Time evolution of correlations in strongly interacting fermions after a quantum quench

Using the adaptive time-dependent density matrix renormalization group, we study the time evolution of density correlations of interacting spinless fermions on a one-dimensional lattice after a sudden change in the interaction strength. Over a broad range of model parameters, the correlation function exhibits a characteristic light-cone-like time evolution representative of a ballistic transport of information. Such behavior is observed both when quenching an insulator into the metallic region and also when quenching within the insulating region. However, when a metallic state beyond the quantum critical point is quenched deep into the insulating regime, no indication for ballistic transport is observed. Instead, stable domain walls in the density correlations emerge during the time evolution, consistent with the predictions of the Kibble-Zurek mechanism.Comment: Published version; minor changes, references adde

Persistent supersolid phase of hard-core bosons on the triangular lattice

We study hard-core bosons with unfrustrated hopping ($t$) and nearest neighbour repulsion ($U$) on the triangular lattice. At half-filling, the system undergoes a zero temperature ($T$) quantum phase transition from a superfluid phase at small $U$ to a supersolid at $U_c \approx 4.45$ in units of $2t$. This supersolid phase breaks the lattice translation symmetry in a characteristic $\sqrt{3} \times \sqrt{3}$ pattern, and is remarkably stable--indeed, a smooth extrapolation of our results indicates that the supersolid phase persists for arbitrarily large $U/t$.Comment: 4 pages, 5 figures, two column forma

Comment on "Novel Superfluidity in a Trapped Gas of Fermi Atoms with Repulsive Interaction Loaded on an Optical Lattice"

In a recent letter Machida et al. [Phys. Rev. Lett. 93, 200402 (2004)] concluded that in a trapped gas of fermions with repulsive interactions a superfluid phase appears around the Mott-insulator at the center of the trap. They base their conclusion on a negative binding energy, and a large weight for a singlet formed by particles located at opposite sides of the Mott-insulator. We show here that the observed effects are not related to superfluidity.Comment: Revtex file, 1 page, 1 figure, published versio

The Participation of Members and Non-members in EU Foreign, Security and Defence Policy

The idea of a common policy is, firstly, that it includes all Member States, and, secondly that it should include EU members only. The present contribution aims to assess how the Union has attempted to overcome the tension between the ambition to create a common foreign policy as a clear Union policy, and the need to pragmatically accept the fact that not all Member States are always onboard (and that third states sometimes are). The notion of ‘EU membership’ is thus approached from two different angles: 1. to what extent does EU membership entail the demand that all Member States agree to and implement CFSP decisions; and 2. to what extent is it legally possible for third states to participate in CFSP

An Experimental 11.5 T Nb3Sn LHC Type of Dipole Magnet

As part of the magnet development program for the LHC an experimental 1 m long 11.5 T single aperture Nb3Sn dipole magnet has been designed and is now under construction. The design is focused on full utilisation of the high current density in the powder tube Nb3Sn. A new field optimisation has led to a different winding layout and cable sizes as compared to the reference LHC design. Another important feature of the design is the implementation of a shrink fit ring collar system. An extensive study of the critical current of the Nb3Sn cables as a function of the transverse stress on the cables shows a permanent degradation by the cabling process of about 20%, still leaving a safety margin at the operation field of 11.5 T of 15%. A revised glass/mica glass insulation system is applied which improves the thermal conductivity of the windings as well as the impregnation process considerably. This paper describes various design and production details of the magnet system as well as component test

Valence Bond Solids and Their Quantum Melting in Hard-Core Bosons on the Kagome Lattice

Using large scale quantum Monte Carlo simulations and dual vortex theory we analyze the ground state phase diagram of hard-core bosons on the kagome lattice with nearest neighbor repulsion. In contrast to the case of a triangular lattice, no supersolid emerges for strong interactions. While a uniform superfluid prevails at half-filling, two novel solid phases emerge at densities $\rho=1/3$ and $\rho=2/3$. These solids exhibit an only partial ordering of the bosonic density, allowing for local resonances on a subset of hexagons of the kagome lattice. We provide evidence for a weakly first-order phase transition at the quantum melting point between these solid phases and the superfluid.Comment: 4 pages, 7 figure