5,505 research outputs found
Fate of the Peak Effect in a Type-II Superconductor: Multicriticality in the Bragg-Glass Transition
We have used small-angle-neutron-scattering (SANS) and ac magnetic
susceptibility to investigate the global magnetic field H vs temperature T
phase diagram of a single crystal Nb in which a first-order transition of
Bragg-glass melting (disordering), a peak effect, and surface superconductivity
are all observable. It was found that the disappearance of the peak effect is
directly related to a multicritical behavior in the Bragg-glass transition.
Four characteristic phase boundary lines have been identified on the H-T plane:
a first-order line at high fields, a mean-field-like continuous transition line
at low fields, and two continuous transition line associated with the onset of
surface and bulk superconductivity. All four lines are found to meet at a
multicritical point.Comment: 4 figure
Josephson Vortex States in Intermediate Fields
Motivated by recent resistance data in high superconductors in fields
{\it parallel} to the CuO layers, we address two issues on the Josephson-vortex
phase diagram, the appearances of structural transitions on the observed first
order transition (FOT) curve in intermediate fields and of a lower critical
point of the FOT line. It is found that some rotated pinned solids are more
stable than the ordinary rhombic pinned solids with vacant interlayer spacings
and that, due to the vertical portion in higher fields of the FOT line, the FOT
tends to be destroyed by creating a lower critical point.Comment: 12 pages, 3 figures. To appear in J.Phys.Soc.Jpn. 71, No.2 (February,
2002
Evidence for vortex staircases in the whole angular range due to competing correlated pinning mechanisms
We analyze the angular dependence of the irreversible magnetization of
YBaCuO crystals with columnar defects inclined from the c-axis. At
high fields a sharp maximum centered at the tracks' direction is observed. At
low fields we identify a lock-in phase characterized by an angle-independent
pinning strength and observe an angular shift of the peak towards the c-axis
that originates in the material anisotropy. The interplay among columnar
defects, twins and ab-planes generates a variety of staircase structures. We
show that correlated pinning dominates for all field orientations.Comment: 9 figures, 4 figure
Hall Anomaly and Vortex-Lattice Melting in Superconducting Single Crystal YBa2Cu3O7-d
Sub-nanovolt resolution longitudinal and Hall voltages are measured in an
ultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order
vortex-lattice melting transition are observed simultaneously. Changes in the
dynamic behavior of the vortex solid and liquid are correlated with features of
the Hall conductivity sxy. With the magnetic field oriented at an angle from
the twin-boundaries, the Hall conductivity sharply decreases toward large
negative values at the vortex-lattice melting transition.Comment: 6 pages, 2 figures included, Postscript, to appear in Phys. Rev. Let
Hysteretic creep of elastic manifolds
We study the dynamic response of driven systems in the presence of quenched
disorder. A simple heuristic model for hysteretic creep of elastic manifolds is
proposed and evaluated numerically. It provides a qualitative explanation of
the phenomenology observed in experiments on high-temperature superconductors.Comment: 4 pages Revtex + epsf, plus 3 figures postscrip
Scanning Tunneling Spectroscopy in MgB2
We present scanning tunneling microscopy measurements of the surface of
superconducting MgB2 with a critical temperature of 39K. In zero magnetic field
the conductance spectra can be analyzed in terms of the standard BCS theory
with a smearing parameter Gamma. The value of the superconducting gap is 5.2
meV at 4.2 K, with no experimentally significant variation across the surface
of the sample. The temperature dependence of the gap follows the BCS form,
fully consistent with phonon-mediated superconductivity in this novel
superconductor. The application of a magnetic field induces strong
pair-breaking as seen in the conductance spectra in fields up to 6 T.Comment: 4 pages, 4 figure
Microwave device investigations
Materials, devices and novel schemes for generation, amplification and detection of microwave and millimeter wave energy are studied. Considered are: (1) Schottky-barrier microwave devices; (2) intermodulation products in IMPATT diode amplifiers; and (3) harmonic generation using Read diode varactors
Controlled Dynamics of Interfaces in a Vibrated Granular Layer
We present experimental study of a topological excitation, {\it interface},
in a vertically vibrated layer of granular material. We show that these
interfaces, separating regions of granular material oscillation with opposite
phases, can be shifted and controlled by a very small amount of an additional
subharmonic signal, mixed with the harmonic driving signal. The speed and the
direction of interface motion depends sensitively on the phase and the
amplitude of the subharmonic driving.Comment: 4 pages, 6 figures, RevTe
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