645 research outputs found
Conductance measurement of spin-orbit coupling in the two-dimensional electron systems with in-plane magnetic field
We consider determination of spin-orbit (SO) coupling constants for the
two-dimensional electron gas from measurements of electric properties in
rotated in-plane magnetic field. %Due to the interplay Due to the SO coupling
the electron backscattering is accompanied by spin precession and spin mixing
of the incident and reflected electron waves. The competition of the external
and SO-related magnetic fields produces a characteristic conductance dependence
on the in-plane magnetic field value and orientation which, in turn, allows for
determination of the absolute value of the effective spin-orbit coupling
constant as well as the ratio of the Rashba and Dresselhaus SO contributions.Comment: 4 pages + supplementary material
Interference features in scanning gate conductance maps of quantum point contacts with disorder
We consider quantum point contacts (QPCs) defined within disordered
two-dimensional electron gases as studied by scanning gate microscopy. We
evaluate the conductance maps in the Landauer approach and wave function
picture of electron transport for samples with both low and high electron
mobility at finite temperatures. We discuss the spatial distribution of the
impurities in the context of the branched electron flow. We reproduce the
surprising temperature stability of the experimental interference fringes far
from the QPC. Next, we discuss -- previously undescribed -- funnel-shaped
features that accompany splitting of the branches visible in previous
experiments. Finally, we study elliptical interference fringes formed by an
interplay of scattering by the point-like impurities and by the scanning probe.
We discuss the details of the elliptical features as functions of the tip
voltage and the temperature, showing that the first interference fringe is very
robust against the thermal widening of the Fermi level. We present a simple
analytical model that allows for extraction of the impurity positions and the
electron gas depletion radius induced by the negatively charged tip of the
atomic force microscope, and apply this model on experimental scanning gate
images showing such elliptical fringes
Large variation in the boundary-condition slippage for a rarefied gas flowing between two surfaces
We study the slippage of a gas along mobile rigid walls in the sphere-plane
confined geometry and find that it varies considerably with pressure. The
classical no-slip boundary condition valid at ambient pressure changes
continuously to an almost perfect slip condition in a primary vacuum. Our study
emphasizes the key role played by the mean free-path of the gas molecules on
the interaction between a confined fluid and solid surfaces and further
demonstrates that the macroscopic hydrodynamics approach can be used with
confidence even in a primary vacuum environment where it is intuitively
expected to fail
Casimir force measurements in Au-Au and Au-Si cavities at low temperature
We report on measurements of the Casimir force in a sphere-plane geometry
using a cryogenic force microscope to move the force probe in situ over
different materials. We show how the electrostatic environment of the
interacting surfaces plays an important role in weak force measurements and can
overcome the Casimir force at large distance. After minimizing these parasitic
forces, we measure the Casimir force between a gold-coated sphere and either a
gold-coated or a heavily doped silicon surface in the 100-400 nm distance
range. We compare the experimental data with theoretical predictions and
discuss the consequence of a systematic error in the scanner calibration on the
agreement between experiment and theory. The relative force over the two
surfaces compares favorably with theory at short distance, showing that this
Casimir force experiment is sensitive to the dielectric properties of the
interacting surfaces.Comment: accepted for publication in Physical Review
Flow of evaporating, gravity-driven thin liquid films over topography
The effect of topography on the free surface and solvent concentration profiles of an evaporating thin film of liquid flowing down an inclined plane is considered. The liquid is assumed to be composed of a resin dissolved in a volatile solvent with the associated solvent concentration equation derived on the basis of the well-mixed approximation. The dynamics of the film is formulated as a lubrication approximation and the effect of a composition-dependent viscosity is included in the model. The resulting time-dependent, nonlinear, coupled set of governing equations is solved using a full approximation storage multigrid method.
The approach is first validated against a closed-form analytical solution for the case of a gravity-driven, evaporating thin film flowing down a flat substrate. Analysis of the results for a range of topography shapes reveal that although a full-width, spanwise topography such as a step-up or a step-down does not affect the composition of the film, the same is no longer true for the case of localized topography, such as a peak or a trough, for which clear nonuniformities of the solvent concentration profile can be observed in the wake of the topography
Inhomogeneous superconductivity induced in a weak ferromagnet
Under certain conditions, the order parameter induced by a superconductor (S)
in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g.
in the so-called pi-coupling in S/F/S junctions. In principle, the
inhomogeneous state can be induced at T_c as function of the F-layer thickness
d_F in S/F bilayers and multilayers, which should result in a dip-like
characteristic of T_c(d_F). We show the results of measurements on the S/F
system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where
such effects might be expected. We find that the critical thickness for the
occurrence of superconductivity is still relatively high, even for these weak
ferromagnets. The resulting dip then is intrinsically shallow and difficult to
observe, which explains the lack of a clear signature in the T_c(d_F) data.Comment: 4 pages, 4 figures. To be publishedin Physica C (proceedings of the
Second Euroconference on Vortex Matter in Superconductors, Crete, 2001
Formation of quantum dots in the potential fluctuations of InGaAs heterostructures probed by scanning gate microscopy
The disordered potential landscape in an InGaAs/InAlAs two-dimensional
electron gas patterned into narrow wires is investigated by means of scanning
gate microscopy. It is found that scanning a negatively charged tip above
particular sites of the wires produces conductance oscillations that are
periodic in the tip voltage. These oscillations take the shape of concentric
circles whose number and diameter increase for more negative tip voltages until
full depletion occurs in the probed region. These observations cannot be
explained by charging events in material traps, but are consistent with Coulomb
blockade in quantum dots forming when the potential fluctuations are raised
locally at the Fermi level by the gating action of the tip. This interpretation
is supported by simple electrostatic simulations in the case of a disorder
potential induced by ionized dopants. This work represents a local
investigation of the mechanisms responsible for the disorder-induced
metal-to-insulator transition observed in macroscopic two-dimensional electron
systems at low enough density
The NF-κB subunit c-Rel regulates Bach2 tumour suppressor expression in B-cell lymphoma
The REL gene, encoding the NF-κB subunit c-Rel, is frequently amplified in B-cell lymphoma and functions as a tumour-promoting transcription factor. Here we report the surprising result that c-rel–/– mice display significantly earlier lymphomagenesis in the c-Myc driven, Eμ-Myc model of B-cell lymphoma. c-Rel loss also led to earlier onset of disease in a separate TCL1-Tg-driven lymphoma model. Tumour reimplantation experiments indicated that this is an effect intrinsic to the Eμ-Myc lymphoma cells but, counterintuitively, c-rel–/– Eμ-Myc lymphoma cells were more sensitive to apoptotic stimuli. To learn more about why loss of c-Rel led to earlier onset of disease, microarray gene expression analysis was performed on B cells from 4-week-old, wild-type and c-rel–/– Eμ-Myc mice. Extensive changes in gene expression were not seen at this age, but among those transcripts significantly downregulated by the loss of c-Rel was the B-cell tumour suppressor BTB and CNC homology 2 (Bach2). Quantitative PCR and western blot analysis confirmed loss of Bach2 in c-Rel mutant Eμ-Myc tumours at both 4 weeks and the terminal stages of disease. Moreover, Bach2 expression was also downregulated in c-rel–/– TCL1-Tg mice and RelA Thr505Ala mutant Eμ-Myc mice. Analysis of wild-type Eμ-Myc mice demonstrated that the population expressing low levels of Bach2 exhibited the earlier onset of lymphoma seen in c-rel–/– mice. Confirming the relevance of these findings to human disease, analysis of chromatin immunoprecipitation sequencing data revealed that Bach2 is a c-Rel and NF-κB target gene in transformed human B cells, whereas treatment of Burkitt's lymphoma cells with inhibitors of the NF-κB/IκB kinase pathway or deletion of c-Rel or RelA resulted in loss of Bach2 expression. These data reveal a surprising tumour suppressor role for c-Rel in lymphoma development explained by regulation of Bach2 expression, underlining the context-dependent complexity of NF-κB signalling in cancer
pi-Junction behavior and Andreev bound states in Kondo quantum dots with superconducting leads
We investigate the temperature- and coupling-dependent transport through
Kondo dot contacts with symmetric superconducting s-wave leads. For finite
temperature T we use a superconducting extension of a selfconsistent auxiliary
boson scheme, termed SNCA, while at T=0 a perturbative renormalization group
treatment is applied. The finite-temperature phase diagram for the 0--pi
transition of the Josephson current in the junction is established and related
to the phase-dependent position of the subgap Kondo resonance with respect to
the Fermi energy. The conductance of the contact is evaluated in the zero-bias
limit. It approaches zero in the low-temperature regime, however, at finite T
its characteristics are changed through the coupling- and temperature-dependent
0--pi transition.Comment: 12 pages, 12 figure
Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island
We explore transport across an ultra-small Quantum Hall Island (QHI) formed
by closed quan- tum Hall edge states and connected to propagating edge channels
through tunnel barriers. Scanning gate microscopy and scanning gate
spectroscopy are used to first localize and then study a single QHI near a
quantum point contact. The presence of Coulomb diamonds in the spectroscopy
con- firms that Coulomb blockade governs transport across the QHI. Varying the
microscope tip bias as well as current bias across the device, we uncover the
QHI discrete energy spectrum arising from electronic confinement and we extract
estimates of the gradient of the confining potential and of the edge state
velocity.Comment: 13 pages, 3 figure
- …