Research

Topological Insulators

(a) The conducting surface states of a topological insulator and (b) the corresponding bulk and surface band structure. Adapted from Tokura et al., Nature Review Physics 1, 126 (2019).

Topological insulators (TIs) possess a unique electronic band structure, in which the valence band and the conduction band hybridize and become inverted due to strong spin-orbit coupling, making the interior of TIs insulating while the surface is conductive. The conducting surface states form a Dirac cone that is protected by time-reversal symmetry and hence are spin-polarized, making TIs a valuable material for spintronic applications. Moreover, if the time reversal symmetry of the surface band is broken by ferromagnetic order, a magnetic gap may be open at the Dirac point, leading to more intriguing physics such as the quantum anomalous Hall effect, and axion insulator state. However, to date the manifestation of these quantum states still requires delicate material engineering and demanding experimental conditions. We aim to improve this situation by scrutinizing the phase transition mechanism and quantum transport behavior in TIs, as well as exploring new materials that potentially can show these exotic quantum states with more accessible approaches. 

2D materials

Since the discovery of graphene in 2004, condensed matter physics has undergone a revolutionary change owing to the new physics revealed in the study of graphene. The knowledge gained from graphene research inspired the study of other van der Waals (vdW) materials, leading to a whole new research area - 2D materials. Nowadays, all the 2D vdW materials together become a big group that span a broad spectrum of electronic properties, from insulators to semiconductors to semimetals, rendering 2D materials much more promising for future technology applications. 

In 2018, it was found that the insulator-superconductor transition can be realized in the twisted bilayer graphene (TBG). This discovery rapidly changed the landscape of graphene research. Later, scientists further realized ferromagnetic phase and the quantum anomalous Hall state in TBG. The key of these phase transitions is the strong correlation of charge carriers. Twistronics, a new research field studying the interactions between strong correlated behavior and band topology thus rises as an substantial subject in condensed matter physics.

We will focus on the study of transport properties in various 2D materials and related variants, including alloyed, doped, functionalized graphene and transition metal dichalcogenides as well as stacked vdW heterostructures. 

A stacked van der Waals heterostructure. Adapted from Geim & Grigorieva, Nature 499, 419 (2013).



(a) The phase diagram of a magic angle twisted bilayer graphene. A transition from a Mott insulator to a superconductor can be observed. (b) A hysteretic loop caused by the ferromagnetic order forming in twisted bilayer graphene. The figures are adapted from Cao et al., Nature 556, 43 (2018) and Sharpe et al., Science 365, 605 (2019).