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Büchler Group

Cold atomic gases in optical lattices

Selected Projects

FCS of the staggered magnetization on a disc-shaped subsystem of Ns = 80 sites for repulsive Hubbard interaction at half-filling. 

Full Counting Statistics for Interacting Fermions with Determinantal Quantum Monte Carlo Simulations

We present a method for computing the full probability distribution function of quadratic observables for the Fermi-Hubbard model within the framework of determinantal quantum Monte Carlo. Especially, in cold atoms experiments with single site resolution, such full counting statistics can be obtained from repeated projective measurements. We demonstrate, that the full counting statistics can provide important information on the size of preformed pairs. Furthermore, we compute the full counting statistics of the staggered magnetization in the repulsive Hubbard model at half filling and find excellent agreement with recent experimental results. We show that current experiments are capable of probing the difference between the Hubbard model and the limiting Heisenberg model.

Microscopic derivation of Hubbard parameters for cold atomic gases

We study the exact solution for two atomic particles in an optical lattice interacting via a Feshbach resonance. The analysis includes the influence of all higher bands, as well as the proper renormalization of molecular energy in the closed channel. Using an expansion in Bloch waves, we show that the problem reduces to a simple matrix equation, which can be solved numerically very efficient. This exact solution allows for the precise determination of the parameters in the Hubbard model and the two-particle bound state energy. We identify the regime, where a single band Hubbard model fails to describe the scattering of the atoms as well as the bound states.

Exact bound states for two particles in a three-dimensional optical lattice, and comparison with the prediction based on a simple derivation within the Hubbard model (green line), and the result by replacing each well by a harmonic trapping potential (blue line).