535 research outputs found
Bond orbital description of the strain induced second order optical susceptibility in silicon
We develop a theoretical model, relying on the well established sp3
bond-orbital theory, to describe the strain-induced in
tetrahedrally coordinated centrosymmetric covalent crystals, like silicon. With
this approach we are able to describe every component of the
tensor in terms of a linear combination of strain gradients and only two
parameters and which can be estimated theoretically. The
resulting formula can be applied to the simulation of the strain distribution
of a practical strained silicon device, providing an extraordinary tool for
optimization of its optical nonlinear effects. By doing that, we were able not
only to confirm the main valid claims known about in strained
silicon, but also estimate the order of magnitude of the generated
in that device
Activated escape of a self-propelled particle from a metastable state
We study the noise-driven escape of active Brownian particles (ABPs) and
run-and-tumble particles (RTPs) from confining potentials. In the small noise
limit, we provide an exact expression for the escape rate in term of a
variational problem in any dimension. For RTPs in one dimension, we obtain an
explicit solution, including the first sub-leading correction. In two
dimensions we solve the escape from a quadratic well for both RTPs and ABPs. In
contrast to the equilibrium problem we find that the escape rate depends
explicitly on the full shape of the potential barrier, and not only on its
height. This leads to a host of unusual behaviors. For example, when a particle
is trapped between two barriers it may preferentially escape over the higher
one. Moreover, as the self-propulsion speed is varied, the escape route may
discontinuously switch from one barrier to the other, leading to a dynamical
phase transition
Nonlocal stationary probability distributions and escape rates for an active Ornstein-Uhlenbeck particle
We evaluate the steady-state distribution and escape rate for an Active
Ornstein-Uhlenbeck Particle (AOUP) using methods from the theory of large
deviations. The calculation is carried out both for small and large memory
times of the active force in one-dimension. We compare our results to those
obtained in the literature about colored noise processes, and we emphasize
their relevance for the field of active matter. In particular, we stress that
contrary to equilibrium particles, the invariant measure of such an active
particle is a non-local function of the potential. This fact has many
interesting consequences, which we illustrate through two phenomena. First,
active particles in the presence of an asymmetric barrier tend to accumulate on
one side of the potential -a ratchet effect that was missing is previous
treatments. Second, an active particle can escape over a deep metastable state
without spending any time at its bottom
Effective driven dynamics for one-dimensional conditioned Langevin processes in the weak-noise limit
In this work we focus on fluctuations of time-integrated observables for a
particle diffusing in a one-dimensional periodic potential in the weak-noise
asymptotics. Our interest goes to rare trajectories presenting an atypical
value of the observable, that we study through a biased dynamics in a
large-deviation framework. We determine explicitly the effective
probability-conserving dynamics which makes rare trajectories of the original
dynamics become typical trajectories of the effective one. Our approach makes
use of a weak-noise path-integral description in which the action is minimised
by the rare trajectories of interest. For `current-type' additive observables,
we find the emergence of a propagative trajectory minimising the action for
large enough deviations, revealing the existence of a dynamical phase
transition at a fluctuating level. In addition, we provide a new method to
determine the scaled cumulant generating function of the observable without
having to optimise the action. It allows one to show that the weak-noise and
the large-time limits commute in this problem. Finally, we show how the biased
dynamics can be mapped in practice to an effective driven dynamics, which takes
the form of a driven Langevin dynamics in an effective potential. The
non-trivial shape of this effective potential is key to understand the link
between the dynamical phase transition in the large deviations of current and
the standard depinning transition of a particle in a tilted potential
Encouraging self-reflection in social work students: Using Personal Construct methods
We report on a research study in which we introduced undergraduate social work students in England to two methods originating in Personal Construct Theory (PCT) as a way of encouraging in-depth self-reflection. The methods were first piloted with four students and subsequently used with two large classes of second year students. The students often found the methods challenging, but many felt that these enabled them to reflect upon social work practice and upon their own assumptions, values and behaviour in a fresh and thought-provoking way. We argue that such methods could usefully be added to existing methods for encouraging reflexivity in social work students. Teaching materials and instructions for delivering our teaching session using these methods are available online at http://www.hud.ac.uk/research/researchcentres/capr/projects/personal-construct-methods-in-reflective-practice
Optical Gain in Carbon Nanotubes
Semiconducting single-wall carbon nanotubes (s-SWNTs) have proved to be
promising material for nanophotonics and optoelectronics. Due to the
possibility of tuning their direct band gap and controlling excitonic
recombinations in the near-infrared wavelength range, s-SWNT can be used as
efficient light emitters. We report the first experimental demonstration of
room temperature intrinsic optical gain as high as 190 cm-1 at a wavelength of
1.3 {\mu}m in a thin film doped with s-SWNT. These results constitute a
significant milestone toward the development of laser sources based on carbon
nanotubes for future high performance integrated circuits.Comment: 4 figure
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