We demonstrate the first experimental realization of a well‐controlled fermionic spinor gas with interaction‐driven spin oscillations. By properly choosing the initial spin states we can change the effective length of the atomic spin from 1/2 to 9/2. The control over the magnetic field allows to initialize and stop spin dynamics and to select the number of involved levels. We extract the microscopic interaction parameters and find excellent agreement with a two particle model including all spin‐dependent interactions. By tuning the depth of the optical lattice, we investigated the transition from on‐site dominated to quantum manybody spin dynamics, where spins diffuse throughout the lattice producing highly entangled states. For this case, we observe a new form of melting of a band insulator.

arXiv:1203.0948v1

We present a universal method to create a tunable, artificial vector gauge potential for neutral particles trapped in an optical lattice. The necessary Peierls phase of the hopping parameters between neighboring lattice sites is generated by applying a suitable periodic inertial force such that the method does not rely on any internal structure of the particles. We experimentally demonstrate the realization of such artificial potentials, which generate ground state superfluids at arbitrary non-zero quasi-momentum. We furthermore investigate possible implementations of this scheme to create tuneable magnetic fluxes, going towards model systems for strong-field physics.

arXiv:1203.0049 (2012)

We show that multi-orbital and density-induced tunneling have significant impact on the phase diagram of bosonic atoms in optical lattices. Off-site interactions lead to density-induced hopping, so-called bond-charge interactions and can reach the same order of magnitude as conventional tunneling. In addition, interaction-induced higher-band processes also give rise to strongly modified tunneling, on-site, and bond-charge interactions. The derived phase diagram deviates substantially from the single-band Bose-Hubbard predictions indicating strong changes of the SF-MI transition point.

New J. Phys. 14, 033021 (2012)

We perform a detailed experimental study of the band excitations and tunneling properties of ultracold fermions in optical lattices. Employing a novel multi-band spectroscopy for fermionic atoms we can measure the full band structure and tunneling energy with high accuracy. In an attractive Bose-Fermi mixture we observe a significant reduction of the fermionic tunneling energy, which depends on the relative atom numbers. We attribute this to an interaction-induced increase of the lattice depth due to self-trapping of the atoms.

Phys. Rev. Lett. 107, 135303 (2011)

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Article at Physical Review Focus

We present the first realization of a large scale quantum simulator for classical magnetism in a triangular optical lattice. Via a specific modulation of the optical lattice, we can tune the couplings in different directions independently, even from ferromagnetic to antiferromagnetic. Thereby we are able to study a rich phase diagram. This includes observations of magnetic phenomena like Néel order and spin frustration which arises due to the competition between spin-interactions and the geometry of the lattice. We directly observe spontaneous symmetry breaking caused by frustration.

Science 333, 996-999 (2011)

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We observe a novel multi-orbital superfluid phase with a complex order parameter in binary spin mixtures. In this unconventional superfluid, the local phase angle of the complex order parameter is continuously twisted between neighboring lattice sites. The nature of this twisted superfluid quantum phase is an interaction-induced admixture of the p-orbital favored by the graphene-like band structure of the hexagonal optical lattice used in the experiment. We observe a second-order quantum phase transition between the normal superfluid and the twisted superfluid phase which is accompanied by a symmetry breaking in momentum space.

Nature Physics 8, 71–75 (2012)

In solid state materials  hexagonal structures play an extremely important role and lead to novel physics like that of carbon nanotubes or graphene. We have for the first time realized a sample of ultracold atoms in a spin-dependent optical lattice with hexagonal symmetry. We show that combined effects of the lattice and interactions between atoms lead to a forced antiferromagnetic Néel order. We could also demonstrate that the coexistence of two components – one Mott-insulating and the other one superfluid – leads to the formation of a forced supersolid.

Nature Physics 7, 434–440 (2011)

We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting condensate of ultracold atoms in an optical lattice. By the comparison of our experimental data with a spatially resolved, time-dependent dynamic Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime. A systematic shift of the sound and amplitude modes' resonance frequencies due to the finite Bragg beam intensity is observed.

Phys. Rev. Lett. 106, 205303 (2011)

We have investigated head-on collision of dark solitons generated in an elongated Bose-Einstein condensate. No discernable interaction can be recorded, in full agreement with the fundamental theoretical concepts of solitons as mutually transparent quasiparticles. Our soliton generation technique allows for the creation of solitons with different depth; hence, they can be distinguished and their trajectories be followed.
Phys. Rev. Lett. 101, 120406 (2008)

We have observed long-lived dark and dark-bright solitons with lifetimes of up to several seconds as well as their dynamics in highly stable optically trapped 87Rb Bose-Einstein condensates. In particular, our detailed studies of dark and dark-bright soltion oscillations reveal the particle-like nature of these collective excitations for the first time.
Nature Physics 4, 496 - 501 (2008)