133 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
Weyl semimetallic state in the Rashba-Hubbard model
We investigate the Hubbard model with the Rashba spin-orbit coupling on a
square lattice. The Rashba spin-orbit coupling generates two-dimensional Weyl
points in the band dispersion. In a system with edges along [11] direction,
zero-energy edge states appear, while no edge state exists for a system with
edges along an axis direction. The zero-energy edge states with a certain
momentum along the edges are predominantly in the up-spin state on the right
edge, while they are predominantly in the down-spin state on the left edge.
Thus, the zero-energy edge states are helical. By using a variational Monte
Carlo method for finite Coulomb interaction cases, we find that the Weyl points
can move toward the Fermi level by the correlation effects. We also investigate
the magnetism of the model by the Hartree-Fock approximation and discuss weak
magnetic order in the weak-coupling region.Comment: 9 pages, 11 figure
Multipole ordering in f-electron systems
In order to investigate multipole ordering in f-electron systems from a
microscopic viewpoint, we study the so-called \Gamma_8 models on three kinds of
lattices, simple cubic (sc), bcc, and fcc, based on a j-j coupling scheme with
f-electron hopping integrals through (ff\sigma) bonding. From the \Gamma_8
model, we derive an effective model for each lattice structure by using the
second-order perturbation theory with respect to (ff\sigma). By further
applying mean-field theory to the effective model, we find a \Gamma_{3g}
antiferro-quadrupole transition for the sc lattice, a \Gamma_{2u}
antiferro-octupole transition for the bcc lattice, and a longitudinal triple-q
\Gamma_{5u} octupole transition for the fcc lattice.Comment: 2 pages, 3 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
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