Seeing Electronic Correlations in Quantum Materials

June 5th, 2019 HERMANN SUDEROW Universidad Autónoma de Madrid

Understanding strong electronic correlations and how to make useful devices from that (such as a room temperature superconductor or a protected quantum computer) is considered by many as one of the most important unsolved issues in condensed matter physics. As often happens in such cases, it is quite difficult even to properly define the problem... There are many different materials, heavy fermions, cuprates, pnictides or chalcogenides where electron correlations produce weird properties and that are now termed "quantum materials". Among the most peculiar properties are unconventional and topological superconductors emerging from an electronic state which does not follow the simple Fermi liquid theory (that maps correlations into a modified effective electron bandmass). To produce these properties, we need to tune the material into a "sweet spot" by modifying some parameter, that can be charge, magnetic field or strain. The "sweet spot" is often at a quantum phase transition occurring at zero temperature between two ground states when modifying the tuning parameter. At the quantum phase transition, contrary to simple intuition, quantum fluctuations dominate thermal smearing and produce the new unconventional electronic properties. I will show how to visualize electronic properties of quantum materials using Scanning Tunneling Microscopes (STM) at very low temperatures. With such microscopes, we see electronic wavefunctions and their interference pattern. I will first discuss a couple of simple examples, including 2H-NbSe2 (a well-known charge ordered superconductor) and WTe2 (a layered semimetal whose bandstructure has topologically non-trivial properties), and then show interference experiments in heavy fermions and in pnictides at a quantum critical point.

Wednesday, June 5, 2019, 12:00. ICFO’s Seminar Room

Hosted by Prof. Jens Biegert