Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract No. W-31-109-ENG-38. Work at Portland State University was supported by NSF Grant No. CHE-9632815 and the Petroleum Research Fund ACS-PRF 31099-AC1.
Physical Review B
Organic superconductors, Charge transfer, Thin films -- Thermal properties, Expansion (Heat) -- Measurement
We report high-resolution measurements of the coefficient of thermal expansion, α=l⁻¹ x (δl/δΤ), on single crystals of the organic superconductors β″-(ET)₂SF₅CH₂CF₂SO₃ and κ-(ET)₂Cu(NCS)₂. For both salts we find large and highly anisotropic phase-transition anomalies at Tc. Combining these data with literature results on the specific heat via the Ehrenfest relation, the uniaxial pressure coefficients of Tc can be determined. Most remarkably, a strikingly similar in-plane vs out-of-plane anisotropy is found for both compounds: the strong suppression of Tc observed in hydrostatic-pressure experiments is dominated by a huge negative uniaxial stress effect perpendicular to the conducting planes. Therefore we expect that an increase of Tc in this class of superconductors can be obtained by enlarging the distance between the conducting layers. Application of magnetic fields perpendicular to the planes for the β″-(ET)₂SF₅CH₂CF₂SO₃ salt were found to result in pronounced superconducting fluctuation effects and scaling behavior in α(Τ,B). Owing to the pronounced phase-transition anomalies in α(Τ,B) at Τc, our measurements allow for an accurate determination of the upper critical fields. We find Bc2⊥(0)=(1.4±0.2) T and Bc2‖(0)=(10.4±0.5) for fields perpendicular and parallel to the conducting planes, respectively.
Müller, J., Lang, M., Steglich, F., Schlueter, J. A., Kini, A. M., Geiser, U., Mohtasham, J., Winter, R. W., Gard, G. L., Sasaki, T., Toyota, N. (2000). Comparative Thermal-Expansion Study of β″-(ET)₂SF₅CH₂CF₂SO₃ and κ-(ET)₂Cu(NCS)₂: Uniaxial Pressure Coefficients of Tc and Upper Critical Fields. Physical Review B, 61, 11739.