444 research outputs found
Catalogue of lunar craters cross sections. I - Craters with peaks Research report no. 16
Lunar craters with centrally located peaks - tables and profile graph
Two-body correlations and the superfluid fraction for nonuniform systems
We extend the one-body phase function upper bound on the superfluid fraction
in a periodic solid (a spatially ordered supersolid) to include two-body phase
correlations. The one-body current density is no longer proportional to the
gradient of the one-body phase times the one-body density, but rather it
depends also on two-body correlation functions. The equations that
simultaneously determine the one-body and two-body phase functions require a
knowledge of one-, two-, and three-body correlation functions. The approach can
also be extended to disordered solids. Fluids, with two-body densities and
two-body phase functions that are translationally invariant, cannot take
advantage of this additional degree of freedom to lower their energy.Comment: 13 page
Spin Pumping of Current in Non-Uniform Conducting Magnets
Using irreversible thermodynamics we show that current-induced spin transfer
torque within a magnetic domain implies spin pumping of current within that
domain. This has experimental implications for samples both with conducting
leads and that are electrically isolated. These results are obtained by
deriving the dynamical equations for two models of non-uniform conducting
magnets: (1) a generic conducting magnet, with net conduction electron density
n and net magnetization ; and (2) a two-band magnet, with up and down
spins each providing conduction and magnetism. For both models, in regions
where the equilibrium magnetization is non-uniform, voltage gradients can drive
adiabatic and non-adiabatic bulk spin torques. Onsager relations then ensure
that magnetic torques likewise drive adiabatic and non-adiabatic currents --
what we call bulk spin pumping. For a given amount of adiabatic and
non-adiabatic spin torque, the two models yield similar but distinct results
for the bulk spin pumping, thus distinguishing the two models. As in the recent
spin-Berry phase study by Barnes and Maekawa, we find that within a domain wall
the ratio of the effective emf to the magnetic field is approximately given by
, where P is the spin polarization. The adiabatic spin torque
and spin pumping terms are shown to be dissipative in nature.Comment: 13 pages in pdf format; 1 figur
Is the electrostatic force between a point charge and a neutral metallic object always attractive?
We give an example of a geometry in which the electrostatic force between a
point charge and a neutral metallic object is repulsive. The example consists
of a point charge centered above a thin metallic hemisphere, positioned concave
up. We show that this geometry has a repulsive regime using both a simple
analytical argument and an exact calculation for an analogous two-dimensional
geometry. Analogues of this geometry-induced repulsion can appear in many other
contexts, including Casimir systems.Comment: 7 pages, 7 figure
Speech Communication
Contains reports on seven research projects.Contract AF19(604)-2061 with Air Force Cambridge Research CenterContract N5ori-07861 with the Navy (Office of Naval Research)National Science Foundatio
Low-Frequency Electromagnetic Response Function for Strong-Coupling Superconductors
Journals published by the American Physical Society can be found at http://journals.aps.org
Smecticlike phase for modulated XY spins in two dimensions
The row model for frustrated XY spins on a triangular lattice in 2D is used
to study incommensurate{IC}) spiral and commensurate{C} antiferromagnetic (AF)
phases, in the regime where a C-IC transition occurs. Using fluctuating
boundary conditions and specific histogram techniques, a detailed Monte Carlo
(MC) study reveals more structure in the phase diagram than found in previous
MC simulations of the full parameter space. On the (C) side, equilibrium
configurations consist of alternating stripes of spiral phases of opposite
chirality separated by walls of the (C) phase. For this same parameter regime,
thermodynamic quantities are computed analytically using the NSCHA, a
generalization of the self consistent harmonic approximation appropriate for
chiral systems. On the commensurate side of the (C)-(IC) boundary, NSCHA
predicts an instability of the (C) phase. This suggests that the state is
spatially inhomogeneous, consistent with the present MC result: it resembles
the smectic-A phase of liquid crystals, and its existence implies that the
Lifshitz point is at for modulated XY spins in 2D. The connection
between frustrated XY systems and the vortex state of strong type II
superconductors suggests that the smectic phase may correspond to a vortex
liquid phase of superconducting layers.Comment: Single Postscript file containing 24 pages of text and 8 figures. To
appear in May 1 issue of Phys. Rev. B, Vol. 5
Thermal phase diagrams of columnar liquid crystals
In order to understand the possible sequence of transitions from the
disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene
(HHTT), we study a three-dimensional planar model with octupolar interactions
inscribed on a triangular lattice of columns. We obtain thermal phase diagrams
using a mean-field approximation and Monte Carlo simulations. These two
approaches give similar results, namely, in the quasi one-dimensional regime,
as the temperature is lowered, the columns order with a linear polarization,
whereas helical phases develop at lower temperatures. The helicity patterns of
the helical phases are determined by the exact nature of the frustration in the
system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe
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Enhanced 99Tc retention in glass waste form using Tc(IV)-incorporated Fe minerals
Technetium (99Tc) immobilization by doping into iron oxide mineral phases may alleviate the problems with Tc volatility during vitrification of nuclear waste. Because reduced Tc, Tc(IV), substitutes for Fe(III) in the crystal structure by a process of Tc reduction from Tc(VII) to Tc(IV) followed by co-precipitation of Fe oxide minerals, two Tc-incorporated Fe minerals (Tc-goethite and Tc-magnetite/maghemite) were prepared and tested for Tc retention in glass melt samples at temperatures between 600 and 1000 °C. After being cooled, the solid glass specimens prepared at different temperatures at 600, 800, and 1000 °C were analyzed for Tc oxidation state using Tc K-edge XANES. In most samples, Tc was partially (<60%) oxidized from Tc(IV) to Tc(VII) as the melt temperature increased up to 600 °C. However, most of Tc(IV) was completely (>95%) oxidized to Tc(VII) at temperature above 800 °C. Tc retention in glass melt samples prepared using Tc-incorporated Fe minerals were slightly higher (∼10%) than in glass prepared using KTcO4 because of limited and delayed Tc volatilization
Phase transitions in a frustrated XY model with zig-zag couplings
We study a new generalized version of the square-lattice frustrated XY model
where unequal ferromagnetic and antiferromagnetic couplings are arranged in a
zig-zag pattern. The ratio between the couplings can be used to tune the
system, continuously, from the isotropic square-lattice to the
triangular-lattice frustrated XY model. The model can be physically realized as
a Josephson-junction array with two different couplings, in a magnetic field
corresponding to half-flux quanta per plaquette. Mean-field approximation,
Ginzburg-Landau expansion and finite-size scaling of Monte Carlo simulations
are used to study the phase diagram and critical behavior. Depending on the
value of , two separate transitions or a transition line in the
universality class of the XY-Ising model, with combined and U(1)
symmetries, takes place. In particular, the phase transitions of the standard
square-lattice and triangular-lattice frustrated XY models correspond to two
different cuts through the same transition line. Estimates of the chiral
() critical exponents on this transition line deviate significantly from
the pure Ising values, consistent with that along the critical line of the
XY-Ising model. This suggests that a frustrated XY model or Josephson-junction
array with a zig-zag coupling modulation can provide a physical realization of
the XY-Ising model critical line.Comment: 11 pages, 9 figures, RevTex, to appear in Phys. Rev.
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