Advanced Seminar on Condensed Matter Physics

Wintersemester 2015/2016

go to Sommersemester 2016
Prof. Dr. Rüdiger Klingeler
INF 227, SR 1.403
Friday 11:15-13:00 h

13.11.2015 11:15
Kiryl Batvinyeu, KIP
INF 227, SR 1.403
4.12.2015 11:15
Prof. Dr. Martin Dressel, University of Stuttgart, 1st Institute of Physics
KIP 1.403

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.

11.12.2015 11:15-13:00 h
Johannes Werner, KIP
INF 227, SR 1.403
18.12.2015 11:15 h
Dr. Christian Reitz, Karlsruher Institut für Technologie, Institut für Nanotechnologie
INF 227, SR 1.403
15.1.2016 11:15
Dr. Alexander Tsirlin, Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg, Germany
INF 227, SR 1.403

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:

  1. real-world manifestations of the Heisenberg model on the frustrated square lattice in vanadium oxides;

  2. novel physics of the Heisenberg model on the kagome lattice probed in copper minerals;

  3. 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.

29.1.2016 11:15
Kaustav Dey, IISER Pune
INF 227, SR 1.403
5.2.2016 11:15
Dr. Christoph Meingast, Institut für Festkörperphysik, Karlsruher Institut für Technologie
KIP 1.403