965 research outputs found
Thermal nature of de Sitter spacetime and spontaneous excitation of atoms
We consider, in de Sitter spacetime, both freely falling and static two-level
atoms in interaction with a conformally coupled massless scalar field in the de
Sitter-invariant vacuum, and separately calculate the contributions of vacuum
fluctuations and radiation reaction to the atom's spontaneous excitation rate.
We find that spontaneous excitations occur even for the freely falling atom as
if there is a thermal bath of radiation at the Gibbons-Hawking temperature and
we thus recover, in a different physical context, the results of Gibbons and
Hawking that reveals the thermal nature of de Sitter spacetime. Similarly, for
the case of the static atom, our results show that the atom also perceives a
thermal bath which now arises as a result of the intrinsic thermal nature of de
Sitter spacetime and the Unruh effect associated with the inherent acceleration
of the atom.Comment: 11 page
Group expansions for impurities in superconductors
A new method is proposed for practical calculation of the effective
interaction between impurity scatterers in superconductors, based on algebraic
properties of related Nambu matrices for Green functions. In particular, we
show that the density of states within the s-wave gap can have a non-zero
contribution (impossible either in Born and in T-matrix approximation) from
non-magnetic impurities with concentration , beginning from order.Comment: 5 pages, 1 figur
Effects of spin-elastic interactions in frustrated Heisenberg antiferromagnets
The Heisenberg antiferromagnet on a compressible triangular lattice in the
spin- wave approximation is considered. It is shown that the interaction
between quantum fluctuations and elastic degrees of freedom stabilizes the low
symmetric L-phase with a collinear Neel magnetic ordering. Multi-stability in
the dependence of the on-site magnetization on an unaxial pressure is found.Comment: Revtex, 4 pages, 2 eps figure
Pair production of neutralinos via gluon-gluon collisions
The production of a neutralino pair via gluon-gluon fusion is studied in the
minimal supersymmetric model(MSSM) at proton-proton colliders. The numerical
analysis of their production rates are carried out in the mSUGRA scenario. The
results show that this cross section may reach about 80 femto barn for
pair production and 23 femto barn
for pair production with suitable
input parameters at the future LHC collider. It shows that this loop mediated
process can be competitive with the quark-antiquark annihilation process at the
LHC.Comment: LaTex file, l4 pages, 5 EPS figure
Effect of Pivot Point on Aerodynamic Force and Vortical Structure of Pitching Flat Plate Wings
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106512/1/AIAA2013-792.pd
Entanglement between a qubit and the environment in the spin-boson model
The quantitative description of the quantum entanglement between a qubit and
its environment is considered. Specifically, for the ground state of the
spin-boson model, the entropy of entanglement of the spin is calculated as a
function of , the strength of the ohmic coupling to the environment,
and , the level asymmetry. This is done by a numerical
renormalization group treatment of the related anisotropic Kondo model. For
, the entanglement increases monotonically with , until it
becomes maximal for . For fixed , the entanglement
is a maximum as a function of for a value, .Comment: 4 pages, 3 figures. Shortened version restricted to groundstate
entanglemen
Singularities in the Fermi liquid description of a partially filled Landau level and the energy gaps of fractional quantum Hall states
We consider a two dimensional electron system in an external magnetic field
at and near an even denominator Landau level filling fraction. Using a
fermionic Chern--Simons approach we study the description of the system's low
energy excitations within an extension of Landau's Fermi liquid theory. We
calculate perturbatively the effective mass and the quasi--particle interaction
function characterizing this description. We find that at an even denominator
filling fraction the fermion's effective mass diverges logarithmically at the
Fermi level, and argue that this divergence allows for an {\it exact}
calculation of the energy gaps of the fractional quantized Hall states
asymptotically approaching these filling fractions. We find that the
quasi--particle interaction function approaches a delta function. This singular
behavior leads to a cancelation of the diverging effective mass from the long
wavelength low frequency linear response functions at even denominator filling
fractions.Comment: 46 pages, RevTeX, 5 figures included in a uuencoded postscript file.
Minor revisions relative to the original version. The paper will be published
in the Physical Review B, and can be retrieved from the World Wide Web, in
http://cmtw.harvard.edu/~ster
Higgs Signal for h to aa at Hadron Colliders
We assess the prospect of observing a neutral Higgs boson at hadron colliders
in its decay to two spin-zero states, a, for a Higgs mass of 90-130 GeV, when
produced in association with a W or Z boson. Such a decay is allowed in
extensions of the MSSM with CP-violating interactions and in the NMSSM, and can
dominate Higgs boson final states, thereby evading the LEP constraints on
standard Higgs boson production. The light spin-zero state decays primarily via
a to bb and tau+tau-, so this signal channel retains features distinct from the
main backgrounds. Our study shows that at the Tevatron, there may be potential
to observe a few events in the bb tau+tau- or bbbb channels with relatively
small background, although this observation would be statistically limited. At
the LHC, the background problem is more severe, but with cross sections and
integrated luminosities orders of magnitude larger than at the Tevatron, the
observation of a Higgs boson in this decay mode would be possible. The channel
h to aa to bbbb would provide a large statistical significance, with a
signal-to-background ratio on the order of 1:2. In these searches, the main
challenge would be to retain the adequate tagging efficiency of b's and tau's
in the low p_T region.Comment: Version to be published in JHEP. 20 pages, 5 figure
Strain dependence of the acoustic properties of amorphous metals below 1K: Evidence for the interaction between tunneling states
We have conducted a thorough study of the acoustic properties between 10^-4
and 1 Kelvin for the amorphous metal Zr_x Cu_1-x (x=0.3 and x=0.4), by
measuring the relative change of sound velocity dv/v and internal friction Q^-1
as a function of temperature and also of the applied strain, in both
superconducting and normal state. We have found that when plotted versus the
ratio of strain energy to thermal energy, all measurements display the same
behavior: a crossover from a linear regime of ``independent'' tunneling systems
at very low strains and/or high enough temperatures to a nonlinear regime where
dv/v and Q^-1 depend on applied strain and the tunneling systems cannot be
considered as independent.Comment: 4 pages, 4 figures (submitted to PRL
Qubit Disentanglement and Decoherence via Dephasing
We consider whether quantum coherence in the form of mutual entanglement
between a pair of qubits is susceptible to decay that may be more rapid than
the decay of the coherence of either qubit individually. An instance of
potential importance for solid state quantum computing arises if embedded
qubits (spins, quantum dots, Cooper pair boxes, etc.) are exposed to global and
local noise at the same time. Here we allow separate phase-noisy channels to
affect local and non-local measures of system coherence. We find that the time
for decay of the qubit entanglement can be significantly shorter than the time
for local dephasing of the individual qubits.Comment: REVTeX, 9 pages, 1 figure, v2 with minor changes, reference adde
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