Hypersound attenuation and phase velocities in evaporated polycrystalline films of silver,copper and constantan between 1 and 24 GHz

A model for a recombination instability in a semiconductor far from equilibrium is analysed. It is based upon the simultaneous impact ionization of ground state and excited donors at low temperature. The number and the stability of spatially homogeneous steady states and their dependence upon external control parameters is investigated. For certain values of these parameters bistability andS-shaped current-voltage characteristics are found. Nonequilibrium phase transitions of first order between low and high conductivity states can be induced by varying the applied voltage. If certain generation coefficients are negligible, the transition changes from first to second order above some tricritical value of the other parameters.     

Mechanism of charge transfer in n-CdS/p-CdTe heterostructures with a thick layer of the CdTe1 ? xSx solid solution

The current-voltage and capacitance-voltage characteristics of the n-CdS/p-CdTe heterosystem are investigated. An analysis of the results obtained has demonstrated that a high-resistance i layer (the CdTe_{1 − x} S _x solid solution), which is inhomogeneous not only in the electrical conductivity but also in the composition, is formed at the n-CdS/p-CdTe heterointerface. The thicknesses of the solid-solution layers are estimated from the capacitance-voltage characteristics, and regions with different types of conduction are found to exist in the intermediate layer. It is shown that the ambipolar diffusion and drift in the high-resistance CdTe_{1 − x} S _x solid-solution layers are directed toward each other, thus resulting in the appearance of a sublinear portion in the current-voltage characteristics described by the dependence V = exp(Iaw). The observation of a sublinear portion in the current-voltage characteristics measured in both the forward and reverse electric current directions over a wide temperature range 77–323 K suggests that the diffusion-drift regime can occur in different parts of the i layer, depending on the electric current density and the ambient temperature. The changes observed in the electric current and capacitance, as well as in the current-voltage and capacitance-voltage characteristics, after ultrasonic irradiation indicate that, in the CdTe_{1 − x} S _x solid solutions, there exist metastable states, which most probably decay under ultrasonic irradiation and then again form solid solutions with more stable states. Original Russian Text ? Kh.Kh. Ismoilov, A.M. Abdugafurov, Sh.A. Mirsagatov, A.Yu. Leĭderman, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 11, pp. 1953–1957.     

Nonuniform electron states near cleaved bismuth surface

Using scanning tunneling microscopy, we have detected nonuniform electron states on cleaved bismuth surfaces, which manifest themselves in the form of a random relief with a characteristic lateral size of 1–2 nm and a vertical size of a fraction of an angström. This relief is due to variations in current-voltage characteristics when the tip is moved over the sample surface. Features on the current-voltage characteristics associated with states near the surface have been observed in a voltage range of ?0.4 to +0.4 V. The nonuniform states are tentatively associated with defects generated in the process of crystal cleavage.     

Switching behavior of current filaments inp-germanium connected in parallel

Impact ionization breakdown in extrinsic germanium at low temperatures as a phase transition of a non-conducting to a conducting state is associated with a region of negative differential resistance, typically resulting in a current-voltage characteristic of multiple hystereses and switching between different conducting states. As an experimental model, we investigate the parallel combination of two devices each with a perfectlyS-shaped characteristic. The behavior of this simple combination can serve for understanding the complexity of more general cases. Further, we observed an interesting switching behavior between both devices which cannot be understood as an instability of the individual devices or in terms of a local stability analysis of the coupled system.     

Analysis of current-voltage characteristics of inhomogeneous Schottky diodes at low temperatures

Two approaches of Gaussian distribution of barrier heights in inhomogeneous Schottky diodes have been analyzed by comparing the results for consistency between the two. For this the current-voltage characteristics of inhomogeneous Schottky diodes have been generated by using analytically solved thermionic-emission diffusion equation incorporating Gaussian distribution of barrier heights and by direct numerical integration over a barrier height range. The differences in the results obtained in two approaches are discussed and it is shown that the two approaches yield current-voltage characteristics with slightly different features. The discrepancies in the results obtained in two approaches are attributed to the same series resistance assumed for all elementary barriers of the distribution. It is shown that assigning same series resistance to all barrier of the distribution in numerical integration approach causes current saturation at low bias and inhibits intersection of current-voltage curves from being observable which otherwise occurs in the curves obtained using analytical equation. The paper deals with these aspects in details.     

Nonlinear Instability of Inhomogeneous Steady States Solutions to the HMF Model

Journal of Statistical Physics - In this work we prove the nonlinear instability of inhomogeneous steady states solutions to the Hamiltonian mean field (HMF) model. We first study the linear...     

Stochastic models of firstorder nonequilibrium phase transitions in chemical reactions

The steady states of a simple nonlinear chemical system kept far from equilibrium are analyzed. A standard macroscopic analysis shows that the nonlinearity introduces an instability causing a transition analogous to a thermodynamic first-order phase transition. Near this transition the system exhibits hysteresis between two alternative steady states. Fluctuations are introduced into this model using a stochastic master equation. The solution of this master equation is unique, preventing two alternative exactly stable states. However, a quasi-hysteresis occurs involving transitions between alternative metastable steady states on a time scale that is longer than that of the fluctuations around the mean steady state values by a factor of the forme , where ø is the height of a generalized thermodynamic potential barrier between the two states. In the thermodynamic limit this time scale tends to infinity and we have essentially two alternative stable steady states.     

Reply to a note by T. G. Shepherd

Summary In a recent note by Shepherd it is claimed that the main conclusions discussed in a paper by Petroniet al. about certain instability mechanisms for steady equivalent barotropic flows are incorrect and, although no criticism is made regarding the analytical treatment, two counterexamples are presented to support this thesis. In addition to this it is asserted that the envisaged necessity for energy and enstrophy to be transferred in both spatial directions in a normal-mode instability is ?extremely well known?. Here we demonstrate that Shepherd's conclusions are without foundation and confirm the validity and originality of Petroniet al. results by showing that:a) the first counterexample describes a situation that is in fact provided for by Petroniet al.'s analysis;b) the second counterexample is not dynamically realizable and, even if it were, the related conclusions would not apply to flows defined in doubly periodic or bounded domains such as those considered by Petroniet al.; c) the mechanism cited above is not known at all in the specific context dealt with by Petroniet al. We emphasize this by pointing out that for steady equivalent barotropic flows the range of parameters for which such an instability mechanism is allowed complements exactly the range for which the steady states are stable according to Benziet al.     

Anomalies in the electrophysical, thermal, and elastic properties of layered ferroelectric semiconductor TlGaSe2: Instability in the electronic subsystem

The temperature dependences of the permittivity, dark current, and current-voltage characteristics of the layered ferroelectric semiconductor TlGaSe₂ were measured over the range 77–300 K. The ε(T) curve measured perpendicular to the layers at low frequencies is discovered for the first time to exhibit an anomalous dip with clearly defined boundaries at ～150 and ～200 K. Electrical instability in the form of low-frequency oscillations of current in current-voltage characteristics of a sample in the same temperature range are also observed experimentally for the first time. It is shown that this instability occurs only if an electric field is applied to a sample using potential-controlling contactless electrodes in the form of thin mica spacer layers. The nature of the instability and its influence on various physical properties of the layered TlGaSe₂ crystal are discussed.     

Recombination instability and domainisation inp-Ge(Au)

Results of an experimental study of short and longp-Ge(Au) samples are presented and compared qualitatively with data furnished by the one-dimensional theoretical model describing the evolution of domain instability in a two-parametric space ‘voltageV-emission coefficient β’. According to the theory, single-, double-, and multi-subdomain states and order-disorder-order transitions via intermittency have been observed in the system with increasing voltage. Three operation regimes of the system are found, depending on the region of the parametric space: ohmic, quenched (pulsing) and transit-time. In short samples, a second portion with S-switching is revealed in the current-voltage characteristic.     

Singular Limits of Spatially Inhomogeneous Convection-reaction-diffusion Equations

We study convection-reaction-diffusion equations which have spatially inhomogeneous steady states. When the coefficients of the reaction terms are much larger than the diffusion coefficients, sharp interfaces appear between two stable inhomogeneous steady states. By using the method of matched asymptotic expansions, we derive the equations of the motion of such interfaces, which depend on the mean curvature of the interfaces and depend on the inhomogeneous coefficients locally. This work is partially supported by NNSF of China and SRF for ROCS, SEM (China).     

有扩散不稳定性的四阶反应-扩散系统的空间周期结构

本文利用分叉理论研究一个含参数的有扩散不稳定性的四阶反应-扩散系统的空间周期结构问题,其中扩散项服从Cahn-Hillard广义扩散定律。首先通过稳定性和奇异性的分析得到从空间均匀定态产生空间周期定态的判别准则,然后利用奇异摄动方法获得这个空间周期定态的二阶近似解。本文的理论结果很好地符合文献[3]中的数值结果。 关键词：     

行波管放大器中辐射场的极限环振荡和混沌

以行波管放大器中辐射场的非线性不稳定阈值分析为基础，对辐射场从频率分叉到混沌的演化过程和不同区域这些非线性不稳定态的时间特性和频率特性进行了研究.在“软”非线性区域，辐射场表现为极限环振荡和频率分岔，频谱是离散的且相对于载波频率是不对称的.这种不稳定是阵发性的，适当的调节控制参量，可使器件工作在所需的定态或极限环振荡态上；在“硬”非线性区域，辐射场表现为非周期的随机振荡和频率混沌，频谱连续且频带很宽，场幅值较大的成分集中在接近于零频的低频范围里.这种不稳定是连续性的，不能通过调节参量来消除. 关键词： 行波管 辐射场 分岔 极限环 混沌     

Field electron emission form single-walled carbon nanotubes with deposited cesium atoms

It has been found that deposition g of cesium atoms on single-walled carbon nanotubes covered with potassium atoms not only drastically increases emission current but also considerably changes the shape of current-voltage characteristics of field electron emission, namely, the characteristics become nonlinear in Fowler-Nordheim coordinates. It has been assumed that this effect is associated with the fact that field electron emission in these layers comes from single-walled carbon nanotubes, which have p-type conductivity after potassium treatment, while deposition of cesium leads to the formation of p-n junctions near nanotube tips. Part of the applied voltage drops in p-n junction, thus causing a nonlinearity of current-voltage characteristics.     

Interface resistive switching effects in bulk manganites

A physical mechanism driving the resistance switching in heterocontacts, formed by a metal counterelectrode and electrically conducting bulk perovskite manganites, is discussed. The nature of the inelastic, charge-hopping transport inside insulating and strongly inhomogeneous metal/manganite interfaces is studied theoretically. Comparison with measured current-voltage characteristics for a La_0.67Ca_0.33MnO₃/Ag heterostructure in a high-resistance state reveals the presence of one or more charge traps along a conduction path within the interface. In a low-resistance state the main charge-transferring events are direct tunneling ones. The analysis of electrical noise measurements for a La_0.82Ca_0.18MnO₃ single crystal in three different charge-transport regimes shows scattering centers with a broad, flat spectrum of excitation states, independent of manganite electrical and/or magnetic characteristics. All of these results are consistent with an oxygen-drift model for a bistable resistance state in perovskites.      

Light-induced bistability in spin transition solids leading to thermal and optical hysteresis

We investigate the behaviour of photo-excitable, bistable systems, under permanent light irradiation, in presence of relaxation towards the non-excited state. Cooperativity causes bistability of the steady state, leading to light-induced thermal and optical hysteresis (LITH and LIOH). The light-induced instability is expected to induce demixtion, i.e. the coexistence of domains of the two stable steady states. Such effects are evidenced by magnetic and reflectivity measurements on the spin-crossover solid solution: , with x= 0.3, 0.5, 0.85. Experimental data are in quantitative agreement with a simple macroscopic model which includes a non-linear relaxation term in the master equation. Received: 5 June 1997 / Accepted: 17 July 1998     

Mean Field Magnetic Phase Diagrams for the Two Dimensional <Emphasis Type="Italic">t</Emphasis> — <Emphasis Type="Italic">t</Emphasis>′ — <Emphasis Type="Italic">U</Emphasis> Hubbard Model

We study the ground state phase diagram of the two dimensional t — t′ — U Hubbard model concentrating on the competition between antiferro-, ferro-, and paramagnetism. It is known that unrestricted Hartree–Fock- and quantum Monte Carlo calculations for this model predict inhomogeneous states in large regions of the parameter space. Standard mean field theory, i.e., Hartree–Fock theory restricted to homogeneous states, fails to produce such inhomogeneous phases. We show that a generalization of the mean field method to the grand canonical ensemble circumvents this problem and predicts inhomogeneous states, represented by mixtures of homogeneous states, in large regions of the parameter space. We present phase diagrams which differ considerably from previous mean field results but are consistent with, and extend, results obtained with more sophisticated methods. PACS: 71.10.Fd, 05.70.Fh, 75.50.Ee     

Stability of generation-recombination induced dissipative structures in semiconductors

A class of generation-recombination models involving one type of carriers and several impurity levels is studied. Far from equilibrium, impact ionization can lead to bistability of the homogeneous steady state resulting inS-shaped current-voltage characteristics, and to stationary spatial structures. It is shown for large samples that kink-shaped coexistence profiles are stable, while plane depletion or accumulation layers and cylindrical current filaments are unstable under constant voltage conditions, but can be stabilized by a constant current. The unstable mode is calculated analytically for wide and for narrow depletion layers. The critical slowing-down of this mode is established explicity as a function of the external electric field; it occurs when the threshold or holding field of the switching transitions, or the coexistence field of homogeneous phases is approached. It is shown that the hysteresis cycle of the switching transitions can be shortened by sufficiently large localized fluctuations inducing the nucleation of current layers or filaments.     

Convective and absolute instabilities in the subcritical Ginzburg-Landau equation

We study the nature of the instability of the homogeneous steady states of the subcritical real Ginzburg-Landau equation in the presence of group velocity. The shift of the absolute instability threshold of the trivial steady state, induced by the destabilizing cubic nonlinearities, is confirmed by the numerical analysis of the evolution of its perturbations. It is also shown that the dynamics of these perturbations is such that finite size effects may suppress the transition from convective to absolute instability. Finally, we analyze the instability of the subcritical middle branch of steady states, and show, analytically and numerically, that this branch may be convectively unstable for sufficiently high values of the group velocity. Received 17 December 1998     

Kelvin-Helmholtz instability in a rotating ideally conducting inhomogeneous plasma

The Kelvin-Helmholtz instability in sheared magnetohydrodynamic flow of an ideally conducting rotating inhomogeneous compressible plasma is investigated. The asymptotic behaviour inx of the Kelvin-Helmholtz eigenfunctions for the case of finite compressibility in the presence of rotation is discussed and instability condition is derived. In the incompressible limit, a dispersion relation is derived which has been solved numerically and discussed in detail. It is found that the inhomogeneous system is unstable in an incompressible plasma.