206 research outputs found
Scanning-probe spectroscopy of semiconductor donor molecules
Semiconductor devices continue to press into the nanoscale regime, and new
applications have emerged for which the quantum properties of dopant atoms act
as the functional part of the device, underscoring the necessity to probe the
quantum structure of small numbers of dopant atoms in semiconductors[1-3].
Although dopant properties are well-understood with respect to bulk
semiconductors, new questions arise in nanosystems. For example, the quantum
energy levels of dopants will be affected by the proximity of nanometer-scale
electrodes. Moreover, because shallow donors and acceptors are analogous to
hydrogen atoms, experiments on small numbers of dopants have the potential to
be a testing ground for fundamental questions of atomic and molecular physics,
such as the maximum negative ionization of a molecule with a given number of
positive ions[4,5]. Electron tunneling spectroscopy through isolated dopants
has been observed in transport studies[6,7]. In addition, Geim and coworkers
identified resonances due to two closely spaced donors, effectively forming
donor molecules[8]. Here we present capacitance spectroscopy measurements of
silicon donors in a gallium-arsenide heterostructure using a scanning probe
technique[9,10]. In contrast to the work of Geim et al., our data show
discernible peaks attributed to successive electrons entering the molecules.
Hence this work represents the first addition spectrum measurement of dopant
molecules. More generally, to the best of our knowledge, this study is the
first example of single-electron capacitance spectroscopy performed directly
with a scanning probe tip[9].Comment: In press, Nature Physics. Original manuscript posted here; 16 pages,
3 figures, 5 supplementary figure
Direct observation of micron-scale ordered structure in a two-dimensional electron system
We have applied a novel scanned probe method to directly resolve the interior
structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling
geometry. We find that the application of a perpendicular magnetic field can
induce surprising density modulations that are not static as a function of the
field. Near six and four filled Landau levels, stripe-like structures emerge
with a characteristic wave length ~2 microns. Present theories do not account
for ordered density modulations on this length scale.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Chaos in Andreev Billiards
A new type of classical billiard - the Andreev billiard - is investigated
using the tangent map technique. Andreev billiards consist of a normal region
surrounded by a superconducting region. In contrast with previously studied
billiards, Andreev billiards are integrable in zero magnetic field, {\it
regardless of their shape}. A magnetic field renders chaotic motion in a
generically shaped billiard, which is demonstrated for the Bunimovich stadium
by examination of both Poincar\'e sections and Lyapunov exponents. The issue of
the feasibility of certain experimental realizations is addressed.Comment: ReVTeX3.0, 4 pages, 3 figures appended as postscript file (uuencoded
with uufiles
Anisotropy and periodicity in the density distribution of electrons in a quantum-well
We use low temperature near-field optical spectroscopy to image the electron
density distribution in the plane of a high mobility GaAs quantum well. We find
that the electrons are not randomly distributed in the plane, but rather form
narrow stripes (width smaller than 150 nm) of higher electron density. The
stripes are oriented along the [1-10 ] crystal direction, and are arranged in a
quasi-periodic structure. We show that elongated structural mounds, which are
intrinsic to molecular beam epitaxy, are responsible for the creation of this
electron density texture.Comment: 10 pages, 3 figure
Label-free imaging flow cytometry for analysis and sorting of enzymatically dissociated tissues
Biomedical research relies on identification and isolation of specific cell types using molecular biomarkers and sorting methods such as fluorescence or magnetic activated cell sorting. Labelling processes potentially alter the cells’ properties and should be avoided, especially when purifying cells for clinical applications. A promising alternative is the label-free identification of cells based on physical properties. Sorting real-time deformability cytometry (soRT-DC) is a microfluidic technique for label-free analysis and sorting of single cells. In soRT-FDC, bright-field images of cells are analyzed by a deep neural net (DNN) to obtain a sorting decision, but sorting was so far only demonstrated for blood cells which show clear morphological differences and are naturally in suspension. Most cells, however, grow in tissues, requiring dissociation before cell sorting which is associated with challenges including changes in morphology, or presence of aggregates. Here, we introduce methods to improve robustness of analysis and sorting of single cells from nervous tissue and provide DNNs which can distinguish visually similar cells. We employ the DNN for image-based sorting to enrich photoreceptor cells from dissociated retina for transplantation into the mouse eye
Modifying the surface electronic properties of YBa2Cu3O7-delta with cryogenic scanning probe microscopy
We report the results of a cryogenic study of the modification of
YBa2Cu3O7-delta surface electronic properties with the probe of a scanning
tunneling microscope (STM). A negative voltage applied to the sample during STM
tunneling is found to modify locally the conductance of the native degraded
surface layer. When the degraded layer is removed by etching, the effect
disappears. An additional surface effect is identified using Scanning Kelvin
Probe Microscopy in combination with STM. We observe reversible surface
charging for both etched and unetched samples, indicating the presence of a
defect layer even on a surface never exposed to air.Comment: 6 pages, 4 figures. To appear in Superconductor Science and
Technolog
Modeling Subsurface Charge Accumulation Images of a Quantum Hall Liquid
Subsurface Charge Accumulation imaging is a cryogenic scanning probe
technique that has recently been used to spatially probe incompressible strips
formed in a two-dimensional electron system (2DES) at high magnetic fields. In
this paper, we present detailed numerical modeling of these data. At a basic
level, the method produces results that agree well with the predictions of
models based on simple circuit elements. Moreover, the modeling method is
sufficiently advanced to simulate the spatially resolved measurements. By
comparing directly the simulations to the experimentally measured data, we can
extract quantitatively local electronic features of the 2DES. In particular, we
deduce the electron density of states inside the incompressible strips and
electrical resistance across them.Comment: 11 pages, 3 figure
Imaging Inter-Edge State Scattering Centers in the Quantum Hall Regime
We use an atomic force microscope tip as a local gate to study the scattering
between edge channels in a 2D electron gas in the quantum Hall regime. The
scattering is dominated by individual, microscopic scattering centers, which we
directly image here for the first time. The tip voltage dependence of the
scattering indicates that tunneling occurs through weak links and localized
states.Comment: 4 pages, 5 figure
Random Matrix Model for Superconductors in a Magnetic Field
We introduce a random matrix ensemble for bulk type-II superconductors in the
mixed state and determine the single-particle excitation spectrum using random
matrix theory. The results are compared with planar tunnel junction experiments
in PbBi thin films. More low energy states appear than in the
Abrikosov-Gor'kov-Maki or Ginzburg-Landau descriptions, consistent with
observations.Comment: 4 pages, 1 postscript figure, to appear in Phys. Rev. Let
Imaging Coulomb Islands in a Quantum Hall Interferometer
In the Quantum Hall regime, near integer filling factors, electrons should
only be transmitted through spatially-separated edge states. However, in
mesoscopic systems, electronic transmission turns out to be more complex,
giving rise to a large spectrum of magnetoresistance oscillations. To explain
these observations, recent models put forward that, as edge states come close
to each other, electrons can hop between counterpropagating edge channels, or
tunnel through Coulomb islands. Here, we use scanning gate microscopy to
demonstrate the presence of quantum Hall Coulomb islands, and reveal the
spatial structure of transport inside a quantum Hall interferometer. Electron
islands locations are found by modulating the tunneling between edge states and
confined electron orbits. Tuning the magnetic field, we unveil a continuous
evolution of active electron islands. This allows to decrypt the complexity of
high magnetic field magnetoresistance oscillations, and opens the way to
further local scale manipulations of quantum Hall localized states
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