79 research outputs found
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
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
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.
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
Phonon dispersion in 1-layer cuprate HgBa2CuO4+d
We investigate the low energy acoustical and optical modes in
using inelastic x-ray scattering (IXS). The
experimental phonon dispersion and the dynamical structure factor are compared
with an atomic shell model, and the set of the atomic potentials obtained are
discussed. Our results are also compared with those obtained by Raman
spectroscopy and with density-of-state data measured by inelastic neutron
scattering
Collective Excitations and Thermodynamics of Disordered State: New Insights into an Old Problem
K.T. is grateful to EPSRC and V.V.B. to RFBR for financial
suppor
Myosin IIA Modulates T Cell Receptor Transport and CasL Phosphorylation during Early Immunological Synapse Formation
Activation of T cell receptor (TCR) by antigens occurs in concert with an elaborate multi-scale spatial reorganization of proteins at the immunological synapse, the junction between a T cell and an antigen-presenting cell (APC). The directed movement of molecules, which intrinsically requires physical forces, is known to modulate biochemical signaling. It remains unclear, however, if mechanical forces exert any direct influence on the signaling cascades. We use T cells from AND transgenic mice expressing TCRs specific to the moth cytochrome c 88–103 peptide, and replace the APC with a synthetic supported lipid membrane. Through a series of high spatiotemporal molecular tracking studies in live T cells, we demonstrate that the molecular motor, non-muscle myosin IIA, transiently drives TCR transport during the first one to two minutes of immunological synapse formation. Myosin inhibition reduces calcium influx and colocalization of active ZAP-70 (zeta-chain associated protein kinase 70) with TCR, revealing an influence on signaling activity. More tellingly, its inhibition also significantly reduces phosphorylation of the mechanosensing protein CasL (Crk-associated substrate the lymphocyte type), raising the possibility of a direct mechanical mechanism of signal modulation involving CasL
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