Barbara Capogrosso-Sansone

Monte Carlo study of the two-dimensional Bose-Hubbard model

One of the most promising applications of ultracold gases in optical lattices is the possibility to use them as quantum emulators of more complex condensed matter systems. We provide benchmark calculations, based on exact quantum Monte Carlo simulations, for the emulator to be tested against. We report results for the ground state phase diagram of the two-dimensional Bose-Hubbard model at unity filling factor. We precisely trace out the critical behavior of the system and resolve the region of small insulating gaps,

Quantum phases of cold polar molecules in 2D optical lattices.

\ensuremath{\Delta}⪡J

Ultracold dipolar gases in optical lattices

. The critical point is found to be

Expansion of a Quantum Gas Released from an Optical Lattice

{(J/U)}_{c}=0.05974(3)

Critical entropies for magnetic ordering in bosonic mixtures on a lattice

, in perfect agreement with the high-order strong-coupling expansion method of Elstner and Monien [Phys. Rev. B 59, 12184 (1999)]. In addition, we present data for the effective mass of particle and hole excitations inside the insulating phase and obtain the critical temperature for the superfluid-normal transition at unity filling factor.

Phase diagram of one-dimensional hard-core bosons with three-body interactions

We study the quantum phases of hard-core bosonic polar molecules on a two-dimensional square lattice interacting via repulsive dipole-dipole interactions. In the limit of small tunneling, we find evidence for a devils staircase, where Mott solids appear at rational fillings of the lattice. For finite tunneling, we establish the existence of extended regions of parameters where the ground state is a supersolid, obtained by doping the solids either with particles or vacancies. We discuss the effects of finite temperature and finite-size confining potentials as relevant to experiments.

The Beliaev technique for a weakly interacting Bose gas

This tutorial is a theoretical work, in which we study the physics of ultra-cold dipolar bosonic gases in optical lattices. Such gases consist of bosonic atoms or molecules that interact via dipolar forces, and that are cooled below the quantum degeneracy temperature, typically in the nK range. When such a degenerate quantum gas is loaded into an optical lattice produced by standing waves of laser light, new kinds of physical phenomena occur. Then, these systems realize extended Hubbard-type models, and can be brought to a strongly correlated regime. The physical properties of such gases, dominated by the long-range, anisotropic dipole?dipole interactions, are discussed using the mean-field approximations and exact quantum Monte Carlo techniques (the worm algorithm).

On-site number statistics of ultracold lattice bosons

We analyze the interference pattern produced by ultracold atoms released from an optical lattice, commonly interpreted as the momentum distributions of the trapped quantum gas. We show that for finite times of flight the resulting density distribution can, however, be significantly altered, similar to a near-field diffraction regime in optics. We illustrate our findings with a simple model and realistic quantum Monte Carlo simulations for bosonic atoms and compare the latter to experiments.

First-Order Phase Transitions in Optical Lattices with Tunable Three-Body Onsite Interaction

We perform a numeric study (Worm algorithm Monte Carlo simulations) of ultracold two-component bosons in two- and three-dimensional optical lattices. At strong enough interactions and low enough temperatures the system features magnetic ordering. We compute critical temperatures and entropies for the disappearance of the Ising antiferromagnetic and the xy-ferromagnetic order and find that the largest possible entropies per particle are {approx} 0.5k{sub B}. We also estimate (optimistically) the experimental hold times required to reach equilibrium magnetic states to be on a scale of seconds. Low critical entropies and long hold times render the experimental observations of magnetic phases challenging and call for increased control over heating sources.

Demixing Effects in Mixtures of Two Bosonic Species

B. Capogrosso-Sansone, S. Wessel, H.P. Büchler, P. Zoller, and G. Pupillo Department of Physics, University of Massachusetts, Amherst, MA 01003, US Institute for Theoretical Physics III, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany Institute for Theoretical Physics, University of Innsbruck, 6020 Innsbruck, Austria and Institute for Quantum Optics and Quantum Information, 6020 Innsbruck, Austria (Dated: December 15, 2008)