Coupling between Phonons and Intrinsic Josephson Oscillations in Cuprate Superconductors

Abstract

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 ...

Long Josephson junction in a resonant cavity

“…This system is formally analogous to the junction-resonant cavity system, in that both involve coupling between a junction and a harmonic oscillator mode. Helm et al 20,21 have developed a model describing the coupling between longitudinal phonons and intrinsic Josephson oscillations in cuprate superconductors, which, when sufficiently anisotropic, behave like stacks of underdamped Josephson junctions. [22][23][24] These theories have also been extended into the quantum regime.…”

Temperature and magnetic field effects on the carrier density and Hall mobility of boron-doped Tl–Ba–Ca–Cu–O superconductor

“…The authors reported that the contribution from acoustic phonons to the resistivity was more pronounced than the electron-phonon interactions. For Tl-2223 samples, it was explained by Helm et al [24] that the subgap structures in the current-voltage characteristics corresponds to the frequencies of longitudinal c-axis phonons with large oscillator strength. Similarly, the c-axis infrared response studies on Tl-2223 [25] resulted in the observation of two longitudinal phonons located at 400 and 620 cm −1 and an in-plane optical plasmon at ∼1.0 eV.…”

The Behavior of Subgap Structures of Intrinsic Josephson Junctions in $({\rm Bi},{\rm Pb})_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{\rm y}$Under Magnetic Field and Microwave Irradiation

“…And then, two different mechanisms to explain them have been proposed. One is the resonant coupling mechanism between infrared-active optical phonons and oscillating Josephson currents in the IJJs, proposed by Helm et al [10]. Another is the phonon assisted tunneling mechanism due to the interference of Raman-active optical phonons with ac Josephson currents in the IJJs, proposed by Maksimov et al [11].…”