Julian Struck

Quantum Simulation of Frustrated Classical Magnetism in Triangular Optical Lattices

An optical lattice of trapped atoms provides a tractable and tunable setup to study complex magnetic interactions. Magnetism plays a key role in modern technology and stimulates research in several branches of condensed matter physics. Although the theory of classical magnetism is well developed, the demonstration of a widely tunable experimental system has remained an elusive goal. Here, we present the realization of a large-scale simulator for classical magnetism on a triangular lattice by exploiting the particular properties of a quantum system. We use the motional degrees of freedom of atoms trapped in an optical lattice to simulate a large variety of magnetic phases: ferromagnetic, antiferromagnetic, and even frustrated spin configurations. A rich phase diagram is revealed with different types of phase transitions. Our results provide a route to study highly debated phases like spin-liquids as well as the dynamics of quantum phase transitions.

Tunable gauge potential for neutral and spinless particles in driven optical lattices.

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 nonzero quasimomentum. We furthermore investigate possible implementations of this scheme to create tunable magnetic fluxes, going towards model systems for strong-field physics.

Non-Abelian gauge fields and topological insulators in shaken optical lattices

Time-periodic driving like lattice shaking offers a low-demanding method to generate artificial gauge fields in optical lattices. We identify the relevant symmetries that have to be broken by the driving function for that purpose and demonstrate the power of this method by making concrete proposals for its application to two-dimensional lattice systems: We show how to tune frustration and how to create and control band touching points like Dirac cones in the shaken kagome lattice. We propose the realization of a topological and a quantum spin Hall insulator in a shaken spin-dependent hexagonal lattice. We describe how strong artificial magnetic fields can be achieved for example in a square lattice by employing superlattice modulation. Finally, exemplified on a shaken spin-dependent square lattice, we develop a method to create strong non-abelian gauge fields.

Multi-component quantum gases in spin-dependent hexagonal lattices

Ultracold quantum gases in optical lattices have been used to study a wide range of many-body effects. Nearly all experiments so far, however, have been performed in cubic optical lattice structures. Now a ‘honeycomb’ lattice structure has been realized. The approach promises insight into materials with hexagonal crystal symmetries, such as graphene or carbon nanotubes.

Engineering Ising-XY spin-models in a triangular lattice using tunable artificial gauge fields

A quantum gas trapped in an optical lattice of triangular symmetry can now be driven from a paramagnetic to an antiferromagnetic state by a tunable artificial magnetic field.

Quantum phase transition to unconventional multi-orbital superfluidity in optical lattices

The behaviour of molecules and solids is governed by the interplay of electronic orbitals. Superfluidity, in contrast, is typically considered a single-orbital effect. Now, a combined experimental and theoretical study provides evidence for a multi-orbital superfluid, with a complex order parameter, occurring in a binary spin mixture of atoms trapped in an hexagonal optical lattice.

Spin-orbit coupling in periodically driven optical lattices

We propose a method for the emulation of artificial spin orbit coupling in a system of ultracold, neutral atoms trapped in a tight-binding lattice. This scheme does not involve near-resonant laser fields, avoiding the heating processes connected to the spontaneous emission of photons. In our case, the necessary spin dependent tunnel matrix elements are generated by a rapid, spin dependent, periodic force, which can be described in the framework of an effective, time averaged Hamiltonian. An additional radio frequency coupling between the spin states leads to a mixing of the spin bands.

Spinor Bose-Einstein condensates in triangular optical lattices

The physics of quantum degenerate cold gases in optical lattices has rapidly grown to an extremely dynamic field over the past few years. In the experimental realizations, however, mainly cubic symmetries (or their two- and one-dimensional projections) are considered. We have implemented an optical lattice with an underlying triangular symmetry in order to investigate strongly correlated ultra-cold atoms in a novel experimental geometry. Exhibiting an explicit polarization dependence (compare figure 1) the optical lattice realized here should allow for the creation and analysis of thus far unexplored magnetic phases. Experiments attending to the quantum phase transition from a superfluid to a Mott-insulating state in a three- as well as in a two-dimensional system with triangular symmetry have been performed. Similarities as well as differences to the findings obtained in cubic lattices will be discussed and can be attributed to the inherent differences in the crucial lattice parameters such as tunneling energy J and on-site interaction U.