2,951 research outputs found
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
Mean curvature flow in a Ricci flow background
Following work of Ecker, we consider a weighted Gibbons-Hawking-York
functional on a Riemannian manifold-with-boundary. We compute its variational
properties and its time derivative under Perelman's modified Ricci flow. The
answer has a boundary term which involves an extension of Hamilton's Harnack
expression for the mean curvature flow in Euclidean space. We also derive the
evolution equations for the second fundamental form and the mean curvature,
under a mean curvature flow in a Ricci flow background. In the case of a
gradient Ricci soliton background, we discuss mean curvature solitons and
Huisken monotonicity.Comment: final versio
Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids
We studied experimentally the effect of a stripe-like domain structure in a
ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a
superconducting Pb microbridge. The system was designed in such a way that the
bridge is oriented perpendicular to the domain walls. It is demonstrated that
depending on the ratio between the amplitude of the nonuniform magnetic field
B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the
superconducting material, the regions of the reverse-domain superconductivity
in the H-T plane can be isolated or can overlap (H is the external magnetic
field, T is temperature). The latter case corresponds to the condition
B_0/H_{c2}<1 and results in the formation of superconductivity above the
magnetic domains of both polarities. We discovered the regime of edge-assisted
reverse-domain superconductivity, corresponding to localized superconductivity
near the edges of the bridge above the compensated magnetic domains. Direct
verification of the formation of inhomogeneous superconducting states and
external-field-controlled switching between normal state and inhomogeneous
superconductivity were obtained by low-temperature scanning laser microscopy.Comment: 11 pages, 12 figure
Dynamics and transformations of Josephson vortex lattice in layered superconductors
We consider dynamics of Josephson vortex lattice in layered superconductors
with magnetic, charge (electrostatic) and charge-imbalance (quasiparticle)
interactions between interlayer Josephson junctions taken into account. The
macroscopic dynamical equations for interlayer Josephson phase differences,
intralayer charge and electron-hole imbalance are obtained and used for
numerical simulations. Different transformations of the vortex lattice
structure are observed. It is shown that the additional dissipation due to the
charge imbalance relaxation leads to the stability of triangular lattice.Comment: 9 pages, 3 eps figures, to be published in Phys. Rev.
Safe abstractions of data encodings in formal security protocol models
When using formal methods, security protocols are usually modeled at a high level of abstraction. In particular, data encoding and decoding transformations are often abstracted away. However, if no assumptions at all are made on the behavior of such transformations, they could trivially lead to security faults, for example leaking secrets or breaking freshness by collapsing nonces into constants. In order to address this issue, this paper formally states sufficient conditions, checkable on sequential code, such that if an abstract protocol model is secure under a Dolev-Yao adversary, then a refined model, which takes into account a wide class of possible implementations of the encoding/decoding operations, is implied to be secure too under the same adversary model. The paper also indicates possible exploitations of this result in the context of methods based on formal model extraction from implementation code and of methods based on automated code generation from formally verified model
Dynamic structure selection and instabilities of driven Josephson lattice in high-temperature superconductors
We investigate the dynamics of the Josephson vortex lattice in layered
high-T superconductors at high magnetic fields. Starting from coupled
equations for superconducting phases and magnetic field we derive equations for
the relative displacements [phase shifts] between the planar Josephson arrays
in the layers. These equations reveal two families of steady-state solutions:
lattices with constant phase shifts between neighboring layers, starting from
zero for a rectangular configuration to for a triangular configuration,
and double-periodic lattices. We find that the excess Josephson current is
resonantly enhanced when the Josephson frequency matches the frequency of the
plasma mode at the wave vector selected by the lattice structure. The regular
lattices exhibit several kinds of instabilities. We find stability regions of
the moving lattice in the plane lattice structure - Josephson frequency. A
specific lattice structure at given velocity is selected uniquely by boundary
conditions, which are determined by the reflection properties of
electromagnetic waves generated by the moving lattice. With increase of
velocity the moving configuration experiences several qualitative
transformations. At small velocities the regular lattice is stable and the
phase shift between neighboring layers smoothly decreases with increase of
velocity, starting from for a static lattice. At the critical velocity
the lattice becomes unstable. At even higher velocity a regular lattice is
restored again with the phase shift smaller than . With increase of
velocity, the structure evolves towards a rectangular configuration.Comment: 28 pages, 12 figures, submitted to Phys. Rev.
Viral expression and molecular profiling in liver tissue versus microdissected hepatocytes in hepatitis B virus - associated hepatocellular carcinoma.
Background: The molecular mechanisms whereby hepatitis B virus (HBV) induces hepatocellular carcinoma (HCC)
remain elusive. We used genomic and molecular techniques to investigate host-virus interactions by studying multiple areas of the same liver from patients with HCC.
Methods: We compared the gene signature of whole liver tissue (WLT) versus laser capture-microdissected (LCM)
hepatocytes along with the intrahepatic expression of HBV. Gene expression profiling was performed on up to 17 WLT specimens obtained at various distances from the tumor center from individual livers of 11 patients with HCC and on selected LCM samples. HBV markers in liver and serum were determined by real-time polymerase chain reaction (PCR)and confocal immunofluorescence.
Results: Analysis of 5 areas of the liver showed a sharp change in gene expression between the immediate perilesional area and tumor periphery that correlated with a significant decrease in the intrahepatic expression of HB surface antigen (HBsAg). The tumor was characterized by a large preponderance of down-regulated genes, mostly involved in the
metabolism of lipids and fatty acids, glucose, amino acids and drugs, with down-regulation of pathways involved in the activation of PXR/RXR and PPARα/RXRα nuclear receptors, comprising PGC-1α and FOXO1, two key regulators
critically involved not only in the metabolic functions of the liver but also in the life cycle of HBV, acting as essential transcription factors for viral gene expression. These findings were confirmed by gene expression of microdissected
hepatocytes. Moreover, LCM of malignant hepatocytes also revealed up-regulation of unique genes associated with
cancer and signaling Pathways, including two novel HCC-associated cancer testis antigen genes, NUF2 and TTK.
Conclusions: Integrated gene expression profiling of whole liver tissue with that of microdissected hepatocytes
demonstrated that HBV-associated HCC is characterized by a metabolism switch-off and by a significant reduction in
HBsAg. LCM proved to be a critical tool to validate gene signatures associated with HCC and to identify genes that may play a role in hepatocarcinogenesis, opening new perspectives for the discovery of novel diagnostic markers and therapeutic targets
Broken symmetry of row switching in 2D Josephson junction arrays
We present an experimental and theoretical study of row switching in
two-dimensional Josephson junction arrays. We have observed novel dynamic
states with peculiar percolative patterns of the voltage drop inside the
arrays. These states were directly visualized using laser scanning microscopy
and manifested by fine branching in the current-voltage characteristics of the
arrays. Numerical simulations show that such percolative patterns have an
intrinsic origin and occur independently of positional disorder. We argue that
the appearance of these dynamic states is due to the presence of various
metastable superconducting states in arrays.Comment: 4 Pages, 6 Figure
Localization of superconductivity in superconductor-electromagnet hybrids
We investigate the nucleation of superconductivity in a superconducting Al
strip under the influence of the magnetic field generated by a current-carrying
Nb wire, perpendicularly oriented and located underneath the strip. The
inhomogeneous magnetic field, induced by the Nb wire, produces a spatial
modulation of the critical temperature T_c, leading to a controllable
localization of the superconducting order parameter (OP) wave function. We
demonstrate that close to the phase boundary T_c(B_ext) the localized OP
solution can be displaced reversibly by either applying an external
perpendicular magnetic field B_ext or by changing the amplitude of the
inhomogeneous field.Comment: 10 pages, 6 figure
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