958 research outputs found
Mesophases in Nearly 2D Room-Temperature Ionic Liquids
Computer simulations of (i) a [C12mim][Tf2N] film of nanometric thickness
squeezed at kbar pressure by a piecewise parabolic confining potential reveal a
mesoscopic in-plane density and composition modulation reminiscent of
mesophases seen in 3D samples of the same room-temperature ionic liquid (RTIL).
Near 2D confinement, enforced by a high normal load, relatively long aliphatic
chains are strictly required for the mesophase formation, as confirmed by
computations for two related systems made of (ii) the same [C12mim][Tf2N]
adsorbed at a neutral solid surface and (iii) a shorter-chain RTIL
([C4mim][Tf2N]) trapped in the potential well of part i. No in-plane modulation
is seen for ii and iii. In case ii, the optimal arrangement of charge and
neutral tails is achieved by layering parallel to the surface, while, in case
iii, weaker dispersion and packing interactions are unable to bring aliphatic
tails together into mesoscopic islands, against overwhelming entropy and
Coulomb forces. The onset of in-plane mesophases could greatly affect the
properties of long-chain RTILs used as lubricants.Comment: 24 pages 10 figure
Static friction on the fly: velocity depinning transitions of lubricants in motion
The dragging velocity of a model solid lubricant confined between sliding
periodic substrates exhibits a phase transition between two regimes,
respectively with quantized and with continuous lubricant center-of-mass
velocity. The transition, occurring for increasing external driving force F_ext
acting on the lubricant, displays a large hysteresis, and has the features of
depinning transitions in static friction, only taking place on the fly.
Although different in nature, this phenomenon appears isomorphic to a static
Aubry depinning transition in a Frenkel-Kontorova model, the role of particles
now taken by the moving kinks of the lubricant-substrate interface. We suggest
a possible realization in 2D optical lattice experiments.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let
Ab-initio self-energy corrections in systems with metallic screening
The calculation of self-energy corrections to the electron bands of a metal
requires the evaluation of the intraband contribution to the polarizability in
the small-q limit. When neglected, as in standard GW codes for semiconductors
and insulators, a spurious gap opens at the Fermi energy. Systematic methods to
include intraband contributions to the polarizability exist, but require a
computationally intensive Fermi-surface integration. We propose a numerically
cheap and stable method, based on a fit of the power expansion of the
polarizability in the small-q region. We test it on the homogeneous electron
gas and on real metals such as sodium and aluminum.Comment: revtex, 14 pages including 5 eps figures v2: few fixe
Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet
The low-energy spectra of the single-mode h x (G+H) linear Jahn-Teller model
is studied by means of exact diagonalization. Both eigenenergies and
photoemission spectral intensities are computed. These spectra are useful to
understand the vibronic dynamics of icosahedral clusters with partly filled
orbital quintet molecular shells, for example C60 positive ions.Comment: 14 pages revte
Interplay of Orbital Degeneracy and Superconductivity in a Molecular Conductor
We study electron propagation in a molecular lattice model. Each molecular
site involves doubly degenerate electronic states coupled to doubly degenerate
molecular vibration, leading to a so--called E-e type of Jahn-Teller
Hamiltonian. For weak electron-phonon coupling and in the anti-adiabatic limit
we find that the orbital degeneracy induces an intersite pairing mechanism
which is absent in the standard non-degenerate polaronic model. In this limit
we analyse the model in the presence of an additional on-site repulsion and we
determine, within BCS mean field theory, the region of stability of
superconductivity. In one dimension, where powerful analytical techniques are
available, we are able to calculate the phase diagram of the model both for
weak and for strong electron-phonon coupling.Comment: 11 pages, REVTEX style, 3 compressed figures adde
A simple approach to the correlation of rotovibrational states in four-atomic molecules
The problem of correlation between quantum states of four-atomic molecules in
different geometrical configurations is reviewed in detail. A general, still
simple rule is obtained which allows one to correlate states of a linear
four-atomic molecule with those of any kind of non-linear four-atomic molecule.Comment: 16 pages (+8 figures), Postscript (ready to print!
Dynamical Jahn-Teller Effect and Berry Phase in Positively Charged Fullerene I. Basic Considerations
We study the Jahn-Teller effect of positive fullerene ions C
and C. The aim is to discover if this case, in analogy with the
negative ion, possesses a Berry phase or not, and what are the consequences on
dynamical Jahn-Teller quantization. Working in the linear and spherical
approximation, we find no Berry phase in C, and
presence/absence of Berry phase for coupling of one hole to an
/ vibration. We study in particular the special equal-coupling case
(), which is reduced to the motion of a particle on a 5-dimensional
sphere. In the icosahedral molecule, the final outcome assesses the
presence/absence of a Berry phase of for the hole coupled to
/ vibrations. Some qualitative consequences on ground-state symmetry,
low-lying excitations, and electron emission from C are spelled out.Comment: 31 pages (RevTeX), 3 Postscript figures (uuencoded
Axial collective excitations of a degenerate Fermi gas in the BEC to unitarity crossover
We show that, with reasonable hypotheses leading to a simple modeling,a link
can be obtained from experiments on the axial low frequency collective modes
between the molecular scattering length and the energy parameter of the gas at the unitarity limit. We also point out that, in
order to reach the range where the features of the Bose limit can be clearly
seen, experiments have to go to more dilute situations than have been achieved
presently.Comment: 5 pages, revtex, 2 figure
Vitis vinifera - a chemotaxonomic approach: Pollen wall proteins
The electrophoretic pattern of the pollen wall proteins from clones of cv. Nebbiolo grown in different areas show a geographical clustering. Vermentino, Pigato and Favorita are found virtually identical both by morphological and biochemical criteria
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