561 research outputs found
Ehrenfest-time dependence of weak localization in open quantum dots
Semiclassical theory predicts that the weak localization correction to the
conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time
exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys.
Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak
localization in the open quantum kicked rotator that confirm this prediction.
Our results disagree with the `effective random matrix theory' of transport
through ballistic chaotic cavities.Comment: 4 pages, 2 figure
Positive Cross Correlations in a Normal-Conducting Fermionic Beam Splitter
We investigate a beam splitter experiment implemented in a normal conducting
fermionic electron gas in the quantum Hall regime. The cross-correlations
between the current fluctuations in the two exit leads of the three terminal
device are found to be negative, zero or even positive depending on the
scattering mechanism within the device. Reversal of the cross-correlations sign
occurs due to interaction between different edge-states and does not reflect
the statistics of the fermionic particles which `antibunch'.Comment: 4 pages, 4 figure
Dynamic generation of orbital quasiparticle entanglement in mesoscopic conductors
We propose a scheme for dynamically creating orbitally entangled
electron-hole pairs through a time-dependent variation of the electrical
potential in a mesoscopic conductor. The time-dependent potential generates a
superposition of electron-hole pairs in two different orbital regions of the
conductor, a Mach-Zehnder interferometer in the quantum Hall regime. The
orbital entanglement is detected via violation of a Bell inequality, formulated
in terms of zero-frequency current noise. Adiabatic cycling of the potential,
both in the weak and strong amplitude limit, is considered.Comment: 4 pages, 2 figures; references update
A Model To Determine Capital Needs For Efficient Boer Goat Farming
The purpose of this study is to determine the optimal capital needs to establish a Boer goat farm. A study was performed to simulate the financial performance of 55 scenarios, with varying farm sizes and varying levels of capital employed. These results formed the basis to create a data envelopment analysis model, where farm sizes and capital employed are used as input variables versus eight financial performance indicators as output variables. The study found that the scenarios are technically efficient, but only 23 of the 55 scenarios are fully scale efficient, with six scenarios operating on a scale that is too small and 26 operating on a scale that is too large, which implies that 32 of the scenarios did not achieve economies of scale. Furthermore, nine of the ten farm sizes used are not scale efficient when using 100% of their capital needs. The study therefore concludes that the financial success of starting a South African (SA) Boer goat farm is locked up in the natural growth of the herd, with the practical implication that aspirating SA Boer goat farmers will be most efficient when using a limited amount of capital and not immediately stock the farm at the maximum level of animals, but to stock it only partly and wait for the gain in the growth of the herd
Estimating Scale Efficiency Of Platinum-Mining Companies Environmental Performance: A South African Perspective
The purpose of the study is to develop a data envelopment analysis (DEA) model to estimate the relative scale efficiency of platinum-mining companies environmental performance. South African platinum-mines were used to demonstrate the model, which uses environmental performance indicators as the input variables in order to generate mineral extraction and financial performances as the output variables. The input variables considered were greenhouse gas emissions, water usage and energy usage, while the output variables were platinum production, return on equity and return on assets. The contribution of the study is that a DEA model was developed that could identify relatively efficient companies that could act as benchmarks with regard to environmental issues in the mining sector. A further contribution is that the study concluded that platinum-mining companies tend not to achieve economies of scale, where the companies that are relatively larger in size tend to operate on a scale that is too large and companies that are physically relatively smaller in size tend to operate on a scale that is too small
Classical limit of transport in quantum kicked maps
We investigate the behavior of weak localization, conductance fluctuations,
and shot noise of a chaotic scatterer in the semiclassical limit. Time resolved
numerical results, obtained by truncating the time-evolution of a kicked
quantum map after a certain number of iterations, are compared to semiclassical
theory. Considering how the appearance of quantum effects is delayed as a
function of the Ehrenfest time gives a new method to compare theory and
numerical simulations. We find that both weak localization and shot noise agree
with semiclassical theory, which predicts exponential suppression with
increasing Ehrenfest time. However, conductance fluctuations exhibit different
behavior, with only a slight dependence on the Ehrenfest time.Comment: 17 pages, 13 figures. Final versio
Quantum state tomography with quantum shotnoise
We propose a scheme for a complete reconstruction of one- and two-particle
orbital quantum states in mesoscopic conductors. The conductor in the transport
state continuously emits orbital quantum states. The orbital states are
manipulated by electronic beamsplitters and detected by measurements of average
currents and zero frequency current shotnoise correlators. We show how, by a
suitable complete set of measurements, the elements of the density matrices of
the one- and two-particle states can be directly expressed in terms of the
currents and current correlators.Comment: 4 pages, 2 figure
Shot Noise and Full Counting Statistics from Non-equilibrium Plasmons in Luttinger-Liquid Junctions
We consider a quantum wire double junction system with each wire segment
described by a spinless Luttinger model, and study theoretically shot noise in
this system in the sequential tunneling regime. We find that the
non-equilibrium plasmonic excitations in the central wire segment give rise to
qualitatively different behavior compared to the case with equilibrium
plasmons. In particular, shot noise is greatly enhanced by them, and exceeds
the Poisson limit. We show that the enhancement can be explained by the
emergence of several current-carrying processes, and that the effect disappears
if the channels effectively collapse to one due to, {\em e.g.}, fast plasmon
relaxation processes.Comment: 9 pages; IOP Journal style; several changes in the tex
Management of Mine Explosion Risks
The management of explosions in the underground mining environment has been a high priority for managers since the turn of the century. In the early 1900s explosions, worldwide, reached catastrophic dimensions. In a number of large explosions more than 1000 people have died. In the early years of research, the basics of why explosions occurred were investigated and the participation of coal dust in underground coal mine explosions was proved. Taffanel, a French researcher, was one of the pioneers in establishing the use of stone dust and was instrumental in the design and development of the first stone-dust barriers. Throughout the century, research on explosions has continued in different parts of the world. Since explosions still occur in underground coal mines, as recent events in the Ukraine have illustrated, a certain problem area still exists. This area can be regarded as managerial. This paper gives an overview of the progress made by management in implementing explosion-prevention strategies and presents a new approach to explosion management in terms of an explosion management model, based on sound risk management principles. These principles allow the manager to close the loop in normal explosion management by focusing on all aspects of risk. Including auditing and monitoring to adequately address and manage the residual risk
Molecular states in carbon nanotube double quantum dots
We report electrical transport measurements through a semiconducting
single-walled carbon nanotube (SWNT) with three additional top-gates. At low
temperatures the system acts as a double quantum dot with large inter-dot
tunnel coupling allowing for the observation of tunnel-coupled molecular states
extending over the whole double-dot system. We precisely extract the tunnel
coupling and identify the molecular states by the sequential-tunneling line
shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
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