921 research outputs found
Local quasiparticle density of states of superconducting SmFeAsOF single crystals: Evidence for spin-mediated pairing
We probe the local quasiparticles density-of-states in micron-sized
SmFeAsOF single-crystals by means of Scanning Tunnelling
Spectroscopy. Spectral features resemble those of cuprates, particularly a
dip-hump-like structure developed at energies larger than the gap that can be
ascribed to the coupling of quasiparticles to a collective mode, quite likely a
resonant spin mode. The energy of the collective mode revealed in our study
decreases when the pairing strength increases. Our findings support
spin-fluctuation-mediated pairing in pnictides.Comment: 11 pages, 4 figure
Comment on ``Quasiparticle Spectra around a Single Vortex in a d-wave Superconductor''
In a recent Letter Morita, Kohmoto and Maki analyzed the structure of
quasiparticle states near a single vortex in a d-wave superconductor using an
approximate version of the Bogoliubov - de Gennes theory. Their principal
result is the existence of a bound state within the core region at finite
energy with full rotational symmetry, which they assert explains the recent
scanning tunneling microscopy results on YBCO single crystals. Here we argue
that the approximation used in this work is fundamentally inadequate for the
description of a d-wave vortex and that the obtained circular symmetry of the
local density of states is an unphysical artifact of this approximation.Comment: 1 page REVTeX, to appear in PR
Strong-coupling analysis of scanning tunneling spectra in BiSrCaCuO
We study a series of spectra measured in the superconducting state of
optimally-doped Bi-2223 by scanning tunneling spectroscopy. Each spectrum, as
well as the average of spectra presenting the same gap, is fitted using a
strong-coupling model taking into account the band structure, the BCS gap, and
the interaction of electrons with the spin resonance. After describing our
measurements and the main characteristics of the strong-coupling model, we
report the whole set of parameters determined from the fits, and we discuss
trends as a function of the gap magnitude. We also simulate angle-resolved
photoemission spectra, and compare with recent experimental results.Comment: Published versio
Monitoring Epiphytic Lichen Biodiversity to Detect Environmental Quality and Air Pollution: the Case Study of Roccamonfina Park (Campania Region - Italy)
In this study, lichen distribution was examined in a district of Campania region to monitor the evolution of air quality at landscape scale. A new methodological approach was tried by overlapping the results of biomonitoring samplings to land cover maps, in order to highlight the relations between air pollution and land use patterns. The main objectives of this work were the following: i) to evaluate air quality on the studied area with the aid of IBL index, and reporting geo-referenced data on a thematic map; ii) to relate lichen distribution and biodiversity to land use spatial patterns; iii) to put the basis for a comparative analysis focussed on changes induced by the present socio-economic
evolution of the plain, from agriculture to industrial and tertiary; iv) to provide a reproducible protocol for monitoring air quality, identifying clusters of lichen species linked to particular land use models and formulating previsions about environmental quality on areas characterized by similar dynamics
Polaronic signature in the metallic phase of La0.7Ca0.3MnO3 films detected by scanning tunneling spectroscopy
In this work we map tunnel conductance curves with nanometric spatial
resolution, tracking polaronic quasiparticle excitations when cooling across
the insulator-to-metal transition in La0.7Ca0.3MnO3 films. In the insulating
phase the spectral signature of polarons, a depletion of conductance at low
bias flanked by peaks, is detected all over the scanned surface. These features
are still observed at the transition and persist on cooling into the metallic
phase. Polaron-binding energy maps reveal that polarons are not confined to
regions embedded in a highly-conducting matrix but are present over the whole
field of view both above and below the transition temperature.Comment: 10 pages, 4 figure
Vortex lattice structure in a d_{x^2-y^2}-wave superconductor
The vortex lattice structure in a d_{x^2-y^2}-wave superconductor is
investigated near the upper critical magnetic field in the framework of the
Ginzburg Landau theory extended by including the correction terms such as the
higher order derivatives derived from the Gor'kov equation. On lowering
temperature, the unit cell shape of the vortex lattice gradually varies from a
regular triangular lattice to a square lattice through the shape of an
isosceles triangle. As for the orientation of the vortex lattice, the base of
an isosceles triangle is along the a axis or the b axis of the crystal. The
fourfold symmetric structure around a vortex core is also studied in the vortex
lattice case. It is noted that these characteristic features appear even in the
case the induced s-wave order parameter is absent around the vortex of the
d_{x^2-y^2}-wave superconductivity. We also investigate the effect of the
induced s-wave order parameter. It enhances (suppresses) these characteristic
features of the d_{x^2-y^2}-wave superconductor when the s-wave component of
the interaction is attractive (repulsive).Comment: 20 pages, RevTex, 9 figures in 3 PS-files and 5 GIF-file
Theory of vortex excitation imaging via an NMR relaxation measurement
The temperature dependence of the site-dependent nuclear spin relaxation time
T_1 around vortices is studied in s-wave and d-wave superconductors.Reflecting
low energy electronic excitations associated with the vortex core, temperature
dependences deviate from those of the zero-field case, and T_1 becomes faster
with approaching the vortex core. In the core region, T_1^{-1} has a new peak
below T_c. The NMR study by the resonance field dependence may be a new method
to prove the spatial resolved vortex core structure in various superconductors.Comment: 5 pages, 3 figure
Impurity induced resonant state in a pseudogap state of a high temperature superconductor
We predict a resonance impurity state generated by the substitution of one Cu
atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c
superconductor. The precise microscopic origin of the pseudogap is not
important for this state to be formed, in particular this resonance will be
present even in the absence of superconducting fluctuations in the normal
state. In the presence of superconducting fluctuations, we predict the
existence of a counterpart impurity peak on a symmetric bias.
The nature of impurity resonance is similar to the previously studied
resonance in the d-wave superconducting state.Comment: 4 pages, 2 figure
Operation of a 1-Liter-Volume Gaseous Argon Scintillation Counter
We have built a gas-phase argon ionization detector to measure small nuclear
recoil energies (< 10 keVee). In this paper, we describe the detector response
to X-ray and gamma calibration sources, including analysis of pulse shapes,
software triggers, optimization of gas content, and energy- and
position-dependence of the signal. We compare our experimental results against
simulation using a 5.9-keV X-ray source, as well as higher-energy gamma sources
up to 1332 keV. We conclude with a description of the detector, DAQ, and
software settings optimized for a measurement of the low-energy nuclear
quenching factor in gaseous argon. This work was performed under the auspices
of the U.S. Department of Energy by Lawrence Livermore National Laboratory in
part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344.
Funded by Lab-wide LDRD. LLNL-JRNL-415990-DRAFT.Comment: 29 pages, single-column, double-spaced, 21 figure
Vortex Structure in Superconducting Stripe States
The vortex structure in superconducting stripe states is studied according to
the Bogoliubov-de Gennes theory on the two-dimensional Hubbard model with
nearest-neighbor sites pairing interaction. The vortex is trapped at the
outside region of the stripe line, where the superconductivity is weak. The
superconducting coherence length along the stripe direction becomes long. There
are no eminent low-energy electronic states even near the vortex core. These
characters resemble the Josephson vortex in layered superconductors under a
parallel field.Comment: LaTeX 5 pages (using jpsj macros) with 3 figure
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