146 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
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
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
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
Photodoping and in-gap interface states across the metal-insulator transition in LaAlO3/SrTiO3 heterostructures
By using scanning tunneling microscopy/spectroscopy we show that the interface between LaAlO3 and SrTiO3 band insulators is characterized by in-gap interface states. These features were observed in insulating as well as conducting LaAlO3/SrTiO3 bilayers. The data show how the interface density of states evolves across the insulating to metal transition, demonstrating that nanoscale electronic inhomogeneities in the system are induced by spatially localized electrons
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
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
Imaging the essential role of spin-fluctuations in high-Tc superconductivity
We have used scanning tunneling spectroscopy to investigate short-length
electronic correlations in three-layer Bi2Sr2Ca2Cu3O(10+d) (Bi-2223). We show
that the superconducting gap and the energy Omega_dip, defined as the
difference between the dip minimum and the gap, are both modulated in space
following the lattice superstructure, and are locally anti-correlated. Based on
fits of our data to a microscopic strong-coupling model we show that Omega_dip
is an accurate measure of the collective mode energy in Bi-2223. We conclude
that the collective mode responsible for the dip is a local excitation with a
doping dependent energy, and is most likely the (pi,pi) spin resonance.Comment: 4 pages, 4 figure
Theory of vortex lattice effects on STM spectra in d-wave superconductors
Theory of scanning tunneling spectroscopy of low energy quasiparticle (QP)
states in vortex lattices of d-wave superconductors is developed taking account
of the effects caused by an extremely large extension of QP wavefunctions in
the nodal directions and the band structure in the QP spectrum. The oscillatory
structures in STM spectra, which correspond to van Hove singularities are
analysed. Theoretical calculations carried out for finite temperatures and
scattering rates are compared with recent experimental data for high
temperature cuprates.Comment: 4 pages, 3 eps figures, M2S-HTSC-VI conference paper, using Elsevier
style espcrc2.st
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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