248 research outputs found
Collective effects in charge transfer within a hybrid organic-inorganic system
A collective electron transfer (ET) process was discovered by studying the
current noise in a field effect transistor with light-sensitive gate formed by
nanocrystals linked by organic molecules to its surface. Fluctuations in the ET
through the organic linker are reflected in the fluctuations of the transistor
conductivity. The current noise has an avalanche character. Critical exponents
obtained from the noise power spectra, avalanche distributions, and the
dependence of the average avalanche size on avalanche duration are consistent
with each other. A plausible model is proposed for this phenomenonComment: 15 pages 4 figures. Accepted for publication in Physical Review
Letter
Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet
We report Hall sensor measurements of the magnetic relaxation of Mn
acetate as a function of magnetic field applied along the easy axis of
magnetization. Data taken at a series of closely-spaced temperatures between
0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order''
transition between thermally-assisted relaxation and magnetic decay via quantum
tunneling.Comment: 4 pages, including 7 figure
Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO
We report on magnetic measurements carried out in a textured
YBaCuO and YBa(CuAg)O (at
0.02) crystals. The so-called fishtail-effect (FE) or second
magnetization peak has been observed in a wide temperature range
0.4~~0.8 for . The origin of the FE arises for
the competition between surface barrier and bulk pinning. This is confirmed in
a non-monotonically behavior of the relaxation rate . The value
for Ag-doped crystals is larger than for the pure one due to the presence of
additional pinning centers, above all on silver atoms.Comment: 6 pages, 6 figure
V-I characteristics in the vicinity of order-disorder transition in vortex matter
The shape of the V-I characteristics leading to a peak in the differential
resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2
is investigated. r_d is large when measured by dc current. However, for a small
Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and
displays a large out-of-phase signal. In contrast, the ac response increases
with frequency in the absence of dc bias. These surprisingly opposite phenomena
and the peak in r_d are shown to result from a dynamic coexistence of two
vortex matter phases rather than from the commonly assumed plastic depinning.Comment: 12 pages 4 figures. Accepted for publication in PRB rapi
Crystallization of the ordered vortex phase in high temperature superconductors
The Landau-Khalatnikov time-dependent equation is applied to describe the
crystallization process of the ordered vortex lattice in high temperature
superconductors after a sudden application of a magnetic field. Dynamic
coexistence of a stable ordered phase and an unstable disordered phase, with a
sharp interface between them, is demonstrated. The transformation to the
equilibrium ordered state proceeds by movement of this interface from the
sample center toward its edge. The theoretical analysis dictates specific
conditions for the creation of a propagating interface, and provides the time
scale for this process.Comment: 8 pages and 3 figures; to be published in Phys. Rev. B (Rapid
Communications section
Weiss Oscillations in Surface Acoustic Wave Propagation
The interaction of a surface acoustic wave (SAW) with a a two-dimensional
electron gas in a periodic electric potential and a classical magnetic field is
considered. We calculate the attenuation of the SAW and its velocity change and
show that these quantities exhibit Weiss oscillations.Comment: 4 pages REVTEX, 2 figures included as eps file
Plasticity in current-driven vortex lattices
We present a theoretical analysis of recent experiments on current-driven
vortex dynamics in the Corbino disk geometry. This geometry introduces
controlled spatial gradients in the driving force and allows the study of the
onset of plasticity and tearing in clean vortex lattices. We describe plastic
slip in terms of the stress-driven unbinding of dislocation pairs, which in
turn contribute to the relaxation of the shear, yielding a nonlinear response.
The steady state density of free dislocations induced by the applied stress is
calculated as a function of the applied current and temperature. A criterion
for the onset of plasticity at a radial location in the disk yields a
temperature-dependent critical current that is in qualitative agreement with
experiments.Comment: 11 pages, 4 figure
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The Survival Benefits of Antiretroviral Therapy in South Africa
Background. We sought to quantify the survival benefits attributable to antiretroviral therapy (ART) in South Africa since 2004. Methods. We used the Cost-Effectiveness of Preventing AIDS ComplicationsâInternational model (CEPAC) to simulate 8 cohorts of human immunodeficiency virus (HIV)âinfected patients initiating ART each year during 2004â2011. Model inputs included cohort-specific mean CD4+ T-cell count at ART initiation (112â178 cells/”L), 24-week ART suppressive efficacy (78%), second-line ART availability (2.4% of ART recipients), and cohort-specific 36-month retention rate (55%â71%). CEPAC simulated survival twice for each cohort, once with and once without ART. The sum of the products of per capita survival differences and the total numbers of persons initiating ART for each cohort yielded the total survival benefits. Results. Lifetime per capita survival benefits ranged from 9.3 to 10.2 life-years across the 8 cohorts. Total estimated population lifetime survival benefit for all persons starting ART during 2004â2011 was 21.7 million life-years, of which 2.8 million life-years (12.7%) had been realized by December 2012. By 2030, benefits reached 17.9 million life-years under current policies, 21.7 million life-years with universal second-line ART, 23.3 million life-years with increased linkage to care of eligible untreated patients, and 28.0 million life-years with both linkage to care and universal second-line ART. Conclusions. We found dramatic past and potential future survival benefits attributable to ART, justifying international support of ART rollout in South Africa
Economic performance or electoral necessity? Evaluating the system of voluntary income to political parties
Whilst the public funding of political parties is the norm in western democracies, its comprehensive introduction has been resisted in Britain. Political and electoral arrangements in Britain require parties to function and campaign on a regular basis, whilst their income follows cycles largely related to general elections. This article shows that the best predictor of party income is the necessity of a well-funded general election campaign rather than party performance. As a result, income can only be controlled by parties to a limited degree, which jeopardises their ability to determine their own financial position and fulfil their functions as political parties
Instabilities and disorder-driven first-order transition of the vortex lattice
Transport studies in a Corbino disk geometry suggest that the Bragg glass
phase undergoes a first-order transition into a disordered solid. This
transition shows a sharp reentrant behavior at low fields. In contrast, in the
conventional strip configuration, the phase transition is obscured by the
injection of the disordered vortices through the sample edges, which results in
the commonly observed vortex instabilities and smearing of the peak effect in
NbSe2 crystals. These features are found to be absent in the Corbino geometry,
in which the circulating vortices do not cross the sample edges.Comment: 12 pages 3 figures. Accepted for publication in Physical Review
Letter
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