Introduction. Two research themes of our group are explained below for non-experts. Both themes are driven by the desire to understand quantum matter, substances found in nature or synthesized in laboratories with spectacular, counterintuitive, but immensely useful macroscopic properties. Comprehending the collective behaviors of a large number of quantum particles interacting strongly with each other is a notoriously hard, fundamental science challenge. Many believe that the ability to coherently control quantum matter will lead to a paradigm shift in e.g. designing functional materials, computing and communication, and metrology.

A. Many body physics of ultracold quantum gases

Ultracold atoms refer to quantum degenerate gases of alkali atoms (such as Rubidium, Potassium or Lithium) confined in vacuum by laser beams and chilled to nano-Kelvin temperatures. They are the latest breeds of quantum matter, and literally the coolest stuff ever created by mankind. These systems provide well-controlled settings to advance our fundamental understanding on the collective behaviors of strongly interacting quantum particles. Pioneering works leading to these achievements were awarded Nobel Prize in 1997 (for "development of methods to cool and trap atoms with laser light") and 2001 (for "achievement of Bose-Einstein condensation in dilute gases of alkali atoms").

We are particularly interested in the quantum phases of Fermi gases in new parameter regimes brought by ongoing cold atoms experiments. Examples include an "exotic" form of superfluidity know as modulated superfluid, the orbital ordering pattern of atoms on higher bands, dipolar Fermi gases, and topological phases of cold atoms.


>> One-dimensional Fermi gas with spin imbalance. We constructed the effective field theory from the Bethe ansatz solution (PRA, 2008; JLT 2010), and examined its exact thermodynamics (PRL 2009). [Image courtesy of Prof. R. Hulet, Rice Univ. Liao et al, see Nature 467 (2010)]


>> Phase diagram of two-component dipolar Fermi gases from renormalization group analysis. arXiv:1209.2671. Our earlier work predicted a new phase of matter, bond order solid, in single component dipolar Fermi gases (PRL, 2012).


>> A topological ladder without artificial gauge fields or spin orbit coupling. Nature Communications 4, 1523 (2013). doi:10.1038/ncomms2523

B. Superconducting heterostructures; Quantum transport

Superconductivity is a hall mark macroscopic quantum mechanical phenomenon. At low temperatures, many materials become superconductors with vanishing electrical resistance. Moreover, (weak) magnetic field gets expelled from the bulk. A conventional superconductors can be thought as a "perfect" quantum fluid of pairs of electrons, loosely bound together by some attractive interaction between electrons, all sharing the same quantum state. Superconductor has become a leading competitor in building new of quantum devices and qubits (the fundamental building block for a quantum computer).

We are interested in superconductors driven out of equilibrium, especially in spatially inhomogeneous systems as in devices. The research is motivated by the recent success in fabricating superconducting nanostructures and circuits for electronic, spintronic, as well as quantum computing applications. More broadly, our group investigates the transport properties of mesoscopic to nano-scale quantum devices.
>> The spectrum of a superconductor-topological insulator heterostructure, movie made by Mahmoud Lababidi. Lababidi and Zhao, PRB 86, 161108(R) (2012)


>> Complex band structure of Bi2Se3, a topological insulator. Zhao et al, PRB 82, 205331 (2010).