Interplane corrugations in quasi-one-dimensional Fermi surface sections of deuterated and undeuterated k-(BEDT-TTF) 2 Cu(NCS) 2 $ A. Narduzzo * , R.S. Edwards, A. Ardavan, J. Singleton Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK Abstract Recently a small but definite isotope effect has been found on deuteration of the organic superconductor k-(BEDT-TTF) 2 Cu(NCS) 2 [Physica C 351 (2000) 251]. We have investigated the alterations in band-structure brought about by deuteration using a millimetre-wave technique which is sensitive to the corrugations of the quasi-one-dimensional (Q1D) sheets of the Fermi surface [J. Phys. 13 (2001) 2235]. We have mapped the interplane corrugations of the Q1D Fermi surface sections and discuss the dependence of the warping components of these Q1D sheets on the deuteration. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Organic superconductor; Magnetic resonance; Bandstructure k-(BEDT-TTF) 2 Cu(NCS) 2 is an organic charge transfer salt, a highly anisotropic metal. It becomes a superconductor below T c ¼ 10:4 K. It is a model material for investigating strongly correlated electron systems. Its Fermi surface con- sists of quasi-two-dimensional (Q2D) closed pockets and Q1D corrugated sheets. The closed sections can be inves- tigated using traditional techniques such as Shubnikov–de Haas oscillations [3]. The investigation of Fermi surface traversal resonances is an extremely powerful method to understand the topology of the Q1D sections [2]. Quasiparticles move along trajectories on the Fermi sur- face perpendicular to an applied magnetic field B. Consider the corrugations of a Q1D sheet resulting from the super- position of a number of periodical deformations some of which are along different directions (warping axes). When B is tilted with respect to the sheet’s warping axis the compo- nent of the carrier velocity parallel to the sheet oscillates with an angular frequency o res . A resonant absorption occurs when the frequency of the incoming photons matches o res (Fermi surface traversal resonance, FTR [5]). Samples are placed inside a rectangular cavity, resonating in the TE 102 mode, at about 70 GHz. The cavity can rotate with respect to the external magnetic field B. The sample acts as a perturbation of the cavity resonance, and is placed in an oscillating magnetic field antinode so that the induced currents flow in the low conductivity direction. The skin depth is given by d ¼ 1 ffiffiffiffiffiffiffiffiffiffiffi pmsn p (1) where m is the relative magnetic permeability, s the con- ductivity of the metal, and n the frequency of the radiation. The skin depth in our case is of the order of the sample’s thickness. Our measurements are therefore mainly a probe of the bulk interplane ac magnetoconductivity. The source and detector of electromagnetic radiation is a millimetre-wave vector network analyser (MVNA) [4]. Fig. 1(a) shows the recorded normalised cavity transmis- sion as the magnetic field B is swept. Measurements were taken at a temperature of 1.4 K. The traces correspond to different angles y between the external field and the normal to the highly conducting layers of the sample. The first dip to appear at low fields is due to a superconducting transition; the second visible absorption feature is a FTR. The relevant parameters for FTR are the component of B in the plane of the Q1D sheet, and the angle it forms with the warping axis. Fig. 1(b) shows the data taken for four different azimuthal orientations of the sample within the cavity plotted in this reduced frame of reference. The solid lines correspond to fits to the data of the expression o B k ¼ A sinðc þ c 0 Þ (2) Synthetic Metals 133–134 (2003) 129–130 $ Yamada Conference LVI, The Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001—Abstract Number B7Mon. * Corresponding author. Tel.: þ44-1865-272366; fax: þ44-1865-272400. E-mail address: a.narduzzo@physics.ox.ac.uk (A. Narduzzo). 0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0379-6779(02)00329-6