24,430 research outputs found
An H-Theorem for the Lattice Boltzmann Approach to Hydrodynamics
The lattice Boltzmann equation can be viewed as a discretization of the
continuous Boltzmann equation. Because of this connection it has long been
speculated that lattice Boltzmann algorithms might obey an H-theorem. In this
letter we prove that usual nine-velocity models do not obey an H-theorem but
models that do obey an H-theorem can be constructed. We consider the general
conditions a lattice Boltzmann scheme must satisfy in order to obey an
H-theorem and show why on a lattice, unlike the continuous case, dynamics that
decrease an H-functional do not necessarily lead to a unique ground state.Comment: 6 pages, latex, no figures, accepted for publication in Europhys.
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Weak localization in ferromagnetic (Ga,Mn)As nanostructures
We report on the observation of weak localization in arrays of (Ga,Mn)As
nanowires at millikelvin temperatures. The corresponding phase coherence length
is typically between 100 nm and 200 nm at 20 mK. Strong spin-orbit interaction
in the material is manifested by a weak anti-localization correction around
zero magnetic field.Comment: 5 pages, 3 figure
Apprentice pay in Britain, Germany and Switzerland: Institutions, market forces and market power
This is the accepted version of the original publication in the European Journal of Industrial Relations, which is available online at http://ejd.sagepub.com/content/19/3/201.The pay of metalworking apprentices is high in Britain, middling in Germany and low in Switzerland. We analyse these differences using fieldwork evidence and survey data, drawing on both economic and institutionalist theories. Several institutional attributes influence apprentice pay, partly by affecting supply and demand in markets for training places. Institutional support for apprenticeship training appears to involve important complementarities in both Germany and Switzerland, in contrast to Britain’s less coherent and more market-driven approach.We thank the Hans-Böckler-Stiftung, Anglo-German Foundation, SKOPE (Oxford), the Swiss federal government (OPET/SERI) and WZB (Berlin) for financial support
Charge transfer and coherence dynamics of tunnelling system coupled to a harmonic oscillator
We study the transition probability and coherence of a two-site system,
interacting with an oscillator. Both properties depend on the initial
preparation. The oscillator is prepared in a thermal state and, even though it
cannot be considered as an extended bath, it produces decoherence because of
the large number of states involved in the dynamics. In the case in which the
oscillator is intially displaced a coherent dynamics of change entangled with
oscillator modes takes place. Coherency is however degraded as far as the
oscillator mass increases producing a increasingly large recoherence time.
Calculations are carried on by exact diagonalization and compared with two
semiclassical approximations. The role of the quantum effects are highlighted
in the long-time dynamics, where semiclassical approaches give rise to a
dissipative behaviour. Moreover, we find that the oscillator dynamics has to be
taken into account, even in a semiclassical approximation, in order to
reproduce a thermally activated enhancement of the transition probability
Phase coherent transport in (Ga,Mn)As
Quantum interference effects and resulting quantum corrections of the
conductivity have been intensively studied in disordered conductors over the
last decades. The knowledge of phase coherence lengths and underlying dephasing
mechanisms are crucial to understand quantum corrections to the resistivity in
the different material systems. Due to the internal magnetic field and the
associated breaking of time-reversal symmetry quantum interference effects in
ferromagnetic materials have been scarcely explored. Below we describe the
investigation of phase coherent transport phenomena in the newly discovered
ferromagnetic semiconductor (Ga,Mn)As. We explore universal conductance
fluctuations in mesoscopic (Ga,Mn)As wires and rings, the Aharonov-Bohm effect
in nanoscale rings and weak localization in arrays of wires, made of the
ferromagnetic semiconductor material. The experiments allow to probe the phase
coherence length L_phi and the spin flip length L_SO as well as the temperature
dependence of dephasing.Comment: 22 pages, 10 figure
Design of 12.5 kA current leads for the Large Hadron Collider using high temperature superconductor material
The Large Hadron Collider will be equipped with about 8000 superconducting magnets. Some 2600 current leads will feed the currents ranging from 25 to 12500 A. CERN aims to reduce the consumption of liquid helium, using high temperature superconductors in these leads. A development of leads for 12.5 kA is being conducted in collaboration with Oxford Instruments. The design options for these leads are described. A test rig and prototype lead have been made according to one of the options. Electrical contact tests are in progress on BSCCO-2212 and YBCO-123 samples. In the first run, the prototype carried 13000 A
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