163 research outputs found
Multiband superconductivity in NbSe_2 from heat transport
The thermal conductivity of the layered s-wave superconductor NbSe_2 was
measured down to T_c/100 throughout the vortex state. With increasing field, we
identify two regimes: one with localized states at fields very near H_c1 and
one with highly delocalized quasiparticle excitations at higher fields. The two
associated length scales are most naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc
The advent of high transverse-field muon spin rotation (TF-muSR) has led to
recent muSR investigations of the magnetic-field response of cuprates above the
superconducting transition temperature T_c. Here the results of such
experiments on hole-doped cuprates are reviewed. Although these investigations
are currently ongoing, it is clear that the effects of high field on the
internal magnetic field distribution of these materials is dependent upon a
competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the
response to the external field above Tc is dominated by heterogeneous spin
magnetism. However, the magnetism that dominates the observed inhomogeneous
line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely
different kind of magnetism in the heavily overdoped regime. The origin of the
magnetism above x ~ 0.19 is currently unknown, but its presence hints at a
competition between superconductivity and magnetism that is reminiscent of the
underdoped regime. In contrast, the width of the internal field distribution of
underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of
superconducting carriers. This observation suggests that the magnetic response
above Tc is not dominated by electronic moments, but rather inhomogeneous
fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference
Evidence for Static Magnetism in the Vortex Cores of Ortho-II YBaCuO
Evidence for static alternating magnetic fields in the vortex cores of
underdoped YBaCuO is reported. Muon spin rotation measurements
of the internal magnetic field distribution of the vortex state of
YBaCuO in applied fields of T and T reveal a
feature in the high-field tail of the field distribution which is not present
in optimally doped YBaCuO and which fits well to a model with
static magnetic fields in the vortex cores. The magnitude of the fields is
estimated to be 18(2) G and decreases above K. We discuss possible
origins of the additional vortex core magnetism within the context of existing
theories.Comment: Submitted to PRL; corresponding author: [email protected]
Expansion of the Vortex Cores in YBa2Cu3O6.95 at Low Magnetic Fields
Muon spin rotation spectroscopy has been used to measure the effective size
of the vortex cores in optimally doped YBa2Cu3O6.95 as a function of
temperature and magnetic field deep in the superconducting state. While the
core size at H=2T is close to 20 angstroms and consistent with that measured by
STM at 6T, we find a striking increase in the core size at lower magnetic
fields, where it approaches an extraordinarily large value of about 100
angstroms. This suggests that the average value of the superconducting
coherence length in cuprate superconductors may be larger than previously
thought at low magnetic fields.Comment: 9 pages, 4 figures, 1 text fil
Determination of Optical Properties of a Field Emission Gun Coupled with a Linear Accelerator for High Voltage Microscopy
The electron optical properties of a field emission gun plus a twenty-stage linear accelerator system have been determined in the case where the entrance pupil of the system is defined by one of the electrode apertures.
The first part of the calculation concerns the electrical and geometrical parameters of triode and tetrode field emission guns which can give a fixed location of the source and a small spherical aberration coefficient.
The second part is related to the possibility of placing a tetrode field emission gun at the top of the linear accelerator of the scanning high voltage electron microscope which is being constructed in the Toulouse Laboratory. Interesting conditions can be obtained when the electrical parameters are fixed as follows: either Vi/Ve= 0 or Vi/Ve= 20 (Ve is the extracting voltage, Vi is the potential of the second anode of the gun) whatever the accelerating voltage may be
Diffusion imaging with a multi-echo MISSTEC sequence
An imaging method is presented to measure the water-diffusion coefficient. The sequence (MISSTEC) uses the simultaneous acquisition of a spin echo and several stimulated echoes with the same intensity except for diffusion weighting. The optimal number of stimulated echoes was calculated to minimize the diffusion coefficient error (D). D values obtained in vitro and in vivo were in good agreement with those from the spin-echo sequence (IntraVoxel Incoherent Motion [IVIM] method). The total acquisition time is half that of the classic IVIM method
Field Induced Reduction of the Low Temperature Superfluid Density in YBa2Cu3O6.95
A novel high magnetic field (8 T) spectrometer for muon spin rotation has
been used to measure the temperature dependence of the in-plane magnetic
penetration depth in YBa2Cu3O6.95. At low H and low T, the penetration depth
exhibits the characteristic linear T dependence associated with the energy gap
of a d_x^2-y^2-wave superconductor. However, at higher fields the penetration
depth is essentially temperature independent at low T. We discuss possible
interpretations of this surprising new feature in the low-energy excitation
spectrum.Comment: 8 pages, 4 figures, 1 RevTex file and 4 postscript figure
Field dependence of the vortex core size in a multi-band superconductor
The magnetic field dependence of the vortex core size in the multi-band
superconductor NbSe2 has been determined from muon spin rotation measurements.
The spatially extended nature of the quasiparticle core states associated with
the smaller gap leads to a rapid field-induced shrinkage of the core size at
low fields, while the more tightly bound nature of the states associated with
the larger gap leads to a field-independent core size for fields greater than 4
kOe. A simple model is proposed for the density of delocalized core states that
establishes a direct relationship between the field-induced reduction of the
vortex core size and the corresponding enhancement of the electronic thermal
conductivity. We show that this model accurately describes both NbSe2 and the
single-band superconductor V3Si.Comment: 4 pages, 4 figures. Version accepted for publication in Physical
Review Letter
Nonlocal Effects and Shrinkage of the Vortex Core Radius in YNi2B2C Probed by muSR
The magnetic field distribution in the vortex state of YNi2B2C has been
probed by muon spin rotation (muSR). The analysis based on the London model
with nonlocal corrections shows that the vortex lattice has changed from
hexagonal to square with increasing magnetic field H. At low fields the vortex
core radius, rho_v(H), decreases with increasing H much steeper than what is
expected from the sqrt(H) behavior of the Sommerfeld constant gamma(H),
strongly suggesting that the anomaly in gamma(H) primarily arises from the
quasiparticle excitations outside the vortex cores.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
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