Dramatically decreased magnetoresistance in non-stoichiometric WTe2 crystals.

Magnetoresistance (MR) is the change of electrical resistance under the application of a magnetic field. The materials with large MR can generate great potential applications in magnetic sensors1, magnetic information storage2, and so on. Recently, giant (~1.5 × 105% at 2 K and 9 Tesla magnetic field) and non-saturated MR is observed in two-dimensional (2D) layered transition-metal dichalcogenides WTe23. It leads to a series of works to study the novel physical properties of WTe2, such as superconductivity with Tc as high as 7 K under external mechanical pressure4, and possible quantum spin Hall effect in monolayer WTe2 with bulk electronic energy band-gap as large as 100.0 meV5. Certainly, the origin of extremely large MR is not only an important physical problem, but also valuable to device application of WTe2. As it was proposed, the main physical origin of giant MR is attributed to the nearly perfect compensation of electron and hole pockets3. This opinion is supported by subsequent Fermi surface determination by angle-resolved photoemission spectroscopy (ARPES)6, as well as suppressed MR under external mechanical pressure7. However, recent work based on detailed ARPES claims that there are more subtle details in electronic band structure and abstract Fermi surface morphology in WTe28. Quantum oscillation of MR (Shubnikov-de-Haas oscillation) substantiates that there are multiple fermion pockets9. Some studies find evidences for spin-orbit split bands in WTe2. Spin-orbit split bands suppress the inter- and intra-band backscattering10. In addition, a latest study implies that the large MR in WTe2 also may be related to the crystal quality or carrier mobility11, and the more apparent decreased effect of MR to aliovalent doping (Re and Ta) over simple isovalent substitution (Mo-doping) and the different growth method also support it12. The physical origin of extremely large MR observed in WTe2 therefore is still an open question. In this work, considering the above-mentioned confusion, we intentionally introduce the non-stoichiometry and isovalent doping Mo in WTe2 to investigate the dependence among electron-hole asymmetry, carrier mobility and MR. Our systematic MR and Hall effect measurements substantiate the extremely large MR in stoichiometric WTe2. But large MR is disappeared in non-stoichiometric and isovalent Mo substitution WTe2 crystals. Based on analysis of magnetic-field dependent MR, both enlarged electron-hole concentration asymmetry and decreased mobility synergistically lead to the decreased MR in doped WTe2. From the viewpoint of real application, our result suggests that significant MR is strongly dependent on the stoichiometry of WTe2.