197 research outputs found
Charge transfer fluctuation, wave superconductivity, and the Raman phonon in the Cuprates: A detailed analysis
The Raman spectrum of the phonon in the superconducting cuprate
materials is investigated theoretically in detail in both the normal and
superconducting phases, and is contrasted with that of the phonon. A
mechanism involving the charge transfer fluctuation between the two oxygen ions
in the CuO plane coupled to the crystal field perpendicular to the plane is
discussed and the resulting electron-phonon coupling is evaluated. Depending on
the symmetry of the phonon the weight of different parts of the Fermi surface
in the coupling is different. This provides the opportunity to obtain
information on the superconducting gap function at certain parts of the Fermi
surface. The lineshape of the phonon is then analyzed in detail both in the
normal and superconducting states. The Fano lineshape is calculated in the
normal state and the change of the linewidth with temperature below T is
investigated for a pairing symmetry. Excellent agreement is
obtained for the phonon lineshape in YBaCuO. These
experiments, however, can not distinguish between and a
highly anisotropic -wave pairing.Comment: Revtex, 21 pages + 4 postscript figures appended, tp
The Pairing Mechanism in HTSC investigated by Electronic Raman Scattering
By means of electronic Raman scattering we investigated the symmetry of the
energy gap Delta(k), its temperature dependence and its variation with doping
of well characterized Bi2Sr2CaCu2O8+delta single crystals. The oxygen content
delta was varied between the under- and the overdoped regime by subsequently
annealing the same single crystal in Ar and O2, respectively. The symmetry
analysis of the polarized electronic Raman scattering is consistent with a
d_x^2-y^2-wave symmetry of the energy gap in both regimes. The gap ratio
2Delta_max/k_BT_c and its temperature dependence changes with doping similar to
predictions of theories based on paramagnon coupling.Comment: 3 pages, LaTeX, 2 ps figures available on request to
[email protected]
Trajectory matching of ozonesondes and MOZAIC measurements in the UTLS – Part 2: Application to the global ozonesonde network
Both balloon-borne electrochemical ozonesondes and MOZAIC (measurements of
ozone, water vapour, carbon monoxide and nitrogen oxides by in-service Airbus
aircraft) provide very valuable data sets for ozone studies in the upper
troposphere/lower stratosphere (UTLS). Although MOZAIC's highly accurate
UV-photometers are regularly inspected and recalibrated annually, recent
analyses cast some doubt on the long-term stability of their ozone analysers.
To investigate this further, we perform a 16 yr comparison (1994–2009) of
UTLS ozone measurements from balloon-borne ozonesondes and MOZAIC. The
analysis uses fully three-dimensional trajectories computed from ERA-Interim
(European Centre for Medium-Range Weather Forecasts Re-analysis) wind fields
to find matches between the two measurement platforms. Although different
sensor types (Brewer-Mast and Electrochemical Concentration Cell ozonesondes)
were used, most of the 28 launch sites considered show considerable
differences of up to 25% compared to MOZAIC in the mid-1990s, followed
by a systematic tendency to smaller differences of around 5–10% in
subsequent years. The reason for the difference before 1998 remains unclear,
but observations from both sondes and MOZAIC require further examination to
be reliable enough for use in robust long-term trend analyses starting before
1998. According to our analysis, ozonesonde measurements at tropopause
altitudes appear to be rather insensitive to changing the type of the
Electrochemical Concentration Cell ozonesonde, provided the cathode sensing
solution strength remains unchanged. Scoresbysund (Greenland) showed
systematically 5% higher readings after changing from Science Pump
Corporation sondes to ENSCI Corporation sondes, while a 1.0% KI cathode
electrolyte was retained
VECHR: A Dataset for Explainable and Robust Classification of Vulnerability Type in the European Court of Human Rights
Recognizing vulnerability is crucial for understanding and implementing
targeted support to empower individuals in need. This is especially important
at the European Court of Human Rights (ECtHR), where the court adapts
Convention standards to meet actual individual needs and thus ensures effective
human rights protection. However, the concept of vulnerability remains elusive
at the ECtHR and no prior NLP research has dealt with it. To enable future
research in this area, we present VECHR, a novel expert-annotated multi-label
dataset comprising of vulnerability type classification and explanation
rationale. We benchmark the performance of state-of-the-art models on VECHR
from both prediction and explainability perspectives. Our results demonstrate
the challenging nature of the task with lower prediction performance and
limited agreement between models and experts. Further, we analyze the
robustness of these models in dealing with out-of-domain (OOD) data and observe
overall limited performance. Our dataset poses unique challenges offering
significant room for improvement regarding performance, explainability, and
robustness.Comment: Accepted to EMNLP 202
C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}
We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.Comment: 12 pages, REVTEX, 3 postscript figure
Relation between the superconducting gap energy and the two-magnon Raman peak energy in Bi2Sr2Ca{1-x}YxCu2O{8+\delta}
The relation between the electronic excitation and the magnetic excitation
for the superconductivity in Bi2Sr2Ca{1-x}YxCu2O{8+\delta} was investigated by
wide-energy Raman spectroscopy. In the underdoping region the B1g scattering
intensity is depleted below the two-magnon peak energy due to the "hot spots"
effects. The depleted region decreases according to the decrease of the
two-magnon peak energy, as the carrier concentration ncreases. This two-magnon
peak energy also determines the B1g superconducting gap energy as
from under to overdoping hole concentration.Comment: 10 pages, 4 figure
Raman study of carrier-overdoping effects on the gap in high-Tc superconducting cuprates
Raman scattering in the heavily overdoped (Y,Ca)Ba_2Cu_3O_{7-d} (T_c = 65 K)
and Bi_2Sr_2CaCu_2O_{8+d} (T_c = 55 K) crystals has been investigated. For the
both crystals, the electronic pair-breaking peaks in the A_{1g} and B_{1g}
polarizations were largely shifted to the low energies close to a half of
2Delta_0, Delta_0 being the maximum gap. It strongly suggests s-wave mixing
into the d-wave superconducting order parameter and the consequent
manifestation of the Coulomb screening effect in the B_{1g}-channel. Gradual
mixing of s-wave component with overdoping is not due to the change of crystal
structure symmetry but a generic feature in all high-T_c superconducting
cuprates.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B, Rapid
communicaito
Electronic Raman scattering in YBCO and other superconducting cuprates
Superconductivity induced structures in the electronic Raman spectra of
high-Tc superconductors are computed using the results of ab initio LDA-LMTO
three-dimensional band structure calculations via numerical integrations of the
mass fluctuations, either in the whole 3D Brillouin zone or limiting the
integrations to the Fermi surface. The results of both calculations are rather
similar, the Brillouin zone integration yielding additional weak structures
related to the extended van Hove singularities. Similar calculations have been
performed for the normal state of these high-Tc cuprates. Polarization
configurations have been investigated and the results have been compared to
experimental spectra. The assumption of a simple d_(x^2-y^2)-like gap function
allows us to explain a number of experimental features but is hard to reconcile
with the relative positions of the A1g and B1g peaks.Comment: 14 pages, LaTeX (RevTeX), 5 PostScript figures, uses multicol.sty,
submitted to PR
No Far-Infrared-Spectroscopic Gap in Clean and Dirty High-T Superconductors
We report far infrared transmission measurements on single crystal samples
derived from BiSrCaCuO. The impurity scattering rate of
the samples was varied by electron-beam irradiation, 50MeV O ion
irradiation, heat treatment in vacuum, and Y doping. Although substantial
changes in the infrared spectra were produced, in no case was a feature
observed that could be associated with the superconducting energy gap. These
results all but rule out ``clean limit'' explanations for the absence of the
spectroscopic gap in this material, and provide evidence that the
superconductivity in BiSrCaCuO is gapless.Comment: 4 pages and 3 postscript figures attached. REVTEX v3.0. Accepted for
publication in Phys. Rev. Lett. IRDIRT
Multiple Andreev Reflection and Giant Excess Noise in Diffusive Superconductor/Normal-Metal/Superconductor Junctions
We have studied superconductor/normal metal/superconductor (SNS) junctions
consisting of short Au or Cu wires between Nb or Al banks. The Nb based
junctions display inherent electron heating effects induced by the high thermal
resistance of the NS boundaries. The Al based junctions show in addition
subharmonic gap structures in the differential conductance dI/dV and a
pronounced peak in the excess noise at very low voltages V. We suggest that the
noise peak is caused by fluctuations of the supercurrent at the onset of
Josephson coupling between the superconducting banks. At intermediate
temperatures where the supercurrent is suppressed a noise contribution ~1/V
remains, which may be interpreted as shot noise originating from large multiple
charges.Comment: 7 pages, 7 figures, extended versio
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