2,359 research outputs found
4d Simplicial Quantum Gravity Interacting with Gauge Matter Fields
The effect of coupling non-compact gauge fields to four dimensional
simplicial quantum gravity is studied using strong coupling expansions and
Monte Carlo simulations. For one gauge field the back-reaction of the matter on
the geometry is weak. This changes, however, as more matter fields are
introduced. For more than two gauge fields the degeneracy of random manifolds
into branched polymers does not occur, and the branched polymer phase seems to
be replaced by a new phase with a negative string susceptibility exponent
and fractal dimension .Comment: latex2e, 10 pages incorporating 2 tables and 3 figures (using epsf
Charge and spin state readout of a double quantum dot coupled to a resonator
State readout is a key requirement for a quantum computer. For
semiconductor-based qubit devices it is usually accomplished using a separate
mesoscopic electrometer. Here we demonstrate a simple detection scheme in which
a radio-frequency resonant circuit coupled to a semiconductor double quantum
dot is used to probe its charge and spin states. These results demonstrate a
new non-invasive technique for measuring charge and spin states in quantum dot
systems without requiring a separate mesoscopic detector
Discontinuity Theorem for First Order Phase Transitions. Implications for QCD
A first order phase transition leading to deconfinement and chiral
restoration is a likely possibility for QCD, at least in some region of the
temperature-density plane. A signal for a unique transition is that the order
parameters for such transitions (which can be understood in terms of symmetries
only in limiting situations of very massive or massless quarks) are both
discontinuous at the same critical temperature. We show that such a situation
can be understood on a precise thermodynamical basis because of a general
relation among discontinuities which holds for first order transitions. We
derive the result by a generalization of the Clausius-Clapeyron equation and
also through the effective action formalism. We illustrate the theorem in an
elementary example.Comment: 10 pages, LATEX, UGVA-DPT 1992/12/79
The Strong-Coupling Expansion in Simplicial Quantum Gravity
We construct the strong-coupling series in 4d simplicial quantum gravity up
to volume 38. It is used to calculate estimates for the string susceptibility
exponent gamma for various modifications of the theory. It provides a very
efficient way to get a first view of the phase structure of the models.Comment: LATTICE98(surfaces), 3 pages, 4 eps figure
The SU(3) deconfining phase transition with Symanzik action
We report on the determination of the deconfining temperature in SU(3) pure
gauge theory, using the Symanzik tree level improved action, on lattices of
size 3 x 12^3, 4 x 16^3, 5 x 20^3, 6 x24^3. We find that the asymptotic scaling
violation pattern is similar to the one observed using the Wilson action. We
conclude that the irrelevant operators do not affect, in the range of couplings
considered, the lattice beta function. An analysis based on an effective
coupling formulation shows an apparent improvement.Comment: 8 pages, report IFUP-TH 12/9
Neural mechanisms for voice recognition
We investigated neural mechanisms that support voice recognition in a training paradigm with fMRI. The same listeners were trained on different weeks to categorize the mid-regions of voice-morph continua as an individual's voice. Stimuli implicitly defined a voice-acoustics space, and training explicitly defined a voice-identity space. The predefined centre of the voice category was shifted from the acoustic centre each week in opposite directions, so the same stimuli had different training histories on different tests. Cortical sensitivity to voice similarity appeared over different time-scales and at different representational stages. First, there were short-term adaptation effects: Increasing acoustic similarity to the directly preceding stimulus led to haemodynamic response reduction in the middle/posterior STS and in right ventrolateral prefrontal regions. Second, there were longer-term effects: Response reduction was found in the orbital/insular cortex for stimuli that were most versus least similar to the acoustic mean of all preceding stimuli, and, in the anterior temporal pole, the deep posterior STS and the amygdala, for stimuli that were most versus least similar to the trained voice-identity category mean. These findings are interpreted as effects of neural sharpening of long-term stored typical acoustic and category-internal values. The analyses also reveal anatomically separable voice representations: one in a voice-acoustics space and one in a voice-identity space. Voice-identity representations flexibly followed the trained identity shift, and listeners with a greater identity effect were more accurate at recognizing familiar voices. Voice recognition is thus supported by neural voice spaces that are organized around flexible ‘mean voice’ representations
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Tracer concentration profiles measured in central London as part of the REPARTEE campaign
There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent’s Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London
Bias spectroscopy and simultaneous SET charge state detection of Si:P double dots
We report a detailed study of low-temperature (mK) transport properties of a
silicon double-dot system fabricated by phosphorous ion implantation. The
device under study consists of two phosphorous nanoscale islands doped to above
the metal-insulator transition, separated from each other and the source and
drain reservoirs by nominally undoped (intrinsic) silicon tunnel barriers.
Metallic control gates, together with an Al-AlOx single-electron transistor,
were positioned on the substrate surface, capacitively coupled to the buried
dots. The individual double-dot charge states were probed using source-drain
bias spectroscopy combined with non-invasive SET charge sensing. The system was
measured in linear (VSD = 0) and non-linear (VSD 0) regimes allowing
calculations of the relevant capacitances. Simultaneous detection using both
SET sensing and source-drain current measurements was demonstrated, providing a
valuable combination for the analysis of the system. Evolution of the triple
points with applied bias was observed using both charge and current sensing.
Coulomb diamonds, showing the interplay between the Coulomb charging effects of
the two dots, were measured using simultaneous detection and compared with
numerical simulations.Comment: 7 pages, 6 figure
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