310 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
Closed-form weak localization magnetoconductivity in quantum wells with arbitrary Rashba and Dresselhaus spin-orbit interactions
We derive a closed-form expression for the weak localization (WL) corrections
to the magnetoconductivity of a 2D electron system with arbitrary Rashba
and Dresselhaus (linear) and (cubic) spin-orbit
interaction couplings, in a perpendicular magnetic field geometry. In a system
of reference with an in-plane axis chosen as the high spin-symmetry
direction at , we formulate a new algorithm to calculate the
three independent contributions that lead to WL. The antilocalization is
counterbalanced by the term associated with the spin-relaxation along
, dependent only on . The other term is generated by
two identical scattering modes characterized by spin-relaxation rates which are
explicit functions of the orientation of the scattered momentum. Excellent
agreement is found with data from GaAs quantum wells, where in particular our
theory correctly captures the shift of the minima of the WL curves as a
function of . This suggests that the anisotropy of the effective
spin relaxation rates is fundamental to understanding the effect of the SO
coupling in transport.Comment: 5 pages, 2 figure
The Little Ice Age history of the Glacier des Bossons (Mont Blanc massif, France): a new high-resolution glacier length curve based on historical documents
Historical and proxy records document that there is a substantial asynchronous development in temperature, precipitation and glacier variations between European regions during the last few centuries. The causes of these temporal anomalies are yet poorly understood. Hence, highly resolved glacier reconstructions based on historical evidence can give valuable insights into past climate, but they exist only for few glaciers worldwide. Here, we present a new reconstruction of length changes for the Glacier des Bossons (Mont Blanc massif, France), based on unevaluated historical material. More than 250 pictorial documents (drawings, paintings, prints, photographs, maps) as well as written accounts have been critically analysed, leading to a revised picture of the glacier's history, especially from the mid-eighteenth century up to the 1860s. Very important are the drawings by Jean-Antoine Linck, Samuel Birmann and Eugène Viollet-le Duc, which depict meticulously the glacier's extent during the vast advance and subsequent retreat during the nineteenth century. The new glacier reconstruction extends back to AD 1580 and proves maxima of the Glacier des Bossons around 1610/1643, 1685, 1712, 1777, 1818, 1854, 1892, 1921, 1941, and 1983. The Little Ice Age maximum extent was reached in 1818. Until the present, the glacier has lost about 1.5km in length, and it is now shorter than at any time during the reconstruction period. The Glacier des Bossons reacts faster than the nearby Mer de Glace (glacier reconstruction back to AD 1570 available). The Mont Blanc area is, together with the valley of Grindelwald in the Swiss Alps (two historical glacier reconstructions available back to AD 1535, and 1590, respectively), among the two regions that are probably best-documented in the world regarding historical glacier dat
An Evaluation of Selected Retouching Media for Acrylic Emulsion Paint
In this study, polar and non-polar retouching media were analyzed to assess their applicability and reversibility on acrylic emulsion paint films (Golden and Schmincke acrylic paints). Acrylic emulsion paints are very sensitive to a variety of solvents. Only water, short-chain alcohols and aliphatic hydrocarbons are considered suitable for their treatment. Therefore, the retouching media used in this study were chosen for their solubility in each of these solvents. Distilled water and ethanol were used in order to test the reversibility of the polar retouching. Noctane, nhexane and diethyl ether, which offer weak dispersive interactions but different vapor pressures, were employed for swab removal of the non-polar retouching. Extraction tests with different polar and non-polar solvents, showed which components were leached out of the acrylic paint film sample during swab removal of retouching media. Gloss measurements and photomicrographs taken of the paint film samples before and after the application of the retouching displayed variations when compared to untreated reference samples. Both measurements were taken again after reversibility tests in order to demonstrate any changes in morphology and gloss of the paint film samples
Magnetic cooling for microkelvin nanoelectronics on a cryofree platform
We present a parallel network of 16 demagnetization refrigerators mounted on
a cryofree dilution refrigerator aimed to cool nanoelectronic devices to
sub-millikelvin temperatures. To measure the refrigerator temperature, the
thermal motion of electrons in a Ag wire -- thermalized by a spot-weld to one
of the Cu nuclear refrigerators -- is inductively picked-up by a
superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise
thermometer as well as other thermometers are used to characterize the
performance of the system, finding magnetic field independent heat-leaks of a
few nW/mol, cold times of several days below 1 mK, and a lowest temperature of
150 microK of one of the nuclear stages in a final field of 80 mT, close to the
intrinsic SQUID noise of about 100 microK. A simple thermal model of the system
capturing the nuclear refrigerator, heat leaks, as well as thermal and Korringa
links describes the main features very well, including rather high refrigerator
efficiencies typically above 80%.Comment: 4 color figures, including supplementary inf
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
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
- …