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 ($1/R^2$ vs. $1/R^3$) 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