13 research outputs found
Tunneling images of a 2D electron system in a quantizing magnetic field
We have applied a scanning probe method, Subsurface Charge Accumulation (SCA)
imaging, to resolve the local structure of the interior of a semiconductor
two-dimensional electron system (2DES) in a tunneling geometry. Near magnetic
fields corresponding to integer Landau level filling, submicron scale spatial
structure in the out-of-phase component of the tunneling signal becomes
visible. In the images presented here, the structure repeats itself when the
filling factor is changed from nu=6 to nu=7. Therefore, we believe the images
reflect small modulations in the 2DES density caused by the disorder in the
sample.Comment: 2 pages, 2 color figures, submitted to LT23 proceeding
Modeling single- and multiple-electron resonances for electric-field-sensitive scanning probes
We have developed a modeling method suitable to analyze single- and
multiple-electron resonances detected by electric-field-sensitive scanning
probe techniques. The method is based on basic electrostatics and a numerical
boundary-element approach. The results compare well to approximate analytical
expressions and experimental data.Comment: 10 pages, 4 figure
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.
Trionic Optical Potential for Electrons in Semiconductors
Laser-induced optical potentials for atoms have led to remarkable advances in
precision measurement, quantum information, and towards addressing fundamental
questions in condensed matter physics. Here, we describe analogous optical
potentials for electrons in quantum wells and wires that can be generated by
optically driving the transition between a single electron and a three-body
electron-exciton bound state, known as a trion. The existence of a bound trion
state adds a term to the ac Stark shift of the material proportional to the
light intensity at the position of the electron. According to our theoretical
calculations, this shift can be large relative to the thermal equilibrium
temperature of the electron, resulting in a relatively strong optical potential
that could be used to trap, guide, and manipulate individual electrons within a
semiconductor quantum well or wire. These potentials can be thought of as
artificial nano-structures on the scale of 100 nm that can be spin-dependent
and reconfigurable in real-time. Our results suggest the possibility of
integrating ultrafast optics and gate voltages in new resolved-carrier
semiconductor opto-electronic devices, with potential applications in fields
such as nano-electronics, spintronics, and quantum information processingComment: Article and Supplemental Materials; This is a preprint of the
original submission to Nature Physic
A global-scale screening of non-native aquatic organisms to identify potentially invasive species under current and future climate conditions
The threat posed by invasive non-native species worldwide requires a global approach to identify which introduced species are likely to pose an elevated risk of impact to native species and ecosystems. To inform policy, stakeholders and management decisions on global threats to aquatic ecosystems, 195 assessors representing 120 risk assessment areas across all six inhabited continents screened 819 non-native species from 15 groups of aquatic organisms (freshwater, brackish, marine plants and animals) using the Aquatic Species Invasiveness Screening Kit. This multi-lingual decision-support tool for the risk screening of aquatic organisms provides assessors with risk scores for a species under current and future climate change conditions that, following a statistically based calibration, permits the accurate classification of species into high-, medium- and low-risk categories under current and predicted climate conditions. The 1730 screenings undertaken encompassed wide geographical areas (regions, political entities, parts thereof, water bodies, river basins, lake drainage basins, and marine regions), which permitted thresholds to be identified for almost all aquatic organismal groups screened as well as for tropical, temperate and continental climate classes, and for tropical and temperate marine ecoregions. In total, 33 species were identified as posing a âvery high riskâ of being or becoming invasive, and the scores of several of these species under current climate increased under future climate conditions, primarily due to their wide thermal tolerances. The risk thresholds determined for taxonomic groups and climate zones provide a basis against which area-specific or climate-based calibrated thresholds may be interpreted. In turn, the risk rankings help decision-makers identify which species require an immediate ârapidâ management action (e.g. eradication, control) to avoid or mitigate adverse impacts, which require a full risk assessment, and which are to be restricted or banned with regard to importation and/or sale as ornamental or aquarium/fishery enhancement