Magnetic phase transition in coherently coupled Bose gases in optical lattices

We describe the ground state of a gas of bosonic atoms with two coherently coupled internal levels in a deep optical lattice in a one dimensional geometry. In the single-band approximation this system is described by a Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating phase which can be either paramagnetic or ferromagnetic. We characterize the quantum phase transitions at unit filling by means of a density matrix renormalization group technique and compare it with a mean-field approach. The presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ model in a transverse field and the numerical results compare very well with the analytical results obtained from the spin model. In the superfluid regime quantum fluctuations strongly modify the phase transition with respect to the well established mean-field three dimensional classical bifurcation.Comment: 6 pages, 3 figure

TARGET2 Unlimited: monetary policy implications of asymmetric liquidity management within the Euro area. CEPS Policy Brief No. 248, 13 July 2011

This paper analyses the implications of a continued divergence of TARGET2 balances for monetary policy in the euro area. The accumulation of TARGET2 claims (liabilities) would make the ECB’s liquidity management asymmetric once the TARGET2 claims in core countries have crowded out central bank credit in those regions. Then while providing scarce liquidity to banks in countries with TARGET2 liabilities, the ECB will need to absorb excess liquidity in countries with TARGET2 claims. We discuss three alternatives and their implications for absorbing excess liquidity in core regions: 1) using market-based measures might accelerate the capital flight from periphery to core countries and would add to the accumulation of risky assets by the ECB; 2) conducting non-market based measures, such as imposing differential (unremunerated) reserve requirements, would distort banking markets and would support the development of shadow banking; and 3) staying passive would lead to decreasing interest rates in core Europe entailing inflationary pressure and overinvestment in those regions and possibly future instability of the banking system

Astrometry with "Carte du Ciel" plates, San Fernando zone. II. CdC-SF: a precise proper motion catalogue

The historic plates of the "Carte du Ciel", an international cooperative project launched in 1887, offer valuable first-epoch material for the determination of absolute proper motions. We present the CdC-SF, an astrometric catalogue of positions and proper motions derived from the "Carte du Ciel" plates of the San Fernando zone, photographic material with a mean epoch of 1901.4 and a limiting magnitude of V~16, covering the declination range of -10deg < declination < -2deg. Digitization has been made using a conventional flatbed scanner. Special techniques have been developed to handle the combination of plate material and the large distortion introduced by the scanner. The equatorial coordinates are on the ICRS defined by Tycho-2, and proper motions are derived using UCAC2 as second-epoch positions. The result is a catalogue with positions and proper motions for 560000 stars, covering 1080 degrees squared. The mean positional uncertainty is 0.20" (0.12" for well-measured stars) and the proper-motion uncertainty is 2.0 mas/yr (1.2 mas/yr for well-measured stars). The proper motion catalogue CdC-SF is effectively a deeper extension of Hipparcos, in terms of proper motions, to a magnitude of 15.Comment: 10 pages, 13 figures, 6 tables. Accepted for publication in A&

TARGET2 Unlimited: Monetary Policy Implications of Asymmetric Liquidity Management within the Euro Area

This paper analyses the implications of a continued divergence of TARGET2 balances for monetary policy in the euro area. The accumulation of TARGET2 claims (liabilities) would make ECB’s liquidity management asymmetric once the TARGET2 claims in core countries have crowded out central bank credit in those regions. Then while providing scarce liquidity to banks in countries with TARGET2 liabilities, the ECB will need to absorb excess liquidity in countries with TARGET2 claims. We discuss three alternatives and its implications to absorb excess liquidity in core regions: (1) Using market based measures might accelerate the capital flight from periphery to core countries and would add to the accumulation of risky assets by the ECB. (2) Conducting non-market based measures such as imposing differential (unremunerated) reserve requirements would distort banking markets and would support the development of shadow banking. (3) Staying passive would lead to decreasing interest rates in core Europe entailing inflationary pressure and overinvestment in those regions and possibly future instability of the banking system.TARGET2 balances, monetary policy, euro area, Eurosystem, excess liquidity

Dipolar condensates confined in a toroidal trap: ground state and vortices

We study a Bose-Einstein condensate of 52Cr atoms confined in a toroidal trap with a variable strength of s-wave contact interactions. We analyze the effects of the anisotropic nature of the dipolar interaction by considering the magnetization axis to be perpendicular to the trap symmetry axis. In the absence of a central repulsive barrier, when the trap is purely harmonic, the effect of reducing the scattering length is a tuning of the geometry of the system: from a pancake-shaped condensate when it is large, to a cigar-shaped condensate for small scattering lengths. For a condensate in a toroidal trap, the interaction in combination with the central repulsive Gaussian barrier produces an azimuthal dependence of the particle density for a fixed radial distance. We find that along the magnetization direction the density decreases as the scattering length is reduced but presents two symmetric density peaks in the perpendicular axis. For even lower values of the scattering length we observe that the system undergoes a dipolar-induced symmetry breaking phenomenon. The whole density becomes concentrated in one of the peaks, resembling an origin-displaced cigar-shaped condensate. In this context we also analyze stationary vortex states and their associated velocity field, finding that this latter also shows a strong azimuthal dependence for small scattering lengths. The expectation value of the angular momentum along the z direction provides a qualitative measure of the difference between the velocity in the different density peaks.Comment: 9 pages, 12 figure

Instability of the superfluid flow as black-hole lasing effect

We show that the instability leading to the decay of the one-dimensional superfluid flow through a penetrable barrier are due to the black-hole lasing effect. This dynamical instability is triggered by modes resonating in an effective cavity formed by two horizons enclosing the barrier. The location of the horizons is set by $v(x)=c(x)$, with $v(x),c(x)$ being the local fluid velocity and sound speed, respectively. We compute the critical velocity analytically and show that it is univocally determined by the horizons configuration. In the limit of broad barriers, the continuous spectrum at the origin of the Hawking-like radiation and of the Landau energetic instability is recovered.Comment: 18 pages, 3 figure