J. Phys. IV France 114 (2004) 341
Glass-like transition in -(ET) 2Cu[ N(CN) 2] Br at T g 75 K - implications for the superconducting ground-state propertiesJ. Müller1, 1, M. Lang2, F. Steglich1 and J.A. Schlueter3
1 Max-Planck-Institute for Chemical Physics of Solids, 01187 Dresden, Germany
2 J.W. Goethe University, FOR 412, 60054 Frankfurt, Germany
3 Argonne National Laboratory, Illinois 60439, USA
High-resolution measurements of the linear coefficients of thermal expansion, (T), on -(ET) 2Cu[ N(CN) 2] Br reveal large, step-like anomalies at temperatures around 75 K. These have been identified as a kinetic, glass-like transition, where below a certain temperature T a characteristic disorder in the orientational degrees of freedom of the donor molecule's ethylene endgroups becomes frozen. The phenomenology of this relaxation process provides a natural explanation for the observed time and cooling-rate dependences of the ground-state properties in this system. Furthermore, the data suggest that the degree of disorder in the sample can be varied in a controlled way by controlling the cooling rate as well as the applied pressure. Key words. Organic superconductors - BEDT-TTF salts - thermal expansion - disorder.
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