COUPLING BETWEEN PHONONS AND INTRINSIC JOSEPHSON OSCILLATIONS IN CUPRATE SUPERCONDUCTORS

The transport properties of highly anisotropic cuprate superconductors in c direction can well be described by a stack of Josephson junctions formed by nonsuperconducting material between adjacent superconducting copper oxide multilayers [1]. Recently the observation of subgap structures in the current-voltage (I-V ) characteristic of intrinsic Josephson junctions in the highTc superconductors Tl2Ba2Ca2Cu3O101d (TBCCO) and Bi2Sr2CaCu2O81d (BSCCO) has been reported [2‐ 4]. Each individual branch of the I-V curve shows a structure which can be traced back to the I-V characteristic of one single Josephson junction in the resistive state. These structures seem to be an intrinsic effect, as they have been observed both in step edge junctions (TBCCO) and mesatype stacks (BSCCO) of different sizes. The characteristic voltages are completely independent of temperature and external magnetic fields, which rules out any relation to the superconducting gap, vortex flow, or the thermal excitation of quasiparticles. It was shown that the pattern of one junction can be described phenomenologically by a resistively shunted junction (RSJ) model by assuming ad hoc a special structure for the current-voltage characteristic of the quasiparticles [3]. It was argued that such a structure might result from peaks in the quasiparticle density of states due to Andreev reflection between normal and superconducting regions. Several alternative approaches, including the modulation of the tunneling distance due to Raman-active phonons, have been mentioned in [2], but up to now all suggestions failed to explain the main features of the effect. In this paper we want to discuss a different mechanism involving phonons by assuming that the local electric field oscillations produced by the Josephson effect in a single junction excite infrared active c-axis phonons. In the following we will present a simple model where we couple the nonlinear currentphase relation of one junction to a local oscillator. The analytical and numerical solution of this model provides a very good quantitative explanation of the experimental data. It is shown that the peaks in the subgap structure of the dc current-voltage characteristic correspond to zeros of the dielectric function of the barrier material, i.e., to longitudinal optical phonons. In the minimal version of the RSJ model the total current (density) I › jc sin g1s 0 E 1 Ÿ