In topological insulators (TIs), metallic surface conductance saturates the insulating bulk resistance with decreasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating from metallic surface modes protected by time reversal symmetry (TRS). Such characteristic has been found in several materials including Bi2Te2Se, SmB6 etc. Recently, similar behavior has been observed in metallic compound LaSb, accompanying an extremely large magetoresistance (XMR). Shubnikov-de Hass (SdH) oscillation at low temperatures further confirms the metallic behavior of plateau region under magnetic fields. LaSb [1] has been proposed by the authors as a possible topological semimetal (TSM), while negative magnetoresistance is absent at this moment. Here, high quality single crystals of NbAs2/TaAs2 with inversion symmetry have been grown and the resistivity under magnetic field is systematically investigated. Both of them exhibit metallic behavior under zero magnetic field, and a metal-to-insulator transition occurs when a nonzero magnetic field is applied, resulting in XMR (1.0×10 5 % for NbAs2 and 7.3×10 5 % for TaAs2 at 2.5 K & 14 T). With temperature decreased, a resistivity plateau emerges after the insulator-like regime and SdH oscillation has also been observed in NbAs2 and TaAs2.

We performed comparable polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals.Fifteen Raman-active phonon modes (10Ag+5Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were well assigned, and all the symmetry-allowed Raman modes (11A1+6A2) in the low-symmetry phase Td-MoTe2 (10 K) and 12 Raman phonons (8A1+4A2) in Td-WTe2 were observed and identified. The present work provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance (MR) in this class of materials.

YSb crystals are grown and the transport properties under magnetic field are measured. The resistivity exhibits metallic behavior under zero magnetic field and the low temperature resistivity shows a clear upturn once a moderate magnetic field is applied. The upturn is greatly enhanced by increasing magnetic field, finally resulting in a metal-to-insulator-like transition. With temperature further decreased, a resistivity plateau emerges after the insulator-like regime. At low temperature (2.5 K) and high field (14 T), the transverse magnetoresistance (MR) is quite large (3.47 × 10 4 %). In addition, Shubnikov-de Haas (SdH) oscillation has also been observed in YSb. Periodic behavior of the oscillation amplitude reveals the related information about Fermi surface and two major oscillation frequencies can be obtained from the FFT spectra of the oscillations. The trivial Berry phase extracted from SdH oscillation, band structure revealed by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations demonstrate that YSb is a topologically trivial material. 71.30.+h, 72.15.Eb * These authors contributed equally to this paper †

We report the synthesis of high quality single crystals of BaMnBi2 and investigate the transport properties of the samples. The Hall data reveals electron-type carriers and a mobility µ(5K) = 1500cm 2 /V s. The temperature dependence of magnetization displays behavior that is different from CaMnBi2 or SrMnBi2, which suggests the possible different magnetic structure of BaMnBi2. Angle-dependent magnetoresistance reveals the quasitwo-dimensional Fermi surface. A crossover from semiclassical MR∼H 2 dependence in low field to MR∼H dependence in high field is observed in transverse magnetoresistance. Our results indicate the anisotropic Dirac fermion states in BaMnBi2.

We present the first experimental evidence of an intriguing phase transition between distinct topological states in the novel type-II Weyl semimetal MoTe2. We observe anomalies in Raman phonon frequencies and linewidths as well as electronic quasielastic peaks around 70 K, which together with structural, thermodynamic measurements and electron-phonon coupling (EPC) calculation demonstrate a temperature-induced transition between two topological phases previously identified by contrasting spectroscopic measurements. Analysis of experimental data suggests electron-phonon coupling as the main driving mechanism for the change of key topological characters in the electronic structure of MoTe2. We also find the phase transition sensitive to sample conditions distinguished by synthesis methods. These discoveries of temperature and material condition dependent topological phase evolution and transition in MoTe2 advance fundamental understanding of the underlying physics and enable an effective approach to tuning Weyl semimetal states for technological applications.PACS numbers: 63.20. Kr, 74.62.Bf, 71.90.+q, Recent years have seen the dramatic rise of a new class of quantum materials whose electronic states exhibit symmetry protected topological orders [1,2]. Such electronic states are insensitive to local decoherence processes, thus offering great promise for constructing quantum computing and high-speed electronic and spintronics devices. Distinct topological states, such as topological insulators, Dirac semimetals, and type-I and type-II Weyl semimetals, have been theoretically [1][2][3][4][5][6][7][8] proposed and experimentally [9-18] realized in real materials. A central task in this research field is to unravel the material and environment (e.g., pressure, temperature, etc.) conditions conducive to the existence of topologically ordered phases. To this end, it is essential to be able to induce and control the phase transition that allows an effective manipulation of the unique properties of the topological states. Significant progress has been made in understanding transitions between topologically trivial and nontrivial states. Recent theoretical studies have shown that strain [19,20], phonon [21,22] and/or disorder [23] can induce topological phase transitions that greatly influence electronic states and properties. For instance, when a topological transition occurs, topological surface states dramatically change [8][9][10][11][12][13], greatly impacting the novel transport behaviors [15][16][17][18]24], and phonon modes strongly coupled to electrons may also behave anomalously [25,26]. A recent experiment revealed that a structural transition can act as a switch of the topological phase transition [27]. Meanwhile, however, transitions between distinct topologically ordered states have remained largely unexplored, especially on the experimental front.Molybdenum ditelluride (MoTe 2 ), a type-II Weyl semimetal (WSM) [19,20,28], offers an excellent platform to probe distinct topological phases and possible transiti...