933 research outputs found
Superlattice with hot electron injection: an approach to a Bloch oscillator
A semiconductor superlattice with hot electron injection into the miniband is
considered. The injection changes the stationary distribution function and
results in a qualitative change of the frequency behaviour of the differential
conductivity. In the regime with Bloch oscillating electrons and injection into
the upper part of the miniband the region of negative differential conductivity
is shifted from low frequencies to higher frequencies. We find that the dc
differential conductivity can be made positive and thus the domain instability
can be suppressed. At the same time the high-frequency differential
conductivity is negative above the Bloch frequency. This opens a new way to
make a Bloch oscillator operating at THz frequencies.Comment: RevTeX, 8 pages, 2 figures, to be published in Phys. Rev. B, 15
Januar 200
Direct measurement of decoherence for entanglement between a photon and stored atomic excitation
Violations of a Bell inequality are reported for an experiment where one of
two entangled qubits is stored in a collective atomic memory for a user-defined
time delay. The atomic qubit is found to preserve the violation of a Bell
inequality for storage times up to 21 microseconds, 700 times longer than the
duration of the excitation pulse that creates the entanglement. To address the
question of the security of entanglement-based cryptography implemented with
this system, an investigation of the Bell violation as a function of the
cross-correlation between the generated nonclassical fields is reported, with
saturation of the violation close to the maximum value allowed by quantum
mechanics.Comment: 4 pages, 3 figures. Minor changes. Published versio
Symmetry-breaking and chaos in electron transport in semiconductor superlattices
We study the motion of electrons in a single miniband of a semiconductor
superlattice driven by THz electric field polarized along the growth direction.
We work in the semiclassical balance-equation model, including different
elastic and inelastic scattering rates, and incorporating the self-consistent
electric field generated by electron motion. We explore regions of complex
dynamics, which can include chaotic behaviour and symmetry-breaking. We
estimate the magnitudes of dc current and dc voltage that spontaneously appear
in regions of broken-symmetry for parameters characteristic of modern
semiconductor superlattices. This work complements PRL 80(1998)2669 [
cond-mat/9709026 ].Comment: 4 pages, 3 figures, RevTEX, EPS
Automatic Assignment of EC Numbers
A wide range of research areas in molecular biology and medical biochemistry require a reliable enzyme classification system, e.g., drug design, metabolic network reconstruction and system biology. When research scientists in the above mentioned areas wish to unambiguously refer to an enzyme and its function, the EC number introduced by the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB) is used. However, each and every one of these applications is critically dependent upon the consistency and reliability of the underlying data for success. We have developed tools for the validation of the EC number classification scheme. In this paper, we present validated data of 3788 enzymatic reactions including 229 sub-subclasses of the EC classification system. Over 80% agreement was found between our assignment and the EC classification. For 61 (i.e., only 2.5%) reactions we found that their assignment was inconsistent with the rules of the nomenclature committee; they have to be transferred to other sub-subclasses. We demonstrate that our validation results can be used to initiate corrections and improvements to the EC number classification scheme
Higher education and unemployment in Europe : an analysis of the academic subject and national effects
This paper examines the impact of an academic degree and field of study on short and long-term unemployment across Europe (EU15). Labour Force Survey (LFS) data on over half a million individuals are utilised for that purpose. The harmonized LFS classification of level of education and field of study overcomes past problems of comparability across Europe. The study analyses (i) the effect of an academic degree at a European level, (ii) the specific effect of 14 academic subjects and (iii) country specific effects. The results indicate that an academic degree is more effective on reducing the likelihood of short-term than long-term unemployment. This general pattern even though it is observed for most of the academic subjects its levels show significant variation across disciplines and countries
Generalized drift-diffusion model for miniband superlattices
A drift-diffusion model of miniband transport in strongly coupled
superlattices is derived from the single-miniband Boltzmann-Poisson transport
equation with a BGK (Bhatnagar-Gross-Krook) collision term. We use a consistent
Chapman-Enskog method to analyze the hyperbolic limit, at which collision and
electric field terms dominate the other terms in the Boltzmann equation. The
reduced equation is of the drift-diffusion type, but it includes additional
terms, and diffusion and drift do not obey the Einstein relation except in the
limit of high temperatures.Comment: 4 pages, 3 figures, double-column revtex. To appear as RC in PR
Characterization of growth and metabolism of the haloalkaliphile Natronomonas pharaonis
Natronomonas pharaonis is an archaeon adapted to two extreme conditions: high salt concentration and alkaline pH. It has become one of the model organisms for the study of extremophilic life. Here, we present a genome-scale, manually curated metabolic reconstruction for the microorganism. The reconstruction itself represents a knowledge base of the haloalkaliphile's metabolism and, as such, would greatly assist further investigations on archaeal pathways. In addition, we experimentally determined several parameters relevant to growth, including a characterization of the biomass composition and a quantification of carbon and oxygen consumption. Using the metabolic reconstruction and the experimental data, we formulated a constraints-based model which we used to analyze the behavior of the archaeon when grown on a single carbon source. Results of the analysis include the finding that Natronomonas pharaonis, when grown aerobically on acetate, uses a carbon to oxygen consumption ratio that is theoretically near-optimal with respect to growth and energy production. This supports the hypothesis that, under simple conditions, the microorganism optimizes its metabolism with respect to the two objectives. We also found that the archaeon has a very low carbon efficiency of only about 35%. This inefficiency is probably due to a very low P/O ratio as well as to the other difficulties posed by its extreme environment
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