Session B4: Invited Session: Cold Atoms on Higher Orbital Bands

Unconventional superfluidity in higher bands of an optical lattice

Atoms trapped in optical lattices have been used successfully to study many-body phenomena. However, the shape that bosonic ground-state wavefunctions can take is limited, apparently compromising the usefulness of this approach. Such limitations, however, do not apply to excited states of bosons. The study of atomic superfluids realized in higher Bloch bands, where orbital degrees of freedom are essential, can bring the world of optical lattices closer to relevant condensed matter systems. I will discuss our observations of long coherence times, chiral superfluid order and topological features in higher bands in a square optical lattice.   

Beyond Standard Fermi Hubbard Models

In my talk I will focus on novel physics and novel quantum phases that are expected in a system of ultracold fermionic atoms with long range interactions, such dipolar ones. I will discuss various terms in the Hubbard model that, normally neglected, have to be included in the theory. These terms involve both lowest band physics, as well as higher bands. I will describe several exemplary effects that new terms may lead to: spontaneous breaking of symmetries, such as time-reversal, smectic-like metal phases, spontaneous formation of exotic lattices and 3D textures.   

Higher orbital physics and artificial gauge fields with ultracold quantum gases

Recently the physics of quantum gases in higher orbitals attracted a lot of attention, theoretically and experimentally. We report on studies of a new type of superfluid described by a complex order parameter, resulting from an interaction-induced hybridization of the two lowest orbitals for a binary spin-mixture. As a main result we observe a quantum phase transition between the normal superfluid and this unconventional superfluid phase, where the local phase angle of the complex order parameter is continuously twisted between neighboring lattice sites [1]. In addition we discuss new experimental work on the creation of artificial gauge potentials for neutral atoms in 1D and 2D lattices, which do not rely on the internal structure of the atoms. Via a time-dependent driving of the optical lattice we have full control over amplitude and phase of the complex valued hopping parameters. In a 2D triangular lattice, we demonstrate the realization of gauge invariant staggered fluxes [2]. Our system consists of an array of tubes filled with bosonic atoms having a well-defined local phase. The phase distribution obtained in presence of large amplitude staggered fluxes -- where frustration plays a key role - obeys two fundamental symmetries, the discrete Ising symmetry (Z2) and a continuous global phase symmetry (U(1)). Via the full control of the staggered gauge fields [3], we are able to break the Ising symmetry on purpose which means lifting the degeneracy of the two possible Ising states, in analogy to a longitudinal homogenous magnetic field in the standard Ising-Spin model. The measurements reveal ``textbook like'' magnetization curves with the well known dependence on both, the external magnetic field and the temperature. We observe a thermally driven phase transition from an ordered Ising (ferromagnetic) to an unordered (paramagnetic) state. Future directions to combine orbital physics and gauge fields will be discussed.\\[4pt] [1] Soltan-Panahi et al., Nature Physics 8, 75 (2012)\\[0pt] [2] Struck et al., Science 333, 996 (2011)\\[0pt] [3] Struck et al., PRL 108, 225304 (2012)   

Orbital physics in one dimensional optical lattices

We explore orbital physics of fermions and bosons in one dimensional optical lattices. In a system of one dimensional $p$-orbital bosons, various phases, including anti-ferro-orbital Mott, anti-ferro-orbital superfluid and para-orbital superfluid, have been found. Signatures of phase transitions, in particular time-reversal symmetry breaking, in time-of-flight image are predicted. A fermionic ladder system composed of $s$ and $p$ orbitals is proposed, and we find a topological state featuring fractional defects. An equivalent of spin-orbit coupling naturally arises, not requiring artificial gauge field, in this quantum orbital ladder when the $s$ and $p$ orbital states are identified as a pseudo-spin 1/2. Extending this ladder system to two dimensions we find a flat-band protected by parity. The flat-band makes it plausible to study strongly correlated physics in this system. We also discuss the connection of this fermionic ladder to frustrated $\pi$ flux models and spin-orbital coupled fermions.   

Generation and exploration of the Spin-Orbit coupled Bose gas

To generate an artificial gauge field with ultracold quantum gas becomes a very hot topic in last few years and will continue to be attractive for ultracold atomic and condensed matter physics in the coming future. Many interesting and important topics such as Fractional Quantum Hall effect, Spin-orbit coupling and Topological insulator are connected to this topic very closely. Here we present our recent experimental progress of the synthesized gauge potential and the spin-orbit coupled Bose-Einstein condensate (BEC) in optical dipole trap. Raman coupling technique and a bias magnetic field is applied to tune the structure of the gauge potential and spin-orbit coupling. Several fundamental properties of spin-orbit coupled BEC is experimentally studied including the properties of collective dipole oscillation, the stability of excited dressed state, the critical temperature of spin-orbit coupled Bose gas and the formation of magnetic order during evaporative cooling. These studies enrich the knowledge of this field and further explorations are also in planning.