6,372 research outputs found
Perspectives on the Budget Surplus
This paper provides alternative measures of federal budget surpluses over 10-year and long-term horizons. Official baseline budget forecasts are based on a series of statutory requirements that may be at variance with reasonable expectation. More plausible notions of current policy toward discretionary spending, taxes and retirement trust funds imply that surpluses over the next 10 years will be substantially smaller than the baseline forecasts indicate. Properly accounting for long-term imbalances in social security and the rest of the budget implies that, under plausible definitions of current policy, the federal government faces a long-term shortfall.
Quantum vortex tunneling in thin films
Cuprate films offer a unique opportunity to observe vortex tunneling effects,
due to their unusually low superfluid density and short coherence length. Here,
we measure the magnetoresistance (\textit{MR}) due to vortex motion of a long
meander line of a superconducting film made of underdoped
. At low temperatures (\textit{T}), the \textit{MR}
shows a significant deviation from Arrhenius activation. The data is consistent
with two dimensional Variable Range Hopping (VRH) of single vortices, i.e.
. The VRH temperature scale depends on the
vortex tunneling rates between pinning sites. We discuss its magnitude with
respect to estimated parameters of the meander thin film.Comment: 5 figure
A Schwinger-boson approach to the kagome with Dzyaloshinskii-Moriya interactions: phase diagram and dynamical structure factors
We have obtained the zero-temperature phase diagram of the kagome
antiferromagnet with Dzyaloshinskii-Moriya interactions in Schwinger-boson
mean-field theory. We find quantum phase transitions (first or second order)
between different topological spin liquids and Neel ordered phases (either the
state or the so-called Q=0 state). In the regime of
small Schwinger-boson density, the results bear some resemblances with exact
diagonalization results and we briefly discuss some issues of the mean-field
treatment. We calculate the equal-time structure factor (and its angular
average to allow for a direct comparison with experiments on powder samples),
which extends earlier work on the classical kagome to the quantum regime. We
also discuss the dynamical structure factors of the topological spin liquid and
the Neel ordered phase.Comment: 8 pages, 9 figure
Spin pumping by a field-driven domain wall
We calculate the charge current in a metallic ferromagnet to first order in
the time derivative of the magnetization direction. Irrespective of the
microscopic details, the result can be expressed in terms of the conductivities
of the majority and minority electrons and the non-adiabatic spin transfer
torque parameter . The general expression is evaluated for the specific
case of a field-driven domain wall and for that case depends strongly on the
ratio of and the Gilbert damping constant. These results may provide an
experimental method to determine this ratio, which plays a crucial role for
current-driven domain-wall motion.Comment: 4 pages, 1 figure v2: some typos corrected v3: published versio
Fractional Quantum Hall Effect and Featureless Mott Insulators
We point out and explicitly demonstrate a close connection that exists
between featureless Mott insulators and fractional quantum Hall liquids. Using
magnetic Wannier states as the single-particle basis in the lowest Landau level
(LLL), we demonstrate that the Hamiltonian of interacting bosons in the LLL
maps onto a Hamiltonian of a featureless Mott insulator on triangular lattice,
formed by the magnetic Wannier states. The Hamiltonian is remarkably simple and
consists only of short-range repulsion and ring-exchange terms.Comment: 7 pages, 1 figure. Published version
Vortex Dynamics and Hall Conductivity of Hard Core Bosons
Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum
spin model representation, appropriately gauged on the torus to allow for an
external magnetic field. We find strong lattice effects near half filling. An
effective quantum mechanical description of the vortex degrees of freedom is
derived. Using semiclassical and numerical analysis we compute the vortex
hopping energy, which at half filling is close to magnitude of the boson
hopping energy. The critical quantum melting density of the vortex lattice is
estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed
from the Chern numbers of the low energy eigenstates. At zero temperature, it
reverses sign abruptly at half filling. At precisely half filling, all
eigenstates are doubly degenerate for any odd number of flux quanta. We prove
the exact degeneracies on the torus by constructing an SU(2) algebra of
point-group symmetries, associated with the center of vorticity. This result is
interpreted as if each vortex carries an internal spin-half degree of freedom
('vspin'), which can manifest itself as a charge density modulation in its
core. Our findings suggest interesting experimental implications for vortex
motion of cold atoms in optical lattices, and magnet-transport of short
coherence length superconductors.Comment: 15 pages, 15 figure
Collective modes and superflow instabilities of strongly correlated Fermi superfluids
We study the superfluid phase of the one-band attractive Hubbard model of
fermions as a prototype of a strongly correlated s-wave fermion superfluid on a
lattice. We show that the collective mode spectrum of this superfluid exhibits,
in addition to the long wavelength sound mode, a sharp roton mode over a wide
range of densities and interaction strengths. We compute the sound velocity and
the roton gap within a generalized random phase approximation (GRPA) and show
that the GRPA results are in good agreement, at strong coupling, with a spin
wave analysis of the appropriate strong-coupling pseudospin model. We also
investigate, using this two-pronged approach, the breakdown of superfluidity in
the presence of a supercurrent. We find that the superflow can break down at a
critical flow momentum via several distinct mechanisms - depairing, Landau
instabilities or dynamical instabilities - depending on the dimensionality, the
interaction strength and the fermion density. The most interesting of these
instabilities is a charge modulation dynamical instability which is distinct
from previously studied dynamical instabilities of Bose superfluids. The charge
order associated with this instability can be of two types: (i) a commensurate
checkerboard modulation driven by softening of the roton mode at the Brillouin
zone corner, or, (ii) an incommensurate density modulation arising from
superflow-induced finite momentum pairing of Bogoliubov quasiparticles. We
elucidate the dynamical phase diagram showing the critical flow momentum of the
leading instability over a wide range of fermion densities and interaction
strengths and point out implications of our results for experiments on cold
atom fermion superfluids in an optical lattice.Comment: 14 pages, 10 figures. Corrected 3d phase diagram. References added.
Minor changes in tex
The fully self-consistent quasiparticle random phase approximation and its application to the isobaric analog resonances
A microscopic model aimed at the description of charge-exchange nuclear
excitations along isotopic chains which include open-shell systems, is
developed. It consists of quasiparticle random phase approximation (QRPA) made
on top of Hartree-Fock-Bardeen-Cooper-Schrieffer (HF-BCS). The calculations are
performed by using the Skyrme interaction in the particle-hole channel and a
zero-range, density-dependent pairing force in the particle-particle channel.
At variance with the (many) versions of QRPA which are available in literature,
in our work special emphasis is put on the full self-consistency. Its
importance, as well as the role played by the charge-breaking terms of the
nuclear Hamiltonian, like the Coulomb interaction, the charge symmetry and
charge independence breaking (CSB-CIB) forces and the electromagnetic
spin-orbit, are elucidated by means of numerical calculations of the isobaric
analog resonances (IAR). The theoretical energies of these states along the
chain of the Sn isotopes agree well with the experimental data in the stable
isotopes. Predictions for unstable systems are presented.Comment: 15 pages, 6 figure
A Tonks Giradeau Gas in the Presence of a Local Potential
The physics of a Tonks-Giradeau Gas in the presence of a local potential is
studied. In order to evaluate the single particle density matrix (SPDM) of the
many-body ground state, the Wiger-Jordan transformation is used. The
eigenvector with the largest eigenvalue of the SPDM corresponds to the
"Bose-Einstein Condensate"(BEC) State. We find that the "BEC" state density at
the positon of the local potential decreases, as expected, in the case of a
repulsive potential. For an attractive potential, it decreases or increases
depending on the strength of the potential. The superfluidity of this system is
investigated both numerically and perturbatively. An experimental method for
detecting the effect of an impurity in a Tonks-Giradueau gas is discussed.Comment: 14 pages, 5 figure
Widths of Isobaric Analog Resonances: a microscopic approach
A self-consistent particle-phonon coupling model is used to investigate the
properties of the isobaric analog resonance in Bi. It is shown that
quantitative agreement with experimental data for the energy and the width can
be obtained if the effects of isospin-breaking nuclear forces are included, in
addition to the Coulomb force effects. A connection between microscopic model
predictions and doorway state approaches which make use of the isovector
monopole resonance, is established via a phenomenological ansatz for the
optical potential.Comment: 18 pages, 1 figure. To appear on Phys. Rev. C (tentatively scheduled
for June 1998
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