345 research outputs found
Conductance of Pd-H nanojunctions
Results of an experimental study of palladium nanojunctions in hydrogen
environment are presented. Two new hydrogen-related atomic configurations are
found, which have a conductances of ~0.5 and ~1 quantum unit (2e^2/h). Phonon
spectrum measurements demonstrate that these configurations are situated
between electrodes containing dissolved hydrogen. The crucial differences
compared to the previously studied Pt-H_2 junctions, and the possible
microscopic realizations of the new configurations in palladium-hydrogen
atomic-sized contacts are discussed.Comment: 4 pages, 4 figure
Direct observation of non-local effects in a superconductor
We have used the technique of low energy muon spin rotation to measure the
local magnetic field profile B(z) beneath the surface of a lead film maintained
in the Meissner state (z depth from the surface, z <= 200 nm). The data
unambiguously show that B(z) clearly deviates from an exponential law and
represent the first direct, model independent proof for a non-local response in
a superconductor.Comment: 5 pages, 3 figure
Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-delta thin films
Low-angle grain boundaries with misorientation angles theta < 5 degrees in
optimally doped thin films of YBCO are investigated by magnetooptical imaging.
By using a numerical inversion scheme of Biot-Savart's law the critical current
density across the grain boundary can be determined with a spatial resolution
of about 5 micrometers. Detailed investigation of the spatially resolved flux
density and current density data shows that the current density across the
boundary varies with varying local flux density. Combining the corresponding
flux and current pattern it is found that there exists a universal dependency
of the grain boundary current on the local flux density. A change in the local
flux density means a variation in the flux line-flux line distance. With this
knowledge a model is developped that explains the flux-current relation by
means of magnetic vortex-vortex interaction.Comment: 7 pages, 14 figure
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes
we identify the intrinsic bulk pairing symmetry for both electron and
hole-doped cuprates from the existing bulk- and nearly bulk-sensitive
experimental results such as magnetic penetration depth, Raman scattering,
single-particle tunneling, Andreev reflection, nonlinear Meissner effect,
neutron scattering, thermal conductivity, specific heat, and angle-resolved
photoemission spectroscopy. These experiments consistently show that the
dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave
with eight line nodes, and of anisotropic s-wave in electron-doped cuprates.
The proposed pairing symmetries do not contradict some surface- and
phase-sensitive experiments which show a predominant d-wave pairing symmetry at
the degraded surfaces. We also quantitatively explain the phase-sensitive
experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and
YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure
Kinetic Inductance and Penetration Depth of Thin Superconducting Films Measured by THz Pulse Spectroscopy
We measure the transmission of THz pulses through thin films of YBCO at
temperatures between 10K and 300K. The pulses possess a useable bandwidth
extending from 0.1 -- 1.5 THz (3.3 cm^-1 -- 50 cm^-1). Below T_c we observe
pulse reshaping caused by the kinetic inductance of the superconducting charge
carriers. From transmission data, we extract values of the London penetration
depth as a function of temperature, and find that it agrees well with a
functional form (\lambda(0)/\lambda(T))^2 = 1 - (T/T_c)^{\alpha}, where
\lambda(0) = 148 nm, and \alpha = 2. *****Figures available upon request*****Comment: 7 Pages, LaTe
Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films
The vortex contribution to the dc field (H) dependent microwave surface
impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using
suspended patterned resonators. Z_s(H) is shown to be a direct measure of the
flux density B(H) enabling a very precise test of models of flux penetration.
Three regimes of field-dependent behavior were observed: (1) Initial flux
penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate
fields the flux penetration into the virgin state is in excellent agreement
with calculations based upon the field-induced Bean critical state for thin
film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very
high fields H >>H_s, the flux density is uniform and the measurements enable
direct determination of vortex parameters such as pinning force constants
\alpha_p and vortex viscosity \eta. However hysteresis loops are in
disagreement with the thin film Bean model, and instead are governed by the low
field scale H_i, rather than by H_s. Geometric barriers are insufficient to
account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical
Review B, 600 dpi PostScript file with high resolution figures available at
http://sagar.physics.neu.edu/preprints.htm
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