12,898 research outputs found
Electronic structure and resistivity of the double exchange model
The double exchange (DE) model with quantum local spins S is studied; an
equation of motion approach is used and decoupling approximations analogous to
Hubbard's are made. Our approximate one-electron Green function G is exact in
the atomic limit of zero bandwidth for all S and band filling n, and as n->0
reduces to a dynamical coherent potential approximation (CPA) due to Kubo; we
regard our approximation as a many-body generalisation of Kubo's CPA. G is
calculated self-consistently for general S in the paramagnetic state and for
S=1/2 in a state of arbitrary magnetization. The electronic structure is
investigated and four bands per spin are obtained centred on the atomic limit
peaks of the spectral function. A resistivity formula appropriate to the model
is derived from the Kubo formula and the paramagnetic state resistivity rho is
calculated; insulating states are correctly obtained at n=0 and n=1 for strong
Hund coupling. Our prediction for rho is much too small to be consistent with
experiments on manganites so we agree with Millis et al that the bare DE model
is inadequate. We show that the agreement with experiment obtained by Furukawa
is due to his use of an unphysical density of states.Comment: 20 pages, 8 figures, submitted to J. Phys.: Condens. Matte
Anisotropic spin motive force in multi-layered Dirac fermion system, -(BEDT-TTF)I
We investigate the anisotropic spin motive force in
-(BEDT-TTF)I, which is a multi-layered massless Dirac fermion
system under pressure. Assuming the interlayer antiferromagnetic interaction
and the interlayer anisotropic ferromagnetic interaction, we numerically
examine the spin ordered state of the ground state using the steepest descent
method. The anisotropic interaction leads to the anisotropic spin ordered
state. We calculate the spin motive force produced by the anisotropic spin
texture. The result quantitatively agrees with the experiment.Comment: 6 pages, 6 figures, Proceedings of the International Workshop on
Dirac Electrons in Solids 201
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
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
Variations on the Supersymmetric Q6 Model of Flavor
We observe that a recently proposed supersymmetric model with Q6 flavor
symmetry admits a new CP violating ground state. A new sum rule for the quark
mixing parameters emerges, which is found to be consistent with data. Simple
extensions of the model to the neutrino sector suggest an inverted hierarchical
mass spectrum with nearly maximal CP violation (|delta_{MNS}| simeq pi/2).
Besides reducing the number of parameters in the fermion sector, these models
also provide solutions to the SUSY flavor problem and the SUSY CP problem. We
construct a renormalizable scalar potential that leads to the spontaneous
breaking of CP symmetry and the family symmetry.Comment: 22 pages, 7 figure
Effects of correlated disorder on the magnetism of double exchange systems
We study the effects of short-range correlated disorder arising from chemical
dopants or local lattice distortions, on the ferromagnetism of 3d double
exchange systems. For this, we integrate out the carriers and treat the
resulting disordered spin Hamiltonian within local random phase approximation,
whose reliability is shown by direct comparison with Monte Carlo simulations.
We find large scale inhomogeneities in the charge, couplings and spin
densities. Compared with the homogeneous case, we obtain larger Curie
temperatures () and very small spin stiffnesses (). As a result,
large variations of measured in manganites may be explained
by correlated disorder. This work also provides a microscopic model for
Griffiths phases in double exchange systems.Comment: accepted for publication in Phys. Rev. B (rapid comm.
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
Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors
Applying the dynamical coherent potential approximation to a simple model, we
have systematically studied the carrier states in Mn-type diluted
magnetic semiconductors (DMS's). The model calculation was performed for three
typical cases of DMS's: The cases with strong and moderate exchange
interactions in the absence of nonmagnetic potentials, and the case with strong
attractive nonmagnetic potentials in addition to moderate exchange interaction.
When the exchange interaction is sufficiently strong, magnetic impurity bands
split from the host band. Carriers in the magnetic impurity band mainly stay at
magnetic sites, and coupling between the carrier spin and the localized spin is
very strong. The hopping of the carriers among the magnetic sites causes
ferromagnetism through a {\it double-exchange (DE)-like} mechanism. We have
investigated the condition for the DE-like mechanism to operate in DMS's. The
result reveals that the nonmagnetic attractive potential at the magnetic site
assists the formation of the magnetic impurity band and makes the DE-like
mechanism operative by substantially enhancing the effect of the exchange
interaction. Using conventional parameters we have studied the carrier states
in GaMnAs. The result shows that the ferromagnetism is caused
through the DE-like mechanism by the carriers in the bandtail originating from
the impurity states.Comment: 20 pages, 14 figure
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