870 research outputs found
Non-interacting electrons and the metal-insulator transition in 2D with correlated impurities
While standard scaling arguments show that a system of non-interacting
electrons in two dimensions and in the presence of uncorrelated disorder is
insulating, in this work we discuss the case where inter-impurity correlations
are included. We find that for point-like impurities and an infinite
inter-impurity correlation length a mobility edge exists in 2D even if the
individual impurity potentials are random. In the uncorrelated system we
recover the scaling results, while in the intermediate regime for length scales
comparable to the correlation length, the system behaves like a metal but with
increasing fluctuations, before strong localization eventually takes over for
length scales much larger than the correlation length. In the intermediate
regime, the relevant length scale is not given by the elastic scattering length
but by the inter-impurity correlation length, with important consequences for
high mobility systems.Comment: 4 page
Superconductivity and short range order in metallic glasses FeNiZr
In amorphous superconductors, superconducting and vortex pinning properties
are strongly linked to the absence of long range order. Consequently,
superconductivity and vortex phases can be studied to probe the underlying
microstructure and order of the material. This is done here from resistance and
local magnetization measurements in the superconducting state of
FeNiZr metallic glasses with . Firstly,
we present typical superconducting properties such as the critical temperature
and fields and their dependence on Fe content in these alloys. Then, the
observations of peculiar clockwise hysteresis loops, wide double-step
transitions and large magnetization fluctuations in glasses containing a large
amount of Fe are analyzed to reveal a change in short range order with Fe
content.Comment: 8 pages, 7 figure
Experimental phase diagram of moving vortices
In the mixed state of type II superconductors, vortices penetrate the sample
and form a correlated system due to the screening of supercurrents around them.
Interestingly, we can study this correlated system as a function of density and
driving force. The density, for instance, is controlled by the magnetic field,
B, whereas a current density j acts as a driving force F=jxB on all vortices.
The free motion of vortices is inhibited by the presence of an underlying
potential, which tends to pin the vortices. Hence, to minimize the pinning
strength we studied a superconducting glass in which the depinning current is
10 to 1000 times smaller than in previous studies, which enables us to map out
the complete phase diagram in this new regime. The diagram is obtained as a
function of B, driving current and temperature and led a remarkable set of new
results, which includes a huge peak effect, an additional reentrant depinning
phase and a driving force induced pinning phase.Comment: 4 page
The effect of Semi-Collisional Accretion on Planetary Spins
Planetesimal accretion during planet formation is usually treated as
collisionless. Such accretion from a uniform and dynamically cold disk predicts
protoplanets with slow retrograde rotation. However, if the building blocks of
protoplanets, planetesimals, are small, of order of a meter in size, then they
are likely to collide within the protoplanet's sphere of gravitational
influence, creating a prograde accretion disk around the protoplanet. The
accretion of such a disk results in the formation of protoplanets spinning in
the prograde sense with the maximal spin rate allowed before centrifugal forces
break them apart. As a result of semi-collisional accretion, the final spin of
a planet after giant impacts is not completely random but is biased toward
prograde rotation. The eventual accretion of the remaining planetesimals in the
post giant-impact phase might again be in the semi-collisional regime and
delivers a significant amount of additional prograde angular momentum to the
terrestrial planets. We suggest that in our Solar System, semi-collisional
accretion gave rise to the preference for prograde rotation observed in the
terrestrial planets and perhaps the largest asteroids.Comment: 13 pages, 2 figure
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
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
The quantized Hall effect in the presence of resistance fluctuations
We present an experimental study of mesoscopic, two-dimensional electronic
systems at high magnetic fields. Our samples, prepared from a low-mobility
InGaAs/InAlAs wafer, exhibit reproducible, sample specific, resistance
fluctuations. Focusing on the lowest Landau level we find that, while the
diagonal resistivity displays strong fluctuations, the Hall resistivity is free
of fluctuations and remains quantized at its value, . This is
true also in the insulating phase that terminates the quantum Hall series.
These results extend the validity of the semicircle law of conductivity in the
quantum Hall effect to the mesoscopic regime.Comment: Includes more data, changed discussio
Photoconductivity in AC-driven modulated two dimensional electron gas in a perpendicular magnetic field
In this work we study the microwave photoconductivity of a two-dimensional
electron system (2DES) in the presence of a magnetic field and a
two-dimensional modulation (2D). The model includes the microwave and Landau
contributions in a non-perturbative exact way, the periodic potential is
treated perturbatively. The Landau-Floquet states provide a convenient base
with respect to which the lattice potential becomes time-dependent, inducing
transitions between the Landau-Floquet levels. Based on this formalism, we
provide a Kubo-like formula that takes into account the oscillatory Floquet
structure of the problem. The total longitudinal conductivity and resistivity
exhibit strong oscillations, determined by with
the radiation frequency and the cyclotron frequency. The
oscillations follow a pattern with minima centered at , and maxima centered at , where , is a constant shift
and is the dominant multipole contribution. Negative resistance states
(NRS) develop as the electron mobility and the intensity of the microwave power
are increased. These NRS appear in a narrow window region of values of the
lattice parameter (), around , where is the magnetic
length. It is proposed that these phenomena may be observed in artificially
fabricated arrays of periodic scatterers at the interface of ultraclean
heterostructures.Comment: 20 pages, 8 figure
Delocalization and conductance quantization in one-dimensional systems
We investigate the delocalization and conductance quantization in finite
one-dimensional chains with only off-diagonal disorder coupled to leads. It is
shown that the appearence of delocalized states at the middle of the band under
correlated disorder is strongly dependent upon the even-odd parity of the
number of sites in the system. In samples with inversion symmetry the
conductance equals for odd samples, and is smaller for even parity.
This result suggests that this even-odd behaviour found previously in the
presence of electron correlations may be unrelated to charging effects in the
sample.Comment: submitted to PR
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