29 research outputs found
Image states in metal clusters
The existence of image states in small clusters is shown, using a quantum-mechanical many-body approach. We present image state energies and wave functions for spherical jellium clusters up to 186 atoms, calculated in the GW approximation, where G is the Green's function and W is the dynamically screened Coulomb interaction, which by construction contains the dynamic long-range correlation effects that give rise to image effects. In addition, we find that image states are also subject to quantum confinement. To extrapolate our investigations to clusters in the mesoscopic size range, we propose a semiclassical model potential, which we test against our full GW results
Quenched Dislocation Enhanced Supersolid Ordering
I show using Landau theory that quenched dislocations can facilitate the
supersolid (SS) to normal solid (NS) transition, making it possible for the
transition to occur even if it does not in a dislocation-free crystal. I make
detailed predictions for the dependence of the SS to NS transition temperature
T_c(L), superfluid density %\rho_S(T, L), and specific heat C(T,L) on
temperature T and dislocation spacing L, all of which can be tested against
experiments. The results should also be applicable to an enormous variety of
other systems, including, e.g., ferromagnets.Comment: 5 pages, 2 figure
Relaxation of superflow in a network: an application to the dislocation model of supersolidity of helium crystals
We have considered the dislocation network model for the supersolid state in
He-4 crystals. In difference with uniform 2D and 3D systems, the temperature of
superfluid transition T_c in the network is much smaller than the degeneracy
temperature T_d. It is shown that a crossover into a quasi superfluid state
occurs in the temperature interval between T_c and T_d. Below the crossover
temperature the time of decay of the flow increases exponentially under
decrease of the temperature. The crossover has a continuous character and the
crossover temperature does not depend on the density of dislocations.Comment: Corrected typo
Dislocation-induced superfluidity in a model supersolid
Motivated by recent experiments on the supersolid behavior of He, we
study the effect of an edge dislocation in promoting superfluidity in a Bose
crystal. Using Landau theory, we couple the elastic strain field of the
dislocation to the superfluid density, and use a linear analysis to show that
superfluidity nucleates on the dislocation before occurring in the bulk of the
solid. Moving beyond the linear analysis, we develop a systematic perturbation
theory in the weakly nonlinear regime, and use this method to integrate out
transverse degrees of freedom and derive a one-dimensional Landau equation for
the superfluid order parameter. We then extend our analysis to a network of
dislocation lines, and derive an XY model for the dislocation network by
integrating over fluctuations in the order parameter. Our results show that the
ordering temperature for the network has a sensitive dependence on the
dislocation density, consistent with numerous experiments that find a clear
connection between the sample quality and the supersolid response.Comment: 10 pages, 6 figure
Helical, Angular and Radial Ordering in Narrow Capillaries
To enlighten the nature of the order-disorder and order-order transitions in
block copolymer melts confined in narrow capillaries we analyze peculiarities
of the conventional Landau weak crystallization theory of systems confined to
cylindrical geometry. This phenomenological approach provides a quantitative
classification of the cylindrical ordered morphologies by expansion of the
order parameter spatial distribution into the eigenfunctions of the Laplace
operator. The symmetry of the resulting ordered morphologies is shown to
strongly depend both on the boundary conditions (wall preference) and the ratio
of the cylinder radius and the wave length of the critical order parameter
fluctuations, which determine the bulk ordering of the system under
consideration. In particular, occurrence of the helical morphologies is a
rather general consequence of the imposed cylindrical symmetry for narrow
enough capillaries. We discuss also the ODT and OOT involving some other
simplest morphologies. The presented results are relevant also to other
ordering systems as charge-density waves appearing under addition of an ionic
solute to a solvent in its critical region, weakly charged polyelectrolyte
solutions in poor solvent, microemulsions etc.Comment: 6 pages, 3 figure
Dynamics of Binary Mixtures with Ions: Dynamic Structure Factor and Mesophase Formation
Dynamic equations are presented for polar binary mixtures containing ions in
the presence of the preferential solvation. In one-phase states, we calculate
the dynamic structure factor of the composition accounting for the ion motions.
Microphase separation can take place for sufficiently large solvation asymmetry
of the cations and the anions. We show two-dimensional simulation results of
the mesophase formation with an antagonistic salt, where the cations are
hydrophilic and the anions are hydrophobic. The structure factor S(q) in the
resultant mesophase has a sharp peak at an intermediate wave number on the
order of the Debye-Huckel wave number. As the quench depth is increased, the
surface tension nearly vanishes in mesophases due to an electric double layer.Comment: 24 pages, 10 figures, to appear in Journal of Physics: Condensed
Matte
Retarded Casimir-Polder force on an atom near reflecting microstructures
We derive the fully retarded energy shift of a neutral atom in two different
geometries useful for modelling etched microstructures. First we calculate the
energy shift due to a reflecting cylindrical wire, and then we work out the
energy shift due to a semi-infinite reflecting half-plane. We analyze the
results for the wire in various limits of the wire radius and the distance of
the atom from the wire, and obtain simple asymptotic expressions useful for
estimates. For the half-plane we find an exact representation of the
Casimir-Polder interaction in terms of a single, fast converging integral,
which is easy to evaluate numerically.Comment: 12 pages, 8 figure
Peculiar behavior of the electrical resistivity of MnSi at the ferromagnetic phase transition
The electrical resistivity of a single crystal of MnSi was measured across
its ferromagnetic phase transition line at ambient and high pressures. Sharp
peaks of the temperature coefficient of resistivity characterize the transition
line. Analysis of these data shows that at pressures to ~0.35 GPa these peaks
have fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is
symptomatic that this structure disappears at pressures higher than ~0.35 GPa,
which was identified earlier as a tricritical poin
Bound states of edge dislocations: The quantum dipole problem in two dimensions
We investigate bound state solutions of the 2D Schr\"odinger equation with a
dipole potential originating from the elastic effects of a single edge
dislocation. The knowledge of these states could be useful for understanding a
wide variety of physical systems, including superfluid behavior along
dislocations in solid He. We present a review of the results obtained by
previous workers together with an improved variational estimate of the ground
state energy. We then numerically solve the eigenvalue problem and calculate
the energy spectrum. In our dimensionless units, we find a ground state energy
of -0.139, which is lower than any previous estimate. We also make successful
contact with the behavior of the energy spectrum as derived from semiclassical
considerations.Comment: 6 pages, 3 figures, submitted to PR
Superfluid density near the critical temperature in the presence of random planar defects
The superfluid density near the superconducting transition is investigated in
the presence of spatial inhomogeneity in the critical temperature. Disorder is
accounted for by means of a random term in the conventional
Ginzburg-Landau action for the superconducting order parameter. Focusing on the
case where a low-density of randomly distributed planar defects are responsible
for the variation of , we derive the lowest order correction to the
superfluid density in powers of the defect concentration. The correction is
calculated assuming a broad Gaussian distribution for the strengths of the
defect potentials. Our results are in a qualitative agreement with the
superfluid density measurements in the underdoped regime of high-quality YBCO
crystals by Broun and co-workers.Comment: 14 pages, 7 figures; Accepted to Phys. Rev.