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$_{12}$ 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 YBa$_2$Cu$_3$O$_{7-\delta}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-\delta}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{H}\parallel c$. 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 $R$. The value $H_{max}$ 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 $r$ in the disk yields a temperature-dependent critical current that is in qualitative agreement with experiments.Comment: 11 pages, 4 figure

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 Complications–International model (CEPAC) to simulate 8 cohorts of human immunodeficiency virus (HIV)–infected patients initiating ART each year during 2004–2011. Model inputs included cohort-specific mean CD4+ T-cell count at ART initiation (112–178 cells/µL), 24-week ART suppressive efficacy (78%), second-line ART availability (2.4% of ART recipients), and cohort-specific 36-month retention rate (55%–71%). 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 2004–2011 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