1,580 research outputs found
Fermi surface dichotomy on systems with fluctuating order
We investigate the effect of a dynamical collective mode coupled with
quasiparticles at specific wavevectors only. This coupling describes the
incipient tendency to order and produces shadow spectral features at high
energies, while leaving essentially untouched the low energy quasiparticles.
This allows to interpret seemingly contradictory experiments on underdoped
cuprates, where many converging evidences indicate the presence of charge
(stripe or checkerboard) order, which remains instead elusive in the Fermi
surface obtained from angle-resolved photoemission experiments.Comment: 11 pages, 10 figure
Gutzwiller Charge Phase Diagram of Cuprates, including Electron-Phonon Coupling Effects
Besides significant electronic correlations, high-temperature superconductors
also show a strong coupling of electrons to a number of lattice modes. Combined
with the experimental detection of electronic inhomogeneities and ordering
phenomena in many high-T_c compounds, these features raise the question as to
what extent phonons are involved in the associated instabilities. Here we
address this problem based on the Hubbard model including a coupling to phonons
in order to capture several salient features of the phase diagram of hole-doped
cuprates. Charge degrees of freedom, which are suppressed by the large Hubbard
U near half-filling, are found to become active at a fairly low doping level.
We find that possible charge order is mainly driven by Fermi surface nesting,
with competition between a near-(pi,pi) order at low doping and antinodal
nesting at higher doping, very similar to the momentum structure of magnetic
fluctuations. The resulting nesting vectors are generally consistent with
photoemission and tunneling observations, evidence for charge density wave
(CDW) order in YBa_2Cu_3O_{7-delta} including Kohn anomalies, and suggestions
of competition between one- and two-q-vector nesting.Comment: This is a revised version of arXiv:1207.5715. 25 pages, 5 figures,
plus Supplement [7 pages, 7 figures], available as a pdf [click on other,
then Download Source, & extract pdf file from zip] Manuscript is under
consideration at the NJ
Charge inhomogeneity coexisting with large Fermi surfaces
We discuss how stripes in cuprates can be compatible with a Fermi-liquid-like
Fermi surface and, at the same time, they give rise to a one-dimensional-like
pseudo Fermi surface in the momentum distribution function.Comment: Proceedings of the M2S conference, July 2006, Dresden; 2 pages, 1
figure to appear on Phisica
Advanced Strain-Isolation-Pad Material with Bonded Fibrous Construction
The feasibility of utilizing air lay and liquid lay felt deposition techniques to fabricate strain isolation pad (SIP) materials for the Space Shuttle Orbiter was demonstrated. These materials were developed as candidate replacements for the present needled felt SIP used between the ceramic tiles and the aluminum skin on the undersurface of the Orbiter. The SIP materials that were developed consisted of high temperature aramid fibers deposited by controlled fluid (air or liquid) carriers to form low density unbonded felts. The deposited felts were then bonded at the fiber intersections with a small amount of high temperature polyimide resin. This type of bonded felt construction can potentially eliminate two of the problems associated with the present SIP, viz., transmittal of localized stresses into the tiles and load history dependent mechanical response. However, further work is needed to achieve adequate through thickness tensile strength in the bonded felts
Stripe ordering and two-gap model for underdoped cuprates
The evidence of edge-gaps around the M-points in the metallic state of
underdoped cuprates has triggered a very active debate on their origin. We
first consider the possibility that this spectroscopic feature results from a
quasi-static charge ordering taking place in the underdoped regime. It comes
out that to explain the coexistence of gaps and arcs on the Fermi surface the
charge modulation should be in an eggbox form. In the lack of evidences for
that, we then investigate the local pairing induced by charge-stripe
fluctuations. A proper description of the strong anisotropy of both the
interactions and the Fermi velocities requires a two-gap model for pairing. We
find that a gap due to incoherent pairing forms near the M-points, while
coherence is established by the stiffness of the pairing near the nodal points.
The model allows for a continuos evolution from a pure BCS pairing (over- and
optimally doped regime) to a mixed boson-fermion model (heavily underdoped
regime).Comment: 4 pages, Proceedings of M2S-HTS
Correlation Between Fracture Mechanics Parameters and Fracture Characteristics in Austenites
Fracture surfaces of austenitic samples tested at room temperature show a correlation between fracture characteristics, Δ K-value and R-ratio: 1) at low Δ K-values up to 400 N/mm3/2 and stress ratios of R = 0.1 and R = 0.7, no fatigue striations but fan-like features and flat facets can be seen; 2) fatigue striations, but no facets exist at Δ K-values greater than 900 N/mm3/2 and R = 0.1; 3) at a higher stress ratio of R = 0.7 the facets disappear already at lower Δ K-values of about 650 N/mm3/2.
Applied to failure analysis, this correlation permits an estimate of operating level of fatigue stress as a result of the evaluation of fatigue fracture characteristics.
Fracture characteristics of samples tested at 200°C differ from those tested at room temperature, but do not show any appreciable changes in fracture features dependent on the Δ K-value and R-ratio
Temperature dependence of the collective mode and its influence on the band splitting in bilayer cuprates
The recently observed bilayer splitting in high-T cuprates is analyzed
within a model where the charge carriers are coupled to a phenomenological
bosonic spectrum which interpolates between the marginal Fermi liquid structure
and collective mode type behavior as a function of temperature. We argue that
the origin of the collective mode is probably associated with dynamic
incommensurate charge density waves. Moreover it is shown that the resulting
temperature dependence of the self-energy is in good agreement with
as extracted from angle-resolved photoemission data.Comment: 6 pages, 4 figures, accepted for PR
Phonon renormalization from local and transitive electron-lattice couplings in strongly correlated systems
Within the time-dependent Gutzwiller approximation (TDGA) applied to
Holstein- and SSH-Hubbard models we study the influence of electron
correlations on the phonon self-energy. For the local Holstein coupling we find
that the phonon frequency renormalization gets weakened upon increasing the
onsite interaction for all momenta. In contrast, correlations can enhance
the phonon frequency shift for small wave-vectors in the SSH-Hubbard model.
Moreover the TDGA applied to the latter model provides a mechanism which leads
to phonon frequency corrections at intermediate momenta due to the coupling
with double occupancy fluctuations. Both models display a shift of the
nesting-induced to a instability when the onsite interaction becomes
sufficiently strong and thus establishing phase separation as a generic
phenomenon of strongly correlated electron-phonon coupled systems.Comment: 14 pages, 11 figure
Trial design: how must we move ahead?
Scleroderma is clinically heterogeneous and a variety of plausible mechanisms of disease have been hypothesized. Recent years have witnessed a significant improvement in overall survival although all of the gains in management have been therapies for specific organ involvement, e.g. renal crisis and pulmonary arterial hypertension. Future studies will rely on improved clinical science, which involves structured validation of proposed measures of outcome; development of a combined response index; and further refinement of specific subsets of disease expression. Immunoablation with stem cell reconstitution is an example of aggressive therapy chosen as appropriate for a particularly severe disease subset and in whom the pilot data are encouraging. Good science and clinical ethics force continued consideration of equipoise between risk and benefi
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