We perform a numeric study (Worm algorithm Monte Carlo simulations) of ultracold twocomponent bosons in two-dimensional optical lattices. We study how the Mott insulator to superfluid transition is affected by the presence of a second superfluid bosonic species. We find that, at fixed interspecies interaction, the upper and lower boundaries of the Mott lobe are differently modified. The lower boundary is strongly renormalized even for relatively low filling factor of the second component and moderate (interspecies) interaction. The upper boundary, instead, is affected only for large enough filling of the second component. Whereas boundaries are renormalized we find evidence of polaron-like excitations. Our results are of interest for current experimental setups.PACS numbers: 67.85. Hj, 67.85.Fg, In the last decade, a considerable amount of theoretical and experimental research has been devoted to the objective of using ultracold lattice bosons and fermions to address many outstanding condensed matter problems via Hamiltonian modeling. Single species (Bose) Hubbard models, first introduced for fermions to describe electrons in solids and considered to be the minimal model for high temperature superconductivity, can be experimentally realized with ultracold atoms in optical lattices [1] and have been extensively studied [2,3]. If a second component is introduced, new fascinating phenomena and exotic quantum phases which cannot be accessed with single species atomic gases, become available. Prominent examples include the possibility of realizing quantum magnetic phases [4][5][6], engineering offsite interactions [6][7][8] which would likely lead to supersolid states [9,10], studying disorder/impurities effects [11,12] In the present work we consider a homogeneous system of two-component bosons in a square lattice with repulsive interspecies interaction. We study how the MI-SF transition of the majority component is affected by the presence of a minority superfluid component upon varying the density of the latter and the interspecies interaction. This system can be realized by loading optical lattices with two different atomic species [16,17], or the same atomic species in two different internal energy states [18]. Interspecies interaction strength U ab can be tuned either via Feshbach resonance or by changing the Wannier functions overlap (in the presence of statedependent lattices). Intraspecies interactions U a and U b can also be tuned via Feshabch resonance. If the temperature is low enough, the system is accurately described by the two-component Bose-Hubbard Hamiltonianwhere n The phase diagram of model 1 is very reach. It includes several stable phases which vary from double MI's [24], two independent SF's, and a mixture of one MI and one SF, to less trivial phases like supercounterflow and checkerboard solid [4,5]. In the following we present the first accurate results, based on path integral Monte Carlo simulations by the Worm algorithm [25], for the MI-SF phase diagram of the majority component A. We study...
Motivated by recent experiments on two-component systems, we investigate the ground-state phase diagram of a mixture of two bosonic species by means of path-integral quantum Monte Carlo by the two-worm algorithm. The mixture is trapped in a square lattice at different filling conditions. Various quantum phases are stabilized depending on the interplay between intra-and inter-species interactions and on the filling factors. We show that the ground-state phase diagram at half-filling features a demixed superfluid phase and demixed Mott-Insulator phase when the inter-species interaction becomes greater than the intra-species repulsion, and a double-superfluid phase or a supercounterflow otherwise. We show that demixing, characterized by spatial separation of the two species, can be detected experimentally through the effects of anisotropy revealed by time-of-flight images. We also study how demixing effects depend on the filling factor of the two components. Finally, we found that super-counterflow phase is preserved in the presence of unbalanced populations. arXiv:1507.08877v2 [cond-mat.other] 4 Nov 2015
We present accurate results based on Quantum Monte Carlo simulations of two-component bosonic systems on a square lattice and in the presence of an external harmonic confinement. Starting from hopping parameters and interaction strengths which stabilize the Ising antiferromagnetic phase in the homogeneous case and at half integer filling factor, we study how the presence of the harmonic confinement challenge the realization of such phase. We consider realistic trapping frequencies and number of particles, and establish under which conditions, i.e. total number of particles and population imbalance, the antiferromagnetic phase can be observed in the trap.PACS numbers: 67.60. Bc, 67.85.Hj, 67.85.Fg In recent years great attention has been devoted to mixtures of ultracold atomic gases in optical lattices [1][2][3][4][5][6][7][8]. One of the most remarkable features of such systems is the possibility of stabilizing quantum magnetic phases [9,10]. Though interesting and fundamental on their own, better understanding of magnetic phases is further motivated by applications to quantum-information processing and their relevance to unconventional superconductivity such as high-temperature and heavy-fermion superconductivity. It has been proposed [11] (and recently experimentally investigated [12]) that antiferromagnetic excitations may be responsible for the pairing mechanism in unconventional superconductors. The experimental realization of magnetic Hamiltonians with ultracold atoms would open up the way for direct control over interactions, geometry, and frustration. Recently an Ising antiferromagnetic phase has been realized with singlecomponent ultracold atoms trapped in a one-dimensional optical lattice tilted by a magnetic field [13] (Ising density wave order could also be realized at lower energy scales in specific two-dimensional tilted geometries [14]).Magnetic phases with mixtures of ultracold atoms can be realized at low temperatures, strong interactions and specific filling factors. Experimentally, though, the presence of the external confinement will challenge their realization. Previous theoretical studies, focused on Bose-Bose mixtures, reported accurate results for the groundstate phase boundaries of the Ising antiferromagnetic and the xy-ferromagnetic phases at half-filling factor [15], and for the critical temperature needed for their realization [16]. The temperature scale, determined by spinexchange interactions, results challengingly low (∼ few hundreds pK). Nonetheless, recent experimental efforts have been devoted towards the development of new techniques of refrigeration and thermometry [7,8].The next crucial question to theoretically address is studying the effect of an external trapping potential. In the present work we focus on the Ising antiferromagnetic (AF) phase realized by Bose-Bose mixtures in the strongly interacting regime and at half-filling factor, and study under which conditions it survives the presence of a harmonic confinement. In the local-density approximation, the latter provi...
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