Two-dimensional semiconductors
Transition metal dichalcogenides (TMDs, MX2 with M = Mo, W and X = S, Se) are a new class of two-dimensional (2D) semiconductors. We fabricate atomically thin samples, create heterostructures and devices, and employ optical and electrical probes to study their unique properties. Recent topics of interests include valley- and spin-dependent transport, exciton condensation, and strong correlation physics of moire flat bands.
Terahertz time-domain spectroscopy
Light pulses as short as two optical cycles (~ 5 femtoseconds) can now be produced by modelocked lasers. These pulses have dramatically advanced many areas of ultrafast spectroscopy. One of these is the time-domain spectroscopy of the THz or far-infrared spectral region (1 THz ↔ 300 μm ↔ 33.3 cm-1 ↔ 4.1 meV ↔ 47.6 K). We make use of these ultrashort pulses of propagating electromagnetic radiation to measure conductivity in the THz spectral regime. In particular, when combined with a time-synchronized femtosecond excitation pulse, the method is suitable for the investigation of electronic charge transport under nonequilibrium conditions. These attributes permit THz spectroscopy to circumvent many of the constraints of conventional transport measurement techniques.
Optical and electronic properties of graphene
Graphene is a single layer of carbon atoms arranged in a honeycomb structure. We create single- and few-layer graphene and employ optical spectroscopic techniques to explore their properties.
Non-centrosymmetric superconductivity
We fabricate superconductors of single- or few-layer thickness (such as TMD metal NbSe2) and develop a range of transport measurement techniques to probe their unconventional properties related to broken inversion symmetry. Recent topics of interests include spin and charge transport involving triplet Cooper pairs, and engineering of topological superconductivity via magnetic proximity coupling.
Berry curvature physics
2D materials, such as gapped graphene, monolayer TMDs and Weyl semimetals, possess interesting Berry curvature effects (a geometric effect of their band structures). We study the optical and transport properties of these materials arisen from the Berry curvature effects. We also explore the valley degree of freedom of electrons in these materials as a new type of information carrier for electronic and optoelectronic devices.
2D magnetism
Magnetism in 2D layered materials has attracted much recent interests. We develop techniques to fabricate heterostructures and field effect devices based on these materials, as well as to probe and image magnetic orders and spin dynamics. Recent topics of interests include development of energy-efficient methods to switch magnetism in these materials (e.g. electric field switching and switching by magnetic fluctuations), antiferromagnetic spin dynamics and magnon transport.
