256 research outputs found
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 pages, 4 figure
Low temperature acoustic properties of amorphous silica and the Tunneling Model
Internal friction and speed of sound of a-SiO(2) was measured above 6 mK
using a torsional oscillator at 90 kHz, controlling for thermal decoupling,
non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark
the transition between the linear and non-linear regime. In the linear regime,
excellent agreement with the Tunneling Model was observed for both the internal
friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the
non-linear regime, two different behaviors were observed. Above 10 mK the
behavior was typical for non-linear harmonic oscillators, while below 10 mK a
different behavior was found. Its origin is not understood.Comment: 1 tex file, 6 figure
Out of equilibrium dynamics of a Quantum Heisenberg Spin Glass
We study the out of equilibrium dynamics of the infinite range quantum
Heisenberg spin glass model coupled to a thermal relaxation bath. The SU(2)
spin algebra is generalized to SU(N) and we analyse the large-N limit. The
model displays a dynamical phase transition between a paramagnetic and a glassy
phase. In the latter, the system remains out of equilibrium and displays an
aging phenomenon, which we characterize using both analytical and numerical
methods. In the aging regime, the quantum fluctuation-dissipation relation is
violated and replaced at very long time by its classical generalization, as in
models involving simple spin algebras studied previously. We also discuss the
effect of a finite coupling to the relaxation baths and their possible forms.
This work completes and justifies previous studies on this model using a static
approach.Comment: Minor change
Quantum TAP equations
We derive Thouless-Anderson-Palmer (TAP) equations for quantum disordered
systems. We apply them to the study of the paramagnetic and glassy phases in
the quantum version of the spherical p spin-glass model. We generalize several
useful quantities (complexity, threshold level, etc.) and various ideas
(configurational entropy crisis, etc), that have been developed within the
classical TAP approach, to quantum systems. The analysis of the quantum TAP
equations allows us to show that the phase diagram (temperature-quantum
parameter) of the p spin-glass model should be generic. In particular, we argue
that a crossover from a second order thermodynamic transition close to the
classical critical point to a first order thermodynamic transition close to the
quantum critical point is to be expected in a large class of systems.Comment: 29 pages, 4 fi
Communication Research
Contains reports on seven research projects.Rockefeller FoundationCarnegie Foundatio
Comparing nuclear power trajectories in Germany and the UK: from ‘regimes' to ‘democracies’ in sociotechnical transitions and Discontinuities
This paper focuses on arguably the single most striking contrast in contemporary major energy politics in Europe (and even the developed world as a whole): the starkly differing civil nuclear policies of Germany and the UK. Germany is seeking entirely to phase out nuclear power by 2022. Yet the UK advocates a ‘nuclear renaissance’, promoting the most ambitious new nuclear construction programme in Western Europe.Here,this paper poses a simple yet quite fundamental question: what are the particular divergent conditions most strongly implicated in the contrasting developments in these two countries. With nuclear playing such an iconic role in historical discussions over technological continuity and transformation, answering this may assist in wider understandings of sociotechnical incumbency and discontinuity in the burgeoning field of‘sustainability transitions’. To this end, an ‘abductive’ approach is taken: deploying nine potentially relevant criteria for understanding the different directions pursued in Germany and the UK. Together constituted by 30 parameters spanning literatures related to socio-technical regimes in general as well as nuclear technology in particular, the criteria are divided into those that are ‘internal’ and ‘external’ to the ‘focal regime configuration’ of nuclear power and associated ‘challenger technologies’ like renewables.
It is ‘internal’ criteria that are emphasised in conventional sociotechnical regime theory, with ‘external’ criteria relatively less well explored. Asking under each criterion whether attempted discontinuation of nuclear power would be more likely in Germany or the UK, a clear picture emerges. ‘Internal’ criteria suggest attempted nuclear discontinuation should be more likely in the UK than in Germany– the reverse of what is occurring.
‘External’ criteria are more aligned with observed dynamics –especially those relating to military nuclear commitments and broader ‘qualities of democracy’. Despite many differences of framing concerning exactly what constitutes ‘democracy’, a rich political science literature on this point is unanimous in characterising Germany more positively than the UK. Although based only on a single case,a potentially important question is nonetheless raised as to whether sociotechnical regime theory might usefully give greater attention to the general importance of various aspects of democracy in constituting conditions for significant technological discontinuities and transformations. If so, the policy implications are significant. A number of important areas are identified for future research, including the roles of diverse understandings and specific aspects of democracy and the particular relevance of military nuclear commitments– whose under-discussion in civil nuclear policy literatures raises its own questions of democratic accountability
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