Origin of extremely large magnetoresistance in the candidate type-II Weyl semimetal MoTe2−x

Abstract

The recent observation of extremely large magnetoresistance (MR) in the transition-metal dichalcogenide MoTe2 has attracted considerable interest due to its potential technological applications as well as its relationship with novel electronic states predicted for a candidate type-II Weyl semimetal. In order to understand the origin of the MR, the electronic structure of MoTe2−x (x = 0.08) is systematically tuned by application of pressure and probed via its Hall and longitudinal conductivities. With increasing pressure, a monoclinic-to-orthorhombic (1 T′ to Td) structural phase transition temperature (T*) gradually decreases from 210 K at 1 bar to 58 K at 1.1 GPa, and there is no anomaly associated with the phase transition at 1.4 GPa, indicating that a T = 0 K quantum phase transition occurs at a critical pressure (Pc) between 1.1 and 1.4 GPa. The large MR observed at 1 bar is suppressed with increasing pressure and is almost saturated at 100% for P > Pc. The dependence on magnetic field of the Hall and longitudinal conductivities of MoTe2−x shows that a pair of electron and hole bands are important in the low-pressure Td phase, while another pair of electron and hole bands are additionally required in the high-pressure 1 T′ phase. The MR peaks at a characteristic hole-to-electron concentration ratio (nc) and is sharply suppressed when the ratio deviates from nc within the Td phase. These results establish the comprehensive temperature-pressure phase diagram of MoTe2−x and underscore that its MR originates from balanced electron-hole carrier concentrations.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

“…253 Later, the XMR behaviour was discovered in the Td phase of MoTe 2 . 254,255 Furthermore, the magnetoresistance observed in Td MoTe 2 in high magnetic fields can be tuned by an applied tensile strain (0.5%). 256 At low temperatures and in high magnetic fields, the magnetoresistance variation approaches 30% under uniaxial tensile strain perpendicular to the zigzag chain direction, while the strain along the zigzag chain direction effectively suppresses the magnetoresistance.…”

Electronic band tuning under pressure in MoTe2 topological semimetal

“…14 while magnetoresistance goes down. 22 The same nodes could not be traced in the band dispersions from calculations performed at higher pressures. At pressure values exceeding 1.2 GPa, the 1T′ symmetry is centrosymmetric and no Weyl nodes are expected.…”

“…Above 1 GPa, a broad transition from the T d to 1T′ with apparent phase coexistence is observed. Coupled with the transition to the monoclinic phase under pressure is suppression of the magnetoresistance 22 which may be linked to the disappearance of the Weyl nodes. Shown in Fig.…”

“…Moreover, changes in the Berry curvature may be linked to the decrease in magnetoresistance under pressure. 22 Last, we comment on the prospect of topological superconductivity in the material. While superconductivity is observed in MoTe 2 in the T d and 1T′ phases under a range of pressures, topological superconductivity is not guaranteed unless the following two conditions are satisfied: In (i), the Weyl fermions have to either be responsible or take part in the superconducting pairing.…”

Synthesis of Large-Scale Monolayer 1T′-MoTe2 and Its Stabilization via Scalable hBN Encapsulation

“…In the second metastable polymorph, i.e ., 1T′-MoTe 2 , Mo is in the center of a distorted octahedron of Te atoms, resulting in a monoclinic structure or distorted 1T-MoTe 2 . 13 1T′-MoTe 2 is a semimetal 14 that exhibits large magnetoresistance 15 and superconductivity. 16 1T′-MoTe 2 is also expected to be a type-II Weyl semimetal in bulk form 17 and, as a monolayer, a 2D topological and large-gap quantum spin Hall (QSH) insulator, 18 properties that can be exploited in spintronic and quantum computational applications.…”