Uniaxial strain study of electronic and crystal structures of organic conductors $ S. Kagoshima a,* , R. Kondo a , M. Chusho a , M. Maesato b , T. Mori c , H. Mori d , S. Tanaka d a Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan b Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8224, Japan c Department of Organic and Polymer Materials, Tokyo Institute of Technology, O-Okayama 2-12-1, Meguro-ku, Tokyo 152-0033, Japan d ISTEC-SRL, Shinonome 1-10-13, Koto-ku, Tokyo 135-0062, Japan Abstract Studies of electrical resistance and crystal structures under the uniaxial compression verified that the variety of electronic properties of a- (BEDT-TTF) 2 XHg(SCN) 4 (X ¼ K, NH 4 ), y-(BEDT-TTF) 2 CsZn(SCN) 4 and y-BO 2 Cl(H 2 O) x is systematically controlled by the strain and is explained in terms of the difference in the band structure. One exception, the selenium analogue a-(BEDT-TTF) 2 KHg(SeCN) 4 showed the superconductivity under the uniaxial compression along any direction. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Organic conductor; Uniaxial strain; Uniaxial compression; Electronic structure; Crystal structure; Transport properties The relationship between the crystal structure and elec- tronic properties of organic conductors has been discussed in terms of the volume effect of hydrostatic pressures to the band structure. However, this discussion is insufficient because most of organic conductors suffer anisotropic com- pression under the pressure. By employing the uniaxial strain method for electrical resistance measurements and x-ray structure analyses, we studied this problem in some a- and y-type compounds of BEDT-TTF and BO molecules, which show a wide variety of electronic properties displayed by the quasi-two-dimensional electronic system. To uniaxially compress crystalline samples without Pois- son’s effect, we employed the apparatuses of the ‘‘epoxy method’’ and the ‘‘frozen-oil method’’ developed by us [1]. External forces were applied to the cross-section of the sample-epoxy composite at room temperature or the rod of frozen-oil containing the sample at about 20 K. The created uniaxial strain was of the order of up to 10% in the epoxy method and up to 5% in the frozen-oil method. In x-ray diffraction measurements for structural analyses, we employed the epoxy method where the sample-epoxy composite was put in a tube of beryllium to ensure high transmission of Cu Ka x-ray beam. We used a specially designed goniometer where the x-ray source was rotated while the sample fixed. Details of x-ray apparatuses will be published elsewhere. Sample crystals were prepared by conventional electro- chemical method. For resistance measurements, conven- tional d.c. four-probe method was employed using gold wires of f15 mm and carbon paste. In y-(BEDT-TTF) 2 CsZn, the resistance was measured along the c-axis in the con- ducting plane. In other materials, we measured the resistance perpendicular to the conducting plane. In a-(BEDT-TTF) 2 XHg(SCN) 4 (X ¼ K, NH 4 ), we found that the K-compound, that has no superconducting state under the hydrostatic pressure, shows the superconductivity under the uniaxial compression parallel to either the c-axis in the conducting plane or the b  -axis perpendicular to the plane. The NH 4 -compound having no density-wave state under hydrostatic pressures shows the density-wave state under the uniaxial compression parallel to the a-axis in the conducting plane. Furthermore, the superconducting critical temperature T c was raised from 1.5 up to 6 K by the uniaxial compression parallel to the c-axis [2]. These results are summarized in the phase diagram of Fig. 1. By x-ray diffraction in the NH 4 -compound, we found that the lattice is compressed uniaxially as expected. Structure analyses showed that (1) the compression parallel to the c-axis deforms the Fermi surface into more isotropic shape in the conducting plane, (2) the density-of-states at the Fermi Synthetic Metals 133–134 (2003) 145–146 $ Yamada Conference LVI, The Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001—Abstract Number B12Wed. * Corresponding author. Tel.: þ81-3-5454-6737; fax: þ81-3-5454-4331. E-mail address: kagosima@mail.ecc.u-tokyo.ac.jp (S. Kagoshima). 0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0379-6779(02)00338-7