Metastable Behavior Research Articles

Anomalous aging and strain induced time dependent phenomena in ultra-thin La0.65Ca0.35MnO3 films

We have shown that ultra-thin La0.65Ca0.35MnO3 films exhibit strong metastable behavior. The resistance can vary with time significantly, suggesting that a state of dynamic phase separation exists whereby one phase grows at the expense of another. Physical properties associated with the metastable behavior have been investigated on the films grown on different substrates. We have found that ultra-thin films age much faster than the thicker counterparts and more interestingly the metastability in the resistance of these films enhanced when aged.

A comparison of the M3/M4 metastable defect detected in hydrogen and ICP Ar plasma treated n-GaAs

Defects created by a dc hydrogen plasma have been compared to those observed in n-GaAs exposed to an inductively coupled (ICP) Ar plasma. The reference sample (in the case of H-plasma treated material) contained two prominent native deep level electron traps, possibly M4 and EC−0.56eV, which were both passivated by hydrogen. Plasma treatment also resulted in the formation of a defect observed at 0.58eV (M3) below the conduction band. This defect transforms back into what is believed to be M4 when annealed at 350K for 3h under reverse bias. These two defects compare well with two similar defects observed in the Ar ICP treated samples also showing metastable behavior. Additionally, the electrical characterization of Schottky barrier diodes on n-GaAs, prior to and after hydrogen passivation shows that, depending on the plasma conditions, the plasma ions significantly damage the surface resulting in poor rectifying contacts. The damage is considerably reversed/repaired upon annealing between the room temperature and 573K (300°C).

Investigation of the Unusual Behavior of Metolachlor under Chemical Ionization in a Hybrid 3D Ion Trap Mass Spectrometer

This article describes the strange behavior of the widely used herbicide metolachlor under chemical ionization conditions in a hybrid source ion trap mass spectrometer in gas chromatography/mass spectrometry (GC/MS) coupling. With the use of ammonia as the reagent gas, metolachlor provides a chlorinated ion at m/z 295/297, almost as abundant as the protonated molecule at m/z 284/286, which cannot be isolated to perform tandem mass spectrometry (MS(n)) experiments. Curiously, this ion at m/z = M + 12 is not observed for the herbicides acetochlor and alachlor, which present very similar chemical structures. The chemical structure of the m/z 295/297 ions and the explanation of the observed phenomenon based on the metastable behavior of these ions were elucidated on the basis of experiments including isotopic labeling and modifications of the operating conditions of the ion trap mass spectrometer. This work allows one to give new recommendations for an optimized use of hybrid source ion trap mass spectrometers.

Small random perturbations of a dynamical system with blow-up

We study small random perturbations by additive white-noise of a spatial discretization of a reaction–diffusion equation with a stable equilibrium and solutions that blow up in finite time. We prove that the perturbed system blows up with total probability and establish its order of magnitude and asymptotic distribution. For initial data in the domain of explosion we prove that the explosion time converges to the deterministic one while for initial data in the domain of attraction of the stable equilibrium we show that the system exhibits metastable behavior.

A third measure-metastable state in the dynamics of spontaneous shape change in healthy human's white cells.

Human polymorphonuclear leucocytes, PMN, are highly motile cells with average 12-15 µm diameters and prominent, loboid nuclei. They are produced in the bone marrow, are essential for host defense, and are the most populous of white blood cell types. PMN also participate in acute and chronic inflammatory processes, in the regulation of the immune response, in angiogenesis, and interact with tumors. To accommodate these varied functions, their behavior is adaptive, but still definable in terms of a set of behavioral states. PMN morphodynamics have generally involved a non-equilibrium stationary, spheroid Idling state that transitions to an activated, ellipsoid translocating state in response to chemical signals. These two behavioral shape-states, spheroid and ellipsoid, are generally recognized as making up the vocabulary of a healthy PMN. A third, “random” state has occasionally been reported as associated with disease states. I have observed this third, Treadmilling state, in PMN from healthy subjects, the cells demonstrating metastable dynamical behaviors known to anticipate phase transitions in mathematical, physical, and biological systems. For this study, human PMN were microscopically imaged and analyzed as single living cells. I used a microscope with a novel high aperture, cardioid annular condenser with better than 100 nanometer resolution of simultaneous, mixed dark field and intrinsic fluorescent images to record shape changes in 189 living PMNs. Relative radial roundness, R(t), served as a computable order parameter. Comparison of R(t) series of 10 cells in the Idling and 10 in the Treadmilling state reveals the robustness of the “random” appearing Treadmilling state, and the emergence of behaviors observed in the neighborhood of global state transitions, including increased correlation length and variance (divergence), sudden jumps, mixed phases, bimodality, power spectral scaling and temporal slowing. Wavelet transformation of an R(t) series of an Idling to Treadmilling state change, demonstrated behaviors concomitant with the observed transition.

Metastability of reversible finite state Markov processes

We prove the metastable behavior of reversible Markov processes on finite state spaces under minimal conditions on the jump rates. To illustrate the result we deduce the metastable behavior of the Ising model with a small magnetic field at very low temperature.

Polymer dynamics in the depinned phase: metastability with logarithmic barriers

We consider the stochastic evolution of a (1 + 1)-dimensional polymer in the depinned regime. At equilibrium the system exhibits a double well structure: the polymer lies(essentially) either above or below the repulsive line. As a consequence one expects a metastable behavior with rare jumps between the two phases combined with a fast thermalization inside each phase. However the energy barrier between these two phases is only logarithmic in the system size L and therefore the two relevant time scales are only polynomial in L with no clear-cut separation between them. The whole evolution is governed by a subtle competition between the diffusive behavior inside one phase and the jumps across the energy barriers. In particular the usual scenario in which the tunneling time coincides with the exponential of the energy barrier breaks down. Our main results are: (i) a proof that the mixing time of the system lies between L^{5/2} and L^{5/2+2}; (ii) the identification of two regions associated with the positive and negative phase of the polymer together with the proof of the asymptotic exponentiality of the tunneling time between them with rate equal to a half of the spectral gap.

Frustrated metastable behavior of magnetic and transport properties in charge ordered La1−xCaxMnO3+δ manganites

We have studied the effect of metastable, irreversibility induced by repeated thermal cycles on the electric transport and magnetization of polycrystalline samples of La1-xCaxMnO3 (0.48 ≤ x ≤ 0.55) close to charge ordering. With time and thermal cycling (T < 300 K) there is an irreversible transformation of the low-temperature phase from a partially ferromagnetic and metallic to one that is less ferromagnetic and highly resistive for the composition close to charge ordering (x = 0.50 and 0.52). Irrespective of the actual ground state of the compound, the effect of thermal cycling is toward an increase of the amount of the insulating phase. We have observed 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 observed changes in the resistivity and magnetization as the revived metastable state is cycled. The time changes in the magnetization are logarithmic in general and activation energies are consistent with those expected for electron transfer between Mn ions. Changes induced by thermal cycling can be inhibited by applying magnetic field. These results suggest that oxygen nonstoichiometry results in mechanical strains in this two-phase system, leading to the development of frustrated metastable states which relax toward the more stable charge-ordered and antiferromagnetic microdomains. Our results also suggest that the growth and coexistence of phases gives rise to microstructural tracks and strain accommodation, producing the observed irreversibility.

Dimensionality crossover in vortex dynamics of magnetically coupled F–S–F hybrids

We report on the vortex dynamics in magnetically coupled F–S–F trilayers extracted fromthe analysis of the resistance–current isotherms. The superconducting thin film that isconventionally in the 2D vortex limit exhibits quite different behavior when sandwichedbetween ferromagnetic layers. The value of the dynamic critical exponent strongly increasesin the F–S–F case due to screening of the stray vortex field by the adjacent ferromagneticlayers, leading to an effective dimensional crossover in vortex dynamics. Furthermore, thedirectional pinning by the magnetic stripe domains induces anisotropy in the vortex glasstransition temperature and causes metastable avalanche behavior at strong drivingcurrents.

Chemical deposition methods for Cd-free buffer layers in CI(G)S solar cells: Role of window layers

It is currently possible to prepare Cd-free Cu(In,Ga)Se 2-based solar cells with efficiencies similar or higher than their CdS references. In these cells, higher efficiencies are generally obtained from soft chemical-based techniques giving conformal depositions such as chemical bath deposition (CBD), ion layer gas reaction (ILGAR) or atomic layer deposition (ALD). However most of these devices are characterized by their pronounced transient behaviour. The aim of this paper is to compare these different chemical-based methods (CBD, ALD, ILGAR…) and to try to provide evidence for the dominant influence of the interface between the Cd-free buffer layer and the window layer on the performance and on the metastable electronic behaviour of these solar cells.

Light-induced Open-circuit Voltage Increase in Amorphous Silicon/Microcrystalline Silicon Tandem Solar Cells

ABSTRACTLight-induced metastability of amorphous/microcrystalline (micromorph) silicon tandem solar cell, in which the microcrystalline bottom cell was deposited in a single-chamber system, has been studied under a white light for more than 1000 hours. Two different light-induced metastable behaviors were observed. The first type was the conventional light-induced degradation, where the open-circuit voltage (Voc), fill factor (FF), and short-circuit current density (Jsc) were degraded, hence the efficiency was degraded as well. This phenomenon was observed mainly in the tandem cells with a bottom cell limited current mismatch. The second type was with a light-induced increase in Voc, which sometimes resulted in an increase in efficiency. The second type of light-induced metastability was observed in the tandem cells with a top cell limited current mismatch. The possible mechanisms for these phenomena are discussed.

Investigation of the metastability behavior of CIGS based solar cells with ZnMgO–Zn(S,O,OH) window-buffer layers

In this paper, different electrical characterization techniques are used to investigate the metastability behavior of the ZnO:Al/ZnMgO/Zn(S,O,OH)/Cu(In,Ga)Se2 solar cell. Whereas this device presents a poor conversion efficiency and metastable behavior just after the deposition stage, Thermal Annealing and Light Soaking considerably improved its conversion efficiency. In order to understand the metastable behavior and Thermal Annealing effect of this device, different electrical and electro-optical characterization measurements were carried out including Sub Gap Modulated Photo-Current Spectroscopy. The application of Sub Gap Modulated Photo-Current Spectroscopy measurements performed on an as-deposited device, revealed two deep defects. These defects became shallower and less efficient for the capture of free carriers after Thermal Annealing. Moreover, the photoluminescence spectra exhibit an increase of the measured signal after Thermal Annealing, which is due to a band-to-band recombination. In addition, capacitance–voltage measurements show a hysteresis behavior that is significantly reduced after Light Soaking or Thermal Annealing. We also performed numerical simulations with SCAPS, a software tool of the University of Gent available to the PV research community, to calculate the current–voltage characteristics under AM1.5G illumination and the capacitance–voltage ones under dark conditions. The evolution of the corresponding experimental characteristics from the as-deposited state to different steps during the Thermal Annealing or Light Soaking posttreatment could be well reproduced by varying a high acceptor-like defect density at the buffer/absorber hetero-interface.

Expansion or extinction: deterministic and stochastic two-patch models with Allee effects

We investigate the impact of Allee effect and dispersal on the long-term evolution of a population in a patchy environment. Our main focus is on whether a population already established in one patch either successfully invades an adjacent empty patch or undergoes a global extinction. Our study is based on the combination of analytical and numerical results for both a deterministic two-patch model and a stochastic counterpart. The deterministic model has either two, three or four attractors. The existence of a regime with exactly three attractors only appears when patches have distinct Allee thresholds. In the presence of weak dispersal, the analysis of the deterministic model shows that a high-density and a low-density populations can coexist at equilibrium in nearby patches, whereas the analysis of the stochastic model indicates that this equilibrium is metastable, thus leading after a large random time to either a global expansion or a global extinction. Up to some critical dispersal, increasing the intensity of the interactions leads to an increase of both the basin of attraction of the global extinction and the basin of attraction of the global expansion. Above this threshold, for both the deterministic and the stochastic models, the patches tend to synchronize as the intensity of the dispersal increases. This results in either a global expansion or a global extinction. For the deterministic model, there are only two attractors, while the stochastic model no longer exhibits a metastable behavior. In the presence of strong dispersal, the limiting behavior is entirely determined by the value of the Allee thresholds as the global population size in the deterministic and the stochastic models evolves as dictated by their single-patch counterparts. For all values of the dispersal parameter, Allee effects promote global extinction in terms of an expansion of the basin of attraction of the extinction equilibrium for the deterministic model and an increase of the probability of extinction for the stochastic model.

Mean-Field Dynamics of a Two-Mode Bose-Einstein Condensate Subject to Decoherence

We discuss the dynamics of Bose-Einstein condensates in a double-well potential subject to decoherence (or particle loss). Starting from the full many-body dynamics described by the master equation, an effective Gross-Pitaevskii-like equation is derived in the mean-field approximation. By numerically solving the GP equation, we find that macroscopic quantum self-trapping disappears for strong decoherence, while generalized self-trapping occurs under weak decoherence. The fixed points have been calculated, and we find that an abrupt change from elliptic to an attractor and a repeller occurs, reflecting the metastable behavior of the system around these points.

Metastable nanocrystallization behavior of Ti-based metallic glassy powders during post-heating

The crystallization behavior of Ti-based metallic glassy powders was systematically investigated. The results show a novel metastable nanocrystallization process, i.e. the glassy powders exhibit multi-step phase transition behavior. For the first exothermic event, the entire glassy phase crystallized into metastable nanocrystals, then a face-centered cubic structured Ni2SnZr and a monoclinic structured TiNi0.8Cu0.2 phases formed by a solid state transition (the second exothermic event). These crystallizations are believed to be driven by the synergetic effect of thermodynamics and kinetics.

Metastable magnetization behavior of magnetocaloric R 6Co 1.67Si 3 (R=Tb and Nd) compounds

Magnetic field and time induced steps have been observed in the recently discovered ternary silicide R 6Co 1.67Si 3. Huge relaxation steps are observed across different loops in the low temperature magnetization isotherms. Giant relaxation present in this system indicates the existence of incubation time to get the saturated moment at a certain field. Measurement protocol sensitive magnetization behavior observed in this system may arise from the strong magnetostructural coupling and/or magnetic frustration. Electrical resistivity and magnetoresistance also reflect the magnetic state of the compound. Magnetocaloric effect is found to be large at temperatures close to the magnetic transition temperature.

Metastable phenomena in lutetium nitrate crystal

Abstract The ac conductivities along the c -axis of the lutetium nitrate crystal were measured successively in the frequency range from 0.5 to 10 5 Hz at temperatures from 290 to 210 K. The metastable behaviors of the conductivity were found in the temperature region above 200 K. The fluctuation was given by the successive measurements of the time series of the ac conductivities and represented by non-Gaussian probability distribution function. The effect of the metastable singularity on Raman scattering spectra was observed in the metastable temperature region.

Synthesis of zeolite from Italian coal fly ash: Differences in crystallization temperature using seawater instead of distilled water

In this study Italian coal fly ash was converted into several types of zeolite in laboratory experiments with temperatures of crystallization ranging from 35 up to 90 °C. Distilled and seawater were used during the hydrothermal synthesis process in separate experiments, after a pre-treatment fusion with NaOH. The results indicate that zeolites could be formed from different kind of Italian coal fly ash at low temperature of crystallization using both distilled and seawater. SEM data and the powder patterns of X-ray diffraction analysis show that faujasite, zeolite ZK-5 and sodalite were synthesized when using both distilled and seawater; zeolite A crystallized only using distilled water. In particular the experiments indicate that the synthesis of zeolite X and zeolite ZK-5 takes place at lower temperatures when using seawater (35 and 45 °C, respectively). The formation of sodalite is always competitive with zeolite X which shows a metastable behaviour at higher temperatures (70–90 °C). The chemical composition of the fly ash source could be responsible of the differences on the starting time of synthesized zeolite with distilled water, in any case our data show that the formation of specific zeolites takes place always at lower temperatures when using seawater.

Uniform Estimates for Metastable Transition Times in a Coupled Bistable System

We consider a coupled bistable $N$-particle system on $\mathbb{R}^N$ driven by a Brownian noise, with a strong coupling corresponding to the synchronised regime. Our aim is to obtain sharp estimates on the metastable transition times between the two stable states, both for fixed $N$ and in the limit when $N$ tends to infinity, with error estimates uniform in $N$. These estimates are a main step towards a rigorous understanding of the metastable behavior of infinite dimensional systems, such as the stochastically perturbed Ginzburg-Landau equation. Our results are based on the potential theoretic approach to metastability.

Temperature‐dependent current‐voltage and lightsoaking measurements on Cu(In,Ga)Se2 solar cells with ALD‐Zn1‐xMgxO buffer layers

AbstractIn this paper, lightsoaking and temperature‐dependent current‐voltage (JVT) measurements on Cu(In,Ga)Se2 solar cells with atomic layer deposited Zn1‐xMgxO buffer layers are presented. A range of Mg concentrations are used, from pure ZnO (x=0) to 26% Mg (x=0·26). Since this kind of solar cells exhibit strong metastable behaviour, lightsoaking is needed prior to the JVT‐measurements to enable fitting of these to the one‐diode model. The most prominent effect of lightsoaking cells with Mg‐rich buffer layers is an increased fill factor, while the effect on cells with pure ZnO buffer is mainly to increase Voc·. The activation energy is extracted from JVT‐measurement data by applying three different methods and the ideality factors are fitted to two different models of temperature‐dependence. A buffer layer consisting either of ZnO or Zn1‐xMgxO with a minor Mg content gives solar cells dominated by interface recombination, which probably can be related to a negative conduction band offset. A relatively high Mg content in the buffer layer (x=0·21) leads to solar cells dominated by recombination in the space charge region. The recombination is interpreted as being tunnelling‐enhanced. The situation in between these Mg concentrations is less clear. Before lightsoaking, the sample with x=0·12 has the highest efficiency of 15·3%, while after lightsoaking the sample with x=0·21 holds the best efficiency, 16·1%, exceeding the value for the CdS reference. The Jsc values of the Zn1‐xMgxO cells surpass that of the reference due to the larger bandgap of Zn1‐xMgxO compared to CdS. Copyright © 2009 John Wiley & Sons, Ltd.