729 research outputs found
Superconducting Gap and Pseudogap in Bi-2212
We present results of Raman scattering experiments in differently doped
Bi-2212 single crystals. Below Tc the spectra show pair-breaking features in
the whole doping range. The low frequency power laws confirm the existence of a
-wave order parameter. In the normal state between Tc and T* =
200K we find evidence for a pseudogap in B2g symmetry. Upon doping its effect
on the spectra decreases while its energy scale appears to be unchanged.Comment: 2 pages, 1 EPS figure; LT22 Proceedings to appear in Physica
A Method for Gradient Enhancement of Continuum Damage Models
A method for the regularization of continuum damage material models based on gradient-type enhancement of the free-energy functional is presented. Direct introduction of the gradient of the damage variable would require C1 interpolation of the displacements, which is a complicated task to achieve with quadrilateral elements. Therefore a new variable field is introduced, which makes the model non-local in nature, while preserving C0 interpolation order of the variables at the same time. The strategy is formulated as a pure minimization problem, therefore the LBB-condition does not apply in this case. However, we still take the interpolation of the displacement field one order higher than the interpolation of the field of additional (non-local) variables. That leads to increased accuracy and removes the post-processing step necessary to obtain consistent results in the case of equal interpolation order. Several numerical examples which show the performance of the proposed gradient enhancement are presented. The pathological mesh dependence of the damage model is efficiently removed, together with the difficulties of numerical calculations in the softening range. Calculations predict a development of the damage variable which is mesh-objective for fixed internal material length
Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates
We consider the lightly doped cuprates YCaBaCuO
and LaSrCuO (with ,0.04), where the presence of a
fluctuating nematic state has often been proposed as a precursor of the stripe
(or, more generically, charge-density wave) phase, which sets in at higher
doping. We phenomenologically assume a quantum critical character for the
longitudinal and transverse nematic, and for the charge-ordering fluctuations,
and investigate the effects of these fluctuations in Raman spectra. We find
that the longitudinal nematic fluctuations peaked at zero transferred momentum
account well for the anomalous Raman absorption observed in these systems in
the channel, while the absence of such effect in the channel
may be due to the overall suppression of Raman response at low frequencies,
associated with the pseudogap. While in YCaBaCuO the
low-frequency lineshape is fully accounted by longitudinal nematic collective
modes alone, in LaSrCuO also charge-ordering modes with finite
characteristic wavevector are needed to reproduce the shoulders observed in the
Raman response. This different involvement of the nearly critical modes in the
two materials suggests a different evolution of the nematic state at very low
doping into the nearly charge-ordered state at higher doping.Comment: 12 pages with 10 figures, to appear in Phys. Rev. B 201
Electron-boson glue function derived from electronic Raman scattering
Raman scattering cross sections depend on photon polarization. In the
cuprates nodal and antinodal directions are weighted more strongly in
and symmetry, respectively. On the other hand in angle-resolved
photoemission spectroscopy (ARPES), electronic properties are measured along
well-defined directions in momentum space rather than their weighted averages.
In contrast, the optical conductivity involves a momentum average over the
entire Brillouin zone. Newly measured Raman response data on high-quality
BiSrCaCuO single crystals up to high energies have
been inverted using a modified maximum entropy inversion technique to extract
from and Raman data corresponding electron-boson spectral
densities (glue) are compared to the results obtained with known ARPES and
optical inversions. We find that the spectrum agrees qualitatively
with nodal direction ARPES while the looks more like the optical
spectrum. A large peak around meV in , much less prominent
in , is taken as support for the importance of scattering
at this frequency.Comment: 7 pages, 3 figure
Band and momentum dependent electron dynamics in superconducting as seen via electronic Raman scattering
We present details of carrier properties in high quality single crystals obtained from electronic Raman
scattering. The experiments indicate a strong band and momentum anisotropy of
the electron dynamics above and below the superconducting transition
highlighting the importance of complex band-dependent interactions. The
presence of low energy spectral weight deep in the superconducting state
suggests a gap with accidental nodes which may be lifted by doping and/or
impurity scattering. When combined with other measurements, our observation of
band and momentum dependent carrier dynamics indicate that the iron arsenides
may have several competing superconducting ground states.Comment: 5 pages, 4 figure
Pinpointing Gap Minima in Ba(FeCoAs \textit{via} Band Structure Calculations and Electronic Raman Scattering
A detailed knowledge of the gap structure for the Fe-pnictide superconductors
is still rather rudimentary, with several conflicting reports of either nodes,
deep gap minima, or fully isotropic gaps on the Fermi surface sheets, both in
the plane and along the c-axis. In this paper we present
considerations for electronic Raman scattering which can help clarify the gap
structure and topology using different light scattering geometries. Using
density functional calculations for the Raman vertices, it is shown that the
location of the gap minima may occur on loops stretching over a portion of the
c-axis in Ba(FeCoAs.Comment: 4+ pages, three figure
A balancing act: Evidence for a strong subdominant d-wave pairing channel in
We present an analysis of the Raman spectra of optimally doped based on LDA band structure calculations and the
subsequent estimation of effective Raman vertices. Experimentally a narrow,
emergent mode appears in the () Raman spectra only below
, well into the superconducting state and at an energy below twice the
energy gap on the electron Fermi surface sheets. The Raman spectra can be
reproduced quantitatively with estimates for the magnitude and momentum space
structure of the s pairing gap on different Fermi surface sheets, as
well as the identification of the emergent sharp feature as a
Bardasis-Schrieffer exciton, formed as a Cooper pair bound state in a
subdominant channel. The binding energy of the exciton relative
to the gap edge shows that the coupling strength in this subdominant
channel is as strong as 60% of that in the dominant
channel. This result suggests that may be the dominant pairing
symmetry in Fe-based sperconductors which lack central hole bands.Comment: 10 pages, 6 Figure
Raman-Scattering Detection of Nearly Degenerate -Wave and -Wave Pairing Channels in Iron-Based BaKFeAs and RbFeSe Superconductors
We show that electronic Raman scattering affords a window into the essential
properties of the pairing potential of
iron-based superconductors. In BaKFeAs we observe band
dependent energy gaps along with excitonic Bardasis-Schrieffer modes
characterizing, respectively, the dominant and subdominant pairing channel. The
symmetry of all excitons allows us to identify the subdominant
channel to originate from the interaction between the electron bands.
Consequently, the dominant channel driving superconductivity results from the
interaction between the electron and hole bands and has the full lattice
symmetry. The results in RbFeSe along with earlier ones in
Ba(FeCo)As highlight the influence of the Fermi
surface topology on the pairing interactions.Comment: 5 pages, 4 figure
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