685 research outputs found
Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field
We study a resistively shunted semiconductor superlattice subject to a
high-frequency electric field. Using a balance equation approach that
incorporates the influence of the electric circuit, we determine numerically a
range of amplitude and frequency of the ac field for which a dc bias and
current are generated spontaneously and show that this region is likely
accessible to current experiments. Our simulations reveal that the Bloch
frequency corresponding to the spontaneous dc bias is approximately an integer
multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure
Microwave Photoconductivity in Two-Dimensional Electron Systems due to Photon-Assisted Interaction of Electrons with Leaky Interface Phonons
We calculate the contribution of the photon-assisted interaction of electrons
with leaky interface phonons to the dissipative dc photoconductivity of a
two-dimensional electron system in a magnetic field. The calculated
photoconductivity as a function of the frequency of microwave radiation and the
magnetic field exhibits pronounced oscillations. The obtained oscillation
structure is different from that in the case of photon-assisted interaction
with impurities. We demonstrate that at a sufficiently strong microwave
radiation in the certain ranges of its frequency (or in certain ranges of the
magnetic field) this mechanism can result in the absolute negative
conductivity.Comment: 3 pages, 1 figur
Resonant Photon-Assisted Tunneling Through a Double Quantum Dot: An Electron Pump From Spatial Rabi Oscillations
The time average of the fully nonlinear current through a double quantum dot,
subject to an arbitrary combination of ac and dc voltages, is calculated
exactly using the Keldysh nonequilibrium Green function technique. When driven
on resonance, the system functions as an efficient electron pump due to Rabi
oscillation between the dots. The pumping current is maximum when the coupling
to the leads equals the Rabi frequency.Comment: 6 pages, REVTEX 3.0, 3 postscript figure
Theory of Coherent Time-dependent Transport in One-dimensional Multiband Semiconductor Superlattices
We present an analytical study of one-dimensional semiconductor superlattices
in external electric fields, which may be time-dependent. A number of general
results for the (quasi)energies and eigenstates are derived. An equation of
motion for the density matrix is obtained for a two-band model, and the
properties of the solutions are analyzed. An expression for the current is
obtained. Finally, Zener-tunneling in a two-band tight-binding model is
considered. The present work gives the background and an extension of the
theoretical framework underlying our recent Letter [J. Rotvig {\it et al.},
Phys. Rev. Lett. {\bf 74}, 1831 (1995)], where a set of numerical simulations
were presented.Comment: 15 pages, Revtex 3.0, uses epsf, 2 ps figures attache
Metformin Treatment Attenuates Brain Inflammation and Rescues PACAP/VIP Neuropeptide Alterations in Mice Fed a High-Fat Diet.
High-fat diet (HFD)-induced comorbid cognitive and behavioural impairments are thought to be the result of persistent low-grade neuroinflammation. Metformin, a first-line medication for the treatment of type-2 diabetes, seems to ameliorate these comorbidities, but the underlying mechanism(s) are not clear. Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) are neuroprotective peptides endowed with anti-inflammatory properties. Alterations to the PACAP/VIP system could be pivotal during the development of HFD-induced neuroinflammation. To unveil the pathogenic mechanisms underlying HFD-induced neuroinflammation and assess metformin's therapeutic activities, (1) we determined if HFD-induced proinflammatory activity was present in vulnerable brain regions associated with the development of comorbid behaviors, (2) investigated if the PACAP/VIP system is altered by HFD, and (3) assessed if metformin rescues such diet-induced neurochemical alterations. C57BL/6J male mice were divided into two groups to receive either standard chow (SC) or HFD for 16 weeks. A further HFD group received metformin (HFD + M) (300 mg/kg BW daily for 5 weeks) via oral gavage. Body weight, fasting glucose, and insulin levels were measured. After 16 weeks, the proinflammatory profile, glial activation markers, and changes within the PI3K/AKT intracellular pathway and the PACAP/VIP system were evaluated by real-time qPCR and/or Western blot in the hypothalamus, hippocampus, prefrontal cortex, and amygdala. Our data showed that HFD causes widespread low-grade neuroinflammation and gliosis, with regional-specific differences across brain regions. HFD also diminished phospho-AKT(Ser473) expression and caused significant disruptions to the PACAP/VIP system. Treatment with metformin attenuated these neuroinflammatory signatures and reversed PI3K/AKT and PACAP/VIP alterations caused by HFD. Altogether, our findings demonstrate that metformin treatment rescues HFD-induced neuroinflammation in vulnerable brain regions, most likely by a mechanism involving the reinstatement of PACAP/VIP system homeostasis. Data also suggests that the PI3K/AKT pathway, at least in part, mediates some of metformin's beneficial effects
Energy transitions and uncertainty: creating low carbon investment opportunities in the UK electricity sector
This paper examines how actors in the UK electricity sector are attempting to deliver investment in low carbon generation. Low carbon technologies, because of their relative immaturity, capital intensity and low operational costs, do not readily fit with existing electricity markets and investment templates which were designed for fossil fuel based energy. We analyse key electricity market and infrastructure policies in the UK and highlight how these are aimed at making low carbon technologies ‘investable’ by reducing uncertainty, managing investment risks and repositioning actors within the electricity socio-technical ‘regime’. We argue that our study can inform contemporary debates on the politics and governance of sustainability transitions by empirically investigating the agency of incumbent regime actors in the face of uncertainty and by offering critical insights on the role of markets and finance in shaping socio-technical change
A designer's approach: How can autistic adults with learning disabilities be involved in the design process?
Autistic adults with limited speech and additional learning disabilities who are often excluded from design research are at the heart of this project. These are people whose perceptions, experiences and interactions with their surroundings are unique, but also are people who may not be able to communicate verbally their differences to the remaining 99% of the population. This, in combination with their distinctive cognitive profile, has resulted in a lack of studies involving people living with autism, and consequently their life experiences may neither be heard nor understood and remain largely unexplored. By reflecting upon the ongoing design collaboration between The Helen Hamlyn Centre for Design and the autism charity The Kingwood Trust, this paper reflects on the approach and methods used in three design studies. Particular attention is paid towards the careful selection, adaptation and development of collaborative design methods for autistic adults and their support staff to be involved. By working beyond the boundaries of a neurotypical culture, the project aims to support the greater goal of improving the everyday experiences of people living with autism by breaking down the barriers to participation
An Anisotropic Wormhole:TUNNELLING in Time and Space
We discuss the structure of a gravitational euclidean instanton obtained
through coupling of gravity to electromagnetism. Its topology at fixed is
. This euclidean solution can be interpreted as a tunnelling to
a hyperbolic space (baby universe) at or alternatively as a static
wormhole that joins the two asymptotically flat spaces of a
Reissner--Nordstr\"om type solution with .Comment: PLAIN-TEX, 16 pages (4 figures not included), Report DFTT 2/9
Quantum metastability in a class of moving potentials
In this paper we consider quantum metastability in a class of moving
potentials introduced by Berry and Klein. Potential in this class has its
height and width scaled in a specific way so that it can be transformed into a
stationary one. In deriving the non-decay probability of the system, we argue
that the appropriate technique to use is the less known method of scattering
states. This method is illustrated through two examples, namely, a moving
delta-potential and a moving barrier potential. For expanding potentials, one
finds that a small but finite non-decay probability persists at large times.
Generalization to scaling potentials of arbitrary shape is briefly indicated.Comment: 10 pages, 1 figure
Linear optical absorption spectra of mesoscopic structures in intense THz fields: free particle properties
We theoretically study the effect of THz radiation on the linear optical
absorption spectra of semiconductor structures. A general theoretical
framework, based on non-equilibrium Green functions, is formulated, and applied
to the calculation of linear optical absorption spectrum for several
non-equilibrium mesoscopic structures. We show that a blue-shift occurs and
sidebands appear in bulk-like structures, i.e., the dynamical Franz-Keldysh
effect [A.-P. Jauho and K. Johnsen, Phys. Rev. Lett. 76, 4576 (1996)]. An
analytic calculation leads to the prediction that in the case of superlattices
distinct stable steps appear in the absorption spectrum when conditions for
dynamical localization are met.Comment: 13 Pages, RevTex using epsf to include 8 ps figures. Submitted to
Phys. Rev. B (3 April 97
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