582 research outputs found
Scaling approach for the time-dependent Kondo model
We present a new nonperturbative method to deal with the time-dependent
quantum many-body problem, which is an extension of Wegner's flow equations to
time-dependent Hamiltonians. The formalism provides a scaling procedure for the
set of time-dependent interaction constants. We apply these ideas to a Kondo
model with a ferromagnetic exchange coupling switched on over a time scale
. We show that the asymptotic expectation value of the impurity spin
interpolates continuously between its quenched and adiabatic value
Schwinger's Propagator Is Only A Green's Function
Schwinger used an analytic continuation of the effective action to correctly
compute the particle production rate per unit volume for QED in a uniform
electric field. However, if one simply evaluates the one loop expectation value
of the current operator using his propagator, the result is zero! We analyze
this curious fact from the context of a canonical formalism of operators and
states. The explanation turns out to be that Schwinger's propagator is not
actually the expectation value of the time-ordered product of field operators
in the presence of a time-independent state, although it is of course a Green's
function. We compute the true propagator in the presence of a state which is
empty at where is the lightcone
evolution parameter. Our result can be generalized to electric fields which
depend arbitrarily on .Comment: 18 pages, LaTeX 2 epsilo
Large N limit of SO(N) scalar gauge theory
In this paper we study the large limit of SO(N_c) gauge theory coupled
to a real scalar field following ideas of Rajeev. We see that the phase space
of this resulting classical theory is Sp_1(H)/U(H_+) which is the analog of the
Siegel disc in infinite dimensions. The linearized equations of motion give us
a version of the well-known 't Hooft equation of two dimensional QCD.Comment: 16 pages, no figure
An electron jet pump: The Venturi effect of a Fermi liquid
A three-terminal device based on a two-dimensional electron system is
investigated in the regime of non-equilibrium transport. Excited electrons
scatter with the cold Fermi sea and transfer energy and momentum to other
electrons. A geometry analogous to a water jet pump is used to create a jet
pump for electrons. Because of its phenomenological similarity we name the
observed behavior "electronic Venturi effect".Comment: Journal of Applied Physics Special Topic: Plenary and Invited Papers
from the 30th International Conference on the Physics of Semiconductors,
Seoul, Korea, 2010; http://link.aip.org/link/?JAP/109/10241
Small x divergences in the Similarity RG approach to LF QCD
We study small x divergences in boost invariant similarity renormalization
group approach to light-front QCD in a heavy quark-antiquark state. With the
boost invariance maintained, the infrared divergences do not cancel out in the
physical states, contrary to previous studies where boost invariance was
violated by a choice of a renormalization scale. This may be an indication that
the zero mode, or nontrivial light-cone vacuum structure, might be important
for recovering full Lorentz invariance.Comment: 23 pgs, 1 fig. Revised for publication: typos corrected, improved
discussion of regularizatio
Relaxation of hot electrons in a degenerate two-dimensional electron system: transition to one-dimensional scattering
The energy relaxation channels of hot electrons far from thermal equilibrium
in a degenerate two-dimensional electron system are investigated in transport
experiments in a mesoscopic three-terminal device. We observe a transition from
two dimensions at zero magnetic field to quasi--one-dimensional scattering of
the hot electrons in a strong magnetic field. In the two-dimensional case
electron-electron scattering is the dominant relaxation mechanism, while the
emission of optical phonons becomes more and more important as the magnetic
field is increased. The observation of up to 11 optical phonons emitted per hot
electron allows us to determine the onset energy of LO phonons in GaAs at
cryogenic temperatures with a high precision, \eph=36.0\pm0.1\,meV. Numerical
calculations of electron-electron scattering and the emission of optical
phonons underline our interpretation in terms of a transition to
one-dimensional dynamics.Comment: 15 pages, 9 figure
Gravitating defects of codimension-two
Thin gravitating defects with conical singularities in higher codimensions
and with generalized Israel matching conditions are known to be inconsistent
for generic energy-momentum. A way to remove this inconsistency is proposed and
is realized for an axially symmetric gravitating codimension-two defect in six
dimensional Einstein gravity. By varying with respect to the brane embedding
fields, alternative matching conditions are derived, which are generalizations
of the Nambu-Goto equations of motion of the defect, consistent with bulk
gravity. For a maximally symmetric defect the standard picture is recovered.
The four-dimensional perfect fluid cosmology coincides with conventional FRW in
the case of radiation, but for dust it has rho^{4/3} instead of rho. A
four-dimensional black hole solution is presented having the Schwarzschild form
with a short-distance correction r^{-2}.Comment: Minor changes, to appear in Classical and Quantum Gravit
Towards a covariant model for cosmic self-acceleration
An explicitly covariant formulation is presented of a modified DGP scenario
proposed recently [1], to avoid the instability of the self-accelerating
branch. It is based on the introduction of a bulk scalar field with appropriate
non-minimal coupling to the bulk Einstein-Hilbert term. The method is general
and may be applied to other models as well.Comment: 10 pages, no figures; v2: version published in JHE
On two dimensional coupled bosons and fermions
We study complex bosons and fermions coupled through a generalized Yukawa
type coupling in the large-N_c limit following ideas of Rajeev [Int. Jour. Mod.
Phys. A 9 (1994) 5583]. We study a linear approximation to this model. We show
that in this approximation we do not have boson-antiboson and
fermion-antifermion bound states occuring together. There is a possibility of
having only fermion-antifermion bound states. We support this claim by finding
distributional solutions with energies lower than the two mass treshold in the
fermion sector. This also has implications from the point of view of scattering
theory to this model. We discuss some aspects of the scattering above the two
mass treshold of boson pairs and fermion pairs. We also briefly present a
gauged version of the same model and write down the linearized equations of
motion.Comment: 25 pages, no figure
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