614 research outputs found
Cryogenic scanning force microscopy of quantum Hall samples: Adiabatic transport originating in anisotropic depletion at contact interfaces
Anisotropic magneto resistances and intrinsic adiabatic transport features
are generated on quantum Hall samples based on an (Al,Ga)As/GaAs
heterostructure with alloyed Au/Ge/Ni contacts. We succeed to probe the
microscopic origin of these transport features with a cryogenic scanning force
microscope (SFM) by measuring the local potential distribution within the
two-dimensional electron system (2DES). These local measurements reveal the
presence of an incompressible strip in front of contacts with insulating
properties depending on the orientation of the contact/2DES interface line
relatively to the crystal axes of the heterostructure. Such an observation
gives another microscopic meaning to the term 'non-ideal contact' used in
context with the Landauer-B\"uttiker formalism applied to the quantum Hall
effect.Comment: 5 pages, 4 figure
Density of States of GaAs-AlGaAs Heterostructures Deduced from Temperature Dependend Magnetocapacitance Measurements
Abstract We have analyzed the density of states of a two dimensional electron gas in a GaAs- AlGaAs hetereostructure by measuring the magnetocapacitance in magnetic fields up to 6 Tesla at temperatures below 10 K. The experimental data are well described by a Gaussian-like density of states where the linewidth à is proportional to B
Radiation induced zero-resistance states in GaAs/AlGaAs heterostructures: Voltage-current characteristics and intensity dependence at the resistance minima
High mobility two-dimensional electron systems exhibit vanishing resistance
over broad magnetic field intervals upon excitation with microwaves, with a
characteristic reduction of the resistance with increasing radiation intensity
at the resistance minima. Here, we report experimental results examining the
voltage - current characteristics, and the resistance at the minima vs. the
microwave power. The findings indicate that a non-linear V-I curve in the
absence of microwave excitation becomes linearized under irradiation, unlike
expectations, and they suggest a similarity between the roles of the radiation
intensity and the inverse temperature.Comment: 3 color figures; publishe
Correlated electron tunneling through two separate quantum dot systems with strong capacitive interdot coupling
A system consisting of two independently contacted quantum dots with strong
electrostatic interaction shows interdot Coulomb blockade when the dots are
weakly tunnel coupled to their leads. It is studied experimentally how the
blockade can be overcome by correlated tunneling when tunnel coupling to the
leads increases. The experimental results are compared with numerical
renormalization group calculations using predefined (measured) parameters. Our
results indicate Kondo correlations due to the electrostatic interaction in
this double quantum dot system.Comment: 5 pages, 3 figures, published in Phys. Rev. Lett. Oct. 30t
Compressibility in the Integer Quantum Hall Effect within Hartree-Fock Approximation
Electron-electron interactions seem to play a surprisingly small role in the
description of the integer quantum Hall effect, considering that for just
slightly different filling factors the interactions are of utmost importance
causing the interaction-mediated fractional quantum Hall effect. However,
recent imaging experiments by Cobden et al. and Ilani et al. constitute strong
evidence for the importance of electron-electron interactions even in the
integer effect. The experiments report on measurements of the conductance and
electronic compressibility of mesoscopic MOSFET devices that show disagreement
with predictions from the single particle model. By diagonalising a random
distribution of Gaussian scatterers and treating the interactions in
Hartree-Fock approximation we investigate the role of electron-electron
interactions for the integer quantum Hall effect and find good agreement with
the experimental results.Comment: 4 pages, 4 figures, Proceedings of "Transport in Interacting and
Disordered Systems (TIDS11)", submitted for publication in phys. stat. sol.
(c), including pss style file
Four-terminal magneto-transport in graphene p-n junctions created by spatially selective doping
In this paper we describe a graphene p-n junction created by chemical doping.
We find that chemical doping does not reduce mobility in contrast to
top-gating. The preparation technique has been developed from systematic
studies about influences on the initial doping of freshly prepared graphene. We
investigated the removal of adsorbates by vacuum treatment, annealing and
compensation doping using NH3. Hysteretic behavior is observed in the electric
field effect due to dipolar adsorbates like water and NH3. Finally we
demonstrate spatially selective doping of graphene using patterned PMMA.
4-terminal transport measurements of the p-n devices reveal edge channel mixing
in the quantum hall regime. Quantized resistances of h/e^2, h/3e^2 and h/15e^2
can be observed as expected from theory.Comment: 18 pages, 5 figure
An ultra-bright atom laser
We present a novel, ultra-bright atom-laser and ultra-cold thermal atom beam.
Using rf-radiation we strongly couple the magnetic hyperfine levels of 87Rb
atoms in a magnetically trapped Bose-Einstein condensate. At low rf-frequencies
gravity opens a small hole in the trapping potenital and a well collimated,
extremely bright atom laser emerges from just below the condensate. As opposed
to traditional atom lasers based on weak coupling, this technique allows us to
outcouple atoms at an arbitrarily large rate. We demonstrate an increase in
flux per atom in the BEC by a factor of sixteen compared to the brightest
quasi-continuous atom laser. Furthermore, we produce by two orders of magnitude
the coldest thermal atom beam to date (200 nK).Comment: 20 pages, 9 figures, supplementary material online at
http://www.bec.g
Electrical Transport in High Quality Graphene pnp Junctions
We fabricate and investigate high quality graphene devices with contactless,
suspended top gates, and demonstrate formation of graphene pnp junctions with
tunable polarity and doping levels. The device resistance displays distinct
oscillations in the npn regime, arising from the Fabry-Perot interference of
holes between the two pn interfaces. At high magnetic fields, we observe
well-defined quantum Hall plateaus, which can be satisfactorily fit to
theoretical calculations based on the aspect ratio of the device.Comment: to appear in a special focus issue in New Journal of Physic
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