Abstract:We report a new class of large-gap quantum spin Hall insulators in two-dimensional transition metal dichalcogenides, namely, MX 2 with M=(Mo, W) and X=(S, Se, and Te), whose topological electronic properties are highly tunable by external electric field. We propose a novel topological field effect transistor made of these atomic layer materials and their van der Waals heterostructures. Our device exhibits parametrically enhanced charge-spin conductance through topologically protected transport channels, and can be rapidly switched off by electric field through topological phase transition instead of carrier depletion. Our work provides a practical material platform and device architecture for topological quantum electronics.The discovery of graphene (1) leads to vigorous explorations of two-dimensional (2D) materials (2), revealing a wide range of extraordinary properties (3-5) and functionalities (6, 7). Owing to atomic thickness, 2D materials can be horizontally patterned through chemical and mechanical techniques (8). Moreover, the weak van der Waals (vdW) interaction between adjacent layers enables vertical stacking of different 2D materials, forming vdW heterostructures (9). Although being intensively explored for fundamental research and technological applications, the vast family of 2D materials and their vdW heterostructures have been largely underexploited for topological phases of matter (10, 11), in particular, quantum spin Hall (QSH) insulators (12-18).QSH insulators have an insulating bulk but conducting edge states that are topologically protected from backscattering by time reversal symmetry. Quantized conductance through QSH edge states have been experimentally demonstrated in HgTe/CdTe (15, 16) and InAs/GaSb (19,20) quantum wells. This could in principle provide an alternative route to quantum electronic devices with low dissipation. However, the realization of such QSH-based devices for practical applications is impeded by three critical factors: (a) band gaps of existing QSH insulators are too small, which limits the operating regime to low temperatures. This has motivated an intensive search for large-gap QSH materials (21-25); (b) the small number of conducting channels (e 2 /h per edge) results in a small signal-to-noise ratio; and (c) efficient methods of fast ON/OFF switching are lacking.Here we show that 2D materials provide a practical platform for developing novel topological electronic devices that can potentially overcome the above hurdles. Specifically, based on firstprinciples calculations, we find a new class of large-gap (>0.1eV) QSH insulators in 2D transition metal dichalcogenides (TMDCs) MX 2 with M=(Mo, W) and X = (S, Se, Te). We demonstrate the possibility of a novel vdW-heterostructured topological field effect transistor (vdW-TFET) made of 2D atomic layer materials. Our device exhibits a parametrically enhanced conductance through QSH edge channels in the ON state, and can be rapidly switched OFF via a topological phase transition by applying an electric field. O...

Topological crystalline insulators are new states of matter in which the topological nature of electronic structures arises from crystal symmetries. Here we predict the first material realization of topological crystalline insulator in the semiconductor snTe by identifying its non-zero topological index. We predict that as a manifestation of this non-trivial topology, snTe has metallic surface states with an even number of Dirac cones on high-symmetry crystal surfaces such as {001}, {110} and {111}. These surface states form a new type of high-mobility chiral electron gas, which is robust against disorder and topologically protected by reflection symmetry of the crystal with respect to {110} mirror plane. Breaking this mirror symmetry via elastic strain engineering or applying an in-plane magnetic field can open up a continuously tunable band gap on the surface, which may lead to wide-ranging applications in thermoelectrics, infra-red detection and tunable electronics. Closely related semiconductors PbTe and Pbse also become topological crystalline insulators after band inversion by pressure, strain and alloying.

The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal FeSn, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin-orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.

Three-dimensional topological crystalline insulators were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential for device applications. We demonstrate that thin films of SnTe and Pb1−xSnxSe(Te) grown along the (001) direction are topologically nontrivial in a wide range of film thickness and carry conducting spinfiltered edge states that are protected by the (001) mirror symmetry via a topological invariant. Application of an electric field perpendicular to the film will break the mirror symmetry and generate a band gap in these edge states. This functionality motivates us to propose a novel topological transistor device, in which charge and spin transport are maximally entangled and simultaneously controlled by an electric field. The high on/off operation speed and coupling of spin and charge in such a device may lead to electronic and spintronic applications for topological crystalline insulators. PACS numbers:Crystal structure and symmetry play a fundamental role in determining electronic properties of quantum materials. The interplay between crystallography and electronic topology[1, 2] has advanced our understanding of topological insulators [3][4][5]. More recently, a new type of topological phases termed topological crystalline insulators [6] has been predicted [7] and observed [8][9][10] in three-dimensional materials SnTe and Pb 1−x Sn x Se(Te). A key characteristic of topological crystalline insulators is the presence of metallic boundary states that are protected by crystal symmetry, rather than time-reversal [11]. As a consequence, these states can acquire a band gap under perturbations that break the crystal symmetry [7,12]. This novel property opens up an unprecedented functionality of tuning the charge and spin transport of topological boundary states with high on/off speed by applying an electric field. Here we theoretically demonstrate that thin films of SnTe and Pb 1−x Sn x Se(Te) grown along the (001) direction realize a new, two-dimensional topological crystalline insulator phase that supports spin-filtered edge states with a band gap tunable by electric field effect. Our work may thus enable electronic and spintronic device applications based on topological crystalline insulators.Topological crystalline insulators (TCI) have so far only been realized in three-dimensional materials [7][8][9][10]. In this work, we propose a material realization of a twodimensional (2D) topological crystalline insulator phase, which is topologically distinct from an ordinary insulator in the presence of mirror symmetry about the 2D plane.The topology here is mathematically characterized by two integer topological invariants N + and N − , which are Chern numbers of Bloch states with opposite mirror eigenvalues. While the sum N + + N − determines the quantized Hall conductance, the mirror Chern number [2] defined by N M ≡ (N + − N − )/2 serves as...

Supplementary data are available at Bioinformatics online.

Topological crystalline insulators (TCIs) are new states of matter whose topological distinction relies on the crystal symmetry of periodic solids. The first material realization of TCIs has recently been predicted and observed in IV-VI semiconductor SnTe and related alloys. By combining k • p theory and band structure calculation, we present a unified approach to study topological surface states on various crystal surfaces of these TCI materials based on the electronic structure of the bulk. Depending on the surface orientation, we find two types of surface states with qualitatively different properties. In particular, the (111) surface states consist of four Dirac cones centered at¯ andM, while Dirac cones on (001) and (110) surfaces are located at non-time-reversal-invariant momenta. The latter types of surface states exhibit a Lifshitz transition as a function of Fermi energy, which is accompanied by a Van Hove singularity in the density of states arising from saddle points in the band structure.

Summary Touch perception begins with activation of low-threshold mechanoreceptors (LTMRs) in the periphery. LTMR terminals exhibit tremendous morphological heterogeneity that specifies their mechanical receptivity. In a survey of mammalian skin, we found a preponderance of neurofilament-heavy chain+ circumferential endings associated with hair follicles, prompting us to develop a genetic strategy to interrogate these neurons. Targeted in vivo recordings revealed them to be Aβ Field-LTMRs, identified 50 years ago but largely elusive thereafter. Remarkably, while Aβ Field-LTMRs are highly sensitive to gentle stroking of the skin, they are unresponsive to hair deflection, and they encode skin indentation in the noxious range across large, spotty receptive fields. Individual Aβ Field-LTMRs form up to 180 circumferential endings, making them the most anatomically expansive LTMR identified to date. Thus, Aβ Field-LTMRs are a major mammalian LTMR subtype that forms circumferential endings in hairy skin, and their sensitivity to gentle skin stroking arises through integration across many low-sensitivity circumferential endings.

Monte Carlo simulation is an unbiased numerical tool for studying classical and quantum manybody systems. One of its bottlenecks is the lack of general and efficient update algorithm for large size systems close to phase transition,for which local updates perform badly. In this work, we propose a new general-purpose Monte Carlo method, dubbed self-learning Monte Carlo (SLMC), in which an efficient update algorithm is first learned from the training data generated in trial simulations and then used to speed up the actual simulation. We demonstrate the efficiency of SLMC in a spin model at the phase transition point, achieving a 10-20 times speedup.