The electrodynamic properties of superconductors are of interest from a fundamental side as well as for applications. Since the seminal work of M. Tinkham on the superconducting energy gap in the late 1950s, optical investigations have been established as a powerful method to explore the quasi-particle excitations and their dynamics which yield important information on the density of states, the symmetry of the order parameter, the scattering mechanism, and eventually the glue to superconductivity. In addition, the superconducting condensate is probed, i.e. the Cooper pair density and stiffness. Most recently, it became clear that under certain conditions, also collective modes can be studied: these are the phason excitations (Nambu-Goldstone mode) and the amplitude mode (Higgs mode).
The talk will give a general introduction to the optical properties of superconductors, sketch the theory and highlight some important experimental findings, as well as applications, such as superconducting single photon detectors. In particular we will focus on ultrathin superconducting films, such as InO, Nb, NbN, TiN, and TaN, but also Al, which exhibit a superconductor-insulator transition as disorder or granularity increases. We discuss the possibility of collective low-frequency excitations due to the Higgs mechanism, which become long-lived and well defined in the vicinity of a quantum critical point.
Magnetic systems with low-dimensional and frustrated geometries of exchange interactions can host unusual ground states, such as spin liquids, where no long-range magnetic order occurs even at zero temperature. The spin-liquid ground state has been predicted for several magnetic models in two dimensions, but its experimental observations are scarce.
In this talk, I will discuss the quest for the spin-liquid ground state in bulk magnetic insulators, where quantum magnetic behavior is observed. The talk will cover several simple frustrated geometries in two dimensions:
real-world manifestations of the Heisenberg model on the frustrated square lattice in vanadium oxides;
novel physics of the Heisenberg model on the kagome lattice probed in copper minerals;
Kitaev model on the honeycomb lattice in layered van-der-Waals crystals of a-RuCl3.
I will demonstrate which challenges the bulk materials present, and which material-specific deviations from the ideal models become relevant when low-temperature magnetism is considered.
