Berlin 2012 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 3: Correlated Electrons: Heavy Fermions

TT 3.1: Vortrag

Montag, 26. März 2012, 09:30–09:45, H 3005

Elusive Fulde-Ferrell-Larkin-Ovchinnikov state: Evolution of quasi-particle entropy in high-field superconducting phase in CeCoIn₅ — •Yoshi Tokiwa — I. Physik. Institut, Georg-August Universität Gottingen, Friedrich-Hund-Platz 1, 37077 Göttingen

We have performed a "smoking gun" experiment for Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for CeCoIn₅. Entropy as a function of field is expected to show a steep increase at the transition field from BCS to FFLO superconducting (SC) state, because of the additional quasi-particles in the nodal parts of FFLO SC gap structure. We obtained electronic entropy of CeCoIn₅ by measuring magnetocaloric effect and specific heat at low temperatures down to 100mK and high fields up to 12T. In the high-field SC state for the field along [100], we observed a reduction of entropy, ∼8 mJ/mol·K, which is consistent with a spin density wave (SDW) order without the formation of FFLO state. The anomaly for the SDW transition in magnetocaloric effect and specific heat disappears, when the field is tilted 18^∘ towards [001] from [100] in agreement with the neutron scattering experiment. Our experiment shows negative results for the formation of FFLO state in CeCoIn₅ for all the studied field directions, H∥[100], 18^∘ and [001]. Although the numerous efforts for the experimental realization of FFLO state have been made since its prediction 40 years ago, an unambiguous observation in a solid state system still remains as a challenge in the field of condensed matter physics. This work is done by the collaboration with P. Gegenwart and E. Bauer and supported by DFG through research unit 960 (Quantum phase transitions).