Decay of metastable current states in one-dimensional resonant tunneling devices

Current switching in a double-barrier resonant tunneling structure is studied in the regime where the current-voltage characteristic exhibits intrinsic bistability, so that in a certain range of bias two different steady states of current are possible. Near the upper boundary V_{th} of the bistable region the upper current state is metastable, and because of the shot noise it eventually decays to the stable lower current state. We find the time of this switching process in strip-shaped devices, with the width small compared to the length. As the bias V is tuned away from the boundary value V_{th} of the bistable region, the mean switching time \tau increases exponentially. We show that in long strips \ln\tau \propto (V_{th} -V)^{5/4}, whereas in short strips \ln\tau \propto (V_{th} -V)^{3/2}. The one-dimensional geometry of the problem enables us to obtain analytically exact expressions for both the exponential and the prefactor of \tau. Furthermore, we show that, depending on the parameters of the system, the switching can be initiated either inside the strip, or at its ends.Comment: 12 pages, 5 figures, update to published versio

Difficulties in learning the phonetics of the Chinese language, basic mistakes and means of correction

This article reviews the main difficulties Russian-speaking students encounter while studying the phonetics of the Chinese language, the mistakes that are often made by them at the beginning of learning pronunciation, as well as ways of correcting those mistakes

Longitudinal muon spin relaxation in high purity aluminum and silver

The time dependence of muon spin relaxation has been measured in high purity aluminum and silver samples in a longitudinal 2 T magnetic field at room temperature, using time-differential \musr. For times greater than 10 ns, the shape fits well to a single exponential with relaxation rates of \lambda_{\textrm{Al}} = 1.3 \pm 0.2\,(\textrm{stat.}) \pm 0.3\,(\textrm{syst.})\,\pms and \lambda_{\textrm{Ag}} = 1.0 \pm 0.2\,(\textrm{stat.}) \pm 0.2\,(\textrm{syst.})\,\pms

Stochastic current switching in bistable resonant tunneling systems

Current-voltage characteristics of resonant-tunneling structures often exhibit intrinsic bistabilities. In the bistable region of the I-V curve one of the two current states is metastable. The system switches from the metastable state to the stable one at a random moment in time. The mean switching time \tau depends exponentially on the bias measured from the boundary of the bistable region V_{th}. We find full expressions for \tau (including prefactors) as functions of bias, sample geometry, and in-plane conductivity. Our results take universal form upon appropriate renormalization of the threshold voltage V_{th}. We also show that in large samples the switching initiates inside, at the edge, or at a corner of the sample depending on the parameters of the system.Comment: 21 pages, 5 figure

On the Coupling of Gravitons to Matter

Using relationships between open and closed strings, we present a construction of tree-level scattering amplitudes for gravitons minimally coupled to matter in terms of gauge theory partial amplitudes. In particular, we present examples of amplitudes with gravitons coupled to vectors or to a single fermion pair. We also present two examples with massive graviton exchange, as would arise in the presence of large compact dimensions. The gauge charges are represented by flavors of dynamical scalars or fermions. This also leads to an unconventional decomposition of color and kinematics in gauge theories.Comment: RevTex, 4 page

ENSURING ECONOMIC SECURITY OF RUSSIA IN THE CONTEXT OF AFRICAN DEVELOPMENT TRENDS: PROBLEMS AND PROSPECTS

Abstract Purpose: A new scientific problem is posed and substantiated: the need to take into account the African economic development trends in the system of ensuring the economic security of Russia. In this context, in order to assess the security of the national economy and the competitive position of Russia, the results of scientific studies of the main development trends and threats in Africa, the position of various countries on the African continent are analyzed. Methodology: The authors are based on the analysis of current scientific literature, government official documents, and statistical reports. The main methods of research are analytical review, comparison, systematic approach, cultural and civilizational approach. Result: The article substantiates the position that taking into account the development trends of the African continent is an important component in the system of assessing the national economic security of Russia in the current period and in the future, ensuring competitiveness and strengthening the economic sovereignty of Russia, analyzing the potential of interaction with African countries in the management practice. Applications: This research can be used for the universities, teachers and education students. Novelty/Originality: The authors formulate the main directions of the activation of scientific research and concentration of practical efforts to improve the national economic security of Russia in the context of the development trend of the African continent

Spin noise of a halide perovskite

We report on first observation of spin noise in a strongly birefringent semiconductor -- halide perovskite single crystal MAPbI$_3$. The observed spin noise resonance is ascribed to free holes with a record spin dephasing time of 4 ns. The spin dynamics is found to be affected by the residual light absorption of the crystal providing renormalization of the Larmor frequency. Extended spin noise spectroscopy with rotating magnetic field allowed us not only to evaluate the $g$-factor anisotropy, but also to distinguish two different spin subsystems tentatively associated to twinning of the crystal.Comment: 14 pages, 11 figure

Metastability of (d+n)-dimensional elastic manifolds

We investigate the depinning of a massive elastic manifold with $d$ internal dimensions, embedded in a $(d+n)$-dimensional space, and subject to an isotropic pinning potential $V({\bf u})=V(|{\bf u}|).$ The tunneling process is driven by a small external force ${\bf F}.$ We find the zero temperature and high temperature instantons and show that for the case $1\le d\le 6$ the problem exhibits a sharp transition from quantum to classical behavior: At low temperatures $T<T_{c}$ the Euclidean action is constant up to exponentially small corrections, while for $T> T_{c},$ ${S_{\rm Eucl}(d,T)}/{\hbar} = {U(d)}/{T}.$ The results are universal and do not depend on the detailed shape of the trapping potential $V({\bf u})$. Possible applications of the problem to the depinning of vortices in high-$T_{c}$ superconductors and nucleation in $d$-dimensional phase transitions are discussed. In addition, we determine the high-temperature asymptotics of the preexponential factor for the $(1+1)$-dimensional problem.Comment: RevTeX, 10 pages, 3 figures inserte

Fluctuation corrections to bubble nucleation

The fluctuation determinant which determines the preexponential factor of the transition rate for minimal bubbles is computed for the electroweak theory with $\sin \Theta_W = 0$. As the basic action we use the three-dimensional high-temperature action including, besides temperature dependent masses, the $T \Phi^3$ one-loop contribution which makes the phase transition first order. The results show that this contribution (which has then to be subtracted from the exact result) gives the dominant contribution to the one-loop effective action. The remaining correction is of the order of, but in general larger than the critical bubble action and suppresses the transition rate. The results for the Higgs field fluctuations are compared with those of an approximate heat kernel computation of Kripfganz et al., good agreement is found for small bubbles, strong deviations for large thin-wall bubbles.Comment: 19 pages, LaTeX, no macros, no figure