3,561 research outputs found
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 --
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 Superconductors
I-V characteristics of the high T superconductor
BiSrCaCO 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
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