strong.After thorough analysis,we find that five space groups,namely 51,54,55,57 and 127,can enforce the DNLs in 2D nonmagnetic semimetals,and four type-III magnetic space groups(51.293,54.341,55.355,57.380)plus eight type-IV magnetic space groups(51.299,51.300,51.302,54.348,55.360,55.361,57.387 and 127.396)can enforce the DNLs in 2D antiferromagnetic semimetals.By breaking these symmetries,the different 2D topological phases can be obtained.Furthermore,by the first-principles electronic structure calculations,we predict that monolayer YB4C4 is a good material platform for studying the exotic properties of 2D symmetry-enforced Dirac node-line semimetals.

Quantum anomalous hall effect in collinear antiferromagnetism

The two-dimensional Quantum Hall effect with no external magnetic field is called the Quantum anomalous Hall (QAH) effect. Sofar, experimentally realized QAH insulators all exhibit ferromagnetic order and the QAH effect only occurs at very low temperatures.On the other hand, up to now the QAH effect in collinear antiferromagnetic (AFM) materials has never been reported and thecorresponding mechanism has never been proposed. In this work, we realize the QAH effect by proposing a four-band lattice modelwith static AFM order, which indicates that the QAH effect can be found in AFM materials. Then, as a prototype, we demonstratethat a monolayer CrO can be switched from an AFM Weyl semimetal to an AFM QAH insulator by applying strain, based onsymmetry analysis and the first-principles electronic structure calculations. Our work not only proposes a scenario to search for QAHinsulators in materials, but also reveals a way to considerably increase the critical temperature of the QAH phase.

A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX(X=Cl,Br,I;C=S,Se,Te)

Two-dimensional magnets have received increasing attention since Cr_2Ge_2Te_6 and CrI_3 were experimentally exfoliated and measured in 2017. Although layered ferromagnetic metals were demonstrated at room temperature, a layered ferromagnetic semiconductor with high Curie temperature(Tc) is yet to be unveiled. Here, we theoretically predicted a family of high Tcferromagnetic monolayers, namely MnNX and CrCX(X = Cl, Br and I; C = S, Se and Te). Their Tcvalues were predicted from over 100 K to near 500 K with Monte Carlo simulations using an anisotropic Heisenberg model. Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy, i.e. ～10~1 for effective masses and ～10~2 for carrier mobilities, along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials.Together with the mobility anisotropy, it offers a spin-, dichroism-and mobility-anisotropy locking.These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices.

The melilite-type compound (Sr_(1-x),A_x)_2MnGe_2S_6O(A=K,La) being a room temperature ferromagnetic semiconductor

The seeking of room temperature ferromagnetic semiconductors, which take advantages of both the charge and spin degrees of freedom of electrons to realize a variety of functionalities in devices integrated with electronic, optical, and magnetic storage properties, has been a long-term goal of scientists and engineers. Here, by using the spin-polarized density functional theory calculations, we predict a new series of high temperature ferromagnetic semiconductors based on the melilite-type oxysulfide Sr_2MnGe_2S_6O through hole(K) and electron(La) doping. Due to the lack of strong antiferromagnetic superexchange between Mn ions, the weak antiferromagnetic order in the parent compound Sr_2MnGe_2S_6O can be suppressed easily by charge doping with either p-type or n-type carriers, giving rise to the expected ferromagnetic order. At a doping concentration of 25%, both the hole-doped and electron-doped compounds can achieve a Curie temperature(T_c) above 300 K. The underlying mechanism is analyzed.Our study provides an effective approach for exploring new types of high temperature ferromagnetic semiconductors.

强自旋轨道耦合下稳定的二维狄拉克线半金属

已经研究和报道较多的三维狄拉克节线半金属,具有新颖的准粒子激发现象及有趣的输运性质.然而,在强自旋轨道耦合作用下仍具有稳定的狄拉克线的二维材料却罕有报道.本文发现一个具体的有分数平移的C_(2v)×Z_(2)^(T)对称点群,在强自旋轨道耦合下也能保护稳定的二维狄拉克线.这种对称性可以存在于铋的一种新的同素异形体(砖相)中,也可以推广到具有相同对称性下的其他奇数层以及与铋同族的其他元素,如磷、砷、锑.三层铋砖相结构的价电子总数为30,能够被2整除,但不能被4整除,这正好保证了狄拉克线处于半填充状态而穿过费米能级.有趣的是,三层铋砖相结构的费米能级处除了狄拉克线之外不存在其他多余的态,使得铋砖相结构中出现了干净的狄拉克线.费米能级处干净的狄拉克线不受其他杂态的干扰,这更有利于实验上探测由狄拉克线诱导的相关输运性质.

An experimental setup with controllable tuning parameters for studying the Kondo systems

The Kondo physics has been studied since the 1960 s in condensed matter physics[1,2],which is mainly determined by two competing factors namely the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida(RKKY)interaction.

Distinct superconducting behaviors of pressurized WB2 and ReB2 with different local B layers

The recent discovery of superconductivity up to 32 K in the pressurized MoBreignites the interest in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5 d transition-metal diborides.Here we systematically investigate the responses of both structural and physical properties of WBand ReBto external pressure,which possess different types of boron layers. Similar to MoB, the pressure-induced superconductivity was also observed in WBabove 60 GPa with a maximum Tcof 15 K at 100 GPa, while no superconductivity was detected in ReBin this pressure range. Interestingly, the structures at ambient pressure for both WBand ReBpersist to high pressure without structural phase transitions. Theoretical calculations suggest that the ratio of flat boron layers in this class of transition-metal diborides may be crucial for the appearance of high Tc. The combined theoretical and experimental results highlight the effect of the geometry of boron layers on superconductivity and shed light on the exploration of novel high-Tcsuperconductors in borides.

Three-dimensional acetylenic modified graphene for highperformance optoelectronics and topological materials

Seeking carbon phases with versatile properties is one of the fundamental goals in physics,chemistry,and materials science.Here,based on the first-principles calculations,a family of three-dimensional(3D)graphene networks with abundant and fabulous electronic properties,including rarely reported dipole-allowed truly direct band gap semiconductors with suitable band gaps(1.07–1.87 eV)as optoelectronic/photovoltaic materials and topological nodal-ring semimetals,are proposed through stitching different graphene layers with acetylenic linkages.Remarkably,the optical absorption coefficients in some of those semiconducting carbon allotropes express possibly the highest performance among all of the semiconducting carbon phases known to date.On the other hand,the topological states in those topological nodal-ring semimetals are protected by the time-reversal and spatial symmetry and present nodal rings and nodal helical loops topological patterns.Those newly revealed carbon phases possess low formation energies and excellent thermodynamic stabilities;thus,they not only host a great potential in the application of optoelectronics,photovoltaics,and quantum topological materials etc.,but also can be utilized as catalysis,molecule sieves or Liion anode materials and so on.Moreover,the approach used here to design novel carbon allotropes may also give more enlightenments to create various carbon phases with different applications.