9,657 research outputs found
Coupling Reduction and the Higgs Mass
Assuming the existence of a functional relation among the Standard Model (SM)
couplings gauge and quartic , we determine the mass of the
Higgs particle. Similar considerations for the top and bottom Yukawa couplings
in the minimal supersymmetric SM lead to the prediction of a narrow window for
, one of the main parameters that determine the light Higgs mass.Comment: 17 pages, 16 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.
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
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
Thermopower in the Coulomb blockade regime for Laughlin quantum dots
Using the conformal field theory partition function of a Coulomb-blockaded
quantum dot, constructed by two quantum point contacts in a Laughlin quantum
Hall bar, we derive the finite-temperature thermodynamic expression for the
thermopower in the linear-response regime. The low-temperature results for the
thermopower are compared to those for the conductance and their capability to
reveal the structure of the single-electron spectrum in the quantum dot is
analyzed.Comment: 11 pages, 3 figures, Proceedings of the 10-th International Workshop
"Lie Theory and Its Applications in Physics", 17-23 June 2013, Varna,
Bulgari
Dihedral Families of Quarks, Leptons and Higgs Bosons
We consider finite groups of small order for family symmetry. It is found
that the binary dihedral group Q_6, along with the assumption that the Higgs
sector is of type II, predicts mass matrix of a nearest neighbor interaction
type for quarks and leptons. We present a supersymmetric model based on Q_6
with spontaneously induced CP phases. The quark sector contains 8 real
parameters with one independent phase to describe the quark masses and their
mixing. Predictions in the |V_{ub}|-bar{eta}, |V_{ub}|-sin 2 beta(phi_1) and
|V_{ub}|-|V_{td}/V_{ts}| planes are given. The lepton sector contains also 9
parameters. A normal as well as an inverted spectrum of neutrino masses is
possible, and we compute V_{e3}. We find that |V_{e3}|^2 > 10^{-4} in the case
of a normal spectrum, and |V_{e3}|^2 >8 10^{-4} in the case of an inverted
spectrum. It is also found that Q_6 symmetry forbids all Baryon number
violating terms of d=4, and the contributions to EDMs from the A terms vanish
in this model.Comment: 27 pages, 8 figure
Response-theory for nonresonant hole burning: Stochastic dynamics
Using non-linear response theory the time signals relevant for nonresonant
spectral hole burning are calculated. The step-reponse function following the
application of a high amplitude ac field (pump) and an intermediate waiting
period is shown to be the sum of the equilibrium integrated response and a
modification due to the preparation via ac irradiation. Both components are
calculated for a class of stochastic dipole reorientation models. The results
indicate that the method can be used for a clearcut distinction of
homogeneously and heterogeneously broadened susceptibilities as they occur in
the relaxation of supercooled liquids or other disordered materials. This is
because only in the heterogeneous case is a frequency selective modification of
the response possible.Comment: revised version, 7 pages, 2 figure
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