We focus on the study of exotic states of quantum matter by means of the first-principles methods of computational electronic-structure theory. We are particularly interested in studies of topological phenomena in strongly spin-orbit coupled systems, emergent phenomena at the interface of heterogeneous materials systems, thermopower generation and manipulation in low-dimensional systems, strongly correlated electron systems, and superconductivity. The primary goal of our research is to theoretically predict new physical phenomena in these systems and design materials with advanced electronic and magnetic functionalities. We are also interested in the development of new theoretical approaches and computational techniques to extend the reach and power of the available first-principles methods.

### Exploring quantum degrees of freedom of electrons in solids

Electron as a quantum object has many degrees of freedom, some inherent such as charge, orbital and spin and some acquired from the surrounding environment such as valley pseudospin and lattice pseudospin. The interplay between these degrees of freedom can have profound consequences on the electronic structure of a solid. Our research is focused on the study of this interplay and its resulting quantum phenomena in strongly spin-orbit coupled systems. Bismuth tellurohalides, a group of polar semiconductors made of heavy elements, are one example for which our studies have revealed many novel quantum effects such as the bulk Rashba spin splitting, non-trivial Berry phase, topological phase transition, strong magneto-optical responses and distinct magnetotransport properties, all resulting from the spin-orbit coupling under broken spatial inversion symmetry. Another group of systems, which appear to host an even more complicated interplay between their electronic degrees of freedom, is transition metal dichalcogenides MX2. In these systems which are composed of MX2 layers (where M is a transition metal such as Mo or W and X is a chalcogen such as S or Se), stacking weakly along a certain crystalline direction, in addition to spin-orbit coupling the valley and layer degrees of freedom play an equally important role. Our first-principles studies combined with a variety of experimental techniques have enabled us to unravel the physics behind many unusual quantum phenomena observed in these systems. For example, we have found that as a result of spin-valley locking, the superconductivity in MoS2 prefers an Ising-like pairing leading to an exceptionally high upper critical field. Due to the same mechanism, we have demonstrated that a hidden but yet strong spin polarization can exist in WSe2 and materials alike, despite the presence of a global inversion symmetry. We have also discovered that the electrons accumulated in the energy valleys of WSe2 can form a Fermi liquid regime exhibiting a strong many body effect, manifested by the negative electronic compressibility and tunable valley spin splitting. It’s been also shown that the crossing between the spin-polarized bands of MoTe2 can be topologically protected leading to the formation of type-II Weyl fermions. All together, these unusual properties make transition metal dichalcogenides a promising candidate for a variety of spintronic as well as valleytronic applications.

Hidden spin polarization in bulk 2H-MX_{2} transition metal dichalcogenides.