ELSEVIER Synthetic Metals 70 (1995) 819-820 SqJJlfilTIgl ?ll[ IffdlI?HLS Magnetoresistance anisotropy in the organic superconductor n-(BEDT-TTF)2Cu(NCS)2 M.V, Kartsovnik a'b, A.E. Kovalev a, V.N. Laukhin ¢, H. Ito b, T. Ishiguro b, N.D. Kusheh c, H. Anzai d and G. Saito b ~Institute of Solid State Physics, Ru. Ac. Sci., Chernogolovka, 142432, Russian Federation bFaculty of Science, Kyoto University, Kyoto 606-01, Japan CChernogolovka Institute of Chemical Physics, Ru. Ac. Sci., Chernogolovka, 142432, Russian Federation d Department of Material Science, Himeji Institute of Technology, Kamigory, Hyogo 678-12, Japan We report on systematic studies of the magnetoresistance (MR) anisotropy of tc-(BEDT-TTF):Cu(NCS)2 at ambient pressure and at P =_~ 1.2 kbar. Prominent oscillations of the interplane MR are found under magnetic field rotating in various planes normal to the 2D bc-plane. The oscillations are mainly associated with the existence of the open Fermi surface along the h%K¢-plane, revealing however somewhat complicated behaviour which is most likely due to the specifics of multiply connected Fermi surface. tc-(BEDT-TTF)2Cu(NCS)~, one of the highest- temperature organic superconductors, has been a sub- ject of extensive Fermi surface (FS) studies, revealing strong magnetic quantum oscillations of various fre- quencies (see e.g. [1]). Taking into account the layered crystal structure of the compound, one could also ex- pect the existence of semi-classical angular magnetore- sistance oscillations (AMRO) like those found in other organic conductors [2-4] which have provided an addi- tional direct information about the topology and shape of their FS. However, no AMRO has been reported for this material so far. We have performed detailed studies of interlayer (i.e. normal to the highly-conducting bc-plane) magnetore- sistance (MR) of the title compound as a function of the magnetic field orientation at ambient pressure and at P ="~ 1.2 kbar and found prominent AMRO of a complicated character which is most likely due to the specifics of the multiply connected FS consisting of cylindrical and open parts separated by a small gap as predicted by band structure calculations [5]. The crystals of ~-(BEDT-TTF)2Cu(NCS)2 used for the experiment were grown electrochemically and had the shape of a distorted hexagon with the edges along either the crystal diagonals or b-axis. The samples were oriented under the optical microscope and mounted into a pressure cell which was rotated around two axes in the magnetic field of 11 T. Fig. 1 demonstrates a typical angular dependence of the ambient-pressure MR. The non-monotonous be- haviour resembles AMRO observed in other organic conductors, however the amplitude of the oscillations is rather low and at high angles (above ~_ 600 at 1.5 K and _~ 70 ° at 4.2 K) they are masked by the SC transi- tion and the preceding considerable resistance increase particularly pronounced at 4.2 K. This increase is as- sociated with the resistance upturn which is usually observed in this compound in the R±(T) curves just above the apparent onset of the superconductivity in magnetic field H _1_ b e and attributed to the quasipar- ticle tunneling between the SC 2D layers. The hydrostatic pressure suppresses rapidly T¢ in the present compound. The resistance upturn is expected -q I I I I I I I I 1 0.5! t i: 0_120 0 kbar 4.2 K -60 0 60 1 0 ¢, degrees Fig. 1. Angular dependence of the interplane MR at ambient pressure. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0379-6779(94)02662-I