94 research outputs found
Transport spectroscopy of disordered graphene quantum dots etched into a single graphene flake
We present transport measurements on quantum dots of sizes 45, 60 and 80 nm
etched with an Ar/O2-plasma into a single graphene sheet, allowing a size
comparison avoiding effects from different graphene flakes. The transport gaps
and addition energies increase with decreasing dot size, as expected, and
display a strong correlation, suggesting the same physical origin for both,
i.e. disorder-induced localization in presence of a small confinement gap. Gate
capacitance measurements indicate that the dot charges are located in the
narrow device region as intended. A dominant role of disorder is further
substantiated by the gate dependence and the magnetic field behavior, allowing
only approximate identification of the electron-hole crossover and spin filling
sequences. Finally, we extract a g-factor consistent with g=2 within the error
bars.Comment: 5 pages, 4 (color) figure
Cotunneling Spectroscopy in Few-Electron Quantum Dots
Few-electron quantum dots are investigated in the regime of strong tunneling
to the leads. Inelastic cotunneling is used to measure the two-electron
singlet-triplet splitting above and below a magnetic field driven
singlet-triplet transition. Evidence for a non-equilibrium two-electron
singlet-triplet Kondo effect is presented. Cotunneling allows orbital
correlations and parameters characterizing entanglement of the two-electron
singlet ground state to be extracted from dc transport.Comment: related papers available at http://marcuslab.harvard.ed
Pure hydrogen low-temperature plasma exposure of HOPG and graphene: Graphane formation?
Single- and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to a pure hydrogen low-temperature plasma (LTP). Characterizations include various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence-band structure, which suggests double-sided hydrogenation. With the scanning tunneling microscopy technique, various atomic-scale charge-density patterns were observed, which may be associated with different C-H conformers. Hydrogen-LTP-exposed graphene on SiO₂ has a Raman spectrum in which the D peak to G peak ratio is over 4, associated with hydrogenation on both sides. A very low defect density was observed in the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen-LTP-exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing at over 1000 °C
Hybrid Quantum Dot-2D Electron Gas Devices for Coherent Optoelectronics
We present an inverted GaAs 2D electron gas with self-assembled InAs quantum
dots in close proximity, with the goal of combining quantum transport with
quantum optics experiments. We have grown and characterized several wafers --
using transport, AFM and optics -- finding narrow-linewidth optical dots and
high-mobility, single subband 2D gases. Despite being buried 500 nm below the
surface, the dots are clearly visible on AFM scans, allowing precise
localization and paving the way towards a hybrid quantum system integrating
optical dots with surface gate-defined nanostructures in the 2D gas.Comment: 4 pages, 5 figures (color
Intrinsic Metastabilities in the Charge Configuration of a Double Quantum Dot
We report a thermally activated metastability in a GaAs double quantum dot
exhibiting real-time charge switching in diamond shaped regions of the charge
stability diagram. Accidental charge traps and sensor back action are excluded
as the origin of the switching. We present an extension of the canonical double
dot theory based on an intrinsic, thermal electron exchange process through the
reservoirs, giving excellent agreement with the experiment. The electron spin
is randomized by the exchange process, thus facilitating fast, gate-controlled
spin initialization. At the same time, this process sets an intrinsic upper
limit to the spin relaxation time.Comment: 4 pages, 5 figures (color
Method for Cooling Nanostructures to Microkelvin Temperatures
We propose a new scheme aimed at cooling nanostructures to microkelvin
temperatures, based on the well established technique of adiabatic nuclear
demagnetization: we attach each device measurement lead to an individual
nuclear refrigerator, allowing efficient thermal contact to a microkelvin bath.
On a prototype consisting of a parallel network of nuclear refrigerators,
temperatures of mK simultaneously on ten measurement leads have been
reached upon demagnetization, thus completing the first steps toward ultracold
nanostructures.Comment: 4 pages, 3 (color) figure
Electrical spin protection and manipulation via gate-locked spin-orbit fields
The spin-orbit (SO) interaction couples electron spin and momentum via a
relativistic, effective magnetic field. While conveniently facilitating
coherent spin manipulation in semiconductors, the SO interaction also
inherently causes spin relaxation. A unique situation arises when the Rashba
and Dresselhaus SO fields are matched, strongly protecting spins from
relaxation, as recently demonstrated. Quantum computation and spintronics
devices such as the paradigmatic spin transistor could vastly benefit if such
spin protection could be expanded from a single point into a broad range
accessible with in-situ gate-control, making possible tunable SO rotations
under protection from relaxation. Here, we demonstrate broad, independent
control of all relevant SO fields in GaAs quantum wells, allowing us to tune
the Rashba and Dresselhaus SO fields while keeping both locked to each other
using gate voltages. Thus, we can electrically control and simultaneously
protect the spin. Our experiments employ quantum interference corrections to
electrical conductivity as a sensitive probe of SO coupling. Finally, we
combine transport data with numerical SO simulations to precisely quantify all
SO terms.Comment: 5 pages, 4 figures (color), plus supplementary information 18 pages,
8 figures (color) as ancillary arXiv pd
GaAs Quantum Dot Thermometry Using Direct Transport and Charge Sensing
We present measurements of the electron temperature using gate defined
quantum dots formed in a GaAs 2D electron gas in both direct transport and
charge sensing mode. Decent agreement with the refrigerator temperature was
observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear
demagnetization stages integrated into the sample wires below 1 mK, the device
electron temperature saturates, remaining close to 10 mK. The extreme
sensitivity of the thermometer to its environment as well as electronic noise
complicates temperature measurements but could potentially provide further
insight into the device characteristics. We discuss thermal coupling
mechanisms, address possible reasons for the temperature saturation and
delineate the prospects of further reducing the device electron temperature.Comment: 8 pages, 3 (color) figure
- …