Although the richness of spatial symmetries has led to a rapidly expanding inventory of possible topological crystalline (TC) phases of electrons, physical realizations have been slow to materialize ...due to the practical difficulty in ascertaining band topology in realistic calculations. Here, we integrate the recently established theory of symmetry indicators of band topology into first-principles band-structure calculations, and test it on a database of previously synthesized crystals. On applying our algorithm to just 8 out of the 230 space groups, we are able to efficiently unearth topological materials and predict a diversity of topological phenomena, including: a screw-protected three-dimensional TC insulator, β-MoTe2, with gapped surfaces except for one-dimensional helical hinge states; a rotation-protected TC insulator, BiBr, with coexisting surface Dirac cones and hinge states; non-centrosymmetric Z2 topological insulators undetectable using the well-established parity criterion, AgXO (X = Na, K, Rb); a Dirac semimetal MgBi2O6; a Dirac nodal-line semimetal AgF2; and a metal with three-fold degenerate band crossing near the Fermi energy, AuLiMgSn. Our work showcases how recent theoretical insights into the fundamentals of band structures can aid in the practical goal of discovering new topological materials.Symmetry labels of materials under certain space groups can be used to indicate their band topology. Integrating that into first-principles band-structure calculations, new topological materials with a diversity of topological phenomena are discovered.
Over the past decade, topological materials-in which the topology of electron bands in the bulk material leads to robust, unconventional surface states and electromagnetism-have attracted much ...attention. Although several theoretically proposed topological materials have been experimentally confirmed, extensive experimental exploration of topological properties, as well as applications in realistic devices, has been restricted by the lack of topological materials in which interference from trivial Fermi surface states is minimized. Here we apply our method of symmetry indicators to all suitable nonmagnetic compounds in all 230 possible space groups. A database search reveals thousands of candidate topological materials, of which we highlight 241 topological insulators and 142 topological crystalline insulators that have either noticeable full bandgaps or a considerable direct gap together with small trivial Fermi pockets. Furthermore, we list 692 topological semimetals that have band crossing points located near the Fermi level. These candidate materials open up the possibility of using topological materials in next-generation electronic devices.
Very recently, a novel two-dimension (2D) MXene, MoSi2N4, was successfully synthesized with excellent ambient stability, high carrier mobility, and moderate band gap (2020 Science 369 670). In this ...work, the intrinsic lattice thermal conductivity of monolayer MoSi2N4 is predicted by solving the phonon Boltzmann transport equation based on the first-principles calculations. Despite the heavy atomic mass of Mo and complex crystal structure, the monolayer MoSi2N4 unexpectedly exhibits a quite high lattice thermal conductivity over a wide temperature range between 300 to 800 K. At 300 K, its in-plane lattice thermal conductivity is 224 Wm−1 K−1. The detailed analysis indicates that the large group velocities and small anharmonicity are the main reasons for its high lattice thermal conductivity. We also calculate the lattice thermal conductivity of monolayer WSi2N4, which is only a little smaller than that of MoSi2N4. Our findings suggest that monolayer MoSi2N4 and WSi2N4 are potential 2D materials for thermal transport in future nano-electronic devices.
Symmetry formulated by group theory plays an essential role with respect to the laws of nature, from fundamental particles to condensed-matter systems. Here, by combining symmetry analysis and model ...calculations, we elucidate that the crystallographic symmetry groups of a vast number of magnetic materials with light elements, in which the neglect of relativistic spin-orbit coupling (SOC) is an appropriate approximation, are considerably larger than the conventional magnetic groups. Thus, a symmetry description that involves partially decoupled spin and spatial rotations, dubbed spin group, is required. We derive the classifications of spin point groups describing coplanar and collinear magnetic structures, and the irreducible corepresentations of spin space groups illustrating more energy degeneracies that are disallowed by magnetic groups. One consequence of the spin group is the new antiunitary symmetries that protect SOC-freeZ2topological phases with unprecedented surface-node structures. Our work not only manifests the physical reality of materials with weak SOC, but also sheds light on the understanding of all solids with and without SOC by a unified group theory.
Spin-orbit coupling (SOC), which is the core of many condensed-matter phenomena such as nontrivial band gap and magnetocrystalline anisotropy, is generally considered appreciable only in heavy ...elements. This is detrimental to the synthesis and application of functional materials. Therefore, amplifying the SOC effect in light elements is crucial. Herein, focusing on 3d and 4d systems, we demonstrate that the interplay between crystal symmetry and electron correlation can significantly enhance the SOC effect in certain partially occupied orbital multiplets through the self-consistently reinforced orbital polarization as a pivot. Thereafter, we provide design principles and comprehensive databases, where we list all the Wyckoff positions and site symmetries in all two-dimensional (2D) and three-dimensional crystals that could have enhanced SOC effect. Additionally, we predict nine material candidates from our selected 2D material pool as high-temperature quantum anomalous Hall insulators with large nontrivial band gaps of hundreds of meV. Our study provides an efficient and straightforward way for predicting promising SOC-active materials, relieving the use of heavy elements for next-generation spin-orbitronic materials and devices.
Recently, two-dimensional monolayer MoSi2N4 with hexagonal structure was successfully synthesized in experiment (Hong et al. 2020 Science 369, 670). The fabricated monolayer MoSi2N4 is predicted to ...have excellent mechanical properties. Motived by the experiment, we perform first-principles calculations to investigate the mechanical properties of monolayer MoSi2N4, including its ideal tensile strengths, critical strains, and failure mechanisms. Our results demonstrate that monolayer MoSi2N4 can withstand stresses up to 51.6 and 49.2 GPa along zigzag and armchair directions, respectively. The corresponding critical strains are 26.5% and 17.5%, respectively. For biaxial strain, the ideal tensile strength is 50.6 GPa with a critical strain of 19.5%. Compared with monolayer MoS2, monolayer MoSi2N4 possesses much higher elastic moduli and ideal tensile strengths for both uniaxial and biaxial strains. Interestingly, the critical strain and failure mechanism of zigzag direction in MoSi2N4 are almost the same as those of armchair direction in MoS2, while the critical strain and failure mechanism of armchair direction for MoSi2N4 are similar to the ones of zigzag direction for MoS2. Our work reveals the remarkable mechanical characteristics of monolayer MoSi2N4.
•For biaxial strain, the ideal tensile strength of MoSi2N4 monolayer is 50.6 GPa with a critical strain of 19.5%.•The critical strains of MoSi2N4 monolayer are 26.5% and 17.5% along zigzag and armchair directions, respectively.•The elastic moduli and ideal tensile strengths MoSi2N4 monolayer are much higher than those of MoS2 monolayer.•The failure mechanisms in MoSi2N4 monolayer are interesting.
Valleytronics rooted in the valley degree of freedom is of both theoretical and technological importance as it offers additional opportunities for information storage, as well as electronic, magnetic ...and optical switches. In analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, here we introduce a new member of ferroic family, that is, a ferrovalley material with spontaneous valley polarization. Combining a two-band k·p model with first-principles calculations, we show that 2H-VSe
monolayer, where the spin-orbit coupling coexists with the intrinsic exchange interaction of transition-metal d electrons, is such a room-temperature ferrovalley material. We further predict that such system could demonstrate many distinctive properties, for example, chirality-dependent optical band gap and, more interestingly, anomalous valley Hall effect. On account of the latter, functional devices based on ferrovalley materials, such as valley-based nonvolatile random access memory and valley filter, are contemplated for valleytronic applications.
In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Here, we discover that the nanobelts of Weyl semimetal NbAs maintain a high ...mobility even in the presence of a high sheet carrier density. We develop a growth scheme to synthesize single crystalline NbAs nanobelts with tunable Fermi levels. Owing to a large surface-to-bulk ratio, we argue that a 2D surface state gives rise to the high sheet carrier density, even though the bulk Fermi level is located near the Weyl nodes. A surface sheet conductance up to 5-100 S per □ is realized, exceeding that of conventional 2D electron gases, quasi-2D metal films, and topological insulator surface states. Corroborated by theory, we attribute the origin of the ultrahigh conductance to the disorder-tolerant Fermi arcs. The evidenced low-dissipation property of Fermi arcs has implications for both fundamental study and potential electronic applications.
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