The paper presents a detailed discussion of the current-voltage characteristic of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8ϩ␦ and Tl 2 Ba 2 Ca 2 Cu 3 O 10ϩ␦ . In these materials Josephson tunnel junctions are formed naturally between adjacent superconducting CuO 2 bilayers or trilayers. A typical sample consists of a stack of Josephson junctions. We explicitly show that all junctions inside a given sample have identical tunneling characteristics. We discuss the shape ͑general curvature͒ of the current-voltage characteristic in terms of a superconducting order parameter that has a predominant d x 2 Ϫy 2 symmetry. The I c R n product of the intrinsic Josephson junctions turns out to be 2-3 mV, about 10% of the maximum energy gap ⌬ 0 /e. The currentvoltage characteristic of every individual junction exhibits pronounced structures in the subgap regime. They are best explained by a recently proposed resonant coupling mechanism between infrared active optical c-axis phonons and oscillating Josephson currents.

The recently reported subgap structures in the current-voltage characteristic of intrinsic Josephson junctions in the high-T c superconductors Tl 2 Ba 2 Ca 2 Cu 3 O 101d and Bi 2 Sr 2 CaCu 2 O 81d are explained by the coupling between c-axis phonons and Josephson oscillations. A model is developed where c-axis lattice vibrations between adjacent superconducting multilayers are excited by the Josephson oscillations in a resistive junction. The voltages of the lowest structures correspond well to the frequencies of longitudinal c-axis phonons with large oscillator strength in the two materials, providing a new measurement technique for this quantity. [S0031-9007(97)03697-1] PACS numbers: 74.50. + r, 74.25.Fy, 74.25.Kc 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 high-T c superconductors Tl 2 Ba 2 Ca 2 Cu 3 O 101d (TBCCO) and Bi 2 Sr 2 CaCu 2 O 81d (BSCCO) has been reported [2][3][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 ...

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Standard weak coupling methods are used to study collective modes in the superconducting state of a double-layer system with intralayer and interlayer interaction, as well as a Josephson-type coupling and single particle hopping between the layers by calculating the electronic polarization function perpendicular to the layers. New analytical results are derived for the mode frequencies corresponding to fluctuations of the relative phase and amplitude of the layer order parameters in the case of interlayer pairing and finite hopping t. A new effect is found for finite k-dependent hopping: then the amplitude and phase fluctuations are coupled. Therefore two collective modes may appear in the dynamical c-axis conductivity below the threshold energy for breaking Cooper pairs. With help of numerical calculations we investigate the temperature dependence of the collective modes and show how a plasmon corresponding to charge fluctuations between the layers evolves in the normal state.

We calculate frequency shifts and changes in linewidths of infrared-active phonons within a shell model for the bare phononic system coupled to an electronic doublelayer structure with inter-layer charge transfer. The theoretical concept is applied to YBa 2 Cu 3 O 7 yielding a good description of experimental results in the normal state as well as at the transition to the superconducting state.