In this paper, based on the formulation of O (3) non-linear model, we study two dimensional−flux Hubbard model at half-filling. A quantum non-magnetic insulator is explored near metalinsulator transition that may be a possible candidate of spin liquid state. Such quantum nonmagnetic insulator on square lattice is not induced by frustrations. Instead, it originates from quantum spin fluctuations with relatively small e ective spin-moments. In the strong coupling limit, our results of spin velocity and spin order parameter agree with results obtained from earlier calculations.

We propose a model of coupled local moments and itinerant electrons to describe the iron-based superconductors. Here the spin-density-wave (SDW) order of the itinerant electrons is not driven by Fermi surface nesting but is induced by the Hund's rule coupling with the local moments. The superconducting pairing is of the same origin whose strength can reach strong coupling. The existence of the local moments is supported by various anomalous magnetic properties which are in quantitative agreement with experiments.

In this paper, the quantum phase transition between superfluid state and Mott-insulator state is studied based on an extended Bose-Hubbard model with two-and three-body on-site interactions. By employing the mean-field approximation we find the extension of the insulating 'lobes' and the existence of a fixed point in three dimensional phase space. We investigate the link between experimental parameters and theoretical variables. The possibility to obverse our results through some experimental effects in optically trapped Bose-Einstein Condensates (BEC) is also discussed.

In this paper we study the Macroscopic Quantum Oscillation (MQO) e ect in ferromagnetic single domain magnets with a magnetic field applied along the hard anistropy axis. The level splitting for the ground state, derived with the conventional instanton method, oscillates with the external field and is quenched at some field values. A formula for quantum tunneling at excited levels is also obtained. The existence of topological phase accounts for this kind of oscillation and the corresponding thermodynamical quantities exhibit similar interference e ects which resembles to some extent the electron quantum phase interference induced by gauge potential in the Aharonov- Bohm effect and the-vacuum in Yang-Mills field theory.

In this paper, it is shown how a single stripe and a stripe phase grow from individual holes in the low-doping regime. In an effective low-energy description of the t-J model, i.e., the phase-string model, a hole doped into the spin-ordered phase will induce a dipolar distortion in the background Kou and Weng, Phys. Rev. B 67, 115103 2003. We analyze the hole-dipole configurations with lowest energy under a dipole-dipole interaction and show that these holes tend to arrange themselves into a regular polygon. Such a stable polygon configuration will turn into a stripe as the number of hole dipoles becomes thermodynamically large and eventually a uniform stripe state can be formed, which constitutes an energetically competitive phase at low doping. We also briefly discuss the effect of Zn impurities on individual hole dipoles and stripes.