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Register a new userInteraction-induced chiral p_x pm i p_y superfluid order of bosons in an optical lattice
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The study of superconductivity with unconventional order is complicated in condensed matter systems by their extensive complexity. Optical lattices with their exceptional precision and control allow one to emulate superfluidity avoiding many of the complications of condensed matter. A promising approach to realize unconventional superfluid order is to employ orbital degrees of freedom in higher Bloch bands. In recent work, indications were found that bosons condensed in the second band of an optical chequerboard lattice might exhibit p_x pm i p_y order. Here we present experiments, which provide strong evidence for the emergence of p_x pm i p_y order driven by the interaction in the local p-orbitals. We compare our observations with a multi-band Hubbard model and find excellent quantitative agreement.
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The chiral optical absorption by a single vortex in a p_x pm i p_y-wave superconductor is studied theoretically. The p_x pm i p_y-wave state was recently suggested as the symmetry of the order parameter of Sr_2 Ru O_4 superconductor. Due to the violation of time reversal symmetry, there are two types of vortices whose winding orientation is the same or opposite to the angular momentum of the Cooper pair. In a real material domains with p_x pm i p_y-wave states are expected. However, optical absorption of circular polarized light depends only on the winding of the vortex and has a low energy absorption peak which results in dichroism. Dichroism occurs if superconductivity is realized on a single Fermi surface sheet. However, in the case of several Fermi surface sheets dichroism may disappear, if the both types of carriers are present, electron-like and hole-like. Therefore chiral optical absorption is a possible experiment to detect the orbital dependent superconductivity which was suggested as the superconducting state of Sr_2 Ru O_4.
We show that the dynamics of cold bosonic atoms in a two-dimensional square optical lattice produced by a bichromatic light-shift potential is described by a Bose-Hubbard model with an additional effective staggered magnetic field. In addition to the known uniform superfluid and Mott insulating phases, the zero-temperature phase diagram exhibits a novel kind of finite-momentum superfluid phase, characterized by a quantized staggered rotational flux. An extension for fermionic atoms leads to an anisotropic Dirac spectrum, which is relevant to graphene and high-$T_c$ superconductors.
Geometric frustration of particle motion in a kagome lattice causes the single-particle band structure to have a flat s-orbital band. We probe this band structure by exciting a Bose-Einstein condensate into excited Bloch states of an optical kagome lattice, and then measuring the group velocity through the atomic momentum distribution. We find that interactions renormalize the band structure of the kagome lattice, greatly increasing the dispersion of the third band that, according to non-interacting band theory, should be nearly non-dispersing. Measurements at various lattice depths and gas densities agree quantitatively with predictions of the lattice Gross-Pitaevskii equation, indicating that the observed distortion of band structure is caused by the disortion of the overall lattice potential away from the kagome geometry by interactions.
The electronic states near a surface or a domain wall in the p_x pm i p_y -wave superconductor are studied. This state has been recently suggested as the superconducting state of Sr_2 Ru O_4. The p_x pm i p_y-wave paring state breaks the time reversal symmetry and induces a magnetic field. The obtained temperature dependence of the magnetic field is consistent with the observed mu SR data.
In the previous papers, we studied the bosonic t-J mode and derived an effective field theory, which is a kind of quantum XY model. The bosonic t-J model is expected to be realized by experiments of two-component cold atoms in an optical lattice. In this paper, we consider a similar XY model that describes phase diagram of the t-J model with a mass difference. Phase diagram and critical behavior of the quantum XY model are clarified by means of the Monte-Carlo simulations. Effective field theory that describes the phase structure and low-energy excitations of the quantum XY model is derived. Nambu-Goldstone bosons and the Higgs mode are studied by using the effective field theory and interesting findings are obtained for the system with multiple order, i.e., Bose-Einstein condensations and pseudo-spin symmetry. We also investigate physical properties of the quantum XY model in an effective magnetic field that is realized by rotating the optical lattice, etc. We show that low-energy states of the system strongly depend on the strength of the magnetic field. For some specific strength of the magnetic field, vortex lattice forms and the correlation function of the bosons exhibits solid like behavior, which is a kind of Bose-Einstein condensation.
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