Quantum gases in optical lattices

The experimental realization of correlated quantum phases with ultracold gases in optical lattices and their theoretical understanding has witnessed remarkable progress during the last decade. In this review we introduce basic concepts and tools to describe the many-body physics of quantum gases in optical lattices. This includes the derivation of effective lattice Hamiltonians from first principles and an overview of the emerging quantum phases. Additionally, state-of-the-art numerical tools to quantitatively treat bosons or fermions on different lattices are introduced.Comment: 29 pages, 3 figures. This article will be published as Chapter 2 in "Quantum gas experiments - exploring many-body states", edited by P. Torma and K. Sengstock, Imperial College Press, London, to be published 201

Finite temperature dynamical correlations for the dimerized spin-1/2 chain

We use the density matrix renormalization group method (DMRG) to compute the frequency and momentum resolved spin-spin correlation functions of a dimerized spin-1/2 chain under a magnetic field at finite temperature. The spectral features strongly depend on the regime of the magnetic field. For increasing magnetic fields, the transitions from a gapped spin liquid phase to a Tomonaga-Luttinger liquid, and then to a totally polarized phase, can be identified in the spectra. Compared to the zero temperature case, the finite temperature excitations give rise to additional spectral features that we compute numerically and identify analytically as transitions from thermally excited states. We compute quantitatively the broadening of the dispersion of a single spin-flip excitation due to the temperature and find a strong asymmetric broadening. We discuss the consequences of these findings for neutron experiments on dimerized one dimensional quantum chains.Comment: 13 pages, 14 figure

Propagation front of correlations in an interacting Bose gas

We analyze the quench dynamics of a one-dimensional bosonic Mott insulator and focus on the time evolution of density correlations. For these we identify a pronounced propagation front, the velocity of which, once correctly extrapolated at large distances, can serve as a quantitative characteristic of the many-body Hamiltonian. In particular, the velocity allows the weakly interacting regime, which is qualitatively well described by free bosons, to be distinguished from the strongly interacting one, in which pairs of distinct quasiparticles dominate the dynamics. In order to describe the latter case analytically, we introduce a general approximation to solve the Bose-Hubbard Hamiltonian based on the Jordan-Wigner fermionization of auxiliary particles. This approach can also be used to determine the ground-state properties. As a complement to the fermionization approach, we derive explicitly the time-dependent many-body state in the noninteracting limit and compare our results to numerical simulations in the whole range of interactions of the Bose-Hubbard model.Comment: 16 pages, 7 figure

Slow quench dynamics of Mott-insulating regions in a trapped Bose gas

We investigate the dynamics of Mott-insulating regions of a trapped bosonic gas as the interaction strength is changed linearly with time. The bosonic gas considered is loaded into an optical lattice and confined to a parabolic trapping potential. Two situations are addressed: the formation of Mott domains in a superfluid gas as the interaction is increased, and their melting as the interaction strength is lowered. In the first case, depending on the local filling, Mott-insulating barriers can develop and hinder the density and energy transport throughout the system. In the second case, the density and local energy adjust rapidly whereas long range correlations require longer time to settle. For both cases, we consider the time evolution of various observables: the local density and energy, and their respective currents, the local compressibility, the local excess energy, the heat and single particle correlators. The evolution of these observables is obtained using the time-dependent density-matrix renormalization group technique and comparisons with time-evolutions done within the Gutzwiller approximation are provided.Comment: 15 pages, 13 figure

Organisation und Profession Sozialer Arbeit: Kognitive Vermittlungsprozesse:Eine neo-institutionalistische Betrachtung

Das Verhältnis von Profession und Organisation Sozialer Arbeit wird aus einer – innerhalb des sozialpädagogischen Diskurses bis dato kaum beachteten – innovativen Perspektive, dem neo-institutionalistischen Ansatz, in den Blick genommen. Dabei stellt der familiale Wandel, der sich seit Mitte des 20. Jahrhunderts ereignet hat, den Ausgangspunkt dieser Überlegungen dar, welche den Einfluss der professionellen Thematisierung dieser familialen Transformationsprozesse auf die organisationale Praxis fokussieren. Es werden die Grundlagen des Neo-Institutionalismus und die sozialpädagogische Professionstheorie dargelegt, um die organisations- mit der professionstheoretischen Dimension zu verknüpfen. Auf diese Weise erfolgt eine Erweiterung des professionstheoretischen Zugangs auf der Hintergrundfolie des Neo-Institutionalismus. Auf Basis von Expert_inneninterviews wird eine empirische Analyse der theoretischen Ausführungen, beispielhaft an der Sozialpädagogischen Familienhilfe, vorgenommen. <br/

Controllable manipulation and detection of local densities and bipartite entanglement in a quantum gas by a dissipative defect

We study the complex dynamics of a one-dimensional Bose gas subjected to a dissipative local defect which induces one-body atom losses. In experiments these atom losses occur, for example, when a focused electron or light beam or a single trapped ion is brought into contact with a quantum gas. We discuss how within such setups one can measure or manipulate densities locally and specify the excitations that are induced by the defect. In certain situations the defect can be used to generate entanglement in a controlled way despite its dissipative nature. The careful examination of the interplay between hole excitations and the collapse of the wave function due to nondetection of loss is crucial for the understanding of the dynamics we observe.Comment: 4+ pages, 3 figure