“Terahertz Spectroscopy and Gapless Superconductivity”
Quantum materials lying at the frontier of condensed matter research call for access to increasingly low frequencies in the electromagnetic spectrum. In particular, the terahertz region is a rich arena for probing exotic quasiparticles associated with novel quantum phenomena. Ultrafast femtosecond lasers developed in the late 1990s brought an unexpected opportunity to implement terahertz time-domain spectroscopy (THz-TDS). Here, we introduce the standard method of THz-TDS and illustrate this technique by presenting several case studies in which the distinct advantages of THz-TDS were utilized to obtain key information on representative quantum materials such as topological insulators, superconductors, spin liquid, and low-dimensional van der Waals magnets. In the second part of the talk, I will present our recent findings on gapless superconductivity. This is a peculiar form of superconductivity where infinite d.c. conductivity and the Meissner effect are still intact, but the excitation spectrum lacks a gap.
Hosts: M. Cheng & L. Glazman