883 research outputs found
Localization and delocalization for strong disorder in one-dimensional continuous potentials
In one-dimension and for discrete uncorrelated random potentials, such as
tight binding models, all states are localized for any disorder strength. This
is in contrast to continuous random potentials, where we show here that
regardless of the strength of the random potential, we have delocalization in
the limit where the roughness length goes to zero. This result was obtained by
deriving an expression for the localization length valid for all disorder
strengths. We solved a nonlinear wave equation, whose average over disorder
yields the localization properties of the desired linear wave equation. Our
results, not only explain the origin of the difficulty to observe localization
in certain physical systems, but also show that maximum localization occurs
when the roughness length is comparable to the wavelength, which is relevant
to many experiments in a random medium
Phase diagram of the integer quantum Hall effect in p-type Germanium
We experimentally study the phase diagram of the integer quantized Hall
effect, as a function of density and magnetic field. We used a two dimensional
hole system confined in a Ge/SiGe quantum well, where all energy levels are
resolved, because the Zeeman splitting is comparable to the cyclotron energy.
At low fields and close to the quantum Hall liquid-to-insulator transition, we
observe the floating up of the lowest energy level, but NO FLOATING of any
higher levels, rather a merging of these levels into the insulating state. For
a given filling factor, only direct transitions between the insulating phase
and higher quantum Hall liquids are observed as a function of density. Finally,
we observe a peak in the critical resistivity around filling factor one.Comment: 4 pages, 4 figures, some changes in the tex
Verfassungsrechtliche Zulässigkeit du sozialgerichtlicher Hintergrund von Teilzeitbeschäftigung und Beurlaubung für Beamtinnen und Richterinnen mit Mutterpflichten
The Last Stages of Terrestrial Planet Formation: Dynamical Friction and the Late Veneer
The final stage of terrestrial planet formation consists of the cleanup of
residual planetesimals after the giant impact phase. Dynamically, a residual
planetesimal population is needed to damp the high eccentricities of the
terrestrial planets after the giant impact stage. Geochemically, highly
siderophile element (HSE) abundance patterns inferred for the terrestrial
planets and the Moon suggest that a total of about 0.01 M_Earth of chondritic
material was delivered as `late veneer' by planetesimals to the terrestrial
planets after the end of giant impacts. Here we combine these two independent
lines of evidence for a leftover population of planetesimals and show that: 1)
A residual planetesimal population containing 0.01 M_Earth is able to damp the
eccentricities of the terrestrial planets after giant impacts to their observed
values. 2) At the same time, this planetesimal population can account for the
observed relative amounts of late veneer added to the Earth, Moon and Mars
provided that the majority of the late veneer was delivered by small
planetesimals with radii <10m. These small planetesimal sizes are required to
ensure efficient damping of the planetesimal's velocity dispersion by mutual
collisions, which in turn ensures that the planets' accretion cross sections
are significantly enhanced by gravitational focusing above their geometric
values. Specifically we find, in the limit that the relative velocity between
the terrestrial planets and the planetesimals is significantly less than the
terrestrial planets' escape velocities, that gravitational focusing yields an
accretion ratio Earth/Mars~17, which agrees well with the accretion ratio
inferred from HSEs of 12-23. For the Earth-Moon system, we find an accretion
ratio of ~200, which is consistent with estimates of 150-700 derived from HSE
abundances that include the lunar crust as well as mantle component. (Abridged)Comment: accepted for publication in ApJ, 9 pages, 4 figures; minor
corrections, additional references adde
Recht als probleemoplossing?
De vraag naar de mogelijkheden en beperkingen om het recht meer te laten zijn dan een instrument voor de juridische beslechting van geschillen staat centraal in dit themanummer van Recht der Werkelijkheid
Quantum Dynamics of Spins Coupled by Electrons in 1D Channel
We develop a unified theoretical description of the induced interaction and
quantum noise in a system of two spins (qubits) coupled via a
quasi-one-dimensional electron gas in the Luttinger liquid regime. Our results
allow evaluation of the degree of coherence in quantum dynamics driven by the
induced indirect exchange interaction of localized magnetic moments due to
conduction electrons, in channel geometries recently experimentally studied for
qubit control and measurement.Comment: 2 figures, in REVTE
Localization Properties of the Periodic Random Anderson Model
We consider diagonal disordered one-dimensional Anderson models with an
underlying periodicity. We assume the simplest periodicity, i.e., we have
essentially two lattices, one that is composed of the random potentials and the
other of non-random potentials. Due to the periodicity special resonance
energies appear, which are related to the lattice constant of the non-random
lattice. Further on two different types of behaviors are observed at the
resonance energies. When a random site is surrounded by non-random sites, this
model exhibits extended states at the resonance energies, whereas otherwise all
states are localized with, however, an increase of the localization length at
these resonance energies. We study these resonance energies and evaluate the
localization length and the density of states around these energies.Comment: 4 page
Decoherence in a N-qubit solid-state quantum computer
We investigate the decoherence process for a quantum register composed of N
qubits coupled to an environment. We consider an environment composed of one
common phonon bath and several electronic baths. This environment is relevant
to the implementation of a charge based solid-state quantum computer. We
explicitly compute the time evolution of all off-diagonal terms of the
register's reduced density matrix. We find that in realistic configurations,
"superdecoherence" and "decoherence free subspaces" do not exist for an N-qubit
system. This means that all off-diagonal terms decay like exp(-q(t)N), where
q(t) is of the same order as the decay function of a single qubit.Comment: 11 page
Two-Dimensional Electron Gas in InGaAs/InAlAs Quantum Wells
We designed and performed low temperature DC transport characterization
studies on two-dimensional electron gases confined in lattice-matched
InGaAs/InAlAs quantum wells grown by
molecular beam epitaxy on InP substrates. The nearly constant mobility for
samples with the setback distance larger than 50nm and the similarity between
the quantum and transport life-time suggest that the main scattering mechanism
is due to short range scattering, such as alloy scattering, with a scattering
rate of 2.2 ps. We also obtain the Fermi level at the
InGaAs/InAlAs surface to be 0.36eV above
the conduction band, when fitting our experimental densities with a
Poisson-Schr\"odinger model.Comment: Accepted in Applied Physics Letter
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