Formation Of Metastable States Research Articles

Cinematographic Recording of a Metastable Floating Island in Two- and Three-Dimensional Crystal Growth.

Many chemical reactions go through a cascade of events in which a series of metastable intermediates appear, and crystal nucleation is no exception. Although the consensus on the energetics of nucleation suggests the formation of metastable states preceding the crystal growth, little experimental evidence has been reported for their dynamics at an atomistic level. Operando imaging of two-dimensional nucleation on a defect-free NaCl nanocrystal in carbon nanotubes using a millisecond angstrom-resolution transmission electron microscope revealed the formation of a metastable "floating island" (FI) that migrates thermally on the (100) facet of NaCl as the first intermediate of epitaxy. The speed of the migration at 298 K is estimated to be larger than 0.3 nm ms-1. When a crystal tumbles in a container, a space repeatedly forms between the crystal and the container wall that hosts the FI. Tumbling changes the surface energy repeatedly and promotes the conversion of the FI into a new epitaxial layer. We anticipate that this surface catalysis mechanism found on the nanoscale also operates in bulk heterogeneous nucleation where agitation and attrition accelerate crystallization.

Controlled transitions between metastable states of 2D magnetocapillary crystals.

Magnetocapillary interactions between particles allow to self-assemble floating crystals along liquid interfaces. For a fixed number of particles, different states possessing different symmetrical features, known as metastable states, coexist. In this paper, we demonstrate how to trigger the transition from one state to another, either by rearranging the crystal, or by controlling its growth. First, we show that externally controlled magnetic fields can squeeze the entire crystal to induce structural modifications, that upon relaxation can lead to a modified state. Second, we propose localized laser-induced thermocapillary flows that can be used to guide new particles towards an existing crystal in a desired direction, thus favoring a particular resulting state. The control of the formation of metastable states is a key ingredient to functionalize such assemblies, paving the way to self-assembled microrobots.

Strong response of Pt clusters to the environment and conditions, formation of metastable states, and simple methods to trace the reversible changes.

Subnanometric metal particles, the so-called "clusters", are known to be responsive to their surroundings, but the detection of occurring changes, understanding the causes, and predicting the consequences are still extremely difficult for such small particles. Our study was aimed at estimating the potential of adsorption-based methods for these purposes. Using carbon monoxide as a probing molecule, which readily adsorbs on both bare and H-covered Pt surface, we have probed the adsorption properties of highly dispersed Pt/γ-Al2O3 samples after treatments under different atmospheres and temperatures (H2 or inert gas, 25-500 °C). The combined results of CO-chemisorption measurements, CO TPD, CO TPO, H2-by-CO displacement, and H2 TPD suggest that the system shuttles between two states: one with oxygen vacancies in the support and the other one with redox-active oxygen near the Pt clusters. These extreme states can be reversibly created and deleted, giving rise to innumerable intermediate structures that differ in the amount, binding strength, and/or reactivity of adsorbed species. Two adsorbates could act cooperatively, resulting in hydrogen spillover onto the support and making the adsorbate-metal-support interactions even more complex. Implications for better understanding the dynamic behavior of oxide-supported clusters and nanoparticles are discussed.

Sorption hysteresis on soils and sediments: Using single‐point desorption isotherms to obtain characteristic free energy values

AbstractSorption–desorption hysteresis (SDH) may control distributions of chemicals across diverse environmental phases, including soils and sediments. Formation of metastable states caused by pore deformation or inelastic swelling of a sorbent and their persistence during desorption have been considered in the literature as one reason for “true” SDH. Such metastable states may lead to the lack of closure of the sorption–desorption loop at non‐zero sorbate concentrations, a phenomenon often noted in soil and environmental literature. SDH has often been characterized using single‐point desorption isotherms (DIs) that combine sorbed states reached during single desorption steps started from different points along a sorption isotherm (SI). This work aimed to demonstrate how single‐point DIs could be used to characterize SDH in the liquid phase in terms of the Gibbs free energy accumulated in some non‐relaxed sorbed states belonging to a DI, as compared with the states of the same composition (sorbed concentration) belonging to an SI. Based on the literature data on SIs and single‐point DIs of pesticides and some hydrocarbons on soils, artificial soils, minerals and sediments, these quantities of extra free energy were determined and their changes in the selected sorbate‐sorbent systems were examined. When the extent of SDH decreases with increasing solute concentration, these additional free energies decrease. They may remain constant or even increase, suggesting, in the latter case, that more work is needed to perturb a sorbent structure at higher sorbed concentrations. Magnitudes of calculated extra free energies may be helpful in examining whether irreversible pore deformation in a sorbent and formation of metastable states are the reasons for SDH. This paper proposes a novel approach for quantifying liquid‐phase SDH in cases in which a thermodynamic justification is sought. It advances the ability to predict the fate of chemicals in typical soil/sediment environments.Highlights Desorption from soils and sediments to solutions may be hysteretic due to formed metastable states Hysteresis is quantified in terms of excessive free energies of metastable states Extra free energies are sorbate‐ and sorbent‐dependent, varying across sorption isotherms Single‐point desorption isotherms allow quantification of the free energy excess in metastable states

Observation of Phase Transitions in an Electron–Hole System Associated with Dislocation Cores in Cadmium Telluride

Evolution of the fine structure of dislocation microphotoluminescence lines in cadmium telluride with a decrease in the optical-excitation power density has been demonstrated at low (5 K) temperatures. The character of this evolution for each of the three bands under investigation (Y1 (~1.47 eV), Y2 (~1.49 eV), and Y3 (~1.51 eV)) indicates the existence of a transition from electron–hole plasma to exciton states. It is established that the fine structure of band Y3 at the excitation densities of 0.1–0.01 W/cm2 exhibits signs of formation of metastable states with participation of a dislocation core.

Phase Formation Initiated by Irradiation of the Film (Si) – Substrate (Grade 3 Steel) System with a High-Intensity Pulsed Electron Beam

The physical principles of formation of phase composition and defect substructure of a surface alloy in a low-stability state formed as a result of irradiation of the film (Si) - substrate (Grade 3 steel) system with a high -intensity electron beam are revealed. The formation of a multiphase submicro- and nanocrystalline layer is observed. It is shown that irradiation of metals and alloys with a pulsed electron beam in a regime of the surface-layer melting followed by high-rate crystallization frequently gives rise to the formation of local low-stability regions exhibiting a tendency towards the formation of metastable states up to amorphization of the material. A treatment regime is identified, which allows increasing the material microhardness and wear resistance by about 3 and 7.5 factors, respectively.

Nanomodification of structure of non-ferrous metals in mode of superdeep penetration

Purpose. This paper is devoted to the study of the effect of quenching from the melt on the features of the formation of metastable states and the physical properties of Co − 23 % Be and Co − 50 % Be alloys (at. %). Methodology. Quenching from a liquid state was carried out by a known technique of splat-quenching. Cooling rate estimated by foil thickness was ~ 106…107 K/s. Cast alloy samples were obtained by Tamman furnace using a copper mold with a gap of D=0,5 mm. In this case, the estimated cooling rate was ~ (2…3)·103 K/s. The X-ray diffraction analysis was carried out with the use of the URS-2.0 X-ray equipment in filtered cobalt Кa-radiation and by DRON-2.0 diffractometer in monochromatic copper radiation. Microhardness was measured on the PMT−3 microhardnessmeter at a load of 50 g with an accuracy of ± 30..50 MPa. Magnetic studies were carried out on a vibration sample magnetometer with a magnetic field of up to 500 kA/m by comparing the signal from a splat-quenched sample with a sample of pure nickel, for which the specific magnetization was precisely determined. The accuracy of determining the period of the crystal lattice of the alloy, taking into account the extrapolation of the reflection angle by 90 0, was ± 3·10–4 nm. Findings. The formation of homogeneous highly supersaturated (up to 23 at. %) solid solutions having the composition of the initial liquid (С0) was revealed in Co − Be alloys by the method of quenching from the liquid state. At the maximum cooling rates (107 K/s), which are sufficient to form a solid amorphous state, it is not possible to prevent ordering processes in the congruent equiatomic phase of Co − Be (type CsCl). Originality. It was experimentally confirmed that under the conditions of quenching from a liquid state a significant supercooling of the melt below the decay temperature of the phases of the eutectoid composition b1 in the Co − Be alloys. The positive role of the QLS method in increasing the level of mechanical and hard-magnetic characteristics in rapidly-quenched films is shown. Practical value. Co-Be alloys in the form of splat-quenched films can be used in modern high-tech materials of microelectronics .

Cross sections for formation of 139mCe radioisotope through the 140Ce (n, 2n) reaction over 13.73–14.77 MeV neutrons

Cross sections for the formation of metastable state of 139Ce through 140Ce(n, 2n)139mCe reaction were measured at five neutron energies over 13.73–14.77 MeV range using the activation method and off-line gamma-ray spectrometric technique. These cross sections were in agreement with the corresponding theoretical cross sections estimated over 10–20 MeV neutrons by EMPIRE-3.2 code, using LEVEDEN 4 and strength function GSTRFN 0 parameters, and TALYS-1.8 code using ldmodel 5 and preeqmode 4 parameters. The estimated most probable excitation energies of 139Ce were 1.320–3.877 MeV, respectively over 13.73 MeV and 14.77 MeV neutron energies.

Accumulated Gibbs free energy as a quantitative measure of desorption hysteresis associated with the formation of metastable states

The persistence of metastable states was proposed in the literature as one explanation for sorption-desorption hysteresis (SDH) of organic compounds on soils and sediments. When such metastable states freely exchange sorbate molecules with the surroundings and there is no spontaneous exit of a whole system from that state, it is possible to determine the extra Gibbs free energy (ΔGext) accumulated in a system due to the persistence of metastable states. A novel contribution of this paper is the characterization of SDH, in which the sorption isotherm (SI) and desorption isotherm (DI) do not close a loop, in terms of free energy needed to create "frozen", metastable states. To that end, liquid phase sorption of non-ionized sorbates is considered and by integrating over the sorption-desorption sequence, ΔGext and an integral hysteresis index (IHI) were obtained. Experimental data collected from the literature on aqueous sorption and desorption of polyaromatic hydrocarbons, triazines and ureas were examined on soils, sediments, organic matter-rich sorbents, montmorillonites and fullerene. Positive ΔGext values were obtained to quantify the thermodynamic potential for spontaneous exit from a metastable state that is not implemented due to the kinetic barriers. Relating the ΔGext values to sorbate molecular structure and sorbent properties may allow the prediction of SDH for various chemicals on sorbents in which the sorbate-induced perturbation of a sorbent matrix is believed to be a cause for the formation of persistent metastable states and the appearance of a non-closed sorption-desorption sequence.

Life times of metastable states guide regulatory signaling in transcriptional riboswitches

Transcriptional riboswitches modulate downstream gene expression by a tight coupling of ligand-dependent RNA folding kinetics with the rate of transcription. RNA folding pathways leading to functional ON and OFF regulation involve the formation of metastable states within well-defined sequence intervals during transcription. The kinetic requirements for the formation and preservation of these metastable states in the context of transcription remain unresolved. Here, we reversibly trap the previously defined regulatory relevant metastable intermediate of the Mesoplasma florum 2′-deoxyguanosine (2′dG)-sensing riboswitch using a photocaging-ligation approach, and monitor folding to its native state by real-time NMR in both presence and absence of ligand. We further determine transcription rates for two different bacterial RNA polymerases. Our results reveal that the riboswitch functions only at transcription rates typical for bacterial polymerases (10–50 nt s−1) and that gene expression is modulated by 40–50% only, while subtle differences in folding rates guide population ratios within the structural ensemble to a specific regulatory outcome.

Creation and annealing of metastable defect states in CH3NH3PbI3 at low temperatures

Methylammonium lead iodide (CH3NH3PbI3), an organic-inorganic perovskite widely used for optoelectronic applications, is known to dissociate under illumination with light at photon energies around 2.7 eV and higher. Here, we show that photo-induced dissociation is not limited to ambient temperatures but can be observed even at 5 K. The photo-induced dissociation of N–H bonds results in the formation of metastable states. Photoluminescence (PL) measurements reveal the formation of defect states that are located 100 meV within the bandgap. This is accompanied by a quenching of the band-to-band PL by one order of magnitude. Defect generation is reversible and annealing at 30 K recovers the band-to-band PL, while the light-induced defect states disappear concurrently.

Relationship between the thermomechanical treatment regimes and heterogeneous structure of carbon fiber

The fine structure of carbon fibers obtained at different temperatures of high-temperature treatment was studied. It was shown that the fiber material is heterogeneous. Increasing the temperature treatment under the conditions of the tension load stimulates more active growth of the sizes of the coherent scattering regions oriented parallel to the fiber axis. With an increase in the orientation angle relative to the fiber axis, the size of the coherent scattering regions of the components decreases, as well as the component composition changes toward the formation of metastable states corresponding to large values of d002

Chemical disorder driven reentrant spin cluster glass state formation and associated magnetocaloric properties of Nd2Ni0.94Si2.94.

In this work, we report the synthesis of a new ternary intermetallic compound, Nd2Ni0.94Si2.94, that forms in single phase only in a defect crystal structure. The compound exhibits an antiferromagnetic transition below 7.2 K (TN) followed by a spin cluster freezing behaviour below 2.85 K (Tf), which makes the compound a reentrant spin cluster glass system. The detailed studies of dc and ac magnetization, heat capacity, non-equilibrium dynamical behaviour, viz., aging effect, temperature and field dependent magnetic relaxation and magnetic memory effect establish the compound to be a cluster-glass material below freezing temperature. The interplay between competing exchange coupling (c/a ≃ 1.04 ⇒ JNN ≃ JNNN) and chemical disorder driven variation in the electronic environment among the Nd ions has been argued to be responsible for such a metastable state formation. A considerable value of MCE parameters (-ΔS ∼ 11.4 J kg-1 K-1, RCP ∼ 160 J kg-1 and ΔTad ∼ 5.2 K for a field change of 70 kOe) is obtained for this magnetically frustrated glassy compound.

Bistability Versus Metastability in Driven Dissipative Rydberg Gases

We investigate the possibility of a bistable phase in an open many-body system. To this end we discuss the microscopic dynamics of a continuously off-resonantly driven Rydberg lattice gas in the regime of strong decoherence. Our experimental results reveal a prolongation of the temporal correlations with respect to the lifetime of a single Rydberg excitation and show strong evidence for the formation of finite-sized Rydberg excitation clusters in the steady state. We simulate our data using a simplified and a full many-body rate-equation model. The results are compatible with the formation of metastable states associated with a bimodal counting distribution as well as dynamic hysteresis. A scaling analysis reveals however, that the correlation times remain finite for all relevant system parameters. This suggest that the Rydberg aggregate is composed of many small clusters and all correlation lengths remain finite. This is a strong indication for the absence of a global bistable phase, previously suggested to exist in this system.

Dynamic Error in Strain-Induced Magnetization Reversal of Nanomagnets Due to Incoherent Switching and Formation of Metastable States: A Size-Dependent Study

Modulation of stress anisotropy of magnetostrictive nanomagnets with strain offers an extremely energy-efficient method of magnetization reversal. The reversal process, however, is often incoherent and hence error-prone in the presence of thermal noise at room temperature. Occurrence of incoherent metastable states in the potential landscape of the nanomagnet can further exacerbate the error. Stochastic micromagnetic simulations at room temperature are used to understand and calculate energy dissipations and switching error probabilities in this important magnetization switching methodology. We find that these quantities have an intriguing dependence on nanomagnet size: small nanomagnets perform better owing to the fact that they are more resilient to the formation of metastable states and magnetization dynamics in them is more coherent. However, for a fixed stress anisotropy energy density, smaller nanomagnets will also have poorer resilience against thermal instability. Thus, the challenge in straintronics is to maximize the stress anisotropy energy density by developing materials and processes that yield the largest magnetostriction.

Ortho-positronium observation in the Double Chooz experiment

The Double Chooz experiment measures the neutrino mixing angle θ 13 by detecting reactor $$ {\overline{\nu}}_e $$ via inverse beta decay. The positron-neutron space and time coincidence allows for a sizable background rejection, nonetheless liquid scintillator detectors would profit from a positron/electron discrimination, if feasible in large detector, to suppress the remaining background. Standard particle identification, based on particle dependent time profile of photon emission in liquid scintillator, can not be used given the identical mass of the two particles. However, the positron annihilation is sometimes delayed by the ortho-positronium (o-Ps) metastable state formation, which induces a pulse shape distortion that could be used for positron identification. In this paper we report on the first observation of positronium formation in a large liquid scintillator detector based on pulse shape analysis of single events. The o-Ps formation fraction and its lifetime were measured, finding the values of 44 % ±12 % (sys.) ±5 % (stat.) and 3.68 ns ±0.17 ns (sys.) ±0.15 ns (stat.) respectively, in agreement with the results obtained with a dedicated positron annihilation lifetime spectroscopy setup.

Effects of Neutral, Capped Ends Versus Charged Ends on the Folding of the Trp-cage Miniprotein

Charge-charge interactions have a significant impact on the free energy landscape of proteins. The role addition or subtraction of charged groups can play in the stabilization or destabilization of meta-stable non-native states of proteins remains relatively unstudied in detail. Problematically, current experimental methods can only indirectly characterize the non-native states of a protein. All-atom molecular dynamics simulations provide a useful tool for studying the non-native states of a protein but are limited in scale to the smallest proteins. Therefore, in order to study effects of charges while ensuring adequate sampling, we choose to simulate a small, fast-folding model protein, the Trp-cage, with replica-exchange molecular dynamics. As most simulations use a neutral end, capped sequence while experiments tend to use a charged end, zwitterionic variant, we chose to simulate and compare the folding of the Trp-cage both with neutral, capped ends and in zwitterionic form with a positively charged N terminus and a negatively charged C terminus. We find that while the native state of the protein remains relatively unchanged, the equilibration time for the charged end simulation is much longer, suggesting a rougher folding landscape. Analysis suggests the formation of meta-stable states characterized by the possession of non-native charge pairs. Furthermore, we also report an increase in beta-sheet content that cannot be directly explained by the formation of a specific non-native charge pair, indicating the addition of charges plays a much more complex role than the direct creation of non-native charge pairs. We finally use string methods to determine the most probable transition path between non-native meta-stable states and the native state in order to better explain the factors resulting in the increase in equilibration time for the simulation.

Metastable-state formation as a possible mechanism for the conductance anomalies in mesoscopic structures

We study metastable-state formation in the one-dimensional model of a quantum ballistic contact, in which the contact is represented by a potential barrier with electron-electron interaction localized therein. It is shown that when the interaction parameter exceeds a critical value, a metastable state with spontaneous spin polarization of the barrier is formed. The difference between the grand potentials of the metastable and globally stable states tends to zero at the critical point; therefore, the metastable state manifests itself in transport even at low temperatures and its effect gradually increases with temperature. The main effect is a decrease in the conductance with increasing temperature, which occurs in a certain range of the barrier-region potential, similar to that observed during the formation of the well-known 0.7 conductance anomaly.

Metastability of anatase: size dependent and irreversible anatase-rutile phase transition in atomic-level precise titania

Since crystal phase dominantly affects the properties of nanocrystals, phase control is important for the applications. To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection. Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states. Therefore, we take into account the kinetic control polymerization of TiO6 octahedral units to explain how the crystal phase of the crystal-nucleus-size titania is dependent on which coordination sites, cis- or trans-, react in the TiO6 octahedra, suggesting possibilities for the synthetic phase control of nanocrystals. In short, the dendrimer templates give access to crystal nucleation chemistry. The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

Hybrid Porphyrin–Silicon Nanowire Field-Effect Transistor by Opto-Electrical Excitation

A porphyrin-silicon nanowire (Si-NW) hybrid field-effect transistor is introduced. The hybrid device has separate electrical and optical gates surrounding the Si-NW channel. Porphyrin, a component of chlorophyll, is employed as an optical gate to modulate the potential of the Si-NW channel. Due to the independently formed hybrid gates, both optical and electrical excitation can effectively modulate the device. The exposed porphyrin optical gate responds to the optical excitation, and independently formed electrical gates respond to the electrical excitation. Charge transfer characteristics between a semiconductor channel and the porphyrin optical gate are deeply investigated. Optical, electrical, and opto-electrical excitation methods are employed to analyze the charging and discharging behaviors. Of these methods, opto-electrical excitation enables the strongest charge transfer because the inversion electron formation by an electrical pulse and the photoinduced charge transfer by an optical stimulus are affected simultaneously. Discharging processes, such as rapid discharging, exponential detrapping, and the formation of metastable states are also analyzed.