Variational Monte Carlo study of ferromagnetism in the two-orbital Hubbard model on a square lattice

To understand effects of orbital degeneracy on magnetism, in particular effects of Hund's rule coupling, we study the two-orbital Hubbard model on a square lattice by a variational Monte Carlo method. As a variational wave function, we consider a Gutzwiller projected wave function for a staggered spin and/or orbital ordered state. We find a ferromagnetic phase with staggered orbital order around quarter-filling, i.e., electron number n=1 per site, and an antiferromagnetic phase without orbital order around half-filling n=2. In addition, we find that another ferromagnetic phase without orbital order realizes in a wide filling region for large Hund's rule coupling. These two ferromagnetic states are metallic except for quarter filling. We show that orbital degeneracy and strong correlation effects stabilize the ferromagnetic states.Comment: 4 pages, 2 figure

Ferromagnetism and orbital order in the two-orbital Hubbard model

We investigate spin and orbital states of the two-orbital Hubbard model on a square lattice by using a variational Monte Carlo method at quarter-filling, i.e., the electron number per site is one. As a variational wave function, we consider a Gutzwiller projected wave function of a mean-field type wave function for a staggered spin and/or orbital ordered state. Then, we evaluate expectation value of energy for the variational wave functions by using the Monte Carlo method and determine the ground state. In the strong Coulomb interaction region, the ground state is the perfect ferromagnetic state with antiferro-orbital (AF-orbital) order. By decreasing the interaction, we find that the disordered state becomes the ground state. Although we have also considered the paramagnetic state with AF-orbital order, i.e., purely orbital ordered state, and partial ferromagnetic states with and without AF-orbital order, they do not become the ground state.Comment: 4 pages, 1 figure, accepted for publication in Journal of Physics: Conference Serie

Fulde-Ferrell-Larkin-Ovchinnikov State in the absence of a Magnetic Field

We propose that in a system with pocket Fermi surfaces, a pairing state with a finite total momentum q_tot like the Fulde-Ferrell-Larkin-Ovchinnikov state can be stabilized even without a magnetic field. When a pair is composed of electrons on a pocket Fermi surface whose center is not located at Gamma point, the pair inevitably has finite q_tot. To investigate this possibility, we consider a two-orbital model on a square lattice that can realize pocket Fermi surfaces and we apply fluctuation exchange approximation. Then, by changing the electron number n per site, we indeed find that such superconducting states with finite q_tot are stabilized when the system has pocket Fermi surfaces.Comment: 4 pages, 5 figure

Finite-size scaling for the S=1/2 Heisenberg Antiferromagnetic Chain

Corrections to the asymptotic correlation function in a Heisenberg spin-1/2 antiferromagnetic spin chain are known to vanish slowly (logarithmically) as a function of the distance r or the chain size L. This leads to significant differences with numerical results. We calculate the sub-leading logarithmic corrections to the finite-size correlation function, using renormalization group improved perturbation theory, and compare the result with numerical data.Comment: 7 pages Revtex, 3 figure

Exact Correlation Amplitude for the S=1/2 Heisenberg Antiferromagnetic Chain

The exact amplitude for the asymptotic correlation function in the S=1/2 Heisenberg antiferromagnetic chain is determined: goes to (-1)^r delta^{ab}(ln r)^{1/2}/[(2 pi)^{3/2}r]. The behaviour of the correlation functions for small xxz anisotropy and the form of finite-size corrections to the correlation function are also analysed.Comment: 8 pages, 3 figures, added reference and discussio

Stochastic Transition between Turbulent Branch and Thermodynamic Branch of an Inhomogeneous Plasma

Transition phenomena between thermodynamic branch and turbulent branch in submarginal turbulent plasma are analyzed with statistical theory. Time-development of turbulent fluctuation is obtained by numerical simulations of Langevin equation which contains submarginal characteristics. Probability density functions and transition rates between two states are analyzed. Transition from turbulent branch to thermodynamic branch occurs in almost entire region between subcritical bifurcation point and linear stability boundary.Comment: 10 pages, 8 figures, to be published in J. Phys. Soc. Jp

Magnetization Relaxation and Collective Spin Excitations in Correlated Double--Exchange Ferromagnets

We study spin relaxation and dynamics of collective spin excitations in correlated double--exchange ferromagnets. For this, we introduce an expansion of the Green's functions equations of motion that treats non--perturbativerly all correlations between a given number of spin and charge excitations and becomes exact within a sub--space of states. Our method treats relaxation beyond Fermi's Golden Rule while recovering previous variational results for the spin--wave dispersion. We find that the momentum dependence of the spin--wave dephasing rate changes qualitatively due to the on--site Coulomb interaction, in a way that resembles experiment, and depends on its interplay with the magnetic exchange interaction and itinerant spin lifetime. We show that the collective spin relaxation and its dependence on the carrier concentration depends sensitively on three--body correlations between a spin excitation and a Fermi sea electron and hole. The above spin dynamics can be controlled via the itinerant carrier population.Comment: 13 pages, 10 figures, published in Phys. Rev.