61 research outputs found
Elastic response of [111]-tunneling impurities
We study the dynamic response of a [111] quantum impurity, such as lithium or
cyanide in alkali halides, with respect to an external field coupling to the
elastic quadrupole moment. Because of the particular level structure of a
eight-state system on a cubic site, the elastic response function shows a
biexponential relaxation feature and a van Vleck type contribution with a
resonance frequency that is twice the tunnel frequency . This
basically differs from the dielectric response that does not show relaxation.
Moreover, we show that the elastic response of a [111] impurity cannot be
reduced to that of a two-level system. In the experimental part, we report on
recent sound velocity and internal friction measurements on KCl doped with
cyanide at various concentrations. At low doping (45 ppm) we find the dynamics
of a single [111] impurity, whereas at higher concentrations (4700 ppm) the
elastic response rather indicates strongly correlated defects. Our theoretical
model provides a good description of the temperature dependence of
and at low doping, in particular the relaxation peaks, the absolute
values of the amplitude, and the resonant contributions. From our fits we
obtain the value of the elastic deformation potential eV.Comment: 19 pages, 5 figure
Interacting quantum rotors in oxygen-doped germanium
We investigate the interaction effect between oxygen impurities in
crystalline germanium on the basis of a quantum rotor model. The dipolar
interaction of nearby oxygen impurities engenders non-trivial low-lying
excitations, giving rise to anomalous behaviors for oxygen-doped germanium
(Ge:O) below a few degrees Kelvin. In particular, it is theoretically predicted
that Ge:O samples with oxygen-concentration of 10cm show (i)
power-law specific heats below 0.1 K, and (ii) a peculiar hump in dielectric
susceptibilities around 1 K. We present an interpretation for the power-law
specific heats, which is based on the picture of local double-well potentials
randomly distributed in Ge:O samples.Comment: 13 pages, 11 figures; to be published in Phys. Rev.
Solving spin quantum-master equations with matrix continued-fraction methods: application to superparamagnets
We implement continued-fraction techniques to solve exactly quantum master
equations for a spin with arbitrary S coupled to a (bosonic) thermal bath. The
full spin density matrix is obtained, so that along with relaxation and
thermoactivation, coherent dynamics is included (precession, tunnel, etc.). The
method is applied to study isotropic spins and spins in a bistable anisotropy
potential (superparamagnets). We present examples of static response, the
dynamical susceptibility including the contribution of the different relaxation
modes, and of spin resonance in transverse fields.Comment: Resubmitted to J. Phys. A: Math. Gen. Some rewriting here and there.
Discussion on positivity in App.D3 at request of one refere
Ellipsoidal particles at fluid interfaces
For partially wetting, ellipsoidal colloids trapped at a fluid interface,
their effective, interface--mediated interactions of capillary and
fluctuation--induced type are analyzed. For contact angles different from
90, static interface deformations arise which lead to anisotropic capillary
forces that are substantial already for micrometer--sized particles. The
capillary problem is solved using an efficient perturbative treatment which
allows a fast determination of the capillary interaction for all distances
between and orientations of two particles. Besides static capillary forces,
fluctuation--induced forces caused by thermally excited capillary waves arise
at fluid interfaces. For the specific choice of a spatially fixed three--phase
contact line, the asymptotic behavior of the fluctuation--induced force is
determined analytically for both the close--distance and the long--distance
regime and compared to numerical solutions.Comment: 10 pages, submitted to EPJE (Special Issue Soft Matter Conference
Aachen'07
Thermophoresis of charged colloidal particles
Thermally induced particle flow in a charged colloidal suspension is studied
in a fluid-mechanical approach. The force density acting on the charged
boundary layer is derived in detail. From Stokes' equation with no-slip
boundary conditions at the particle surface, we obtain the particle drift
velocity and the thermophoretic transport coefficients. The results are
discussed in view of previous work and available experimental data.Comment: 9 pages, 2 figure
Quantum gates by coupled asymmetric quantum dots and controlled-NOT-gate operation
A quantum computer based on an asymmetric coupled dot system has been
proposed and shown to operate as the controlled-NOT-gate. The basic idea is (1)
the electron is localized in one of the asymmetric coupled dots. (2)The
electron transfer takes place from one dot to the other when the energy-levels
of the coupled dots are set close. (3)The Coulomb interaction between the
coupled dots mutually affects the energy levels of the other coupled dots. The
decoherence time of the quantum computation and the measurement time are
estimated. The proposed system can be realized by developing the technology of
the single-electron memory using Si nanocrystals and the direct combination of
the quantum circuit and the conventional circuit is possible.Comment: LaTeX, 7 pages, 5 figures, revised content, to appear in Phys. Rev.
Cumulant Expansions and the Spin-Boson Problem
The dynamics of the dissipative two-level system at zero temperature is
studied using three different cumulant expansion techniques. The relative
merits and drawbacks of each technique are discussed. It is found that a new
technique, the non-crossing cumulant expansion, appears to embody the virtues
of the more standard cumulant methods.Comment: 26 pages, LaTe
Preparing for disaster: a comparative analysis of education for critical infrastructure collapse
This article explores policy approaches to educating populations for potential critical infrastructure collapse in five different countries: the UK, the US, Germany, Japan and New Zealand. ‘Critical infrastructure’ is not always easy to define, and indeed is defined slightly differently across countries – it includes entities vital to life, such as utilities (water, energy), transportation systems and communications, and may also include social and cultural infrastructure. The article is a mapping exercise of different approaches to critical infrastructure protection and preparedness education by the five countries. The exercise facilitates a comparison of the countries and enables us to identify distinctive characteristics of each country’s approach. We argue that contrary to what most scholars of security have argued, these national approaches diverge greatly, suggesting that they are shaped more by internal politics and culture than by global approaches
The ‘state of exception’ and disaster education: a multilevel conceptual framework with implications for social justice
The term ‘state of exception’ has been used by Italian political theorist Giorgio Agamben to explain the ways in which emergencies, crises and disasters are used by governments to suspend legal processes. In this paper, we innovatively apply Agamben’s theory to the way in which countries prepare and educate the population for various types of emergencies. We focus on two main aspects of Agamben’s work: first, the paradoxical nature of the state of exception, as both a transient and a permanent part of governance. Second, it is a ‘liminal’ concept expressing the limits of law and where ‘law’ meets ‘not-law’. We consider the relationship between laws related to disasters and emergencies, and case studies of the ways in which three countries (England, Germany and Japan) educate their populations for crisis and disaster. In England, we consider how emergency powers have been orientated around the protection of the Critical National Infrastructure and how this has produced localised ‘states of exception’ and, relatedly, pedagogical anomalies. In Germany, we consider the way in which laws related to disaster and civil protection, and the nature of volunteering for civil protection, produce exceptional spaces for non-German bodies. In Japan, we consider the debate around the absence of emergency powers and relate this to Japanese non-exceptional disaster education for natural disasters. Applying Agamben’s work, we conclude by developing a new, multilevel empirical framework for analysing disaster education with implications for social justice
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