Origin and consequences of the compensation (Meyer-Neldel) law

Entropy and Isokinetic Temperature in Fast Ion Transport

Ion transport in crystalline solids is an essential process for many electrochemical energy converters such as solid-state batteries and fuel cells. Empirical data have shown that ion transport in crystal lattices obeys the Meyer-Neldel Rule (MNR). For similar, closely related materials, when the material properties are changed by doping or by strain, the measured ionic conductivities showing different activation energies intersect on the Arrhenius plot, at an isokinetic temperature. Therefore, the isokinetic temperature is a critical parameter for improving the ionic conductivity. However, a comprehensive understanding of the fundamental mechanism of MNR in ion transport is lacking. Here the physical significance and applicability of MNR is discussed, that is, of activation entropy-enthalpy compensation, in crystalline fast ionic conductors, and the methods for determining the isokinetic temperature. Lattice vibrations provide the excitation energy for the ions to overcome the activation barrier. The multi-excitation entropy model suggests that isokinetic temperature can be tuned by modulating the excitation phonon frequency. The relationship between isokinetic temperature and isokinetic prefactor can provide information concerning conductivity mechanisms. The need to effectively determine the isokinetic temperature for accelerating the design of new fast ionic conductors with high conductivity is highlighted.

Enthalpy-entropy compensation in the slow Arrhenius process

The Meyer-Neldel compensation law, observed in a wide variety of chemical reactions and other thermally activated processes, provides a proportionality between the entropic and the enthalpic components of an energy barrier. By analyzing 31 different polymer systems, we show that such an intriguing behavior is encountered also in the slow Arrhenius process, a recently discovered microscopic relaxation mode, responsible for several equilibration mechanisms both in the liquid and the glassy state. We interpret this behavior in terms of the multiexcitation entropy model, indicating that overcoming large energy barriers can require a high number of low-energy local excitations, providing a multiphonon relaxation process.

The Nature of the Enthalpy-Entropy Compensation and "Exotic" Arrhenius Parameters in the Denaturation Kinetics of Proteins

Protein unfolding is a ubiquitous process responsible for the loss of protein functionality (denaturation), which, in turn, can be accompanied by the death of cells and organisms. The nature of enthalpy-entropy compensation (EEC) in the kinetics of protein unfolding is a subject of debate. In order to investigate the nature of EEC, the "completely loose" transition state (TS) model has been applied to calculate the Arrhenius parameters for the unfolding of polyglycine dimers as a model process. The calculated Arrhenius parameters increase with increasing dimer length and demonstrate enthalpy-entropy compensation. It is shown that EEC results from the linear correlations of enthalpy and entropy of activation with dimer length, which are derived directly from the properties of the transition state. It is shown that EEC in solvated (hydrated, etc.) proteins is a direct consequence of EEC in proteins themselves. The suggested model allows us also to reproduce and explain "exotic" very high values of the pre-exponential factor measured for the proteins unfolding, which are drastically higher than those known for unimolecular reactions of organic molecules. A similar approach can be applied to analyzing the nature of EEC phenomena observed in other areas of chemistry.

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

Solid electrolytes are considered as an ideal substitution of liquid electrolytes, avoiding the potential hazards of volatilization, flammability, and explosion for liquid electrolyte-based rechargeable batteries. However, there are significant performance gaps to be bridged between solid electrolytes and liquid electrolytes; one with a particular importance is the ionic conductivity which is highly dependent on the material types and structures. In this review, the general physical image of ion hopping in the crystalline structure is revisited, by highlighting two main kernels that impact ion migration: ion hopping pathways and skeletons interaction. The universal strategies to effectively improve ionic conductivity of inorganic solid electrolytes are then systematically summarized: constructing rapid diffusion pathways for mobile ions; and reducing resistance of the surrounding potential field. The scoped strategies offer an exclusive view on the working principle of ion movement regardless of the ion species, thus providing a comprehensive guidance for the future exploitation of solid electrolytes.

A β-NMR study of the depth, temperature, and molecular-weight dependence of secondary dynamics in polystyrene: Entropy–enthalpy compensation and dynamic gradients near the free surface

We investigated the depth, temperature, and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and β-detected nuclear magnetic resonance. Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin–lattice relaxation time [Formula: see text] was measured near the free surface and in the bulk. Spin–lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence [Formula: see text]. There is no clear correlation of the average entropy of activation [Formula: see text] and enthalpy of activation [Formula: see text] with MW, but there is a clear correlation between [Formula: see text] and [Formula: see text], i.e., entropy–enthalpy compensation. This results in the average Gibbs energy of activation, [Formula: see text], being approximately independent of MW. Measurements of the temperature dependence of [Formula: see text] as a function of depth below the free surface indicate the inherent entropic barrier, i.e., the entropy of activation corresponding to [Formula: see text] = 0, has an exponential dependence on the distance from the free surface before reaching the bulk value. This results in [Formula: see text] near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a “double-exponential” depth dependence. This model can explain the depth dependence of [Formula: see text] in polystyrene films. The characteristic length of enhanced dynamics is ∼6 nm and approximately independent of MW near room temperature.

Single skyrmion true random number generator using local dynamics and interaction between skyrmions

Magnetic skyrmions are of great interest to both fundamental research and applications in post-von-Neumann computing devices. The successful implementation of skyrmionic devices requires functionalities of skyrmions with effective controls. Here we show that the local dynamics of skyrmions, in contrast to the global dynamics of a skyrmion as a whole, can be introduced to provide effective functionalities for versatile computing. A single skyrmion interacting with local pinning centres under thermal effects can fluctuate in time and switch between a small-skyrmion and a large-skyrmion state, thereby serving as a robust true random number generator for probabilistic computing. Moreover, neighbouring skyrmions exhibit an anti-correlated coupling in their fluctuation dynamics. Both the switching probability and the dynamic coupling strength can be tuned by modifying the applied magnetic field and spin current. Our results could lead to progress in developing magnetic skyrmionic devices with high tunability and efficient controls.

Skyrmions, a kind of topological spin texture, have been considered as having potential for use in spin-based computing. Here, Wang et al. study the local dynamics of skyrmions interacting with pinning sites, demonstrating a skyrmion true random number generator with the magnetic field and spin current as two control parameters.

Explanation of the Compensation Law and the Isokinetic Point in the Electrical Conduction of Crosslinked Polyethylene

Thermally activated direct current (DC) electrical conductivity in low-density polyethylene (LDPE) is known to be subject to the compensation law. Accordingly, the preexponential factor follows a specific relation with activation energy, reducing overall changes in conductivity. This relationship is governed by the Meyer-Neldel temperature. However, there is no published evidence for a corresponding isokinetic point, a temperature where the conductivity of all LDPE samples is the same. Here, it is determined that the compensation law applies to both DC and alternating current (AC) conduction for LDPE and for crosslinked polyethylene (XLPE) without an observed isokinetic point. The potential origins of compensation in polyethylene are discussed as well as reasons for similarity between LDPE and XLPE. It is observed that prolonged water exposure removed the compensation behavior. Meanwhile, preheating samples in the oven prior to measurements modifies the compensation behavior and reduced the spread around the isokinetic point. It is thus deduced that an isokinetic point can be observed in polyethylene but is obscured by contributions from water and other impurities.

Quantitative Evaluations of Hydrogen Diffusivity in V-X (X = Cr, Al, Pd) Alloy Membranes Based on Hydrogen Chemical Potential

Vanadium (V) has higher hydrogen permeability than Pd-based alloy membranes but exhibits poor resistance to hydrogen-induced embrittlement. The alloy elements are added to reduce hydrogen solubility and prevent hydrogen-induced embrittlement. To enhance hydrogen permeability, the alloy elements which improve hydrogen diffusivity in V are more suitable. In the present study, hydrogen diffusivity in V-Cr, V-Al, and V-Pd alloy membranes was investigated in view of the hydrogen chemical potential and compared with the previously reported results of V-Fe alloy membranes. The additions of Cr and Fe to V improved the mobility of hydrogen atoms. In contrast, those of Al and Pd decreased hydrogen diffusivity. The first principle calculations revealed that the hydrogen atoms cannot occupy the first-nearest neighbor T sites (T1 sites) of Al and Pd in the V crystal lattice. These blocking effects will be a dominant contributor to decreasing hydrogen diffusivity by the additions of Al and Pd. For V-based alloy membranes, Fe and Cr are more suitable alloy elements compared with Al and Pd in view of hydrogen diffusivity.

Energy barriers and cooperative motion at the surface of freestanding glassy polystyrene films

We investigate the near-surface relaxation of freestanding atactic polystyrene films with molecular dynamics simulations. As in previous coarse-grained simulations, relaxation times for backbone segments and phenyl rings are linked to their bulk relaxation times via a power-law coupling relation. Variation of the coupling exponent with distance from the surface is consistent with depth-dependent activation barriers. We also quantify a reduction in dynamical heterogeneity at the interface, which can be interpreted in the framework of cooperative models for glassy dynamics.

Entropy-reduced Retention Times in Magnetic Memory Elements: A Case of the Meyer-Neldel Compensation Rule

We compute mean waiting times between thermally activated magnetization reversals in a nanodisk with parameters similar to a free CoFeB layer used in magnetic random access memories. By combining Langer's theory and forward flux sampling simulations, we show that the Arrhenius prefactor can take values up to 10^{21} Hz, orders of magnitude beyond the value of 10^{9}  Hz typically assumed, and varies drastically as a function of material parameters. We show that the prefactor behaves like an exponential of the activation energy, which highlights a case of the Meyer-Neldel compensation rule. This suggests that modeling information retention times with a barrier-independent prefactor in such magnetic storage elements is not justified.

A Review for Consistent Analysis of Hydrogen Permeability through Dense Metallic Membranes

The hydrogen permeation coefficient (ϕ) is generally used as a measure to show hydrogen permeation ability through dense metallic membranes, which is the product of the Fick’s diffusion coefficient (D) and the Sieverts’ solubility constant (K). However, the hydrogen permeability of metal membranes cannot be analyzed consistently with this conventional description. In this paper, various methods for consistent analysis of hydrogen permeability are reviewed. The derivations of the descriptions are explained in detail and four applications of the consistent descriptions of hydrogen permeability are introduced: (1) prediction of hydrogen flux under given conditions, (2) comparability of hydrogen permeability, (3) understanding of the anomalous temperature dependence of hydrogen permeability of Pd-Ag alloy membrane, and (4) design of alloy composition of non-Pd-based alloy membranes to satisfy both high hydrogen permeability together with strong resistance to hydrogen embrittlement.

Diffusion coefficients of polar organic compounds in agarose hydrogel and water and their use for estimating uptake in passive samplers

Diffusion coefficient (D) is an important parameter for prediction of micropollutant uptake kinetics in passive samplers. Passive samplers are nowadays commonly used for monitoring trace organic pollutants in different environmental matrices. Samplers utilising a hydrogel layer to control compound diffusion are gaining popularity. In this work we investigated diffusion of several perfluoroalkyl substances, currently used pesticides, pharmaceuticals and personal care products in 1.5% agarose hydrogel by measuring diffusion coefficients using two methods: a diffusion cell and a sheet stacking technique. Further, diffusion coefficients in water were measured using Taylor dispersion method. The sheet stacking method was used to measure D at 5, 12, 24, and 33 °C in order to investigate temperature effect on diffusion. Median D values ranged from 2.0 to 8.6 × 10^-6 cm² s^-1 and from 2.1 to 8.5 × 10^-6 cm² s^-1 for the diffusion cell and sheet stack methods respectively. For most compounds, the variability between replicates was higher than the difference between values obtained by the two methods. Rising temperature from 10 to 20 °C increases the diffusion rate by the factor of 1.41 ± 0.10 in average. In water, average D values ranged from 3.03 to 10.0 × 10^-6 cm² s^-1 and were comparable to values in hydrogel, but some compounds including perfluoroalkyl substances with a long aliphatic chain could not be evaluated properly due to sorptive interactions with capillary walls in the Taylor dispersion method. Sampling rates estimated using the measured D values were systematically higher than values estimated from laboratory sampler calibration in our previously published study, by the factor of 2.2 ± 1.0 in average.

Diffusion of Organic Molecules as a Function of Temperature in a Sucrose Matrix (a Proxy for Secondary Organic Aerosol)

Knowledge of diffusion coefficients as a function of temperature in secondary organic aerosol (SOA) or proxies of SOA is needed to predict atmospheric chemistry, climate, and air quality. We determined diffusion coefficients as a function of temperature of a fluorescent organic molecule in a sucrose matrix (a proxy for SOA). Diffusion coefficients were a strong function of temperature (e.g., at water activity = 0.43, diffusion coefficients decreased by a factor of ∼40 as the temperature decreased by 20 K). Interestingly, the apparent activation energy for diffusion of the fluorescent organic molecule was similar to the apparent activation for diffusion of water in the sucrose matrix. On the basis of these measurements, the mixing time of organic molecules by diffusion in some types of SOA particles will often be >1 h in the free troposphere, if a sucrose matrix is an accurate proxy for these types of SOA.

Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites

Capacitive techniques, routinely used for solar cell parameter extraction, probe the voltage-modulation of the depletion layer capacitance isothermally as well as under varying temperature. In addition, defect states within the semiconductor band gap respond to such stimuli. Although extensively used, capacitive methods have found difficulties when applied to elucidating bulk defect bands in photovoltaic perovskites. This is because perovskite solar cells (PSCs) actually exhibit some intriguing capacitive features hardly connected to electronic defect dynamics. The commonly reported excess capacitance observed at low frequencies is originated by outer interface mechanisms and has a direct repercussion on the evaluation of band gap defect levels. Starting by updating previous observations on Mott-Schottky analysis in PSCs, it is discussed how the thermal admittance spectroscopy and the deep level transient spectroscopy characterization techniques present spectra with overlapping or even "fake" peaks caused by the mobile ion-related, interfacial excess capacitance. These capacitive techniques, when used uncritically, may be misleading and produce wrong outcomes.

Competing Structural Influences in the Li Superionic Conducting Argyrodites Li6PS5- xSe xBr (0 ≤ x ≤ 1) upon Se Substitution

Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure-property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li₆PS_{5- x}Se _xBr (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li₆PS_{5- x}Se _xBr transport behavior was found to be largely unaffected by the incorporation of Se^2- due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimizing the ionic conductivity of solid ion conductors, local structural influences cannot be ignored and the idea of "the softer the lattice, the better" does not always hold true.

Magnetic states of nanostructures containing Ni2+ ions at the surface of SiO2 nanospheres

Ultra-small magnetic particles containing Ni²⁺ ions were grown at the surface of SiO₂ spheroidal nanoparticles (typical diameter: 50 nm) starting from NiCl₂ solutions. Depending on preparation details, two samples characterized by magnetic sub-nanostructures or lamellar sub-nanoparticles at the SiO₂ nanosphere surface were obtained. The decorated SiO₂ nanospheres were submitted to physico-chemical and magnetic characterization. In both samples, a magnetically blocked phase is observed at low temperature. Below 5 K, discontinuities in isothermal magnetization loops and magnetic relaxation effects suggest the onset of coherent quantum tunneling of nanoparticle magnetization (QTM). Relaxation effects give are described by a field- and temperature-dependent magnetic viscosity S_V(H,T); the total spin number of magnetic units is estimated by fitting the isothermal S_V(H) curve to a model for an assembly of particles with random anisotropy axes. The mean number of aligned spins involved in the low-temperature relaxation is 32 and 15 in the two considered samples. Phonon-assisted QTM plays an increasingly important role with raising temperature and the quantum regime gradually merges with the classical behavior. Above the blocking temperature the magnetic units behave as classical superparamagnetic particles. When the intra-particle ferromagnetic order disappears the Ni²⁺ ions respond individually to the magnetic field.

Entropy-limited topological protection of skyrmions

Magnetic skyrmions are topologically protected whirls that decay through singular magnetic configurations known as Bloch points. We used Lorentz transmission electron microscopy to infer the energetics associated with the topological decay of magnetic skyrmions far from equilibrium in the chiral magnet Fe_1-x Co _x Si. We observed that the lifetime τ of the skyrmions depends exponentially on temperature, [Formula: see text]. The prefactor τ₀ of this Arrhenius law changes by more than 30 orders of magnitude for small changes of the magnetic field, reflecting a substantial reduction of the lifetime of skyrmions by entropic effects and, thus, an extreme case of enthalpy-entropy compensation. Such compensation effects, being well known across many different scientific disciplines, affect topological transitions and, thus, topological protection on an unprecedented level.

The equation of state of polymers. Part III: Relation with the compensation law

The properties of amorphous polymers and of organic compounds under pressure are interpreted in the framework of the modified Van der Walls Equation of State (mVW-EOS) the Vogel-Fulcher-Tamann (VFT) law and of the compensation law. We have shown recently that polymers and organic compounds in amorphous liquid and crystalline states verify the mVW-EOS which depends on three parameters, [Formula: see text] [Formula: see text] and [Formula: see text]. In this paper we compare the characteristic pressure [Formula: see text] of the mVW-EOS to the various pressures [Formula: see text] deduced from thermodynamic and kinetic properties of polymers in the liquid and solid states. [Formula: see text] and [Formula: see text] are: a) the enthalpy and volume change at the melting and glass transitions (the glass being isotropic or oriented and annealed below [Formula: see text] at various aging conditions); b) the activation parameters of individual [Formula: see text] and cooperative [Formula: see text] motions in crystalline liquid and amorphous polymers studied by dielectric or mechanical spectroscopy; and c) the activation parameters of amorphous (solid and liquid) polymers submitted to a deformation depending on the time frequency temperature and strain rate. For a same material, whatever its state and whatever the experimental properties analyzed (dielectric and mechanical relaxation, viscosity, auto-diffusion, yielding under hydrostatic pressure), we demonstrate that [Formula: see text], ([Formula: see text] Grüneisen parameter, [Formula: see text] compressibility). In all polymers and organic compounds (and water), these pressures, weakly dependent on T and P near [Formula: see text] and [Formula: see text] at low pressure are characteristic of the H-H inter-molecular interactions. It is shown that the two empirical Lawson and Keyes relations of the compensation law can be deduced from the mVW-EOS.

Recent Developments in p-Type Oxide Semiconductor Materials and Devices

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

Some New Observations on Activation Energy of Crystal Growth for Thermally Activated Crystallization

Calorimetric study of glass/crystal phase transformation in disordered semiconductors is a significant tool for understanding their crystallization kinetics. Such studies provide the basis for practical application of glasses. Differential scanning calorimetry (DSC) is one of the advanced techniques for the analysis of thermally induced crystallization in glassy or amorphous systems. We are reporting the nonisothermal DSC measurements on four amorphous systems of Se70Te30 alloy with Ag, Cd, Sb, and Zn as chemical modifiers. In general, the rate constant (K) shows Arrhenian dependence on temperature (T), i.e., K = K0 exp (-Eg/RT) where Eg is the activation energy of crystal growth and K0 is called the pre-exponential factor of rate constant. In the present work, an experiment is designed to see the effect of composition on the activation energy of crystal growth. We have found Meyer-Neldel relation (MNR) between Eg and K0 for present systems. Another interesting feature of present work is the observation of further relation between Meyer-Neldel prefactor and Meyer-Neldel energy.

Correlated parameter fit of arrhenius model for thermal denaturation of proteins and cells

Thermal denaturation of proteins is critical to cell injury, food science and other biomaterial processing. For example protein denaturation correlates strongly with cell death by heating, and is increasingly of interest in focal thermal therapies of cancer and other diseases at temperatures which often exceed 50 °C. The Arrhenius model is a simple yet widely used model for both protein denaturation and cell injury. To establish the utility of the Arrhenius model for protein denaturation at 50 °C and above its sensitivities to the kinetic parameters (activation energy E a and frequency factor A) were carefully examined. We propose a simplified correlated parameter fit to the Arrhenius model by treating E a, as an independent fitting parameter and allowing A to follow dependently. The utility of the correlated parameter fit is demonstrated on thermal denaturation of proteins and cells from the literature as a validation, and new experimental measurements in our lab using FTIR spectroscopy to demonstrate broad applicability of this method. Finally, we demonstrate that the end-temperature within which the denaturation is measured is important and changes the kinetics. Specifically, higher E a and A parameters were found at low end-temperature (50 °C) and reduce as end-temperatures increase to 70 °C. This trend is consistent with Arrhenius parameters for cell injury in the literature that are significantly higher for clonogenics (45-50 °C) vs. membrane dye assays (60-70 °C). Future opportunities to monitor cell injury by spectroscopic measurement of protein denaturation are discussed.

Entropy-enthalpy Compensation of Biomolecular Systems in Aqueous Phase: a Dry Perspective

We survey thermodynamic measurements on processes involving biological macromolecules in aqueous solution, which illustrate well the ubiquitous phenomenon of entropy-enthalpy compensation. The processes include protein folding/unfolding and ligand binding/unbinding, with compensation temperatures varying by about 50 K around an average near 293 K. We show that incorporating both near-exact entropy-enthalpy compensation (due to solvent relaxation) and multi-excitation entropy (from vibrational quanta) leads to a compensation temperature in water of about 230 K. We illustrate a general procedure for subtracting solvent and environment-related terms to determine the bare Gibbs free energy changes of chemical processes.

Glass: Kohlrausch exponent, fragility, anharmonicity

The thermodynamical and mechanical properties of (fragile and strong) glass are modeled based on a generalised activation energy relationship log( τ ) = ΔG ( β )/RTn(T') process of glass-forming liquids. This cooperative process involves 1/n(T') elementary β motions of activation Gibbs energy ΔG ( β ) dependent on the equivalent temperature T', the temperature of the liquid in equilibrium having the volume of the glass, function of temperature and aging conditions. From this modified VFT law the relaxation of any properties (V , H , stress, creep) can be calculated and approximated by the Kohlrausch function. This model predicts consistency relationships for: a) the temperature (and aging time) variation of the Kohlrausch exponent; b) the temperature dependence of the stabilisation time domain of strong and fragile glass; c) the linear relation between the activation parameters (E (*) energy, S (*) entropy, V (*) volume) of the α and β transition. The Lawson and Keyes (LK) relations are recalled and it is shown that these relations (somewhat equivalant to the compensation law or Meyer-Neldel rule) are observed generally in glass. Morever the (macroscopic) ratios ΔH/ΔV observed during aging or after a temperature jump and the (microscopic) ratio E (*)/V (*) are found equal to κγ (κ compressibily, γ Grüneisen parameter), in agreement with the LK predictions. From various experiments and in agreement with predictions of this model we conclude that the Grüneisen parameter γ ( B ) (pressure derivative of the bulk modulus) and the Mean Square Displacement (MSD) characterising the anharmonicity of solids (and liquids) are the main parameters which govern the relaxation properties of the glass state. Linear relations between the parameters γ ( B ), the fragility m, and the Kohlrausch exponent n ( g ) at T ( g ) are explained. These correlations underscore a strong relationship between the fragilty of glass formers and the extent of the anharmonicity in the interatomic interactions.

Understanding Phase Change Memory Reliability and Scaling by Physical Models of the Amorphous Chalcogenide Phase

Phase change memory (PCM) devices are based on the electrically-induced change of phase within an active chalcogenide material. PCM features large resistance window, fast threshold/phase switching and high endurance, thus motivating a broad interest as potential Flash replacement and/or nonvolatile storage class memory. Despite the relatively mature progress of research and technology, there is still a wide debate about the ultimate scaling perspective for PCMs. Structural relaxation, crystallization and noise affecting the amorphous chalcogenide phase need to be addressed by accurate physical models for a realistic scaling projection. This work discusses the scaling of PCM devices in terms of the conduction mechanisms and structural stability of the amorphous chalcogenide phase. Resistance window narrowing, current fluctuations, resistance drift and crystallization in the amorphous phase will be explained by a unified model for thermal excitation of the structure by many-phonon phenomena. The downscaling of the reset current, needed to reduce the cell area in memory arrays, and thermal disturb between adjacent cells during reset will be finally addressed to assess the scaling capability of high-density PCM crossbar architectures.

Phase behavior and molecular mobility of n-octylcyanobiphenyl confined to molecular sieves: dependence on the pore size

The molecular dynamics of 4-n-octyl-4'-cyanobiphenyl (8CB) confined inside the pores of a series of AlMCM-41 samples with the same structure, constant composition (SiAl=14.7) but different pore sizes (diameter between 2.3 and 4.6 nm) was investigated by broadband dielectric spectroscopy (10(-2)-10(9) Hz) in a large temperature interval. Two relaxation processes are observed: one has a bulklike behavior and is assigned to the 8CB in the pore center. The relaxation time of the second relaxation process is essentially slower than that of the former one and this process is related to the dynamics of molecules in a surface layer with a paranematic order. Both relaxation processes are specifically influenced by the interaction of the molecules with the surface and by the confinement. Above the clearing temperature the temperature dependence of the relaxation rate of the bulklike process obeys the Vogel-Fulcher-Tammann (VFT) law. The Vogel temperature increases with decreasing pore size. This is explained by increasing influence of paranematic potential of the surface layer with decreasing pore size. The temperature dependence of the relaxation rate of the surface layer follows also the VFT formula and the Vogel temperature decreases with decreasing pore size. This temperature dependence is controlled by both the interaction of the 8CB molecules with the surface via hydrogen bonding and by spatial confinement effects. To discriminate between both effects the data for the surface layer of 8CB confined to the molecular sieves are compared with results concerning 8CB adsorbed as a quasimonolayer on the surface of silica spheres of aerosil. On this basis a confinement parameter is defined and discussed.

Effects of long jumps, reversible aggregation, and Meyer-Neldel rule on submonolayer epitaxial growth

We demonstrate, using kinetic Monte Carlo simulations of submonolayer epitaxial growth, that long jumps and reversible aggregation have a major impact on the evolution of island morphologies. Long jumps are responsible for a supra-Arrhenius behavior of the effective diffusion coefficient as the attachment and detachment kinetics give rise to a bimodal island size distribution that depends on temperature and long jump extent limits. As the islands density increases with temperature, the average size of stable islands reaches a maximum before decreasing. We have also observed that the diffusion coefficient cannot be used alone to predict the evolution of island sizes and morphologies, the relative rate of each process having a major importance. Our theoretical developments are of direct relevance for materials systems such as Au, Pd, Ag, Cu, Ni, H/Si , H/W(110), Co/Ru , and Co/Ru(S), that are known for exhibiting a compensation effect that cannot be contained within experimental uncertainties.

Entropy factor in the hopping frequency for ionic conduction in oxide glasses induced by energetic clustering

The contribution of configurational entropy to the effective hopping frequency of ionic transport in amorphous systems is discussed. The effective rate of ion hopping has been extracted from the onset frequency of the ac conductivity measured in ionically conducting silicate glasses. Both the onset frequency and the dc conductivity exhibit Arrhenius-type thermal activation with similar values for the activation energy, DeltaEa=0.65+/-0.3 eV. The prefactor of the onset frequency results in nu0'=(1.05+/-0.05)x10(11) Hz, which is much lower than characteristic vibrational frequencies (10(13) Hz). Following standard hopping percolation theory, the long-range motion is dominated by a fraction of high-energy barriers that connect clusters of faster sites. The multiplicity of equivalent sites for ion hop entails a retardation of the effective jumping time with respect to the elementary hop. This effect can be assimilated into a negative activation entropy term in the frequency prefactor of the ion hopping rate, which depends on the features of energy clustering and accounts for the wide dispersion of nu0' reported for many conducting glasses. The model implies an effective percolation length of Lc approximately 7 nm, in good agreement with previous works.

Energetic and entropic contributions to surface diffusion and epitaxial growth

For Pd/Pd(111) an exceptionally high barrier (350 meV) for surface self-diffusion and a negative additional energy DeltaE = -53 meV for step-down diffusion are measured. Both findings agree with the proposed role of free-electron-like surface states. Despite the negative value of DeltaE layer-by-layer growth is not observed. This is related to the low preexponential factor for step-down diffusion. Preadsorption of oxygen increases DeltaE but flattens the films. Again this is due to the prefactors for diffusion. The present results demonstrate the importance of entropic effects for diffusion and growth.