1,518 research outputs found
Coulomb-Blockade directional coupler
A tunable directional coupler based on Coulomb Blockade effect is presented.
Two electron waveguides are coupled by a quantum dot to an injector waveguide.
Electron confinement is obtained by surface Schottky gates on single
GaAs/AlGaAs heterojunction. Magneto-electrical measurements down to 350 mK are
presented and large transconductance oscillations are reported on both outputs
up to 4.2 K. Experimental results are interpreted in terms of Coulomb Blockade
effect and the relevance of the present design strategy for the implementation
of an electronic multiplexer is underlined.Comment: 4 pages, 4 figures, to be published in Applied Physics Letter
Metal-insulator transition at B=0 in a dilute two dimensional GaAs-AlGaAs hole gas
We report the observation of a metal insulator transition at B=0 in a high
mobility two dimensional hole gas in a GaAs-AlGaAs heterostructure. A clear
critical point separates the insulating phase from the metallic phase,
demonstrating the existence of a well defined minimum metallic conductivity
sigma(min)=2e/h. The sigma(T) data either side of the transition can be
`scaled' on to one curve with a single parameter (To). The application of a
parallel magnetic field increases sigma(min) and broadens the transition. We
argue that strong electron-electron interactions (rs = 10) destroy phase
coherence, removing quantum intereference corrections to the conductivity.Comment: 4 pages RevTex + 4 figures. Submitted to PRL. Originally posted 22
September 1997. Revised 12 October 1997 - minor changes to referencing,
figure cations and figure
Parallel quantized charge pumping
Two quantized charge pumps are operated in parallel. The total current
generated is shown to be far more accurate than the current produced with just
one pump operating at a higher frequency. With the application of a
perpendicular magnetic field the accuracy of quantization is shown to be 20
ppm for a current of pA. The scheme for parallel pumping presented in
this work has applications in quantum information processing, the generation of
single photons in pairs and bunches, neural networking and the development of a
quantum standard for electrical current. All these applications will benefit
greatly from the increase in output current without the characteristic decrease
in accuracy as a result of high-frequency operation
Weak Field Hall Resistance and Effective Carrier Density Through Metal-Insulator Transition in Si-MOS Structures
We studied the weak field Hall voltage in 2D electron layers in Si-MOS
structures with different mobilities, through the metal-insulator transition.
In the vicinity of the critical density on the metallic side of the transition,
we have found weak deviations (about 6-20 %) of the Hall voltage from its
classical value. The deviations do not correlate with the strong temperature
dependence of the diagonal resistivity rho_{xx}(T). The smallest deviation in
R_{xy} was found in the highest mobility sample exhibiting the largest
variation in the diagonal resistivity \rho_{xx} with temperature (by a factor
of 5).Comment: 4 pages, 4 figures, RevTe
A dynamic localization of 2D electrons at mesoscopic length scales
We have investigated the local magneto-transport in high-quality 2D electron
systems at low carrier densities. The positive magneto-resistance in
perpendicular magnetic field in the strongly insulating regime has been
measured to evaluate the spatial concentration of localized states within a
mesoscopic region of the samples. An independent measurement of the electron
density within the same region shows an unexpected correspondence between the
density of electrons in the metallic regime and that of the localized states in
the insulating phase. We have argued that this correspondence manifests a rigid
distribution of electrons at low densities.Comment: 8 pages (incl 4 figures), double colum
Variation of the hopping exponent in disordered silicon MOSFETs
We observe a complex change in the hopping exponent value from 1/2 to 1/3 as
a function of disorder strength and electron density in a sodium-doped silicon
MOSFET. The disorder was varied by applying a gate voltage and thermally
drifting the ions to different positions in the oxide. The same gate was then
used at low temperature to modify the carrier concentration.
Magnetoconductivity measurements are compatible with a change in transport
mechanisms when either the disorder or the electron density is modified
suggesting a possible transition from a Mott insulator to an Anderson insulator
in these systems.Comment: 6 pages, 5 figure
Characterization of Lubricants on Ball Bearings by FT-IR Using an Integrating Sphere
Fourier Transform-Infrared reflectance microspectroscopy has been used extensively for the examination of coatings on nonplanar surfaces such as ball bearings. While this technique offers considerable advantages, practical application has many drawbacks, some of which are easily overcome by the use of integrating sphere technology. This paper describes the use of an integrating sphere for the quantification of thin layers of lubricant on the surface of ball bearings and the parameters which require optimization in order to obtain reliable data. Several applications of the technique are discussed including determination of lubricant load on 12.7 mm steel ball bearings and the examination of degraded lubricant on post mortem specimens
Evolution of the bilayer nu = 1 quantum Hall state under charge imbalance
We use high-mobility bilayer hole systems with negligible tunneling to
examine how the bilayer nu = 1 quantum Hall state evolves as charge is
transferred from one layer to the other at constant total density. We map
bilayer nu = 1 state stability versus imbalance for five total densities
spanning the range from strongly interlayer coherent to incoherent. We observe
competition between single-layer correlations and interlayer coherence. Most
significantly, we find that bilayer systems that are incoherent at balance can
develop spontaneous interlayer coherence with imbalance, in agreement with
recent theoretical predictions.Comment: 4 pages, 4 figure
MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’
HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M_⊙. The primary’s longitudinal magnetic field〈B_z〉 varies between about +100 and −250 G, suggesting a surface magnetic dipole strength B_d = 850 G. Bayesian analysis of the Stokes V profiles indicates B_d = 650 G for the primary and B_d < 110 G for the secondary. The primary’s line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that star’s chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent with the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete
Parallel pumping of electrons
We present simultaneous operation of ten single-electron turnstiles leading
to one order of magnitude increase in current level up to 100 pA. Our analysis
of device uniformity and background charge stability implies that the
parallelization can be made without compromising the strict requirements of
accuracy and current level set by quantum metrology. In addition, we discuss
how offset charge instability limits the integration scale of single-electron
turnstiles.Comment: 6 pages, 4 figures, 1 tabl
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