73,752 research outputs found
Acoustically driven storage of light in a quantum well
The strong piezoelectric fields accompanying a surface acoustic wave on a
semiconductor quantum well structure are employed to dissociate optically
generated excitons and efficiently trap the created electron hole pairs in the
moving lateral potential superlattice of the sound wave. The resulting spatial
separation of the photogenerated ambipolar charges leads to an increase of the
radiative lifetime by orders of magnitude as compared to the unperturbed
excitons. External and deliberate screening of the lateral piezoelectric fields
triggers radiative recombination after very long storage times at a remote
location on the sample.Comment: 4 PostScript figures included, Physical Review Letters, in pres
Generalized Numerical Renormalization Group for Dynamical Quantities
In this paper we introduce a new approach for calculating dynamical
properties within the numerical renormalization group. It is demonstrated that
the method previously used fails for the Anderson impurity in a magnetic field
due to the absence of energy scale separation. The problem is solved by
evaluating the Green function with respect to the reduced density matrix of the
full system, leading to accurate spectra in agreement with the static
magnetization. The new procedure (denoted as DM-NRG) provides a unifying
framework for calculating dynamics at any temperature and represents the
correct extension of Wilson's original thermodynamic calculation.Comment: 4 pages RevTeX, 6 eps figures include
Numerical Renormalization Group Analysis of Interacting Quantum Dots
Wilson's Numerical Renormalization Group (NRG) is so far the only
nonperturbative technique that can reliably access low-energy properties of
quantum impurity systems. We present a recent extension of the method, the
DM-NRG, which yields highly accurate results for dynamical quantities at
arbitrary frequencies and temperatures. As an application, we determine the
spectrum of a quantum dot in an external magnetic field. Furthermore, we
discuss magnetic impurities with orbital degeneracy, which have been inferred
in recent experiments on quantum dots in an Aharonov-Bohm geometry. It is
demonstrated that for spinless electrons, interference between neighbouring
levels sets the low-energy scale of the system. Switching on an external field
leads to a remarkable crossover into a regime dominated by orbital Kondo
screening. We predict that the broadening-induced level splitting should be
clearly visible in measurements of the optical absorption power. A more general
model including the electron spin is studied within an extended two-band NRG
procedure. We observe competition between interference and Kondo screening,
similar to the situation in two-impurity models (RKKY).Comment: Invited talk at the DPG spring meeting 2001, to appear in Advances in
Solid State Physics 4
Polaronic excitations in CMR manganite films
In the colossal magnetoresistance manganites polarons have been proposed as
the charge carrier state which localizes across the metal-insulator transition.
The character of the polarons is still under debate. We present an assessment
of measurements which identify polarons in the metallic state of
La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We
focus on optical spectroscopy in these films which displays a pronounced
resonance in the mid-infrared. The temperature dependent resonance has been
previously assigned to polaron excitations. These polaronic resonances are
qualitatively distinct in LSMO and LCMO and we discuss large and small polaron
scenarios which have been proposed so far. There is evidence for a large
polaron excitation in LSMO and small polarons in LCMO. These scenarios are
examined with respect to further experimental probes, specifically charge
carrier mobility (Hall-effect measurements) and high-temperature
dc-resistivity.Comment: 16 pages, 10 figure
Unusual Non-Fermi Liquid Behavior of CeLaNiGe Analyzed in a Single Impurity Anderson Model with Crystal Field Effects
CeNiGe exhibits unusual non-Fermi liquid behavior with the largest
ever recorded value of the electronic specific heat
JKmol without showing any evidence of magnetic order. Specific
heat measurements show that the logarithmic increase of the Sommerfeld
coefficient flattens off below 200 mK. In marked contrast, the local
susceptibility levels off well above 200 mK and already becomes
constant below 1 K. Furthermore, the entropy reaches 2ln2 below 20 K
corresponding to a four level system. An analysis of and was
performed in terms of an single impurity Anderson model with
additional crystal electric field (CEF) splitting. Numerical renormalization
group calculations point to a possible consistent description of the different
low temperature scales in and stemming from the
interplay of Kondo effect and crystal field splitting.Comment: 2 pages, 2 figure
The Orbital Order Parameter in La0.95Sr0.05MnO3 probed by Electron Spin Resonance
The temperature dependence of the electron-spin resonance linewidth in
La0.95Sr0.05MnO3 has been determined and analyzed in the paramagnetic regime
across the orbital ordering transition. From the temperature dependence and the
anisotropy of linewidth and -value the orbital order can be unambiguously
determined via the mixing angle of the wave functions of the -doublet. The linewidth shows a similar evolution with temperature as
resonant x-ray scattering results
Dephasing in Rashba spin precession along mutlichannel quantum wires and nanotubes
Coherent Rashba spin precession along interacting multi-mode quantum channels
is investigated, revisiting the theory of coupled Tomonaga-Luttinger liquids.
We identify susceptibilities as the key-parameters to govern exponents and
Rashba precession lengths. In semiconducting quantum wires spins of different
transport channels are found to {\em dephase} in their respective precession
angles with respect to one another, as a result of the interaction. This could
explain the experimental difficulty to realize the Datta Das transistor. In
single walled carbon nanotubes, on the other hand, interactions are predicted
to suppress dephasing between the two flavor modes at small doping.Comment: 6 pages, figures include
Simultaneous current-, force- and work function measurement with atomic resolution
The local work function of a surface determines the spatial decay of the
charge density at the Fermi level normal to the surface. Here, we present a
method that enables simultaneous measurements of local work function and
tip-sample forces. A combined dynamic scanning tunneling microscope and atomic
force microscope is used to measure the tunneling current between an
oscillating tip and the sample in real time as a function of the cantilever's
deflection. Atomically resolved work function measurements on a silicon
(111)-() surface are presented and related to concurrently recorded
tunneling current- and force- measurements.Comment: 8 pages, 4 figures, submitted to Applied Physics Letter
Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate
Dielectric spectroscopy covering more than 18 decades of frequency has been
performed on propylene carbonate in its liquid and supercooled-liquid state.
Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric
response was investigated up to frequencies well into the microscopic regime.
We discuss the alpha-process whose characteristic timescale is observed over 14
decades of frequency and the excess wing showing up at frequencies some three
decades above the peak frequency. Special attention is given to the
high-frequency response of the dielectric loss in the crossover regime between
alpha-peak and boson-peak. Similar to our previous results in other glass
forming materials we find evidence for additional processes in the crossover
regime. However, significant differences concerning the spectral form at high
frequencies are found. We compare our results to the susceptibilities obtained
from light scattering and to the predictions of various models of the glass
transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
Driven Tunneling: Chaos and Decoherence
Chaotic tunneling in a driven double-well system is investigated in absence
as well as in the presence of dissipation. As the constitutive mechanism of
chaos-assisted tunneling, we focus on the dynamics in the vicinity of
three-level crossings in the quasienergy spectrum. The coherent quantum
dynamics near the crossing is described satisfactorily by a three-state model.
It fails, however, for the corresponding dissipative dynamics, because
incoherent transitions due to the interaction with the environment indirectly
couple the three states in the crossing to the remaining quasienergy states.
The asymptotic state of the driven dissipative quantum dynamics partially
resembles the, possibly strange, attractor of the corresponding damped driven
classical dynamics, but also exhibits characteristic quantum effects.Comment: 32 pages, 35 figures, lamuphys.st
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