317 research outputs found
New insight into the physics of iron pnictides from optical and penetration depth data
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.Comment: 34 pages, 10 figures, submitted to New Journal of Physics
(30.01.2009
Transcatheter Closure of Paravalvular Defects Using a Purpose-Specific Occluder
ObjectivesThis study sought to describe a method of paravalvular leak closure using a purpose-specific occlusion device.BackgroundTranscatheter closure of paravalvular leaks has been hampered by technical challenges, the limitations of available imaging modalities, and the lack of closure devices specifically designed for this purpose.MethodsPatients with severe symptomatic paravalvular regurgitation at high risk for repeat surgery underwent transcatheter leak closure. Both left ventricular puncture and retrograde transfemoral approaches were used with fluoroscopic and 3-dimensional transesophageal guidance. A purpose-specific occluder (Vascular Plug III, AGA Medical Corp., Plymouth, Minnesota) was used.ResultsFive patients with severe prosthetic mitral and aortic paravalvular leaks underwent attempted closure. Implantation of the device was successfully accomplished in all. In 1 patient, the plug interfered with closure of a mechanical valve leaflet and was removed and replaced with an alternate device. Complications included pericardial bleeding in 2 patients with a transapical approach. There was no procedural mortality. At a median follow-up of 191 days (interquartile range [IQR] 169 to 203 days) all patients were alive. New York Heart Association functional class fell from 4 (IQR 3 to 4) to 2 (IQR 2 to 3), hemoglobin rose from 89 g/l (IQR 87 to 108 g/l) to 115 g/l (IQR 104 to 118 g/l), creatinine fell from 109 μmol/l (IQR 106 to 132 μmol/l) to 89 μmol/l (IQR 89 to 126 μmol/l). Median echocardiographic follow-up at 58 days (IQR 56 to 70 days) reported residual regurgitation to be reduced from grade 4 to grade 2 (IQR 1.5 to 2.25).ConclusionsClosure of mitral and aortic prosthetic paravalvular leaks with the Vascular Plug III using either a transapical (mitral) or a retrograde (aortic) approach appears promising
Many-body Effects in Angle-resolved Photoemission: Quasiparticle Energy and Lifetime of a Mo(110) Surface State
In a high-resolution photoemission study of a Mo(110) surface state various
contributions to the measured width and energy of the quasiparticle peak are
investigated. Electron-phonon coupling, electron-electron interactions and
scattering from defects are all identified mechanisms responsible for the
finite lifetime of a valence photo-hole. The electron-phonon induced mass
enhancement and rapid change of the photo-hole lifetime near the Fermi level
are observed for the first time.Comment: RevTEX, 4 pages, 4 figures, to be published in PR
Fluctuation effects of gauge fields in the slave-boson t-J model
We present a quantitative study of the charge-spin separation(CSS) phenomenon
in a U(1) gauge theory of the t-J model of high-Tc superconductures. We
calculate the critical temperature of confinement-deconfinement phase
transition below which the CSS takes place.Comment: Latex, 9 pages, 3 figure
Image of the Energy Gap Anisotropy in the Vibrational Spectum of a High Temperature Superconductor
We present a new method of determining the anisotropy of the gap function in
layered high-Tc superconductors. Careful inelastic neutron scattering
measurements at low temperature of the phonon dispersion curves in the (100)
direction in La_(1.85)Sr_(.15)CuO_4 would determine whether the gap is
predominately s-wave or d-wave. We also propose an experiment to determine the
gap at each point on a quasi-two-dimensional Fermi surface.Comment: 12 pages + 2 figures (included
Outcome of Patients After Transcatheter Aortic Valve Embolization
ObjectivesThis study aims to assess the mid- to long-term follow-up of patients after valve embolization at the time of transcatheter aortic valve implantation (TAVI).BackgroundTranscatheter heart valve (THV) embolization is a rare but serious complication during TAVI. Although various techniques have been developed to manage acute complications and reduce periprocedural morbidity/mortality, long-term clinical and hemodynamic consequences after these events are unknown.MethodsPatients who developed THV embolization after TAVI were prospectively assessed. Clinical and echocardiographic characteristics were recorded at baseline and after successful TAVI/surgical aortic valve replacement. The THV migration and strut fractures/degeneration were assessed by computed tomography.ResultsA total of 7 patients had THV embolization, all of which occurred immediately after valve deployment. The embolized THV was repositioned in the aortic arch proximal to the left subclavian artery (n = 2), immediately distal to the left subclavian artery (n = 2), and in the abdominal aorta (n = 3). A second THV was implanted successfully at the same sitting in 4 patients and at the time of a second procedure in 2 patients. Elective conventional aortic valve replacement was performed in 1 patient. Median follow-up was 1,085 days. One patient died during follow-up from an unrelated cause. The remaining 6 survivors were in New York Heart Association functional class I or II at final follow-up. Mid-term computed tomography follow-up (n = 4,591 to 1,548 days) showed that the leaflets of the embolized THV remain open in all phases of the cardiac cycle. There was also no strut fracture or migration of these valves.ConclusionsClinical outcomes remain good when THV embolization is managed effectively. There are no apparent hemodynamic consequences of a second valve placed in the series. These embolized valves remain in a stable position with no evidence of strut fractures at mid-term follow-up
Measuring anisotropic scattering in the cuprates
A simple model of anisotropic scattering in a quasi two-dimensional metal is
studied. Its simplicity allows an analytic calculation of transport properties
using the Boltzmann equation and relaxation time approximation. We argue that
the c-axis magnetoresistance provides the key test of this model of transport.
We compare this model with experiments on overdoped Tl-2201 and find reasonable
agreement using only weak scattering anisotropy. We argue that optimally doped
Tl-2201 should show strong angular-dependent magnetoresistance within this
model and would provide a robust way of determining the in-plane scattering
anisotropy in the cuprates.Comment: 12 pages, 8 figures, typset in REVTeX 4. Version 2; added references
and corrected typo
Change of the Ground State upon Hole Doping Unveiled by Ni Impurity in High- Cuprates
The electronic ground state in high- cuprates where the
superconducting state is suppressed by Ni substitution has been investigated in
LaSrCuNiO from the specific heat and muon spin
relaxation measurements. It has been found that the ground state changes from a
magnetically ordered state with the strong hole-trapping by Ni to a metallic
state with the Kondo effect of Ni with increasing hole-concentration. Moreover,
the analysis of the results has revealed that a phase separation into the
magnetically ordered phase and the metallic phase occurs around the boundary of
two phases.Comment: 11pages, 4 figure
Disorder and chain superconductivity in YBa_2Cu_3O_{7-\delta}
The effects of chain disorder on superconductivity in YBa_2Cu_3O_{7-\delta}
are discussed within the context of a proximity model. Chain disorder causes
both pair-breaking and localization. The hybridization of chain and plane
wavefunctions reduces the importance of localization, so that the transport
anisotropy remains large in the presence of a finite fraction of
oxygen vacancies. Penetration depth and specific heat measurements probe the
pair-breaking effects of chain disorder, and are discussed in detail at the
level of the self-consistent T-matrix approximation. Quantitative agreement
with these experiments is found when chain disorder is present.Comment: 4 pages, 2 figures, submitted to PRB rapid communication
Role of Inter-Electron Interaction in the Pseudo-Gap Opening in High T Tunneling Experiments
The analysis of tunneling experiments showing the pseudogap type behavior is
carried out based on the idea of the renormalization of density of states due
to the inter-electron interaction in the Cooper channel (superconducting
fluctuations contribution in tunneling current). It is demonstrated that the
observed kink of the zero-bias conductance of junctions in
the vicinity of can be explained in terms of fluctuation theory in a
quite wide range of temperature above , using the values of microscopic
parameters of the electron spectrum taken from independent
experiments. The approach proposed also permits to explain qualitatively the
shape of the tunneling anomalies in and gives a correct estimate for
the pseudogap position and amplitude observed in the experiments on
junctions.Comment: 5 pages, 3 figure
- …