9,469 research outputs found
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
Solutions of the Renormalisation Group Equation in Minimal Supersymmetric Standard Model
Renormalisation Group Equation(RGE) for color and top couplings sector of
MSSM has been solved. The mass of the top comes out to be 180.363 10.876
GeV and =. It is conjectured that the masses of the
other 11 fermions and the CKM phase angle can be theoretically
estimated. The results confirm the fact that the quarks and leptons have been
created having equal mass 115 GeV at the MSSM GUT scale GeV
Phase and amplitude of Aharonov-Bohm oscillations in nonlinear three-terminal transport through a double quantum dot
We study three-terminal linear and nonlinear transport through an
Aharonov-Bohm interferometer containing a double quantum dot using the
nonequilibrium Green's function method. Under the condition that one of the
three terminals is a voltage probe, we show that the linear conductance is
symmetric with respect to the magnetic field (phase symmetry). However, in the
nonlinear transport regime, the phase symmetry is broken. Unlike two-terminal
transport, the phase symmetry is broken even in noninteracting electron
systems. Based on the lowest-order nonlinear conductance coefficient with
respect to the source-drain bias voltage, we discuss the direction in which the
phase shifts with the magnetic field. When the higher harmonic components of
the Aharonov-Bohm oscillations are negligible, the phaseshift is a
monotonically increasing function with respect to the source-drain bias
voltage. To observe the Aharonov-Bohm oscillations with higher visibility, we
need strong coupling between the quantum dots and the voltage probe. However,
this leads to dephasing since the voltage probe acts as a B\"{u}ttiker
dephasing probe. The interplay between such antithetic concepts provides a peak
in the visibility of the Aharonov-Bohm oscillations when the coupling between
the quantum dots and the voltage probe changes.Comment: 17 pages, 9 figures, accepted for publication in Physical Review
Zero-th law in structural glasses: an example
We investigate the validity of a zeroth thermodynamic law for non-equilibrium
systems. In order to describe the thermodynamics of the glassy systems, it has
been introduced an extra parameter, the effective temperature which generalizes
the fluctuation-dissipation theorem (FDT) to off-equilibrium systems and
supposedly describes thermal fluctuations around the aging state. In particular
we analyze two coupled systems of harmonic oscillators with Monte Carlo
dynamics. We study in detail two types of dynamics: sequential dynamics, where
the coupling between the subsystems comes only from the Hamiltonian; and
parallel dynamics where there is another source of coupling: the dynamics. We
show how in the first case the effective temperatures of the two interacting
subsystems are different asymptotically due to the smallness of the thermal
conductivity in the aging regime. This explains why, in structural glasses,
different interacting degrees of freedom can stay at different effective
temperatures, and never thermalize.Comment: 10 pages. Contribution to the Proceedings of the ESF SPHINX meeting
`Glassy behaviour of kinetically constrained models' (Barcelona, March 22-25,
2001). To appear in a special issue of J. Phys. Cond. Mat
Weak-field Hall effect and static polarizability of Bloch electrons
A theory of the weak field Hall effect of Bloch electrons based on the
analysis of the forces acting on electrons is presented. It is argued that the
electric current is composed of two contributions, that driven by the electric
field along current flow and the non-dissipative contribution originated in
demagnetization currents. The Hall resistance as a function of the electron
concentration for the tight-binding model of a crystal with square lattice and
body-centered cubic lattice is described in detail. For comparison the effect
of strong magnetic fields is also discussed
Simulation and theory of vibrational phase relaxation in the critical and supercritical nitrogen: Origin of observed anomalies
We present results of extensive computer simulations and theoretical analysis
of vibrational phase relaxation of a nitrogen molecule along the critical
isochore and also along the gas-liquid coexistence. The simulation includes all
the different contributions [atom-atom (AA), vibration-rotation (VR) and
resonant transfer] and their cross-correlations. Following Everitt and Skinner,
we have included the vibrational coordinate () dependence of the interatomic
potential. It is found that the latter makes an important contribution. The
principal important results are: (a) a crossover from a Lorentzian-type to a
Gaussian line shape is observed as the critical point is approached along the
isochore (from above), (b) the root mean square frequency fluctuation shows
nonmonotonic dependence on the temperature along critical isochore, (c) along
the coexistence line and the critical isochore the temperature dependent
linewidth shows a divergence-like -shape behavior, and (d) the value
of the critical exponents along the coexistence and along the isochore are
obtained by fitting. The origin of the anomalous temperature dependence of
linewidth can be traced to simultaneous occurrence of several factors, (i) the
enhancement of negative cross-correlations between AA and VR contributions and
(ii) the large density fluctuations as the critical point (CP) is approached.
The former makes the decay faster so that local density fluctuations are probed
on a femtosecond time scale. A mode coupling theory (MCT) analysis shows the
slow decay of the enhanced density fluctuations near critical point. The MCT
analysis demonstrates that the large enhancement of VR coupling near CP arises
from the non-Gaussian behavior of density fluctuation and this enters through a
nonzero value of the triplet direct correlation function.Comment: 35 pages, 15 figures, revtex4 (preprint form
Exact Analysis of ESR Shift in the Spin-1/2 Heisenberg Antiferromagnetic Chain
A systematic perturbation theory is developed for the ESR shift and is
applied to the spin-1/2 Heisenberg chain. Using the Bethe ansatz technique, we
exactly analyze the resonance shift in the first order of perturbative
expansion with respect to an anisotropic exchange interaction. Exact result for
the whole range of temperature and magnetic field, as well as asymptotic
behavior in the low-temperature limit are presented. The obtained g-shift
strongly depends on magnetic fields at low temperature, showing a significant
deviation from the previous classical result.Comment: 4 pages, 3 figures,to be published in Phys. Rev. Let
Noise Thermal Impedance of a Diffusive Wire
The current noise density S of a conductor in equilibrium, the Johnson noise,
is determined by its temperature T: S=4kTG with G the conductance. The sample's
noise temperature Tn=S/(4kG) generalizes T for a system out of equilibrium. We
introduce the "noise thermal impedance" of a sample as the amplitude of the
oscillation of Tn when heated by an oscillating power. For a macroscopic
sample, it is the usual thermal impedance. We show for a diffusive wire how
this (complex) frequency-dependent quantity gives access to the electron-phonon
interaction time in a long wire and to the diffusion time in a shorter one, and
how its real part may also give access to the electron-electron inelastic time.
These times are not simply accessible from the frequency dependence of S
itself.Comment: 4 pages, 2 figure
Generalized constraints on quantum amplification
We derive quantum constraints on the minimal amount of noise added in linear
amplification involving input or output signals whose component operators do
not necessarily have c-number commutators, as is the case for fermion currents.
This is a generalization of constraints derived for the amplification of
bosonic fields whose components posses c-number commutators.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
Diffusive transport in spin-1 chains at high temperatures
We present a numerical study on the spin and thermal conductivities of the
spin-1 Heisenberg chain in the high temperature limit, in particular of the
Drude weight contribution and frequency dependence. We use the Exact
Diagonalization and the recently developed microcanonical Lanczos method; it
allows us a finite size scaling analysis by the study of significantly larger
lattices. This work, pointing to a diffusive rather than ballistic behavior is
discussed with respect to other recent theoretical and experimental studies
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