5,447 research outputs found
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.
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