6,921 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
Super and Sub-Poissonian photon statistics for single molecule spectroscopy
We investigate the distribution of the number of photons emitted by a single
molecule undergoing a spectral diffusion process and interacting with a
continuous wave laser field. The spectral diffusion is modeled based on a
stochastic approach, in the spirit of the Anderson-Kubo line shape theory.
Using a generating function formalism we solve the generalized optical Bloch
equations, and obtain an exact analytical formula for the line shape and
Mandel's Q parameter. The line shape exhibits well known behaviors, including
motional narrowing when the stochastic modulation is fast, and power
broadening. The Mandel parameter, describing the line shape fluctuations,
exhibits a transition from a Quantum sub-Poissonian behavior in the fast
modulation limit, to a classical super-Poissonian behavior found in the slow
modulation limit. Our result is applicable for weak and strong laser field,
namely for arbitrary Rabi frequency. We show how to choose the Rabi frequency
in such a way that the Quantum sub-Poissonian nature of the emission process
becomes strongest. A lower bound on is found, and simple limiting behaviors
are investigated. A non-trivial behavior is obtained in the intermediate
modulation limit, when the time scales for spectral diffusion and the life time
of the excited state, become similar. A comparison is made between our results,
and previous ones derived based on the semi-classical generalized
Wiener--Khintchine theorem.Comment: 14 Phys. Rev style pages, 10 figure
Generating extremal neutrino mixing angles with Higgs family symmetries
The existence of maximal and minimal mixing angles in the neutrino mixing
matrix motivates the search for extensions to the Standard Model that may
explain these angles. A previous study (C.I.Low and R.R.Volkas,
Phys.Rev.D68,033007(2003)), began a systematic search to find the minimal
extension to the Standard Model that explains these mixing angles. It was found
that in the minimal extensions to the Standard Model which allow neutrino
oscillations, discrete unbroken lepton family symmetries only generate neutrino
mixing matrices that are ruled out by experiment. This paper continues the
search by investigating all models with two or more Higgs doublets, and an
Abelian family symmetry. It is found that discrete Abelian family symmetries
permit, but cannot explain, maximal atmospheric mixing, however these models
can ensure theta_{13}=0.Comment: Minor modifications, references added, typos corrected. LaTeX, 16
page
Microscopic formula for transport coefficients of causal hydrodynamics
The Green-Kubo-Nakano formula should be modified in relativistic
hydrodynamics because of the problem of acausality and the breaking of sum
rules. In this work, we propose a formula to calculate the transport
coefficients of causal hydrodynamics based on the projection operator method.
As concrete examples, we derive the expressions for the diffusion coefficient,
the shear viscosity coefficient, and corresponding relaxation times.Comment: 4 pages, title was modified, final version published in Phys. Rev.
Finite Grand Unified Theories and the Quark Mixing Matrix
In N = 1 super Yang-Mills theories, under certain conditions satisfied by the
spectrum and the Yukawa couplings, the beta functions will vanish to all orders
in perturbation theory. We address the generation of realistic quark mixing
angles and masses in such finite Grand Unified Theories. Working in the context
of finite SUSY SU(5), we present several examples with realistic quark mixing
matrices. Non-Abelian discrete symmetries are found to be important in
satisfying the conditions for finiteness. Our realistic examples are based on
permutation symmetries and the tetrahedral symmetry . These examples
enable us to address questions such as the decay rate of the proton in finite
GUTs.Comment: 16 pages, LaTeX, typos correcte
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
Quadratic short-range order corrections to the mean-field free energy
A method for calculating the short-range order part of the free energy of
order-disorder systems is proposed. The method is based on the apllication of
the cumulant expansion to the exact configurational entropy. Second-order
correlation corrections to the mean-field approximation for the free energy are
calculated for arbitrary thermodynamic phase and type of interactions. The
resulting quadratic approximation for the correlation entropy leads to
substantially better values of transition temperatures for the
nearest-neighbour cubic Ising ferromagnets.Comment: 7 pages, no figures, IOP-style LaTeX, submitted to J. Phys. Condens.
Matter (Letter to the Editor
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