We show that multi-orbital and density-induced tunneling have a significant impact on the phase diagram of bosonic atoms in optical lattices. Off-site interactions lead to density-induced hopping, the so-called bond-charge interactions, which can be identified with an effective tunneling potential 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. We derive an extended occupation-dependent Hubbard model with multiorbitally renormalized processes and compute the corresponding phase diagram. It substantially deviates from the single-band Bose-Hubbard model and predicts strong changes of the superfluid-to-Mott-insulator transition. In general, the presented beyond-Hubbard physics plays an essential role in bosonic lattice systems and has an observable influence on experiments with tunable interactions.

Microscopic spin interaction processes are fundamental for global static and dynamical magnetic properties of many-body systems. Quantum gases as pure and well isolated systems offer intriguing possibilities to study basic magnetic processes including non-equilibrium dynamics. Here, we report on the realization of a well-controlled fermionic spinor gas in an optical lattice with tunable effective spin ranging from 1/2 to 9/2. We observe long-lived intrinsic spin oscillations and investigate the transition from two-body to many-body dynamics. The latter results in a spin-interaction driven melting of a band insulator. Via an external magnetic field we control the system's dimensionality and tune the spin oscillations in and out of resonance. Our results open new routes to study quantum magnetism of fermionic particles beyond conventional spin 1/2 systems.Comment: 9 pages, 5 figure

We show that correlated pair tunneling drives a phase transition to a twisted superfluid with a complex order parameter. This unconventional superfluid phase spontaneously breaks the time-reversal symmetry and is characterized by a twisting of the complex phase angle between adjacent lattice sites. We discuss the entire phase diagram of the extended Bose—Hubbard model for a honeycomb optical lattice showing a multitude of quantum phases including twisted superfluids, pair superfluids, supersolids and twisted supersolids. Furthermore, we show that the nearest-neighbor interactions lead to a spontaneous breaking of the inversion symmetry of the lattice and give rise to dimerized density-wave insulators, where particles are delocalized on dimers. For two components, we find twisted superfluid phases with strong correlations between the species already for surprisingly small pair-tunneling amplitudes. Interestingly, this ground state shows an infinite degeneracy ranging continuously from a supersolid to a twisted superfluid.

We study the dynamics of bosonic atoms in a tilted one-dimensional optical lattice and report on the first direct observation of density-induced tunneling. We show that the interaction affects the time evolution of the doublon oscillation via density-induced tunneling and pinpoint its density-and interaction-dependence. The experimental data for different lattice depths are in good agreement with our theoretical model. Furthermore, resonances caused by second-order tunneling processes are studied, where the density-induced tunneling breaks the symmetric behavior for attractive and repulsive interactions predicted by the Hubbard model. PACS numbers: 37.10.Jk, 03.75.Lm, 67.85.Hj, 75.10.Pq The Hubbard model is the primary description for strongly correlated electrons in solids. It takes into account the interaction between the particles at a lattice site and the tunneling between the sites, whereas other interaction processes are neglected. It was pointed out that these additional interactions may have crucial influence in strongly correlated materials such as superconductors or ferromagnets [1][2][3][4]. Of particular importance is the so-called bond-charge interaction that represents a density-induced tunneling of an electron. In solids, this interaction-driven process cannot be studied systematically due to the lack of direct control over the electron density and the interaction strength. Furthermore, the complexity of the investigated materials hinders a direct observation of this interaction effect. Hence, the role of interaction-induced tunneling has remained an open question in condensed matter physics.Ultracold atoms in optical lattices allow the realization of extremely pure lattice systems without defects and phononic excitations. Furthermore, the unique control of both the lattice potential and the interaction strength permits a systematic study of static and dynamic properties. In optical lattices, density-induced tunneling [5][6][7][8][9] is even more pronounced due to the characteristic shape of the Wannier functions [7]. Several indications for density-induced tunneling have been found: It has a strong influence on the superfluid to Mottinsulator transition in bosonic [7,10,11] and multicomponent systems such as Bose-Fermi mixtures of atoms [6,8,[12][13][14][15]. As a tunneling process, it also modifies the effective band structure, which has also been observed in a Bose-Fermi mixture [16]. A direct observation of density-induced tunneling processes was hindered mainly by the fact that the Mott insulator transition depends only on the ratio of on-site interaction and total tunneling and by the averaging over different on-site occupancies in experimental systems.Here, we report on the direct observation of densityinduced tunneling with ultracold atoms in a tilted one dimensional optical lattice. We study the dynamics of a 1D Mott insulator after quenching the tilt energy E between neighboring sites into resonance with the on-site interaction energy U . We show that the resulting resonant particle o...

We show that off-site processes and multi-orbital physics have a crucial impact on the phase diagram of quantum gas mixtures in optical lattices. In particular, we discuss Bose-Fermi mixtures where the intra- and interspecies interactions induce competing density-induced hopping processes, the so-called bond-charge interactions. Furthermore, higher bands strongly influence tunneling and on-site interactions. We apply a multi-orbital interaction-induced dressing of the lowest band which leads to renormalized hopping processes. These corrections give rise to an extended Hubbard model with intrinsically occupation-dependent parameters. The resulting decrease of the tunneling competes with a decrease of the total on-site interaction energy both affecting the critical lattice depth of the superfluid to Mott insulator transition. In contrast to the standard Bose-Fermi-Hubbard model, we predict a large shift of the transition to shallower lattice depths with increasing Bose-Fermi attraction. The applied theoretical model allows an accurate prediction of the modified tunneling amplitudes and the critical lattice depth both recently observed experimentally