ELSEVIER Synthetic Metals 103 (1999) 190.5-1906 One-and two-dimensional angle-dependent magnetoresistance oscillations ( AMROs) in K-(BEDT-TTF)$u(SCN)2 in fields of up to 33 T M.-S. Namr, 14. M. Honold’, C. Proust2, N. Harrison3, C. H. Mielke3, S. J. Blundelll, J. Singleton’, W. HayesI, M. Kurmoo4, and P. Day4 ’ Oxford University Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom ’ Laboratoire de Physique de la Mat&e Condense’e, Service National des Champs Magne’tiques Pulse’s, INSA, Complexe Scientifique de Rangueil, 31077, Toulouse, France National High Magnetic Field Laboratory, LANL, MS-E536, Los Alamos, New Mexico 87545, U.S.A. 4 The Royal Institution, 21 Albemarle Street, London WlX 4BC, United Kingdom Abstract .4ngle-dependent magnetoresistance oscillations (AMROs) in /+(BEDT-TTF)zCu(SCN)z have been observed in fields of up to 33 T. AMROs due to both open and closed Fermi surface sections have been measured simultaneously for the first time in an organic metal. This provides information concerning the low temperature Fermi surface and demonstrates the absence of a spin-density wave state in this material. Keywords: Angle-dependent magnetoresistance oscillations, organic superconductors, magnetic breakdown, BEDT-TTF. ,+(BEDT-TTF)2Cu(SCN)z is an organic supercon- ductor with a Fermi surface (FS) comprising a quasi- two-dimensional (Q2D) pocket (a pocket) and a pair of quasi-one-dimensional (QlD) sheets separated by a gap across which magnetic breakdown (MB) can occur (Fig- ure 2, inset). The FS and the effect of MB in high fields have been extensively studied using quantum oscillatory techniques [l-3]. To provide further information about the FS shape we have performed measurementsof angle- dependent magnetoresistance oscillations (AMROs) in magnetic fields of up to 33 T at NHMFL, Tallahassee. AMRO experiments have been used to investigate the FSs of a variety of BEDT-TTF salts [4-71. Our data are presented in Figure 1 for several values of the azimuthal angle 4, which defines the plane of ro- tation of the sample. Fine AMROs are observed in the range -80° < 0 < 80°, where 0 is the angle between the magnetic field and the normal to the sample Q2D planes. In contrast to AMROs in the Q: phase BEDT- TTF salts [4], which are dominated by either QlD or Q2D sections, the data in Figure 1 represent a super- position of both kinds of AMROs, together with large amplitude Shubnikov-de Haas oscillations. The mechanism for QlD AMROs leads to periodic minima in the resistance [6,8] at angles tan0 = xv + <, where v is an integer, x = 6 tan(e) is the periodicity and C is a constant; the &dependence of x uniquely identifies the QlD origin of AMRO [8] and has been fitted to the equation ~(4) = xe/(cos($~ - +,I)), yielding y. = 0.798 rt 0.008 and $0 = O” (seeFigure 2). 0 -60 -30 0 Angle (Deb 60 90 FIG. 1. Azimuthal angle (4) dependence of AMROs in K-(BEDT-TTF)sCu(SCN)z at 33 T and 1.6 K. 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00607-9