429 research outputs found
Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots
We have combined direct nanofabrication by local anodic oxidation with
conventional electron-beam lithography to produce a parallel double quantum dot
based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography
methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes
on the same device for optimal controllability. This is illustrated by the
tunability of the interdot coupling in our device. We describe our fabrication
and alignment scheme in detail and demonstrate the tunability in
low-temperature transport measurements.Comment: 4 pages, 3 figure
Non-invasive detection of charge-rearrangement in a quantum dot in high magnetic fields
We demonstrate electron redistribution caused by magnetic field on a single
quantum dot measured by means of a quantum point contact as non-invasive
detector. Our device which is fabricated by local anodic oxidation allows to
control independently the quantum point contact and all tunnelling barriers of
the quantum dot. Thus we are able to measure both the change of the quantum dot
charge and also changes of the electron configuration at constant number of
electrons on the quantum dot. We use these features to exploit the quantum dot
in a high magnetic field where transport through the quantum dot displays the
effects of Landau shells and spin blockade. We confirm the internal
rearrangement of electrons as function of the magnetic field for a fixed number
of electrons on the quantum dot.Comment: 4 pages, 5 figure
Evidence for a Second Order Phase Transition in Glasses at Very Low Temperatures -- A Macroscopic Quantum State of Tunneling Systems
Dielectric measurements at very low temperature indicate that in a glass with
the eutectic composition BaO-AlO-SiO a phase transition occurs at
5.84 mK. Below that temperature small magnetic fields of the order of 10 T
cause noticeable changes of the dielectric constant although the glass is
insensitive to fields up to 20 T above 10 mK. The experimental findings may be
interpreted as the signature of the formation of a new phase in which many
tunneling systems perform a coherent motion resulting in a macroscopic wave
function.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Molecular Marker Analysis as a Guide to the Sources of Fine Organic Aerosols
The molecular composition of fine particulate (D_p ≥ 2 µm) organic aerosol emissions from the most important sources in the Los Angeles area has been determined. Likewise, ambient concentration patterns for more than 80 single organic compounds have been measured at four urban sites (West Los Angeles, Downtown Los Angeles, Pasadena, and Rubidoux) and at one remote offshore site (San Nicolas Island). It has been found that cholesterol serves as a marker compound for emissions from charbroilers and other meat cooking operations. Vehicular exhaust being emitted from diesel and gasoline powered engines can be traced in the Los Angeles atmosphere using fossil petroleum marker compounds such as steranes and pentacyclic triterpanes (e.g., hopanes). Biogenic fine particle emission sources such as plant fragments abraded from leaf surfaces by wind and weather can be traced in the urban atmosphere. Using distinct and specific source organic tracers or assemblages of organic compounds characteristic for the sources considered it is possible to estimate the influence of different source types at any urban site where atmospheric data are available
Modeling the Formation of Secondary Organic Aerosol. 1. Application of Theoretical Principles to Measurements Obtained in the α-Pinene/, β-Pinene/, Sabinene/, Δ 3
Dynamics of the Destruction and Rebuilding of a Dipole Gap in Glasses
After a strong electric bias field was applied to a glass sample at
temperatures in the millikelvin range its AC-dielectric constant increases and
then decays logarithmically with time. For the polyester glass mylar we have
observed the relaxation of the dielectric constant back to its initial value
for several temperatures and histories of the bias field. Starting from the
dipole gap theory we have developed a model suggesting that the change of the
dielectric constant after transient application of a bias field is only partly
due to relaxational processes. In addition, non-adiabatic driving of tunneling
states (TSs) by applied electric fields causes long lasting changes in the
dielectric constant. Moreover, our observations indicate that at temperatures
below 50 mK the relaxation of TSs is caused primarily by interactions between
TSs.Comment: 4 pages, 4 figures, submitted to PR
Correlated Persistent Tunneling Currents in Glasses
Low temperature properties of glasses are derived within a generalized
tunneling model, considering the motion of charged particles on a closed path
in a double-well potential. The presence of a magnetic induction field B
violates the time reversal invariance due to the Aharonov-Bohm phase, and leads
to flux periodic energy levels. At low temperature, this effect is shown to be
strongly enhanced by dipole-dipole and elastic interactions between tunneling
systems and becomes measurable. Thus, the recently observed strong sensitivity
of the electric permittivity to weak magnetic fields can be explained. In
addition, superimposed oscillations as a function of the magnetic field are
predicted.Comment: 4 page
Meteorological influences on respirable fragment release from Chinese elm pollen
Exposure to airborne pollen from certain plants can cause allergic disease, leading to acute respiratory symptoms. Whole pollen grains, 15–90 μ m-sized particles, provoke the upper respiratory symptoms of rhinitis (hay fever), while smaller pollen fragments capable of depositing in the lower respiratory tract have been proposed as the trigger for asthma. In order to understand factors leading to pollen release and fragmentation we have examined the rupture of Chinese elm pollen under controlled laboratory conditions and in the outdoor atmosphere. Within 30 minutes after immersion in water, 70% of fresh Chinese pollen ruptures, rapidly expelling cytoplasm. Chinese elm flowers, placed in a controlled atmosphere chamber, emitted pollen and pollen debris after a sequential treatment of 98% relative humidity followed by drying and a gentle disturbance. Immunologic assays of antigenic proteins specific to elm pollens revealed that fine particulate material (D p < 2 μ m) collected from the chamber contained elm pollen antigens. In a temporal study of the outdoor urban atmosphere during the Chinese elm bloom season of 2004, peak concentrations of pollen and fine pollen fragments occurred at the beginning of the season when nocturnal relative humidity (RH) exceeded 90%. Following later periods of hot dry weather, pollen counts decreased to zero. The Chinese elm pollen fragments also decreased during the hot weather, but later displayed additional peaks following periods of more moderate RH and temperature, indicating that pollen counts underestimate total atmospheric pollen allergen concentrations. Pollen fragments thus increase the biogenic load in the atmosphere in a form that is no longer recognizable as pollen and, therefore, is not amenable to microscopic analysis. This raises the possibility of exposure of sensitive individuals to pollen allergens in the form of fine particles that can penetrate into the lower airways and pose potentially severe health risks.<br /
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 pages, 4 figure
Out of equilibrium dynamics of a Quantum Heisenberg Spin Glass
We study the out of equilibrium dynamics of the infinite range quantum
Heisenberg spin glass model coupled to a thermal relaxation bath. The SU(2)
spin algebra is generalized to SU(N) and we analyse the large-N limit. The
model displays a dynamical phase transition between a paramagnetic and a glassy
phase. In the latter, the system remains out of equilibrium and displays an
aging phenomenon, which we characterize using both analytical and numerical
methods. In the aging regime, the quantum fluctuation-dissipation relation is
violated and replaced at very long time by its classical generalization, as in
models involving simple spin algebras studied previously. We also discuss the
effect of a finite coupling to the relaxation baths and their possible forms.
This work completes and justifies previous studies on this model using a static
approach.Comment: Minor change
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