GaAs photoconductivity indicates native defect structure and metastability of Te complex

We present photoconductivity measurements at 77K and room temperature of various semi-insulating GaAs samples grown with the LEC technique in PBN. The presence of a sharp structure near mid-gap believed to be due to a native defect, is here explained as arising from dislocations in the sample. We also observe the presence of a metastable state in one of our samples containing Cr and Te dopants, and propose a tentative model involving lattice relaxation at a Te related complex.     

Persistent Photoconductivity in electron-irradiated ZnO bulk single crystals: Evaluation of the metastable conductive state by the dual light illumination

Persistent Photoconductivity (PPC) in 30 MeV electron-irradiated n-type ZnO single crystals is studied under the dual light illumination (DLI: the infrared light excitation after the blue illumination). Below 160 K, the remarkable reduction in PPC is observed by the subsequent infrared illumination, suggesting the release of electrons from the perturbed-host state (PHS) as a metastable conductive state to the nonconductive state ( and/or ) via the higher unoccupied states inside the conduction band. Above 160 K, the slight increase in photocurrent is observed by the subsequent illumination, suggesting the photoexcitation of electrons not relevant to the PHS. These results depend on the electron concentration in the PHS.     

Formation of DX-centers in indium doped CdTe

In CdTe, the achievable n-type doping is limited by the formation of DX-centers. A characteristic feature of DX-centers is the ‘persistent photoconductivity (PPC)’ which is created by illumination at low temperatures and caused by a metastable state of the DX-center. The DX-center and the PPC effect in n-type CdTe are theoretically explained by the ‘large lattice relaxation model’. PAC measurements on In doped CdTe using ¹¹¹In/¹¹¹Cd and, in addition, resistivity measurements on the same samples have been performed. Below 150 K, the samples showed a PPC effect that was accompanied by an increase of about 20% of the carrier concentration. This effect is not accompanied by any changes of the observed EFG. Possible explanations of the EFG observed, originally assigned to the DX-center, will be discussed. Finally, first reports on the investigation of DX-centers in CdTe using the radioactive isotope ¹¹⁷Cd decaying to ¹¹⁷In are presented.     

Photoluminescence and persistent photoconductivity of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N/GaN heterostructures

We have investigated the optical properties of Al_xGa_1-xN/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the Al_xGa_1-xN/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of Al_xGa_1-xN/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the Al_xGa_1-xN/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for Al_xGa_1-xN/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail. PACS 72.20.Jv; 72.40.+w; 78.55.Cr     

Ultracold collision properties of metastable alkaline-earth atoms

Ultracold collisions of spin-polarized 24Mg, 40Ca, and 88Sr in the metastable 3P2 excited state are investigated based on molecular potentials obtained from ab initio calculations. We calculate the long-range interaction potentials and estimate the scattering length and the collisional loss rate as a function of magnetic field. The scattering lengths show resonance behavior due to the appearance of a molecular bound state in a purely long-range interaction potential and are positive for magnetic fields below 50 mT. A loss-rate model shows that losses should be smallest near zero magnetic field and for fields slightly larger than the resonance field, where the scattering length is also positive.     

Revival of metastable behavior in phase separated La0.5Ca0.5MnO3+δ

The La_xCa_1−xMnO_3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.     

Lifetime measurement of the (5d6p)3D3 state of barium by dye-laser spectroscopy

We have measured the radiative lifetime of the (5d6p)³D₃ state of barium by Hanle effect. This state was populated by cw dye laser excitation of an atomic beam of Ba in the metastable (5d6s)³D₃ state. The metastable state was populated by a dc discharge. With dye laser excitation, the shape of the Hanle effect curve is strongly modified due to optical pumping of the metastable state. A brief discussion of the expected and observed signal shapes is given. The radiative lifetime of the (5d6p)³D₃ state is measured to be τ = 10.2 ± 1.5 ns.     

Stabilization by dissipation and stochastic resonant activation in quantum metastable systems

In this tutorial paper we present a comprehensive review of the escape dynamics from quantum metastable states in dissipative systems and related noise-induced effects. We analyze the role of dissipation and driving in the escape process from quantum metastable states with and without an external driving force, starting from a nonequilibrium initial condition. We use the Caldeira–Leggett model and a non-perturbative theoretical technique within the Feynman–Vernon influence functional approach in strong dissipation regime. In the absence of driving, we find that the escape time from the metastable region has a nonmonotonic behavior versus the system-bath coupling and the temperature, producing a stabilizing effect in the quantum metastable system. In the presence of an external driving, the escape time from the metastable region has a nonmonotonic behavior as a function of the frequency of the driving, the thermal-bath coupling and the temperature. The quantum noise enhanced stability phenomenon is observed in both systems investigated. Finally, we analyze the resonantly activated escape from a quantum metastable state in the spin-boson model. We find quantum stochastic resonant activation, that is the presence of a minimum in the escape time as a function of the driving frequency. Background and introductory material has been added in the first three sections of the paper to make this tutorial review reasonably self-contained and readable for graduate students and non-specialists from related areas.     

Optical bistability in four-level nonradiative dyes

We describe conditions under which optical bistability may be observed in a Fabry-Perot filled with saturably absorbing dye. Bistability occurs due to an intensity-dependent phase shift arising from nonresonant dispersion caused by an excited metastable state which is excited to the process of saturable absorption. Preliminary experiments confirm the theoretical predictions using BDN.     

Ferromagnetic CrSb film fabricated by pulse-laser deposition and rapidly annealing

CrSb film was fabricated by pulse laser deposition (PLD) on Si (1 0 0) wafer. Strong ferromagnetism was observed in the CrSb film annealed at a high heating/cooling rate of 200 K/s, which can be attributed to the presence of ferromagnetic zinc blende (ZB) CrSb phase. The appearance of metastable ZB-CrSb results from the strong inner stress due to the precipitation of the monoclinic Sb.     

Observation of metastable effects in pseudomorphic heterostructures InCaAs/InAlAs

Abstract

The results of electrical transport measurements (Hall effect and resistivity) performed on In_.65Ga_.35As-In Al_.48 As modulation doped heterostructure (with Si-doped InAlAs donor layer) and on In _.52Al _.48As thick layer are presented as a function of temperature (77–300K range) and hydrostatic pressure (up to 1300 MPa). At low temperature, the persistent photoconductivity (PPC) and metastable donor states occupation effects were observed. Due to the pressure induced decrease of the two-dimensional electron gas concentration the pronounced increase in the amplitude of observed effects was obtained. The above results are discussed in terms of DX-like centers and/or interface/surface states in the QW structure.     

Heterogeneous hole nucleation in electron-charged 4He wetting films

4He wetting layers on Cs exhibit an unusually long-lived metastable state upon undercooling to temperatures well below the wetting temperature T(w) approximately 1.9 K. The decay of this state by homogeneous thermal nucleation of holes is disfavored by the incipient divergence of the free energy barrier separating the metastable thick film from the stable thin-film state. We propose that interface deformations ("dimples") created by electrons bound at the 4He liquid-vapor interface can be used as nuclei for the heterogeneous nucleation of holes. The size and excess free energy of the dimple can be tuned by an applied electric field E which allows the lifetime of the metastable film to be controlled.     

Energy loss of light ions with allowance for the presence of metastable particles in the ion beam

The change in the average ion charge due to the presence of particles in metastable states in beams of helium-like ions has been studied. The charge distributions and average charges have been calculated for ions with nuclear charges Z = 3?7 and a velocity of 3.65 au in nitrogen. The previously obtained data on the cross sections of loss and capture of electrons by metastable ions and ions in the ground state were used in the calculations. It is shown that the presence of metastable particles in ion beams leads to an increase in the average charge and energy loss for all considered ions.     

Persistent photoconductivity in ZnO nanostructures induced by surface oxygen vacancy

ZnO nanowall networks grown on SiO₂/Si substrate were found to exhibit persistent photoconductivity (PPC). The relaxation rate of the persistent photocurrent is enhanced by a higher oxygen level in the ambient suggesting that PPC is closely related to the ZnO surface. Surface modification with hydrogen peroxide can significantly reduce the PPC relaxation time, implying that surface oxygen deficiency is responsible for the effect. The transition between the neutral and the metastable singly ionized states of the surface oxygen vacancy is suggested to account for the phenomenon and it is supported by the temperature and wavelength dependence of the PPC. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hysteresis in a superfluid atom circuit

We study a hysteresis phenomenon in a rotating BEC with a weak link in a quasi-one-dimensional torus by proposing a microscopic theoretical model including a dissipation bath. By analyzing the role of dissipation and the decay rates of all the energy levels, we are able to give a microscopic interpretation of hysteresis recently observed in the experiment and confirm that the hysteresis is the result of the presence of metastable state. In particular, we obtain the hysteresis loops in a quench process just as that in the experiment. We also find that the shape and size of the hysteresis loop change drastically with the strength of the link.     

Metastability in the Hamiltonian mean field model and Kuramoto model

We briefly discuss the state of the art on the anomalous dynamics of the Hamiltonian mean field (HMF) model. We stress the important role of the initial conditions for understanding the microscopic nature of the intriguing metastable quasi-stationary states (QSS) observed in the model and the connections to Tsallis statistics and glassy dynamics. We also present new results on the existence of metastable states in the Kuramoto model and discuss the similarities with those found in the HMF model. The existence of metastability seems to be quite a common phenomenon in fully coupled systems, whose origin could be also interpreted as a dynamical mechanism preventing or hindering synchronization.     

Production of a bose einstein condensate of metastable helium atoms

We recently observed a Bose-Einstein condensate in a dilute gas of ⁴He in the 2³S₁ metastable state. In this article, we describe the successive experimental steps which led to the Bose-Einstein transition at 4.7 μK: loading of a large number of atoms in a MOT, efficient transfer into a magnetic Ioffé-Pritchard trap, and optimization of the evaporative cooling ramp. Quantitative measurements are also given for the rates of elastic and inelastic collisions, both above and below the transition. Received 15 October 2001     

The Asymptotic Behavior of the Ergodic Measure of a Glauber + Kawasaki model

We consider an interacting particle system given by the Glauber + Kawasaki dynamics. It is known that this process has a reaction diffusion equation as hydrodynamic limit. The ergodicity of this process in the presence of a metastable state (double well potential) was recently proved by S. Brassesco et al. In this Letter we prove that, in the limit, as ε → 0, the expected value of each spin converges to the global minimizer of the potential. We also prove decay of correlations of the ergodic measure.AMS Subject Classification (2000). 60K35 (82C22, 82C31)This work was partially supported by CNPq     

Noise-induced stabilization in population dynamics

We investigate a model in which strong noise in a subpopulation creates a metastable state in an otherwise unstable two-population system. The induced metastable state is vortexlike, and its persistence time grows exponentially with the noise strength. A variety of distinct scaling relations are observed depending on the relative strength of the subpopulation noises.