131 research outputs found
Measurement of the energy dependence of phase relaxation by single electron tunneling
Single electron tunneling through a single impurity level is used to probe
the fluctuations of the local density of states in the emitter. The energy
dependence of quasi-particle relaxation in the emitter can be extracted from
the damping of the fluctuations of the local density of states (LDOS). At
larger magnetic fields Zeeman splitting is observed.Comment: 2 pages, 4 figures; 25th International Conference on the Physics of
Semiconductors, Osaka, Japan, September 17-22, 200
On Generalizations of Network Design Problems with Degree Bounds
Iterative rounding and relaxation have arguably become the method of choice
in dealing with unconstrained and constrained network design problems. In this
paper we extend the scope of the iterative relaxation method in two directions:
(1) by handling more complex degree constraints in the minimum spanning tree
problem (namely, laminar crossing spanning tree), and (2) by incorporating
`degree bounds' in other combinatorial optimization problems such as matroid
intersection and lattice polyhedra. We give new or improved approximation
algorithms, hardness results, and integrality gaps for these problems.Comment: v2, 24 pages, 4 figure
Approximating the minimum directed tree cover
Given a directed graph with non negative cost on the arcs, a directed
tree cover of is a rooted directed tree such that either head or tail (or
both of them) of every arc in is touched by . The minimum directed tree
cover problem (DTCP) is to find a directed tree cover of minimum cost. The
problem is known to be -hard. In this paper, we show that the weighted Set
Cover Problem (SCP) is a special case of DTCP. Hence, one can expect at best to
approximate DTCP with the same ratio as for SCP. We show that this expectation
can be satisfied in some way by designing a purely combinatorial approximation
algorithm for the DTCP and proving that the approximation ratio of the
algorithm is with is the maximum outgoing degree of
the nodes in .Comment: 13 page
Zeeman energy and spin relaxation in a one-electron quantum dot
We have measured the relaxation time, T1, of the spin of a single electron
confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic
field, applied parallel to the two-dimensional electron gas in which the
quantum dot is defined, Zeeman splitting of the orbital states is directly
observed by measurements of electron transport through the dot. By applying
short voltage pulses, we can populate the excited spin state with one electron
and monitor relaxation of the spin. We find a lower bound on T1 of 50
microseconds at 7.5 T, only limited by our signal-to-noise ratio. A continuous
measurement of the charge on the dot has no observable effect on the spin
relaxation.Comment: Replaced with the version published in Phys. Rev. Let
Shot noise in resonant tunneling through a zero-dimensional state with a complex energy spectrum
We investigate the noise properties of a GaAs/AlGaAs resonant tunneling
structure at bias voltages where the current characteristic is determined by
single electron tunneling. We discuss the suppression of the shot noise in the
framework of a coupled two-state system. For large bias voltages we observed
super-Poissonian shot noise up to values of the Fano factor .Comment: 4 pages, 4 figures, accepted for Phys. Rev.
Characterization of steady-state fluorescence properties of polystyrene latex spheres using off- and online spectroscopic methods
Fluorescent dyed polystyrene latex spheres (PSLs) are commonly used for
characterization and calibration of instruments detecting fluorescence
signals from particles suspended in the air and other fluids. Instruments
like the Ultraviolet Aerodynamic Particle Sizer (UV-APS) and the Waveband
Integrated Bioaerosol Sensor (WIBS) are widely used for bioaerosol research,
but these instruments present significant technical and physical challenges
requiring careful characterization with standard particles. Many other
research communities use flow cytometry and other instruments that
interrogate fluorescence from individual particles, and these also frequently
rely on fluorescent PSLs as standards. Nevertheless, information about
physical properties of commercially available PSLs provided by each
manufacturer is generally proprietary and rarely available, making their use
in fluorescence validation and calibration very difficult.
This technical note presents an overview of steady-state fluorescence
properties of fluorescent and non-fluorescent PSLs, as well as of
polystyrene-divinylbenzene (PS-DVB) particles, by using on- and offline
spectroscopic techniques. We show that the fluorescence landscape of
PSLs is more complex than the information typically provided by
manufacturers may imply, especially revealing multimodal emission patterns.
Furthermore, non-fluorescent PSLs also exhibit defined patterns of
fluorescent emission originating from a mixture of polystyrene and
detergents, which becomes a crucial point for fluorescence threshold
calibrations and qualitative comparison between instruments. By comparing
PSLs of different sizes, but doped with the same dye, changes in emission
spectra from bulk solutions are not immediately obvious. On a
single-particle scale, however, fluorescence intensity values increase with
increasing particle size. No significant effect in the fluorescence
signatures was detectable by comparing PSLs in dry vs. wet states,
indicating that solvent water may only play a minor role as a fluorescence
quencher.
Because information provided by manufacturers of commercially available PSLs
is generally very limited, we provide the steady-state excitation–emission
matrices (EEMs) of PSLs as open-access data within the Supplement. Detergent
and solvent effects are also discussed in order to provide information not
available elsewhere to researchers in the bioaerosol and other research
communities. These data are not meant to serve as a fundamental library of
PSL properties because of the variability of fluorescent properties between
batches and as a function of particle aging and agglomeration. The data
presented, however, provide a summary of spectral features which are
consistent across these widely used fluorescent standards. Using these
concepts, further checks will likely be required by individual researchers
using specific lots of standards.</p
Strong quantum memory at resonant Fermi edges revealed by shot noise
Studies of non-equilibrium current fluctuations enable assessing correlations
involved in quantum transport through nanoscale conductors. They provide
additional information to the mean current on charge statistics and the
presence of coherence, dissipation, disorder, or entanglement. Shot noise,
being a temporal integral of the current autocorrelation function, reveals
dynamical information. In particular, it detects presence of non-Markovian
dynamics, i.e., memory, within open systems, which has been subject of many
current theoretical studies. We report on low-temperature shot noise
measurements of electronic transport through InAs quantum dots in the
Fermi-edge singularity regime and show that it exhibits strong memory effects
caused by quantum correlations between the dot and fermionic reservoirs. Our
work, apart from addressing noise in archetypical strongly correlated system of
prime interest, discloses generic quantum dynamical mechanism occurring at
interacting resonant Fermi edges.Comment: 6 pages, 3 figure
Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures
Measurements of resonant tunneling through a localized impurity state are
used to probe fluctuations in the local density of states of heavily doped
GaAs. The measured differential conductance is analyzed in terms of correlation
functions with respect to voltage. A qualitative picture based on the scaling
theory of Thouless is developed to relate the observed fluctuations to the
statistics of single particle wavefunctions. In a quantitative theory
correlation functions are calculated. By comparing the experimental and
theoretical correlation functions the effective dimensionality of the emitter
is analyzed and the dependence of the inelastic lifetime on energy is
extracted.Comment: 41 pages, 14 figure
Spectral Intensity Bioaerosol Sensor (SIBS): an instrument for spectrally resolved fluorescence detection of single particles in real time
Primary biological aerosol particles (PBAPs) in the atmosphere are highly relevant
for the Earth system, climate, and public health. The analysis of PBAPs,
however, remains challenging due to their high diversity and large
spatiotemporal variability. For real-time PBAP analysis, light-induced
fluorescence (LIF) instruments have been developed and widely used in
laboratory and ambient studies. The interpretation of fluorescence data from
these instruments, however, is often limited by a lack of spectroscopic
information. This study introduces an instrument – the Spectral Intensity
Bioaerosol Sensor (SIBS; Droplet Measurement Technologies (DMT), Longmont,
CO, USA) – that resolves fluorescence spectra for single particles and thus
promises to expand the scope of fluorescent PBAP quantification and
classification.
The SIBS shares key design components with the latest versions of the
Wideband Integrated Bioaerosol Sensor (WIBS) and the findings presented here
are also relevant for the widely deployed WIBS-4A and WIBS-NEO as well as
other LIF instruments. The key features of the SIBS and the findings of this
study can be summarized as follows.
Particle sizing yields reproducible linear responses for particles in the
range of 300 nm to 20 µm. The lower sizing limit is significantly
smaller than for earlier commercial LIF instruments (e.g., WIBS-4A and the
Ultraviolet Aerodynamic Particle Sizer; UV-APS), expanding the analytical
scope into the accumulation-mode size range.
Fluorescence spectra are recorded for two excitation wavelengths (λex=285 and 370 nm) and a wide range of emission wavelengths
(λmean=302–721 nm) with a resolution of 16
detection channels, which is higher than for most other commercially
available LIF bioaerosol sensors.
Fluorescence spectra obtained for 16 reference compounds confirm that the
SIBS provides sufficient spectral resolution to distinguish major modes of
molecular fluorescence. For example, the SIBS resolves the spectral
difference between bacteriochlorophyll and chlorophyll a and b.
A spectral correction of the instrument-specific detector response is
essential to use the full fluorescence emission range.
Asymmetry factor (AF) data were assessed and were found to provide only
limited analytical information.
In test measurements with ambient air, the SIBS worked reliably and yielded
characteristically different spectra for single particles in the coarse mode
with an overall fluorescent particle fraction of ∼4 %
(3σ threshold), which is consistent with earlier studies in
comparable environments.</ul
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