J. Kronjäger

Ultracold quantum gases in triangular optical lattices

Over recent years, exciting developments in the field of ultracold atoms confined in optical lattices have led to numerous theoretical proposals devoted to the quantum simulation of problems e.g. known from condensed matter physics. Many of those ideas demand experimental environments with non-cubic lattice geometries. In this paper, we report on the implementation of a versatile three-beam lattice allowing for the generation of triangular as well as hexagonal optical lattices. As an important step, the superfluid–Mott insulator (SF–MI) quantum phase transition has been observed and investigated in detail in this lattice geometry for the first time. In addition to this, we study the physics of spinor Bose–Einstein condensates (BEC) in the presence of the triangular optical lattice potential, especially spin changing dynamics across the SF–MI transition. Our results suggest that, below the SF–MI phase transition, a well-established mean-field model describes the observed data when renormalizing the spin-dependent interaction. Interestingly, this opens up new perspectives for a lattice-driven tuning of a spin dynamics resonance occurring through the interplay of the quadratic Zeeman effect and spin-dependent interaction. Finally, we discuss further lattice configurations that can be realized with our setup.

Magnetically tuned spin dynamics resonance.

We present the experimental observation of a magnetically tuned resonance phenomenon in the spin mixing dynamics of ultracold atomic gases. In particular, we study the magnetic field dependence of spin conversion in F=2 (87)Rb spinor condensates in the crossover from interaction dominated to quadratic Zeeman dominated dynamics. We discuss the observations in the framework of spin dynamics as well as matter wave four wave mixing. Furthermore, we show that the validity range of the single mode approximation for spin dynamics is significantly extended at high magnetic field.

Spontaneous Pattern Formation in an Antiferromagnetic Quantum Gas

In this Letter we report on the spontaneous formation of surprisingly regular periodic magnetic patterns in an antiferromagnetic Bose-Einstein condensate (BEC). The structures evolve within a quasi-one-dimensional BEC of 87Rb atoms on length scales of a millimeter with typical periodicities of 20…30  μm, given by the spin healing length. We observe two sets of characteristic patterns which can be controlled by an external magnetic field. We identify these patterns as linearly unstable modes within a mean-field approach and calculate their mode structure as well as time and energy scales, which we find to be in good agreement with observations. These investigations open new prospects for controlled studies of symmetry breaking and complex quantum magnetism in bulk BEC.

Measurement of a mixed-spin-channel Feshbach resonance in {sup 87}Rb

We report on the observation of a mixed-spin-channel Feshbach resonance at the low magnetic field value of 9.09{+-}0.01 G for a mixture of |2,-1> and |1,+1> states in {sup 87}Rb. This mixture is important for applications of multicomponent Bose-Einstein condensates of {sup 87}Rb, e.g., in spin mixture physics and for quantum entanglement. Values for position, height, and width of the resonance are reported and compared to a recent theoretical calculation of this resonance.

Collective magnetism in arrays of spinor Bose-Einstein condensates

We elucidate dipolar magnetic interaction effects in spinor condensates at a low-background field. In particular, we show that arrays of ferromagnetic spinor Bose-Einstein condensates show a rich phase structure that is strongly influenced by the competition of shape anisotropy and higher-order magnetostatic contributions both depending on the condensates form.

Towards a Bose-Fermi mixture experiment in a 2D optical lattice with high optical resolution

This experiment is a novel setup for a very versatile two species Bose-Fermi mixture quantum gas experiment for 87Rb and 40K in optical potentials. Development of improved new technologies are emphasised in this project and cover self-made vacuum chambers realised with different techniques, a miniaturised, ultra stable laser system and a compact oil-cooled coil design for magnetic trapping and transport of ultracold atom clouds.