In-Plane ESR Microwave Conductivity Measurements and Electronic Band Structure Studies of the Organic Superconductor ′′-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 H. Hau Wang,* Michael L. VanZile, John A. Schlueter, Urs Geiser, Aravinda M. Kini, and Paul P. Sche Chemistry and Materials Science DiVisions, Argonne National Laboratory, Argonne, Illinois 60439 H.-J. Koo and M.-H. Whangbo Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 Paul G. Nixon, Rolf W. Winter, and Gary L. Gard Department of Chemistry, Portland State UniVersity, Portland, Oregon 97207-0751 ReceiVed: April 19, 1999 The electronic structure of the organic superconductor ′′-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 (BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) was characterized with the use of electron spin resonance (ESR) spectroscopy and electronic band structure calculations. The room-temperature ESR line width is 24-27 G in the plane of a donor molecule layer (i.e., in the ab-plane) and ∼32 G along the normal to this plane (i.e., along the c*-direction). The ab-plane anisotropy of the microwave conductivity was extracted for the first time from the ESR Dysonian line shape analysis. The in-plane conductivity varies sinusoidally, is maximal along the interstack direction (b-axis), and is minimal along the donor stack direction (a-axis). The Fermi surfaces of the title compound consist of a 2D hole pocket and a pair of 1D wavy lines. The directions for the in-plane conductivity maximum and minimum are in excellent agreement with the electronic band structure calculated for ′′-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 , and the origin of the in-plane conductivity anisotropy lies in the one-dimensional part of the Fermi surface. This is the first time that an organic conductor shows Dysonian ESR line shape due to its 2D and strongly metallic nature, yet the 1D character is revealed simultaneously through the in-plane conductivity anisotropy. 1. Introduction All BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene or ET] radical cation based organic superconductors consist of two- dimensional highly conductive donor layers alternating with charge-compensating and charge-insulating anion layers. Infor- mation concerning in-plane conductivity anisotropy that bears direct correlation to the theoretical Fermi surface is not easily accessible. Conventional four-probe measurements provide conductive information in the plane but only along one direction because of the fact that direct contact is required for the method. In an attempt to obtain in-plane angular-dependent conductivity, we have carried out noncontact electron spin resonance (ESR) measurements on the newly reported organic superconductor ′′-(ET) 2 SF 5 CH 2 CF 2 SO 3 . The in-plane microwave conductivities were extracted for the first time for this class of materials. In this article, we demonstrate that the resulting microwave conductivity anisotropy correlates well with the calculated Fermi surface by extended Hu ¨ckel theory. Organic conductors and superconductors based on organic donor molecules exhibit novel physical properties such as superconductivity, field-induced spin density wave, quantum Hall effect, etc. 1,2 The first organic superconductor, (TMTSF) 2 PF 6 salt, where TMTSF denotes tetramethyltetraselenafulvalene, was realized in 1980 with a superconducting transition temperature, T c of 0.9 K under 12 kbar pressure. 3 The organic superconduc- tors with the highest T c ’s are found in the family of κ-(ET) 2 Cu- [N(CN) 2 ]X, X ) Br (T c is 11.6 K at ambient pressure) and X ) Cl (12.8 K under 0.3 kbar). 4,5 The ET molecules in the oxidized form are nearly flat and tend to form a layer of parallel stacked donor molecules (-phase) or a layer of orthogonally packed dimers (κ-phase). The crystal structures of typical ET salts consist of such two-dimensional donor layers alternating with anion layers. As a result, they are highly conducting within the layer due to S‚‚‚S orbital overlap and poorly conducting normal to the layer. The -phase organic superconductors consist of four isostructural members, -(ET) 2 X (X ) I 3 , IBr 2 , AuI 2 , and *-I 3 ), 6-9 while about 30 κ-phase superconductors are known. 2,10 Recently, we reported a new ambient pressure organic superconductor ′′-(ET) 2 SF 5 CH 2 CF 2 SO 3 , with T c at 5.2 K. 11 In the donor-molecule layers of the ′′-phase, the plane of the ET molecule is tilted away from the donor stacking axis by about 60°, and the nearest-neighbor ET molecules between stacks are almost coplanar. There are three other ′′-like superconductors, i.e., (ET) 3 Cl 2 ‚2H 2 O(T c ) 2K under 16 kbar) 12 and (ET) 4 - (H 2 O)M(C 2 O 4 ) 3 ‚C 6 H 5 CN (T c ) 7.0 K for M ) Fe and T c ) 6.0 K for M ) Cr). 13,14 These ′′-like salts are not isostructural but have common structural features in the donor layers. Herein, we present the normal-state ESR properties, extracted in-plane microwave conductivities, and electronic band structure of the new superconductive ′′-(ET) 2 SF 5 CH 2 CF 2 SO 3 salt. * Author for correspondence. 5493 J. Phys. Chem. B 1999, 103, 5493-5499 10.1021/jp991268j CCC: $18.00 © 1999 American Chemical Society Published on Web 06/11/1999