Phonons in intrinsic Josephson systems with parallel magnetic field

Subgap resonances in the I-V curves of layered superconductors are explained by the coupling between Josephson oscillations and phonons with dispersion in c-direction. In the presence of a magnetic field applied parallel to the layers additional structures due to fluxon motion appear. Their coupling with phonons is investigated theoretically and a shift of the phonon resonances in strong magnetic fields is predicted.Comment: Invited Paper to the "2nd International Symposium on Intrinsic Josephson Effects and Plasma Oscillations in High-Tc Superconductors", 22-24 August 2000, Sendai, Japan, to be published in Physica

Subgap structures in the current-voltage characteristic of the intrinsic Josephson effect due to phonons

A modified RSJ-model for the coupling of intrinsic Josephson oscillations and c-axis phonons in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} is deveoped. This provides a very good explanation for recently reported subgap structures in the I-V-characteristic of the c-axis transport. It turns out that the voltages of these structures coincide with the eigenfrequencies of longitudinal optical phonons, providing a new measurement technique for this quantity. The significantly enhanced microwave emission at the subgap structures in both the GHz and THz region is discussed.Comment: correction of minor misprints, revtex, 3 pages, two postscript figures, aps, epsf, Contributed Paper to the "International Symposion on the Intrinsic Josphson effect and THz Plasma Oscillations", 22-25 February 1997, Sendai, Japan; to be published in Physica

Microscopic theory of the coupling of intrinsic Josephson oscillations and phonons

A microscopic theory for the coupling of intrinsic Josephson oscillations and dispersive phonon branches in layered superconductors is developed. Thereby the effect of phonons on the electronic c-axis transport enters through an effective longitudinal dielectric function. This coupling provides an explanation of recently observed subgap resonances in the $I_{dc}$-$V_{dc}$- curve of anisotropic cuprate superconductors forming a stack of short Josephson junctions. Due to the finite dispersion these resonances can appear at van-Hove-singularities of both optical and acoustical phonon branches, explaining low-voltage structures in the I-V-characteristic, which are not understood in phonon models without dispersion. In long junctions the dispersion of collective electron-phonon modes parallel to the layers is investigated.Comment: 4 pages, 3 figures, 1 table, espcrc2.sty, invited contribution to "Materials and Mechanisms of Superconductivity and High Temperature Superconductors VI - M2S-HTSC-VI", Houston, Texas, 20-25 Feb 2000, to appear in Physica

Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors

The recently reported subgap structures observed in the current-voltage characteristic of intrinsic Josephson junctions in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+\delta} and Bi_2Sr_2CaCu_2O_{8+\delta} 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.Comment: 4 pages, 3 figures, revtex, aps, epsf, psfig. submitted to Physical Review Letters, second version improved in detai

Imaging of Thermal Domains in ultrathin NbN films for Hot Electron Bolometers

We present low-temperature scanning electron microscopy (LTSEM) investigations of superconducting microbridges made from ultrathin NbN films as used for hot electron bolometers. LTSEM probes the thermal structure within the microbridges under various dc current bias conditions, either via electron-beam-induced generation of an unstable hotspot, or via the beam-induced growth of a stable hotspot. Such measurements reveal inhomogeneities on a micron scale, which may be due to spatial variations in the NbN film or film-interface properties. Comparison with model calculations for the stable hotspot regime confirm the basic features of common hot spot models.Comment: 3 pages, 3 figure

Simulation of I-V Hysteresis Branches in An Intrinsic Stack of Josephson Junctions in High TcT_c Superconductors

I-V characteristics of the high T$_c$ superconductor Bi$_2$Sr$_2$Ca$_1$C$_2$O$_8$ shows a strong hysteresis, producing many branches. The origin of hysteresis jumps is studied by use of the model of multi-layered Josephson junctions proposed by one of the authors (T. K.). The charging effect at superconducting layers produces a coupling between the next nearest neighbor phase-differences, which determines the structure of hysteresis branches. It will be shown that a solution of phase motions is understood as a combination of rotating and oscillating phase-differences, and that, at points of hysteresis jumps, there occurs a change in the number of rotating phase-differences. Effects of dissipation are analyzed. The dissipation in insulating layers works to damp the phase motion itself, while the dissipation in superconducting layers works to damp relative motions of phase-differences. Their effects to hysteresis jumps are discussed.Comment: 18 pages, Latex, 8 figures. To be appear in Phys.Rev.B Vol.60(1999

Ratchet effect in dc SQUIDs

We analyzed voltage rectification for dc SQUIDs biased with ac current with zero mean value. We demonstrate that the reflection symmetry in the 2-dimensional SQUID potential is broken by an applied flux and with appropriate asymmetries in the dc SQUID. Depending on the type of asymmetry, we obtain a rocking or a simultaneously rocking and flashing ratchet, the latter showing multiple sign reversals in the mean voltage with increasing amplitude of the ac current. Our experimental results are in agreement with numerical solutions of the Langevin equations for the asymmetric dc SQUID.Comment: 10 pages including 5 Postscript figure

Josephson junctions with negative second harmonic in the current-phase relation: properties of novel varphi-junctions

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions (JJ) based on novel superconductors, e.g., d-wave based or JJ with ferromagnetic barrier. In this situation the role of the second harmonic becomes dominant and it determines the scenario of a 0-pi transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is negative the 0-pi transition becomes continuous and the realization of the so-called varphi junction is possible. We study the unusual properties of such a novel JJ and analyze the possible experimental techniques for their observation.Comment: submitted to PR

Suppression of dissipation in Nb thin films with triangular antidot arrays by random removal of pinning sites

The depinning current Ic versus applied magnetic field B close to the transition temperature Tc of Nb thin films with randomly diluted triangular arrays of antidots is investigated. % Our experiments confirm essential features in Ic(B) as predicted by Reichhardt and Olson Reichhardt [Phys.Rev. B 76, 094512 (2007)]. % We show that, by introducing disorder into periodic pinning arrays, Ic can be enhanced. % In particular, for arrays with fixed density n_p of antidots, an increase in dilution Pd induces an increase in Ic and decrease of the flux-flow voltage for B>Bp=n_p Phi_0.Comment: 5 pages, 4 figure