Publications
Time-resolved terahertz studies of conductivity processes in novel electronic materials
Terahertz (THz) spectroscopy based on femtosecond laser techniques1-12 has emerged as a powerful probe of charge transport and carrier dynamics. The technique makes use of ultrashort pulses of propagating electromagnetic radiation to measure conductivity in the THz spectral regime.
NaSn2As2: An Exfoliatable Layered van der Waals Zintl Phase
The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials.
Gate Tuning of Electronic Phase Transitions in Two-Dimensional NbSe2
Recent experimental advances in atomically thin transition metal dichalcogenide (TMD) metals have unveiled a range of interesting phenomena including the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The atomic thickness of two-dimensional (2D) TMD metals also opens up the possibility for control of these electronic phase transitions by electrostatic gating. Here, we demonstrate reversible tuning of superconductivity and CDW order in model 2D TMD metal NbSe2 by an ionic liquid gate.
Electrical control of the valley Hall effect in bilayer MoS 2 transistors
The valley degree of freedom of electrons in solids has been proposed as a new type of information carrier, beyond the electron charge and spin. The potential of two-dimensional semiconductor transition metal dichalcogenides in valley-based electronic and optoelectronic applications has recently been illustrated through experimental demonstrations of the optical orientation of the valley polarization and of the valley Hall effect in monolayer MoS 2.
Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides
Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX 2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications.
Optical Coherence in Atomic-Monolayer Transition-Metal Dichalcogenides Limited by Electron-Phonon Interactions
We systematically investigate the excitonic dephasing of three representative transition-metal dichalcogenides, namely, MoS2, MoSe2, and WSe2 atomic monolayer thick and bulk crystals, in order to gain a proper understanding of the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy and temperature dependent absorption, combined with theoretical calculations of the phonon spectra, indicate electron-phonon interactions, to be the limiting factor.
Ising pairing in superconducting NbSe2 atomic layers
The properties of two-dimensional transition metal dichalcogenides arising from strong spin-orbit interactions and valley-dependent Berry curvature effects have recently attracted considerable interest. Although single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied, the effects of spin-valley coupling on collective quantum phenomena remain less well understood.
Strongly enhanced charge-density-wave order in monolayer NbSe 2
Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe 2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit.
Effect of surface states on terahertz emission from the Bi2Se3 surface
Three-dimensional topological insulators are materials that behave as an insulator in the interior, but as a metal on the surface with Dirac surface states protected by the topological properties of the bulk wavefunctions. The newly discovered second surface state, located about 1.5eV above the conduction band in Bi2Se3 allows direct photoexcitation of the surface electrons in n-doped samples with a Ti:sapphire femtosecond laser. We have observed efficient THz generation from the Bi2Se3 basal plane upon femtosecond optical excitation.
Charge-neutral disorder and polytypes in heterovalent wurtzite-based ternary semiconductors: The importance of the octet rule
We investigate lattice ordering phenomena for the heterovalent ternaries that are based on the wurtzite lattice, under the constraint that the octet rule be preserved. We show that, with the single exception of a highly symmetric twinned structure, all allowed lattice orderings can be described by a pseudospin model corresponding to the two different stackings of ABAB rows of atoms in the basal plane that occur in the Pna21 and Pmc21 crystal structures.