Carrier Dynamics in κ-Type Organic Superconductors : Time-Resolved Observation(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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The κ-type organic superconductors have unique physical properties, many of which are yet to be understood. Most of their physical properties are closely related to carrier dynamics, which can be directly observed by ultrafast optical measurements in their nonequilibrium states. Time-resolved temperature-dependent reflectivity changes were measured on single crystals of an isostructural pair of organic charge transfer salts κ-(ET)_2Cu[N(CN)_2]X, which are ambient- (X=Br) or high-pressure (X=Cl) superconductors. The latter compound in its ground state is a Mott insulator under ambient pressure. The obtained temperature dependences of relaxation times for both salts indicated the existence of an intermediate electronic state between those of metals and insulators at all measurement temperatures. Such an unusual electronic state coexisted with the superconducting and Mott insulating states in X=Br and X=Cl, respectively. In addition, the results for X=Br revealed the formation of a temperature-dependent well-defined gap in the superconducting transition, while those for X=Cl suggested that the Mott transition in this salt does not depend on electron-phonon interactions, consistent with the theory that the Mott transition should originate from electron correlation.
社団法人日本物理学会の論文
2008-06-15