76 research outputs found
Domain structure of bulk ferromagnetic crystals in applied fields near saturation
We investigate the ground state of a uniaxial ferromagnetic plate with
perpendicular easy axis and subject to an applied magnetic field normal to the
plate. Our interest is the asymptotic behavior of the energy in macroscopically
large samples near the saturation field. We establish the scaling of the
critical value of the applied field strength below saturation at which the
ground state changes from the uniform to a branched domain magnetization
pattern and the leading order scaling behavior of the minimal energy.
Furthermore, we derive a reduced sharp-interface energy giving the precise
asymptotic behavior of the minimal energy in macroscopically large plates under
a physically reasonable assumption of small deviations of the magnetization
from the easy axis away from domain walls. On the basis of the reduced energy,
and by a formal asymptotic analysis near the transition, we derive the precise
asymptotic values of the critical field strength at which non-trivial
minimizers (either local or global) emerge. The non-trivial minimal energy
scaling is achieved by magnetization patterns consisting of long slender
needle-like domains of magnetization opposing the applied fieldComment: 38 pages, 7 figures, submitted to J. Nonlin. Sci
Elastic p-3He and n-3H scattering with two- and three-body forces
We report on a microscopic calculation of n-3H and p-3He scattering employing
the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without
additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H
scattering data is presented. Comparisons are made for the phase shifts and a
selection of measurements in both scattering systems. Differences between our
calculation and the R-matrix results or the experimental data can be attributed
to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX
and the Texas-Los Alamos three-nucleon forces on the phase shifts to be
negligible.Comment: submitted to Phys. Rev.
Massive skyrmions in quantum Hall ferromagnets
We apply the theory of elasticity to study the effects of skyrmion mass on
lattice dynamics in quantum Hall systems. We find that massive Skyrme lattices
behave like a Wigner crystal in the presence of a uniform perpendicular
magnetic field. We make a comparison with the microscopic Hartree-Fock results
to characterize the mass of quantum Hall skyrmions at and investigate
how the low temperature phase of Skyrme lattices may be affected by the
skyrmion mass.Comment: 6 pages and 2 figure
Formulae for zero-temperature conductance through a region with interaction
The zero-temperature linear response conductance through an interacting
mesoscopic region attached to noninteracting leads is investigated. We present
a set of formulae expressing the conductance in terms of the ground-state
energy or persistent currents in an auxiliary system, namely a ring threaded by
a magnetic flux and containing the correlated electron region. We first derive
the conductance formulae for the noninteracting case and then give arguments
why the formalism is also correct in the interacting case if the ground state
of a system exhibits Fermi liquid properties. We prove that in such systems,
the ground-state energy is a universal function of the magnetic flux, where the
conductance is the only parameter. The method is tested by comparing its
predictions with exact results and results of other methods for problems such
as the transport through single and double quantum dots containing interacting
electrons. The comparisons show an excellent quantitative agreement.Comment: 18 pages, 18 figures; to appear in Phys. Rev.
Bcc He as a Coherent Quantum Solid
In this work we investigate implications of the quantum nature of bcc %
He. We show that it is a unique solid phase with both a lattice structure and
an Off-Diagonal Long Range Order of coherently oscillating local electric
dipole moments. These dipoles arise from the local motion of the atoms in the
crystal potential well, and oscillate in synchrony to reduce the dipolar
interaction energy. The dipolar ground-state is therefore found to be a
coherent state with a well defined global phase and a three-component complex
order parameter. The condensation energy of the dipoles in the bcc phase
stabilizes it over the hcp phase at finite temperatures. We further show that
there can be fermionic excitations of this ground-state and predict that they
form an optical-like branch in the (110) direction. A comparison with
'super-solid' models is also discussed.Comment: 12 pages, 8 figure
Scaling Of Chiral Lagrangians And Landau Fermi Liquid Theory For Dense Hadronic Matter
We discuss the Fermi-liquid properties of hadronic matter derived from a
chiral Lagrangian field theory in which Brown-Rho (BR) scaling is incorporated.
We identify the BR scaling as a contribution to Landau's Fermi liquid
fixed-point quasiparticle parameter from "heavy" isoscalar meson degrees of
freedom that are integrated out from a low-energy effective Lagrangian. We show
that for the vector (convection) current, the result obtained in the chiral
Lagrangian approach agrees precisely with that obtained in the
semi-phenomenological Landau-Migdal approach. This precise agreement allows one
to determine the Landau parameter that enters in the effective nucleon mass in
terms of the constant that characterizes BR scaling. When applied to the weak
axial current, however, these two approaches differ in a subtle way. While the
difference is small numerically, the chiral Lagrangian approach implements
current algebra and low-energy theorems associated with the axial response that
the Landau method misses and hence is expected to be more predictive.Comment: 39 pages, latex with 4 eps figure, modified addresses and reference
A quantum Monte Carlo study of the one-dimensional ionic Hubbard model
Quantum Monte Carlo methods are used to study a quantum phase transition in a
1D Hubbard model with a staggered ionic potential (D). Using recently
formulated methods, the electronic polarization and localization are determined
directly from the correlated ground state wavefunction and compared to results
of previous work using exact diagonalization and Hartree-Fock. We find that the
model undergoes a thermodynamic transition from a band insulator (BI) to a
broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since
it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with
no long-range order, we have searched for a second transition to this state by
(i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U.
We find no transition from the BO to MI state, and we propose that the MI state
in 1D is unstable to bond ordering under the addition of any finite ionic
potential. In real 1D systems the symmetric MI phase is never stable and the
transition is from a symmetric BI phase to a dimerized BO phase, with a
metallic point at the transition
Density functional method for nonequilibrium electron transport
We describe an ab initio method for calculating the electronic structure,
electronic transport, and forces acting on the atoms, for atomic scale systems
connected to semi-infinite electrodes and with an applied voltage bias. Our
method is based on the density functional theory (DFT) as implemented in the
well tested Siesta approach (which uses non-local norm-conserving
pseudopotentials to describe the effect of the core electrons, and linear
combination of finite-range numerical atomic orbitals to describe the valence
states). We fully deal with the atomistic structure of the whole system,
treating both the contact and the electrodes on the same footing. The effect of
the finite bias (including selfconsistency and the solution of the
electrostatic problem) is taken into account using nonequilibrium Green's
functions. We relate the nonequilibrium Green's function expressions to the
more transparent scheme involving the scattering states. As an illustration,
the method is applied to three systems where we are able to compare our results
to earlier ab initio DFT calculations or experiments, and we point out
differences between this method and existing schemes. The systems considered
are: (1) single atom carbon wires connected to aluminum electrodes with
extended or finite cross section, (2) single atom gold wires, and finally (3)
large carbon nanotube systems with point defects.Comment: 18 pages, 23 figure
Abelian gauge potentials on cubic lattices
The study of the properties of quantum particles in a periodic potential
subject to a magnetic field is an active area of research both in physics and
mathematics; it has been and it is still deeply investigated. In this review we
discuss how to implement and describe tunable Abelian magnetic fields in a
system of ultracold atoms in optical lattices. After discussing two of the main
experimental schemes for the physical realization of synthetic gauge potentials
in ultracold set-ups, we study cubic lattice tight-binding models with
commensurate flux. We finally examine applications of gauge potentials in
one-dimensional rings.Comment: To appear on: "Advances in Quantum Mechanics: Contemporary Trends and
Open Problems", G. Dell'Antonio and A. Michelangeli eds., Springer-INdAM
series 201
Thorium speciation in seawater
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 100 (2006): 250-268, doi:10.1016/j.marchem.2005.10.024.Since the 1960’s, thorium isotopes occupy a special place in the oceanographer’s toolbox as
tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and
carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of
uranium and radium, their disequilibrium can be used to calculate rates and time scales of
sinking particles. In addition, by ratio-ing particulate 234Th (as well, in principle, other Thnuclides)
to carbon (and other elements), and linking this ratio to the parent-daughter
disequilibrium in the water column, it is possible to calculate fluxes of carbon and other
elements. Most of these applications are possible with little knowledge of the dissolved chemical
properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to
surfaces, i.e., its “particle- or surface-activity”. However, the use of any tracer is hindered by a
lack of knowledge of its chemical properties. Recent observations in the variability of carbon to
234Th ratios in different particle types, as well as of associations of Th(IV) with various marine
organic biomolecules has led to the need for a review of current knowledge and what future
endeavors should be taken to understand the marine chemistry of thorium.The writing of this paper was supported, in parts by NSF (OCE-0351559; OCE-0350758, and
OCE 0354757)
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