115 research outputs found
Dynamical structure factor of a nonlinear Klein-Gordon lattice
The quantum modes of a nonlinear Klein-Gordon lattice have been computed
numerically [L. Proville, Phys. Rev. B 71, 104306 (2005)]. The on-site
nonlinearity has been found to lead to phonon bound states. In the present
paper, we compute numerically the dynamical structure factor so as to simulate
the coherent scattering cross section at low temperature. The inelastic
contribution is studied as a function of the on-site anharmonicity.
Interestingly, our numerical method is not limited to the weak anharmonicity
and permits one to study thoroughly the spectra of nonlinear phonons
Bound State and Order Parameter Mixing Effect by Nonmagnetic Impurity Scattering in Two-band Superconductors
We investigate nonmagnetic impurity effects in two-band superconductors,
focusing on the effects of interband scatterings. Within the Born
approximation, it is known that interband scatterings mix order parameters in
the two bands. In particular, only one averaged energy gap appears in the
excitation spectrum in the dirty limit. [G. Gusman: J. Phys. Chem. Solids {\bf
28} (1967) 2327.] In this paper, we take into account the interband scattering
within the -matrix approximation beyond the Born approximation in the
previous work. We show that, although the interband scattering is responsible
for the mixing effect, this effect becomes weak when the interband scattering
becomes very strong. In the strong interband scattering limit, a two-gap
structure corresponding to two order parameters recovers in the superconducting
density of states. We also show that a bound state appears around a nonmagnetic
impurity depending on the phase of interband scattering potential.Comment: 28pages, 10 figure
Phonon-like and single particle dynamics in liquid lithium
The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been
determined by inelastic x-ray scattering (IXS) in the momentum transfer region
(Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a
phonon-like collective mode evolves towards the single particle dynamics. As a
function of Q, one finds: i) at low Q's, a sound mode with a positive
dispersion of the sound velocity, ii) at intermediate Q's, excitations whose
energy oscillates similarly to phonons in the crystal Brillouin zones, and iii)
at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single
particle dynamics has been reached.Comment: 3 pages and 5 figure
Inelastic X-ray scattering study of the collective dynamics in liquid sodium
Inelastic X-ray scattering data have been collected for liquid sodium at
T=390 K, i.e. slightly above the melting point. Owing to the very high
instrumental resolution, pushed up to 1.5 meV, it has been possible to
determine accurately the dynamic structure factor, , in a wide
wavevector range, nm, and to investigate on the dynamical
processes underlying the collective dynamics. A detailed analysis of the
lineshape of , similarly to other liquid metals, reveals the
co-existence of two different relaxation processes with slow and fast
characteristic timescales respectively. The present data lead to the conclusion
that: i) the picture of the relaxation mechanism based on a simple viscoelastic
model fails; ii) although the comparison with other liquid metals reveals
similar behavior, the data do not exhibit an exact scaling law as the principle
of corresponding state would predict.Comment: RevTex, 7 pages, 6 eps figures. Accepted by Phys. Rev.
Anomalous vacuum energy and stability of a quantum liquid
KT is grateful to EPSRC and V V Brazhkin to RSF 14-22-
00093 for support
Enhanced endothelialization of dacron grafts by external vein wrapping
The efficacy of external vein wrapping of vascular grafts in promoting the development of a luminal endothelial surface was assessed in 12 dogs who underwent thoracoabdominal bypasses with 26-29 cm x 6-mm i.d. double-velour knitted Dacron grafts. In group I (n = 6) 6-cm segments of the grafts were wrapped with autologous jugular vein with its endothelial surface applied against the outside of the graft. In group II (n = 6) the wrap procedure was performed using abdominal wall fascia. The degree and character of graft incorporation was quantitated in all prostheses at 28 days postimplantation. Group I vein wrap prostheses demonstrated uniform endothelial surface coverage in the vein wrap area () that was significantly greater (P P < 0.04). No significant differences existed in endothelial coverage of unwrapped regions of group I and II grafts. This investigation documented that wrapping knitted Dacron grafts with vein enhanced endothelialization of their luminal surface.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25688/1/0000242.pd
Microscopic dynamics in liquid metals: the experimental point of view
The experimental results relevant for the understanding of the microscopic
dynamics in liquid metals are reviewed, with special regards to the ones
achieved in the last two decades. Inelastic Neutron Scattering played a major
role since the development of neutron facilities in the sixties. The last ten
years, however, saw the development of third generation radiation sources,
which opened the possibility of performing Inelastic Scattering with X rays,
thus disclosing previously unaccessible energy-momentum regions. The purely
coherent response of X rays, moreover, combined with the mixed
coherent/incoherent response typical of neutron scattering, provides enormous
potentialities to disentangle aspects related to the collectivity of motion
from the single particle dynamics.
If the last twenty years saw major experimental developments, on the
theoretical side fresh ideas came up to the side of the most traditional and
established theories. Beside the raw experimental results, therefore, we review
models and theoretical approaches for the description of microscopic dynamics
over different length-scales, from the hydrodynamic region down to the single
particle regime, walking the perilous and sometimes uncharted path of the
generalized hydrodynamics extension. Approaches peculiar of conductive systems,
based on the ionic plasma theory, are also considered, as well as kinetic and
mode coupling theory applied to hard sphere systems, which turn out to mimic
with remarkable detail the atomic dynamics of liquid metals. Finally, cutting
edges issues and open problems, such as the ultimate origin of the anomalous
acoustic dispersion or the relevance of transport properties of a conductive
systems in ruling the ionic dynamic structure factor are discussed.Comment: 53 pages, 41 figures, to appear in "The Review of Modern Physics".
Tentatively scheduled for July issu
Endothelial Linings in Prosthetic Vascular Grafts
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73894/1/j.1749-6632.1987.tb33036.x.pd
Anomalous Dispersion of Longitudinal Optical Phonons in Determined by Inelastic X-ray Scattering
The phonon dispersions of along
the direction have been determined by inelastic x-ray scattering.
Compared to the undoped parent compound, the two highest longitudinal phonon
branches, associated with the Cu-O bond-stretching and out-of-plane oxygen
vibration, are shifted to lower energies. Moreover, an anomalous softening of
the bond-stretching band is observed around . These
signatures provide evidence for strong electron-phonon coupling in this
electron-doped high-temperature superconductor.Comment: 4 pages, 4 figure
- …