733 research outputs found
The impact of EU enlargement on migration flows
This document is a report commissioned by the Home Office in order to assess the magnitude of potential migration flows to the UK after the enlargement of the European Union (EU). The countries which are expected to join the EU on 1 May 2004 are Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia. The aim of the report is to provide extensive information that will be helpful in evaluating the migration potential from
these countries to the UK. This is done by (i) describing the socio-economic situation of the accession countries nowadays and since the early 1990s and comparing it to that of the UK and Germany, (ii) critically reviewing the existing literature that attempts to predict the effects of the current EU enlargement on migration flows, (iii) analysing the so-called Southern enlargement (the accessions of Greece, Portugal and Spain) and comparing it to the current one, and (iv) presenting a quantitative analysis of the effect of the current enlargement on migration to the UK
Solitonic Excitations in Linearly Coherent Channels of Bilayer Quantum Hall Stripes
In some range of interlayer distances, the ground state of the
two-dimensional electron gas at filling factor nu =4N+1 with N=0,1,2,... is a
coherent stripe phase in the Hartree-Fock approximation. This phase has
one-dimensional coherent channels that support charged excitations in the form
of pseudospin solitons. In this work, we compute the transport gap of the
coherent striped phase due to the creation of soliton-antisoliton pairs using a
supercell microscopic unrestricted Hartree-Fock approach. We study this gap as
a function of interlayer distance and tunneling amplitude. Our calculations
confirm that the soliton-antisoliton excitation energy is lower than the
corresponding Hartree-Fock electron-hole pair energy. We compare our results
with estimates of the transport gap obtained from a field-theoretic model valid
in the limit of slowly varying pseudospin textures.Comment: 15 pages, 8 figure
Collective Excitations of a Two-Component Bose Condensate at Finite Temperature
We compare the collective modes for Bose-condensed systems with two
degenerate components with and without spontaneous intercomponent coherence at
finite temperature using the time-dependent Hartree-Fock approximation. We show
that the interaction between the condensate and non-condensate in these two
cases results in qualitatively different collective excitation spectra. We show
that at zero temperature the single-particle excitations of the incoherent Bose
condensate can be probed by intercomponent excitations.Comment: 4 pages, 3 figures, Submitted to PR
Theory of Phonon Shakeup Effects on Photoluminescence from the Wigner Crystal in a Strong Magnetic Field
We develop a method to compute shakeup effects on photoluminescence from a
strong magnetic field induced two-dimensional Wigner crystal. Only localized
holes are considered. Our method treats the lattice electrons and the tunneling
electron on an equal footing, and uses a quantum-mechanical calculation of the
collective modes that does not depend in any way on a harmonic approximation.
We find that shakeup produces a series of sidebands that may be identified with
maxima in the collective mode density of states, and definitively distinguishes
the crystal state from a liquid state in the absence of electron-hole
interaction. In the presence of electron-hole interaction, sidebands also
appear in the liquid state coming from short-range density fluctuations around
the hole. However, the sidebands in the liquid state and the crystal state have
different qualitative behaviors. We also find a shift in the main luminescence
peak, that is associated with lattice relaxation in the vicinity of a vacancy.
The relationship of the shakeup spectrum with previous mean-field calculations
is discussed.Comment: 14 pages, uuencoded postscript file for entire paper, also available
at (click phd14) http://rainbow.uchicago.edu/~ldz/paper/paper.htm
Electromagnetic absorption of a pinned Wigner crystal at finite temperatures
We investigate the microwave absorption of a pinned, two-dimensional Wigner
crystal in a strong magnetic field at finite temperatures. Using a model of a
uniform commensurate pinning potential, we analyze thermal broadening of the
electromagnetic absorption resonance. Surprisingly, we find that the pinning
resonance peak should remain sharp even when the temperature is comparable or
greater than the peak frequency. This result agrees qualitatively with recent
experimental observations of the ac conductivity in two-dimensional hole
systems in a magnetically induced insulating state. It is shown, in analogy
with Kohn's theorem, that the electron-electron interaction does not affect the
response of a harmonically pinned Wigner crystal to a spatially uniform
external field at any temperature. We thus focus on anharmonicity in the
pinning potential as a source of broadening. Using a 1/N expansion technique,
we show that the broadening is introduced through the self-energy corrections
to the magnetophonon Green's functions.Comment: 21 pages, 9 eps figure
Diluted Graphene Antiferromagnet
We study RKKY interactions between local magnetic moments for both doped and
undoped graphene. We find in both cases that the interactions are primarily
ferromagnetic for moments on the same sublattice, and antiferromagnetic for
moments on opposite sublattices. This suggests that at sufficiently low
temperatures dilute magnetic moments embedded in graphene can order into a
state analogous to that of a dilute antiferromagnet. We find that in the
undoped case one expects no net magnetic moment, and demonstrate numerically
that this effect generalizes to ribbons where the magnetic response is
strongest at the edge, suggesting the possibility of an unusual spin-transfer
device. For doped graphene we find that moments at definite lattice sites
interact over longer distances than those placed in interstitial sites of the
lattice ( vs. ) because the former support a Kohn anomaly that is
suppressed in the latter due to the absence of backscattering.Comment: 5 pages, two figures include
Collective Excitations, NMR, and Phase Transitions in Skyrme Crystals
At Landau level filling factors near nu =1, quantum Hall ferromagnets form a
Skyrme crystal state with quasi-long-range translational and non-collinear
magnetic order. We develop an effective low energy theory which explains the
presence in these systems of magnetic excitations at low energies below the
Larmor gap (Delta) and which predicts a dramatic enhancement of the nuclear
spin relaxation rate by a factor of 1000. The effective theory predicts a rich
set of quantum and classical phase transitions. Based in part on accurate
time-dependent Hartree-Fock calculations of the ordered state collective
excitation spectrum, we discuss aspects of the T-nu-Delta crystal phase
diagram.Comment: 4 pages REVTEX file and 3 postscript figure
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