Recent progress in two-dimensional transition metal<?A3B2 ACK?>dichalcogenides

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been emerged as a new class of van der Waals materials since the successful isolation of graphene. The strong spin-orbit coupling (SOC) and two-dimensionality give rise to plenty of novel physics including metal-insulator transition, charge density wave (CDW), valleytronics, quantum spin Hall effect, and unconventional superconductivity, which make TMDCs an ideal platform to study the fundamental physics and potential applications. In this review, we firstly introduce the crystal structure of 2D TMDCs materials. Then, we summarize the recent progress in the synthesis, novel physical properties, and applications of 2D TMDCs materials. Finally, a summary and an outlook on the topological superconductivity in this field are presented. 2D TMDCs have a chemical formula of MX2 (M=W, Mo and X=Te, Se, S) with a layered crystal structure. Depending on the coordination environments of M, TMDCs can crystallize in a variety of polytypic structures such as 2 H , 1 T , 1 T ′, and T d phases. In the monolayer 2 H -TMDCs, the breaking of an in-plane mirror symmetry and the presence of the out-of-plane mirror symmetry lead to an Ising spin–orbit coupling (SOC), which serves as an effective out-of-plane field acting on the copper pair and pins the electron spins to out-of-plane directions. This is called Ising superconductivity, which has been observed in gated MoS2, monolayer NbSe2 and TaS2. However, due to the inversion symmetry in monolayer 1 T ′-TMDCs, the introduction of SOC makes them a class of large-gap quantum spin Hall insulators such as monolayer 1 T ′-WTe2. Therefore, if we could consecutively tailor the TMDCs’ structure from 2 H to T d phase, the long-sought topological superconductivity may be realized in one substance by incorporating superconductivity and quantum spin Hall effect together. To explore the extraordinary physics and nanodevice applications, we develop a universal molten-salt-assisted chemical vapor deposition (CVD) method to prepare atomically thin TMDCs, including high-quality 2D superconductors such as monolayer MoTe2 and NbSe2. With the powerful sample growth technique , we demonstrate for the first time that a consecutive structural phase transition from T d to 1 T ′ to 2 H polytype can be realized by increasing the Se concentration in Se-substituted MoTe2. More importantly, the Se-substitution is found to dramatically enhance the superconductivity of the MoTe2 thin film, which is interpreted as the introduction of two-band superconductivity. Furthermore, in bilayer 1 T d-MoTe2, we find that the in-plane upper critical field goes beyond the Pauli paramagnetic limit and shows an emergent two-fold symmetry, which is different from the isotropic in-plane upper critical field in 2 H -TMDCs. We show that this is a result of a new type of asymmetric SOC in 1 T d-TMDCs, which has expanded the well-known Ising SOC. The polytypic structures and strong SOC in 2D TMDCs have led to a variety of novel physics and applications. Recent theoretical works have already shown that 2D TMDCs can be a platform to search for topological superconductivity. With the further development of sample preparation, 2D TMDCs will play an important role in realization of topological quantum computation.