61 research outputs found
Coupling of intrinsic Josephson oscillations in layered superconductors by charge fluctuations
The coupling of Josephson oscillations in layered superconductors is studied
with help of a tunneling Hamiltonian formalism. The general form of the current
density across the barriers between the superconducting layers is derived. The
induced charge fluctuations on the superconducting layers lead to a coupling of
the Josephson oscillations in different junctions. A simplified set of
equations is then used to study the non-linear dynamics of the system. In
particular the influence of the coupling on the current-voltage characteristics
is investigated and upper limits for the coupling strength are estimated from a
comparison with experiments on cuprate superconductors.Comment: To be published in proceedings of SPIE conference San Diego 199
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
Static Charge Coupling of Intrinsic Josephson Junction
A microscopic theory for the coupling of intrinsic Josephson oscillations due
to charge fluctuations on the quasi two-dimensional superconducting layers is
presented. Thereby in close analogy to the normal state the effect of the
scalar potential on the transport current is taken into account consistently.
The dispersion of collective modes is derived and an estimate of the coupling
constant is given. It is shown that the correct treatment of the quasiparticle
current is essential in order to get the correct position of Shapiro steps. In
this case the influence of the coupling on dc-properties like the -curve
is negligible.Comment: 6 pages latex, 5 figures, espcrc2.sty, Invited Contribution to "2nd
International Symposiom on Intrinsic Josephson Effects and Plasma
Oscillations in High-TC Superconductors", 22-24 August, Sendai, Japan, to be
published in Physica
Quantifying trading behavior in financial markets using Google Trends
Crises in financial markets affect humans worldwide. Detailed market data on trading decisions reflect some of the complex human behavior that has led to these crises. We suggest that massive new data sources resulting from human interaction with the Internet may offer a new perspective on the behavior of market participants in periods of large market movements. By analyzing changes in Google query volumes for search terms related to finance, we find patterns that may be interpreted as “early warning signs” of stock market moves. Our results illustrate the potential that combining extensive behavioral data sets offers for a better understanding of collective human behavior
Reflectivity and Microwave Absorption in Crystals with Alternating Intrinsic Josephson Junctions
We compute the frequency and magnetic field dependencies of the reflectivity
in layered superconductors with two alternating intrinsic Josephson
junctions with different critical current densities and quasiparticle
conductivities for the electric field polarized along the c-axis. The parameter
describing the electronic compressibility of the layers and the charge
coupling of neighboring junctions was extracted for the
SmLaSrCuO superconductor from two independent
optical measurements, the fit of the loss function at zero magnetic
field and the magnetic field dependence of the peak positions in .
The experiments are consistent with a free electron value for near
the Josephson plasma frequencies.Comment: 4 pages, 4 postscript figures, misprints in table correcte
Theory for the coupling between longitudinal phonons and intrinsic Josephson oscillations in layered superconductors
In this publication a microscopic theory for the coupling of intrinsic
Josephson oscillations in layered superconductors with longitudinal
c-axis-phonons is developed. It is shown that the influence of lattice
vibrations on the c-axis transport can be fully described by introducing an
effective longitudinal dielectric function. Resonances in the
I-V-characteristic appear at van Hove singularities of both acoustical and
optical longitudinal phonon branches. This provides a natural explanation of
the recently discovered subgap structures in the I-V-characteristic of highly
anisotropic cuprate superconductors. The effect of the phonon dispersion on the
damping of these resonances and the coupling of Josephson oscillations in
different resistive junctions due to phonons are discussed in detail.Comment: submitted to Phys. Rev. B, corrections following referee repor
Quantifying the behavior of stock correlations under market stress
Understanding correlations in complex systems is crucial in the face of turbulence, such as the ongoing financial crisis. However, in complex systems, such as financial systems, correlations are not constant but instead vary in time. Here we address the question of quantifying state-dependent correlations in stock markets. Reliable estimates of correlations are absolutely necessary to protect a portfolio. We analyze 72 years of daily closing prices of the 30 stocks forming the Dow Jones Industrial Average (DJIA). We find the striking result that the average correlation among these stocks scales linearly with market stress reflected by normalized DJIA index returns on various time scales. Consequently, the diversification effect which should protect a portfolio melts away in times of market losses, just when it would most urgently be needed. Our empirical analysis is consistent with the interesting possibility that one could anticipate diversification breakdowns, guiding the design of protected portfolios
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
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