J. Phys. IV France
Volume 114, April 2004
Page(s) 341 - 342

J. Phys. IV France
114 (2004) 341
DOI: 10.1051/jp4:2004114077

Glass-like transition in ${\kappa}
$ -(ET) 2Cu[ N(CN) 2] Br at T g ${\sim }$ 75 K - implications for the superconducting ground-state properties

J. 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, $\alpha _{\rm i}$(T), on ${\kappa}
$ -(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 $_{\rm g}$ 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.

© EDP Sciences 2004