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The heat capacity of a supercooled liquid subjected to a temperature cycle through its glass transition is studied within a kinetic model. In this model, the beta process is assumed to be thermally activated and described by a two-level system. The alpha process is described as a beta relaxation mediated cooperative transition in a double well. The overshoot of the heat capacity during the heating scan is well reproduced and is shown to be directly related to delayed energy relaxation in the double well. In addition, the calculated scan rate dependencies of the glass transition temperature T(g) and the limiting fictive temperature T(f) (L) show qualitative agreement with the known results. Heterogeneity is found to significantly reduce the overshoot of heat capacity. Furthermore, the frequency dependent heat capacity has been calculated within the present framework and found to be rather similar to the experimentally observed behavior of supercooled liquids.

We present results of extensive computer simulations and theoretical analysis of vibrational phase relaxation of a nitrogen molecule along the critical isochore and also along the gas-liquid coexistence. The simulation includes all the different contributions [atom-atom (AA), vibration-rotation (VR) and resonant transfer] and their cross-correlations. Following Everitt and Skinner, we have included the vibrational coordinate ($q$) dependence of the interatomic potential. It is found that the latter makes an important contribution. The principal important results are: (a) a crossover from a Lorentzian-type to a Gaussian line shape is observed as the critical point is approached along the isochore (from above), (b) the root mean square frequency fluctuation shows nonmonotonic dependence on the temperature along critical isochore, (c) along the coexistence line and the critical isochore the temperature dependent linewidth shows a divergence-like $\lambda$-shape behavior, and (d) the value of the critical exponents along the coexistence and along the isochore are obtained by fitting. The origin of the anomalous temperature dependence of linewidth can be traced to simultaneous occurrence of several factors, (i) the enhancement of negative cross-correlations between AA and VR contributions and (ii) the large density fluctuations as the critical point (CP) is approached. The former makes the decay faster so that local density fluctuations are probed on a femtosecond time scale. A mode coupling theory (MCT) analysis shows the slow decay of the enhanced density fluctuations near critical point. The MCT analysis demonstrates that the large enhancement of VR coupling near CP arises from the non-Gaussian behavior of density fluctuation and this enters through a nonzero value of the triplet direct correlation function., Comment: 35 pages, 15 figures, revtex4 (preprint form)

We study a dynamical disorder model for environmental modulation of rate processes where a need of dynamical cooperativity presents an entropy barrier, rather than an energy barrier. The rate depends on a control variable, Q, that describes the collective instantaneous state of the environment and is itself a random walker in finite discrete space with continuous time. We obtain the waiting time distribution for the relaxation by simulating the model. The time dependence of the average survival probability is derived there from and also by a numerical solution through the Liouville-master equation approach to the theoretical problem. We present an analytical treatment of the first passage time problem that is posed by a limiting case of our model. As the rate of the environmental fluctuation,T -1 env, slows down, the decay of the average survival probability is found to be more and more nonexponential in short times, but to change to exponential at longer times. The average survival time,T, exhibits a fractional power law dependence on K(=Tenvk0), where time is scaled in terms of k0-1, k0 being the intrinsic rate coefficient for the relaxation. The mean first passage time in the limiting case of the model exhibits an exponential dependence on the total number of the environmental subsystems N and a non-Arrhenious temperature dependence over the temperature range studied. We note the likely relevance of a part of this result to single molecule spectroscopic studies that reveal a tail in the waiting time distribution at long times.

We have carried out NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture to gain insight into the nature of orientational relaxation (OR) in a viscous liquid. At high pressures when the liquid is highly viscous, the OR is found to occur mainly via correlated hopping, sometimes involving participation of several neighboring atoms, placed in a ring like tunnel. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be gated, restricted primarily by the entropic bottlenecks, with the orientational motion becoming increasingly slower than the translation as the pressure is increased. Orientational relaxation is found to occur with a wide distribution of decay times.

Extensive isothermal-isobaric (NPT) molecular dynamics simulations at many different temperatures and pressures have been carried out in the well-known Kob-Andersen binary mixture model to monitor the effect of pressure (P) and temperature (T) on the dynamic properties such as the viscosity (\eta) and the self-diffusion (Di) coefficients of the binary system. The following results have been obtained: (i) Compared to temperature, pressure is found to have a weaker effect on the dynamical properties. Viscosity and diffusion coefficients are found to vary exponentially with pressure up to a certain high pressure after which the nature of exponential dependence changes. This change is rather sharp. (ii) With temperature, on the other hand, both viscosity and diffusion show super-Arrhenius dependence. Viscosity and diffusion coefficients fit well also to the mode coupling theory (MCT) prediction of a power law dependence on the temperature. The MCT critical temperature (Tc) for both the two dynamical properties are significantly higher than the corresponding critical temperature T\eta 0 obtained by fitting to the Vogel-Fulcher-Tammann (VFT) equation. (iii) The critical temperature for viscosity (T\eta 0) is considerably larger than that for the diffusion coefficients (T D4 0) implying the decoupling between diffusion and viscosity in deeply supercooled liquid. (iv) The nature of the motion of small particles change from continuous to hopping dominated once the larger ones are frozen. (v) The potential energy of the system shows a minimum against density at a relatively high density when the latter is changed by applying pressure at a constant temperature.

Molecular dynamics investigations of methane in one-dimensional channel systems of AlPO4-5 and the carbon nanotube are reported. Methane shows contrasting orientational preference over 1+3 and 2+2 orientations inside AlPO4-5 and the carbon nanotube. In AlPO4-5, 2+2 orientation is preferred in narrow parts of the channel while 1+3 orientation is predominant in the wider part of the channel. In the carbon nanotube 2+2 is preferred over 1+3 orientation throughout the channel. Thus, the orientation of methane can yield insight into methane–host interactions. Simulations with and without allowing for rotation of methane suggest that rotation aids translational mobility of methane in both the channel systems studied. It is shown that molecules with certain orientations are associated with lower mobility, both in AlPO4-5 and carbon nanotube. Further, translational motion exhibits diffusive behavior for methane in AlPO4-5 and superdiffusive behavior in the carbon nanotube. These results suggest that particular orientation or rotation in general can strongly influence translational mobility of methane in one-dimensional channels. It is seen that levitation effects influence rotational motion in addition to the translational motion.

We carry out extensive molecular dynamics ~MD! simulations of translation and rotation of spheroids in the suspension of spheres, for a wide range of values of the aspect ratio, kranging from the extreme limits of long rodlike molecules to the disc-shaped ones. While diffusion is anisotropic in short times ~compared to the orientational correlation time!, it is isotropic in the very long time and the ratio of the two components of short time translational diffusion ~Di and D' , where i and ' refer to the motion parallel and perpendicular to the major axis! scales with the aspect ratio kover a large range of k. For thin discs, the preferred mode of motion is in the plane of the disc. The oblates show a very long-time orientational correlation which is reflected by the slow decay of its time correlation function. The correlation time becomes progressively longer and finally diverges as the spherical limit is approached. This is expected because the angular momentum is conserved in the limit of fully spherical molecules ~ k51!. For both prolates and oblates, the crossover from the anisotropic to the isotropic motion is found to correlate with the orientational correlation time of the spheroid.

The orientation of a solute probe in a binary mixture often exhibits multiple relaxation times at the same solvent viscosity but different compositions [Beddard et al., Nature (London) 294, 145 (1981)]. In order to understand this interesting observation, we have carried out (NPT) molecular dynamics simulation study of rotation of prolate ellipsoids in binary mixtures. The simulations show that for a broad range of model parameters the experimental behavior can be reproduced. The plot of orientational relaxation time versus the solvent viscosity, the latter changed by varying the composition, shows a nonmonotonic viscosity dependence. The nature of the plot is found to depend on the system parameters, especially on the solute solvent interactions. A mode coupling theoretical analysis of this complex dependence of the rotational relaxation time tR on the binary viscosity h is presented. The theory can qualitatively explain the origin of the multiple relaxation time at the same viscosity.

The usual explanation for the observed inverse relation between the orientational correlation time (\tau R) and the self-diffusion (DS) of a tagged solute probe in a viscous liquid is in terms of the hydrodynamic relations which are known to have dubious conceptual validity for small molecules. Here, we present a microscopic derivation of the relation between \tau R and DS. This derivation is based on the general ideas of the mode coupling theory, but uses the time-dependent density functional theory to obtain the torque-torque and force-force time correlation functions on the solute probe. Our analysis shows that the orientational correlation time (\tau R) is inversely proportional to the translational diffusion coefficient (D0) of the solvent molecules. Thus, the viscosity dependence of orientational correlation time enters through the viscosity dependence of the translational diffusion (D0). The same theoretical analysis also shows that the translational diffusion coefficient of the solute probe (DS) is also proportional to the translational diffusion coefficient, D0, of the solvent molecules. This result is in agreement with the recent computer simulation results which show that the product of \tau R and DS is a weak function of the density (hence of the viscosity) of the liquid. The microscopic expressions provide explanation, in terms of the solute-solvent direct correlation functions, the reason for the sensitivity of orientational diffusion to solute-solvent interaction potential.

Brownian dynamics (BD) simulations have been carried out for the time dependent survival probability [Sp(t)] of donor–acceptor pairs embedded at the two ends of an ideal polymer chain. Long distance fluorescence resonance energy transfer (FRET) between the donor and the acceptor is assumed to occur via the Forster mechanism, where the transfer rate k(R) is a function of the distance (R) between the donor and acceptor. For the Rouse chain simulated here, k(R) is assumed to be given by k = kF/[1 + (R/RF)6], where kF is the rate in the limit of zero separation and RF is the Forster radius. The survival probability displays a strong nonexponential decay for the short to intermediate times when RF is comparable to RM [distance at which the R2P(R) is maximum]. The nonexponentiality is also more prominent in the case of highly viscous polymer solutions. It is predicted that the FRET rate can exhibit a fractional viscosity dependence. This prediction can be tested against experiments. We have also compared the BD simulation results with the predictions of the well-known Wilemski–Fixman (WF) theory at the level of survival probability. It is found that the WF theory is satisfactory for the smaller RF values (where the rate is small). However, the agreement becomes progressively poorer as the Forster radius is increased. The latter happens even at intermediate strengths of kF. The present results suggest the need to go beyond the WF theory.

A Monte Carlo study of the orthorhombic(beta), monoclinic(alpha), and liquid phases of toluene in the isobaric isothermal ensemble employing variable shape simulation cell is reported here. The intermolecular potential of Williams and Starr is seen to reproduce the lattice parameters and other known properties reasonably well for the alpha-phase. The beta-phase is not reproduced as well. The structure has been characterized in terms of the radial distribution functions and orientational correlation functions. The transition from the orthorhombic low temperature beta-phase to the high temperature monoclinic alpha-phase has been successfully simulated. The transition is first order and lies between 140 and 145 K in agreement with experiment. The reverse transition from the alpha- to the beta-phase does not take place in agreement with experiment. The liquid phase density and the heat of vaporization are reproduced well. The potential employed predicts an interaction energy which is about 5% in excess of the experimental value. The orientational correlation function and the radial distribution functions are sensitive to the potential and suggest where improvements are possible.

Using a lattice model for adsorption in microporous materials, pure component adsorption isotherms are obtained within a mean field approximation for methane at 300 K and xenon at 300 and 360 K in zeolite NaA. It is argued that the increased repulsive adsorbate-adsorbate interactions at high coverages must play an important role in determining the adsorption behavior. Therefore, this feature is incorporated through a "coverage-dependent interaction'' model, which introduces a free, adjustable parameter. Another important feature, the site volume reduction, has been treated in two ways: a van der Waal model and a 1D hard-rod theory [van Tassel et al., AIChE J. 40, 925 (1994)]; we have also generalized the latter to include all possible adsorbate overlap scenarios. In particular, the 1D hard-rod model, with our coverage-dependent interaction model, is shown to be in best quantitative agreement with the previous grand canonical Monte Carlo isotherms. The expressions for the isosteric heats of adsorption indicate that attractive and repulsive adsorbate-adsorbate interactions increase and decrease the heats of adsorption, respectively. It is concluded that within the mean field approximation, our simple model for repulsive interactions and the 1D hard-rod model for site volume reduction are able to capture most of the important features of adsorption in confined regions. (C) 1999 American Institute of Physics. [S0021-9606(99)70515-5].

We present here a microscopic and self-consistent calculation of the self-diffusion coefficient of a small tagged particle in a dense liquid of much larger particles. In this calculation the solute motion is coupled to both the collective density fluctuation and the transverse current mode of the liquid. The theoretical results are found to be in good agreement with the known computer simulation studies for a wide range of solute solvent size ratio. In addition, the theory can explain the anomalous enhancement of the self-diffusion over the Stokes Einstein value for small solutes, for the first time. Further, we find that for large solutes the crossover to Stokes Einstein behavior occurs only when the solute is 2 3 times bigger than the solvent molecules. The applicability of the present approach to the study of self-diffusion in supercooled liquids is discussed.

We show that newly found BF3-doped polyaniline, though highly conducting, remains amorphous. Magnetic studies reveal many unusual properties, while suggesting that the intrinsic conductivity of this system is significantly larger than all other known forms of conducting polyaniline, establishing it as an interesting class of highly conducting amorphous polymer., 5 pages, 3 figures

We report microwave absorption measurements as a function of temperature (from 290 to 125 K) and magnetic field (from 0 to 0.3 T) in mm-thick parallelepipeds of sintered $V_2O_3$ and $V_2O_3$ containing micron-size Fe precipitates. As before, it turns out that near the metal–insulator (MI) transition, the loss exhibits a sharp peak as a function of temperature. On application of a magnetic field, the peak temperature for ($V_2O_3$ + Fe) changes by a few kelvin, causing a giant magnetoimpedance ( \approx 200% in 0.1 T field) in the neighborhood of the MI transition.

We have investigated the multiferroic and glassy behaviour of metal-organic framework (MOF) material (CH3)(2)NH2Co(CHOO)(3). The compound has perovskite-like architecture in which the metal-formate forms a framework. The organic cation (CH3)(2)NH2+ occupies the cavities in the formate framework in the framework via N-H center dot center dot center dot O hydrogen bonds. At room temperature, the organic cation is disordered and occupies three crystallographically equivalent positions. Upon cooling, the organic cation is ordered which leads to a structural phase transition at 155 K. The structural phase transition is associated with a para-ferroelectric phase transition and is revealed by dielectric and pyroelectric measurements. Further, a PE hysteresis loop below 155 K confirms the ferroelectric behaviour of the material. Analysis of dielectric data reveal large frequency dispersion in the values of dielectric constant and tan delta which signifies the presence of glassy dielectric behaviour. The material displays a antiferromagnetic ordering below 15 K which is attributed to the super-exchange interaction between Co2+ ions mediated via formate linkers. Interestingly, another magnetic transition is also found around 11 K. The peak of the transition shifts to lower temperature with increasing frequency, suggesting glassy magnetism in the sample. (C) 2016 AIP Publishing LLC.

We report x-ray photoelectron spectra from a porous silicon film (PSF) with a photoluminescence peak at 1.8 eV, as a function of argon ion etching time to probe the composition in the surface and the subsurface regions. The results clearly indicate that the surface layer is essentially a fluorine admixed $SiO_2$ phase, while the Si:O:F composition of the subsurface region 2:1:0.2. With the possibility of the existence of hydrogen in this composition it appears that beyond the highly oxidized surface, PSF is a fluorine substituted siloxene derivative, which can be responsible for the visible photoluminescence.

The role of the soft phase (Ni0.8Zn0.2Fe2O4) on the magnetization reversal and coercivity mechanism of the Ni0.8Zn0.2Fe2O4/BaFe12O19 nanocomposite has been investigated. The presence of the interacting field and the disorder in the nanocomposite has been confirmed by the variation of Jr/Jr(∞) vs Jd/Jr(∞) and the irreversible magnetization. To understand the relative strength of the pinning and the nucleation, the magnetic viscosity measurement has been done and the thermal activation volume has been estimated. From the Barbier plot and the activation volume measurement, the dominant mechanism governing the magnetization reversal process has been proposed.

We demonstrate the depinning of Fermi level on both p- and n-type germanium after sulfur passivation by aqueous (NH4)(2)S treatment. Schottky contacts realized using metals with a wide range of work functions produce nearly ideal behavior confirming that the Fermi level is depinned. Examination of the passivated surface using x-ray photoelectron spectroscopy reveals bonding between Ge and sulfur.It is shown that good Ohmic contacts to n-type Ge and a hole barrier height (phi(Bp)) of 0.6 eV to p-type Ge can be achieved after this passivation treatment, with Zr Schottky contacts. This is the highest phi(Bp) reported so far.

Phase separation (PS) in hole-doped cobaltites (La1-xSrxCoxO3) is drawing renewed interest recently. In particular, the magnetic behavior of La0.85Sr0.15CoO3 has been subjected to a controversial debate for the past several years; while some groups show evidence for magnetic PS, others show spin glass (SG) behavior. Here, an attempt is made to resolve the controversy related to ``PS versus SG'' behavior in this compound. We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and the magnetotransport properties of La0.85Sr0.15CoO3 samples. We contemplate that the magnetic PS in La0.85Sr0.15CoO3 is neither intrinsic nor inherent, but it is a consequence of the preparation conditions. It is realized that a low temperature annealed (LTA) sample shows PS whereas the high temperature annealed (HTA) sample shows SG behavior. The Brillouin-like behavior of field cooled dc magnetization and apparently no frequency dependent peak shift in ac susceptibility for the LTA sample characterize it to be of ferromagneticlike whereas a kink in field cooled dc magnetization and a considerable amount (similar to 3 K) of frequency dependent peak shift in the ac susceptibility for the HTA sample characterize it to be of SG state. The magnetotransport properties show that the HTA sample is more semiconducting as compared to the LTA sample. This is interpreted in terms of the presence of isolated as well as coalescing metallic ferromagnetic clusters in the case of LTA sample. The magnetoresistance (MR) at 10 K for the HTA sample exhibits a huge value (similar to 65%) as compared to the LTA sample, and it monotonically decreases with the rise in temperature. Such a high value of MR in the case of HTA sample is strongly believed to be due to the spin dependent part of random potential distribution. Further, the slow decay of remnant magnetization with progress of time and the existence of hysteresis at higher temperatures (up to 200 K) in the case of LTA sample as compared to the HTA sample clearly unveil different magnetic states associated with them.

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time $\tau_c$ at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation $[ \langle \Delta \omega^2(0) \rangle ]$ with $\tau_c$ indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition \chi A dependence of the dephasing time $\tau_v$ is found to be primarily due to the nonmonotonic \chi dependence of $\tau_c$, rather than due to a similar dependence in the amplitude of $\langle \Delta \omega^2(0) \rangle$ . The probability distribution of \Delta \omega shows a markedly non-Gaussian behavior at intermediate composition ( \chi A \simeq 0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity $\eta^*$ with that of $\tau_v$ and $\tau_c$. It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.

Diffusion of a five-site model of methane within porous zeolite A has been investigated by molecular dynamics simulation using potential available in the literature. Equilibrium and dynamical quantities especially those relating to the orientational degrees of freedom of methane have been computed. Methane exhibits interesting orientational preference during intercage diffusion through the eight-membered window which is the rate determining step for diffusion. A predominance of (2+2) orientation in place of (1+3) orientation is seen suggesting a strong translational–orientational coupling. This suggests that orientational motion might strongly influence the translational diffusivity of methane.

Glasses obtained from quenching melts of superconducting bismuth cuprates of the formula Bi2(Ca,Sr)n+1CunO2n+4 with n=1 and 3 exhibit novel dielectric properties. They possess relatively high dielectric constants as well as high electrical conductivity. The novel dielectric properties of these cuprate glasses are likely to be of electronic origin. They exhibit a weak microwave absorption due to the presence of microcrystallites.