TY - JOUR
T1 - Lattice anomalies and Grueneisen parameters in high Tc κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2CuN(CN)2Br
AU - Toyota, N.
AU - Watanabe, Y.
AU - Sasaki, T.
N1 - Funding Information:
tals, and Michael Lang and Maeahito Yoshizawa for enlightening discussions. The present work is partially supported by Grant-in-Aid for Scientific Research from Ministry of Education, Science and Culture of Japan.
PY - 1993/3/29
Y1 - 1993/3/29
N2 - Four-circle X-ray diffraction measurements down to 15K on single crystals of the title superconducting salts (BEDT-TTF)2X have revealed an anomalous temperature dependence of the lattice parameters in a broad range of temperatures centered around 100K, where many anomalies have been observed such as in transport, magnetic and optical properties in both materials. These prominent lattice anomalies, which are clearly featured in broad peaks of the interlayer spacing for X=Cu(NCS)2 and the a-axis (parallel to polymeric chain) for CuN(CN)2Br, are likely to be associated with the respective conformational ordering of the terminal ethylenes. The other lattice parameters change with temperature in a way that the unit cell volume of each compound varies only smoothly with temperature. From the estimated volume expansion coefficient in combination of available specific heat and compressibility data, we evaluate the low-temperature Grueneisen parameter to be 5-6 for both compounds. A thermodynamical analysis within a Bardeen-Cooper-Schrieffer theoretical framework concludes that the well-known high sensitivity of Tc to hydrostatic pressure is caused by the highly volume-sensitive electron-phonon coupling constant, and that the strong lattice anharmonicity plays an essential role in the superconductivity.
AB - Four-circle X-ray diffraction measurements down to 15K on single crystals of the title superconducting salts (BEDT-TTF)2X have revealed an anomalous temperature dependence of the lattice parameters in a broad range of temperatures centered around 100K, where many anomalies have been observed such as in transport, magnetic and optical properties in both materials. These prominent lattice anomalies, which are clearly featured in broad peaks of the interlayer spacing for X=Cu(NCS)2 and the a-axis (parallel to polymeric chain) for CuN(CN)2Br, are likely to be associated with the respective conformational ordering of the terminal ethylenes. The other lattice parameters change with temperature in a way that the unit cell volume of each compound varies only smoothly with temperature. From the estimated volume expansion coefficient in combination of available specific heat and compressibility data, we evaluate the low-temperature Grueneisen parameter to be 5-6 for both compounds. A thermodynamical analysis within a Bardeen-Cooper-Schrieffer theoretical framework concludes that the well-known high sensitivity of Tc to hydrostatic pressure is caused by the highly volume-sensitive electron-phonon coupling constant, and that the strong lattice anharmonicity plays an essential role in the superconductivity.
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U2 - 10.1016/0379-6779(93)90454-5
DO - 10.1016/0379-6779(93)90454-5
M3 - Article
AN - SCOPUS:0027553598
SN - 0379-6779
VL - 56
SP - 2536
EP - 2541
JO - Synthetic Metals
JF - Synthetic Metals
IS - 1
ER -