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TT: Fachverband Tiefe Temperaturen

TT 45: Focused Session: 100 Years of Superconductivity

TT 45.1: Invited Talk

Thursday, March 17, 2011, 10:30–11:00, HSZ 03

Pairing fermions with population imbalance — •Peter Fulde — Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Str. 38, 01187 Dresden

Cooper pairing of fermions starts usually from the premise that the two species forming pairs have equal populations, i.e., that their particle numbers are the same. That need not always be the case, though, and examples to the opposite are known. They concern metals, as well as dense quark matter, nuclear matter or ultracold atoms.

The simplest case is that of a superconductor when a Zeeman term is added to the Hamiltonion. We will show that the Zeeman energy may result in breaking of translational invariance and hence in inhomogeneous superconducting ground states. They are often referred to as FFLO states, since they were first suggested by Fulde and Ferrell and independently by Larkin and Ovchinnikov. Simple arguments will be presented which explain the origin of the symmetry breaking.

The inhomogeneous states are paralleled by the ones caused by a magnetic field acting on the electron orbits, i.e., Abrikosov vortex states. While it took only a few years to verify Abrikosov vortices after they had been predicted, it has taken nearly 40 years to proof the existence of inhomogeneous states caused by population imbalance. In the meantime experiments on CeCoIn₅, and the two-dimensional organic superconductors κ-(BEDT-TTF)₂ · Cu(NCS)₂, λ-(BETS)₂ FeCl₄, λ-(BETS)₂ GaCl₄ and β-(BEDT-TTF)₂ SF₅CH₂CF₂SO₃ provide very strong evidence in some cases, and still disputed ones in others that a FFLO state is forming in high magnetic fields. However, the most important realization of FFLO-like states is found in pi junctions, which seem to be on the verge of important technical applications. We will briefly describe progress which has been made here, based on ideas of Bulaevskii, Buzdin, Demler and others and realized by Ryazanov, Feofanov, Ustinov and others.