"Two hot topics related to quantum critical points: Fermi surface and Charge Density Wave"

Who: Christoph Geibel. MPI CPfS, Dresden, Germany

Place: Donostia International Physics Center. P Manuel Lardizabal 4, Donostia - San Sebastin

Date: Monday, 18 January 2016, 12:00

Quantum critical points (QCP), where a continuous phase transition is steadily tuned from finite temperatures to T = 0 by pressure, chemical doping, or magnetic field, is currently an important topic in solid state physics. In my talk I shall present two research projects addressing very different aspects of quantum critical points.

The first project aims at a precise insight into the hybridization between localized 4f electrons and itinerant conduction electrons in so-called ?heavy fermion? systems, and how this hybridization evolves across a QCP. Heavy fermion systems are intermetallic compounds based on the rare earth elements Ce or Yb. Both elements can be tuned from the usual trivalent state to a tetravalent (Ce) or divalent (Yb) state. This results in a transition from a local moment magnetic state to a non-magnetic state where f degree of freedom becomes itinerant, with a QCP in-between. Accordingly one expects strong changes in the electronic states, e.g. in the Fermi surface, but how these changes occur is yet not clear, neither experimentally nor theoretically. Here our ARPES studies on YbRh2Si2 and related compounds resulted in an unprecedented insight into the interaction between 4f and conduction electrons and its evolution from the magnetic to the non-magnetic regime.

Most QCPs discovered und studied up to now are associated with the disappearance of magnetic order. In contrast QCPs connected with a structural phase transition are rare and poorly studied. In our second project we discovered a system where a continuous Charge Density Wave (CDW) type transition can be steadily tuned to T = 0 by chemical substitution, realizing a structural/CDW quantum critical point. More interestingly we observed a strong enhancement of bulk superconductivity just at this QCP. This is a unique feature among CDW systems which points to a new type of interaction between CDW and superconductivity.

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