4,804 research outputs found
Effective Average Action of Chern-Simons Field Theory
The renormalization of the Chern-Simons parameter is investigated by using an
exact and manifestly gauge invariant evolution equation for the scale-dependent
effective average action.Comment: 14 pages, late
Thermopower of a Kondo-correlated quantum dot
The thermopower of a Kondo-correlated gate-defined quantum dot is studied
using a current heating technique. In the presence of spin correlations the
thermopower shows a clear deviation from the semiclassical Mott relation
between thermopower and conductivity. The strong thermopower signal indicates a
significant asymmetry in the spectral density of states of the Kondo resonance
with respect to the Fermi energies of the reservoirs. The observed behavior can
be explained within the framework of an Anderson-impurity model.
Keywords: Thermoelectric and thermomagnetic effects, Coulomb blockade, single
electron tunneling, Kondo-effect
PACS Numbers: 72.20.Pa, 73.23.HkComment: 4 pages, 4 figures, revised version, changed figure
Gluon Condensation in Nonperturbative Flow Equations
We employ nonperturbative flow equations for an investigation of the
effective action in Yang-Mills theories. We compute the effective action
for constant color magnetic fields and examine Savvidy's
conjecture of an unstable perturbative vacuum. Our results indicate that the
absolute minimum of occurs for B=0. Gluon condensation is described
by a nonvanishing expectation value of the regularized composite operator
which agrees with phenomenological estimates.Comment: 64 pages, late
Quantum Einstein Gravity
We give a pedagogical introduction to the basic ideas and concepts of the
Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum
approach based upon the effective average action, we summarize the state of the
art of the field with a particular focus on the evidence supporting the
existence of the non-trivial renormalization group fixed point at the heart of
the construction. As an application, the multifractal structure of the emerging
space-times is discussed in detail. In particular, we compare the continuum
prediction for their spectral dimension with Monte Carlo data from the Causal
Dynamical Triangulation approach.Comment: 87 pages, 13 figures, review article prepared for the New Journal of
Physics focus issue on Quantum Einstein Gravit
Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
We find considerable evidence supporting the conjecture that four-dimensional
Quantum Einstein Gravity is ``asymptotically safe'' in Weinberg's sense. This
would mean that the theory is likely to be nonperturbatively renormalizable and
thus could be considered a fundamental (rather than merely effective) theory
which is mathematically consistent and predictive down to arbitrarily small
length scales. For a truncated version of the exact flow equation of the
effective average action we establish the existence of a non-Gaussian
renormalization group fixed point which is suitable for the construction of a
nonperturbative infinite cutoff-limit. The truncation ansatz includes the
Einstein-Hilbert action and a higher derivative term.Comment: 18 pages, latex, 3 figure
Observation of the Kondo Effect in a Spin-3/2 Hole Quantum Dot
We report the observation of Kondo physics in a spin- 3/2 hole quantum dot.
The dot is formed close to pinch-off in a hole quantum wire defined in an
undoped AlGaAs/GaAs heterostructure. We clearly observe two distinctive
hallmarks of quantum dot Kondo physics. First, the Zeeman spin-splitting of the
zero-bias peak in the differential conductance is independent of gate voltage.
Second, this splitting is twice as large as the splitting for the lowest
one-dimensional subband. We show that the Zeeman splitting of the zero-bias
peak is highly-anisotropic, and attribute this to the strong spin-orbit
interaction for holes in GaAs.Comment: 4 pages, 4 figure
Spin coherence of holes in GaAs/AlGaAs quantum wells
The carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a
diluted hole gas has been studied by picosecond pump-probe Kerr rotation with
an in-plane magnetic field. For resonant optical excitation of the positively
charged exciton the spin precession shows two types of oscillations. Fast
oscillating electron spin beats decay with the radiative lifetime of the
charged exciton of 50 ps. Long lived spin coherence of the holes with dephasing
times up to 650 ps. The spin dephasing time as well as the in-plane hole g
factor show strong temperature dependence, underlining the importance of hole
localization at cryogenic temperatures.Comment: 5 pages, 4 figures in PostScript forma
Sequential and co-tunneling behavior in the temperature-dependent thermopower of few-electron quantum dots
We have studied the temperature dependent thermopower of gate-defined,
lateral quantum dots in the Coulomb blockade regime using an electron heating
technique. The line shape of the thermopower oscillations depends strongly on
the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from
a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The
latter is attributed to the increasing dominance of cotunneling processes in
the Coulomb blockade regime at low temperatures.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Spin dynamics of electrons and holes in InGaAs/GaAs quantum wells at milliKelvin temperatures
The carrier spin dynamics in a n-doped (In,Ga)As/GaAs quantum well has been
studied by time-resolved Faraday rotation and ellipticity techniques in the
temperature range down to 430 milliKelvin. These techniques give data with very
different spectral dependencies, from which nonetheless consistent information
on the spin dynamics can be obtained, in agreement with theoretical
predictions. The mechanisms of long-lived spin coherence generation are
discussed for the cases of trion and exciton resonant excitation. We
demonstrate that carrier localization leads to a saturation of spin relaxation
times at 45 ns for electrons below 4.5 K and at 2 ns for holes below 2.3 K. The
underlying spin relaxation mechanisms are discussed.Comment: 8 pages, 8 figure
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