MAGNETIC BREAKDOWN AND QUANTUM INTERFERENCE IN THE QUASI-2-DIMENSIONAL SUPERCONDUCTOR KAPPA-(BEDT-TTF)(2)CU(NCS)(2) IN HIGH MAGNETIC-FIELDS

Magnetic breakdown phenomena have been investigated in the longitudina l magnetoresistance of the quasi-two-dimensional (Q2D) superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) in magnetic fields of up to 50 T, well a bove the characteristic breakdown field. The material is of great inte rest because its relatively simple Fermi surface, consisting of a clos ed Q2D pocket and an open Q1D band, is almost identical to the initial hypothetical breakdown network proposed by Pippard. Two frequencies a re expected to dominate the magnetoresistance oscillations: the a freq uency, corresponding to orbits around the closed pocket, and the beta frequency, corresponding to the simplest classical breakdown orbit. Ho wever, a beta - alpha frequency is in fact found to be the dominant hi gh-frequency oscillation in the magnetoresistance. Numerical simulatio ns, employing standard theories for calculating the density of states, indicate that a significant presence of the beta - alpha frequency (f orbidden in the standard theories) can result simply from the frequenc y-mixing effects associated with the pinning of the chemical potential in a quasi-two-dimensional system. While this effect is able to accou nt for the previous experimental observation of beta - alpha frequency oscillations of small amplitude in the magnetization, it cannot expla in why such a frequency dominates the high-field magnetotransport spec trum. Instead we have extended the numerical simulations to include a quantum interference model adapted for longitudinal magnetoresistance in a quasi-two-dimensional conductor. The modified simulations are the n able to account for most of the features of the experimental magneto resistance data.