96 research outputs found
Signatures of nodeless multiband superconductivity and particle-hole crossover in the vortex cores of FeTeSe
Scanning tunneling experiments on single crystals of superconducting
FeTeSe have recently provided evidence for discrete energy
levels inside vortices. Although predicted long ago, such levels are seldom
resolved due to extrinsic (temperature, instrumentation) and intrinsic
(quasiparticle scattering) limitations. We study a microscopic multiband model
with parameters appropriate for FeTeSe. We confirm the
existence of well-separated bound states and show that the chemical disorder
due to random occupation of the chalcogen site does not affect significantly
the vortex-core electronic structure. We further analyze the vortex bound
states by projecting the local density of states on angular-momentum
eigenstates. A rather complex pattern of bound states emerges from the
multiband and mixed electron-hole nature of the normal-state carriers. The
character of the vortex states changes from hole-like with negative angular
momentum at low energy to electron-like with positive angular momentum at
higher energy within the superconducting gap. We show that disorder in the
arrangement of vortices most likely explains the differences found
experimentally when comparing different vortices.Comment: Published versio
A theory of the strain-dependent critical field in Nb3Sn, based on anharmonic phonon generation
We propose a theory to explain the strain dependence of the critical
properties in A15 superconductors. Starting from the strong-coupling formula
for the critical temperature, and assuming that the strain sensitivity stems
mostly from the electron-phonon alpha^2F function, we link the strain
dependence of the critical properties to a widening of alpha^2F. This widening
is attributed to the nonlinear generation of phonons, which takes place in the
anharmonic deformation potential induced by the strain. Based on the theory of
sum- and difference-frequency wave generation in nonlinear media, we obtain an
explicit connection between the widening of alpha^2F and the anharmonic energy.
The resulting model is fit to experimental datasets for Nb3Sn, and the
anharmonic energy extracted from the fits is compared with first-principles
calculations.Comment: 10 pages, 3 figure
Theory of spin-polarized high-resolution electron energy loss spectroscopy from nonmagnetic surfaces with a large spin-orbit coupling
The scattering theory of low-energy (slow) electrons has been developed by
Evans and Mills [Phys. Rev. B 5, 4126 (1972)]. The formalism is merely based on
the electrostatic Coulomb interaction of the scattering electrons with the
charge-density fluctuations above the surface and can describe most of the
interesting features observed in the high-resolution electron energy-loss
spectroscopy experiments. Here we extend this theory by including the
spin-orbit coupling in the scattering process. We discuss the impact of this
interaction on the scattering cross section. In particular, we discuss cases in
which a spin-polarized electron beam is scattered from nonmagnetic surfaces
with a strong spin-orbit coupling. We show that under some assumptions one can
derive an expression for the scattering cross section, which can be used for
numerical calculations of the spin-polarized spectra recorded by spin-polarized
high-resolution electron energy-loss spectroscopy experiments.Comment: 8 pages, 1 figur
Non-Drude universal scaling laws for the optical response of local Fermi liquids
We investigate the frequency and temperature dependence of the low-energy
electron dynamics in a Landau Fermi liquid with a local self-energy. We show
that the frequency and temperature dependencies of the optical conductivity
obey universal scaling forms, for which explicit analytical expressions are
obtained. For the optical conductivity and the associated memory function, we
obtain a number of surprising features that differ qualitatively from the Drude
model and are universal characteristics of a Fermi liquid. Different physical
regimes of scaling are identified, with marked non-Drude features in the regime
where hbar omega ~ kB T. These analytical results for the optical conductivity
are compared to numerical calculations for the doped Hubbard model within
dynamical mean-field theory. For the "universal" low-energy electrodynamics, we
obtain perfect agreement between numerical calculations and analytical scaling
laws. Both results show that the optical conductivity displays a non-Drude
"foot", which could be easily mistaken as a signature of breakdown of the Fermi
liquid, while it actually is a striking signature of its applicability. The
aforementioned scaling laws provide a quantitative tool for the experimental
identification and analysis of the Fermi-liquid state using optical
spectroscopy, and a powerful method for the identification of alternative
states of matter, when applicable.Comment: Published versio
Preeminent role of the Van Hove singularity in the strong-coupling analysis of scanning tunneling spectroscopy for two-dimensional cuprates
In two dimensions the non-interacting density of states displays a Van Hove
singularity (VHS) which introduces an intrinsic electron-hole asymmetry, absent
in three dimensions. We show that due to this VHS the strong-coupling analysis
of tunneling spectra in high- superconductors must be reconsidered. Based
on a microscopic model which reproduces the experimental data with great
accuracy, we elucidate the peculiar role played by the VHS in shaping the
tunneling spectra, and show that more conventional analyses of strong-coupling
effects can lead to severe errors.Comment: 5 pages, 4 figure
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Titanium corrosion in molten glasses : Part 1: Immersion tests and corrosion kinetics
The kinetics and mechanisms of pure titanium corrosion in three glass melts are investigated in the 1050 to 1400 °C temperature range by Immersion tests. The corrosion layers formed at the metal/glass interface, the elemental concentration profiles in the glass and the corrosion rates are determined. Corrosion rates are reported on an Arrhenius diagram for each glass
Can dd excitations mediate pairing ?
The Cu- states in the high- cuprates are often described as a single
band of states, with the other four states having about 2
to 3 eV higher energy due to the lower-than-octahedral crystal field at the
copper sites. However, excitations to these higher energy states observed with
RIXS show indications of strong coupling to doped holes in the
band. This relaunches a decades-old question of the possible role of the
orbital degrees of freedom that once motivated Bednorz and M\"uller to search
for superconductivity in these systems. Here we explore a direction different
from the Jahn-Teller electron-phonon coupling considered by Bednorz and
M\"uller, namely the interaction between holes mediated by excitations.Comment: 6 pages, 4 figure
Revisiting the vortex-core tunnelling spectroscopy in YBaCuO
The observation by scanning tunnelling spectroscopy (STS) of Abrikosov vortex
cores in the high-temperature superconductor YBaCuO (Y123)
has revealed a robust pair of electron-hole symmetric states at finite subgap
energy. Their interpretation remains an open question because theory predicts a
different signature in the vortex cores, characterised by a strong zero-bias
conductance peak. We present STS data on very homogeneous Y123 at 0.4 K
revealing that the subgap features do not belong to vortices: they are actually
observed everywhere along the surface with high spatial and energy
reproducibility, even in the absence of magnetic field. Detailed analysis and
modelling show that these states remain unpaired in the superconducting phase
and belong to an incoherent channel which contributes to the tunnelling signal
in parallel with the superconducting density of states.Comment: Final version with supplementary materia
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Titanium corrosion in molten glasses : Part 2. Electrochemical study and corrosion mechanismus
In situ electrochemical experiments are carried out to determine the mechanisms of pure titanium corrosion in molten glasses. The corrosion layers formed at the metal/glass interface are systematically reported. The formal potentials of the TiIII/TiII and TiIV/TiIII redox couples are respectively positioned at -1.0 V and -0.7 V by coupling square wave voltammetry measurements and polarization of titanium rods. The potential of the BIII/B0 couple is estimated to be near -1.4 V. The results obtained by immersion tests and electrochemical measurements in glasses are compared. They are consistent in terms of corrosion rate variations and morphology of the corrosion scales. The corrosion mechanisms are described in the form of successive redox reactions between the glass and the substrate followed by diffusion of the formed species into the titanium substrate. It is demonstrated that Ti does not form spontaneously protective scales when immersed in molten glass and that protection can not be reached by either anodic or cathodic polarization
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