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The kinetics of crystalline ice nucleation and growth in nonporous, molecular beam deposited amorphous solid water (ASW) films are investigated at temperatures near 140 K. We implement an experimental methodology and corresponding model of crystallization kinetics to decouple growth from nucleation and quantify the temperature dependence and absolute rates of both processes. Nucleation rates are found to increase from ∼3×1013 m-3 s-1 at 134 K to ∼2×1017 m-3 s-1 at 142 K, corresponding to an Arrhenius activation energy of 168 kJ/mol. Over the same temperature range, the growth velocity increases from ∼0.4 to ∼4 Å s-1, also exhibiting Arrhenius behavior with an activation energy of 47 kJ/mol. These nucleation rates are up to ten orders of magnitude larger than in liquid water near 235 K, while growth velocities are ≈109 times smaller. Crystalline ice nucleation kinetics determined in this study differ significantly from those reported previously for porous, background vapor deposited ASW, suggesting the nucleation mechanism is dependent upon film morphology. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Herein we describe a new experimental protocol and develop a corresponding kinetic model of phase change that together enable the decoupling of simultaneous nucleation and growth processes and quantification of their kinetics. Growth is effectively isolated from nucleation by dividing a phase transition into two isothermal stages: prenucleation, where product crystallites nucleate and grow concurrently, and growth, in which transformation is completed essentially entirely by the expansion of these “seed” grains. Using the model, the temperature dependence of growth may be extracted from converted fraction versus time data for specimens that are appreciably larger than the mean crystal grain size. Similarly, the growth velocity can be ascertained from thin films that exhibit thickness-dependent transition kinetics owing to geometric constraints imposed by the specimen size. The combination of grain growth kinetics and information derived from the transformation of unseeded materials then allows the subsequent calculation of the temperature dependence and absolute rates of nucleation. A preliminary assessment of this method (for the crystallization of amorphous solid water) indicates that the model captures the basic physics of “seeded” transformations and that the technique appears to be a viable method of quantifying simultaneous nucleation and growth in some solids. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The kinetics of isothermal crystallization at the free surface of dense, 150-1050 bilayer (BL) (∼55-385 nm) thick amorphous solid water (ASW) films is investigated experimentally, and a model accounting for the observed thickness dependence is proposed. We find that as the ASW film thickness is increased above 150 BL, surface crystallization accelerates, rapidly at first and then more slowly until essentially size-independent kinetics are attained by 1050 BL. The potential origin of this thickness dependence is elucidated by a geometrical model of surface crystallization that we formulated using mechanistic information deduced from available experimental data. This simple mean-field model predicts that as film thickness is reduced below some critical value, the number of grains contributing to surface transformation progressively decreases, forcing each grain to convert a larger surface area and thus slowing crystallization. Good agreement between experimental data and the theory is realized using only two thickness-independent kinetic parameters (per temperature), suggesting that the model describes the basic physics of crystallization in these ASW films. Nucleation and growth rates determined via model fits range from ∼5.5x10[SUP9] cm[SUP-3] s[SUP-1] and ∼0.3 Å/s at 136 K to ∼6.5x10[SUP10] cm[SUP-3] s[SUP-1] and ∼1.5 Å/s at 140 K, corresponding to activation energies of 100 and 68 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

We present a model of phase transformation kinetics at the surface of nonvolatile and volatile solids, with special consideration of finite specimen size in one dimension. For nonvolatile materials in which nucleation occurs randomly and homogeneously throughout the sample, a slowing of surface phase change kinetics is predicted as specimen thickness decreases. This deceleration originates from a reduction in the number of grains contributing to surface transformation and the progressively more two-dimensional growth geometry of each grain as the sample becomes thinner. Sublimation increases the relative impingement velocity of subsurface nucleated grains and the interface, thus accelerating surface phase change (versus nonvolatile materials) in the absence of thickness effects. In thin films, the accelerating influence of sublimation competes with the retarding effect of finite specimen size, and thus transformation kinetics can be faster or slower than for nonvolatile materials, depending upon sample thickness. Finally, the model is fit to experimental data for the crystallization of volatile amorphous solid water films of varying thickness. Good agreement between experiment and theory is found, suggesting that our model captures the essential physics of simultaneous surface phase change and sublimation for finite specimens. [ABSTRACT FROM AUTHOR]

We present a model for the calculation of homoepitaxial film growth rates during silicon deposition on Si(100)-2×1 from disilane. Central to this model is the use of thermalized gaseous disilane adsorption probabilities that have been determined as a function of gas and surface temperature by convoluting supersonic molecular beam adsorption probability data with a Maxwell–Boltzmann distribution of incident kinetic energies and angles. These calculations show that the primary adsorption pathway over the entire range of conditions investigated is the so-called trapping-mediated mechanism, in which dissociative chemisorption occurs via a physisorbed intermediate. A second adsorption mechanism, direct chemisorption, is activated by translational energy and does in fact contribute somewhat to adsorption, but only at high gas and surface temperatures. Hydrogen coverages and silicon film growth rates are calculated from a simple surface decomposition kinetic model together with a phenomenological thermal desorption model and compare favorably to experimental measurements. Under conditions of high flux or low surface temperature, the growth rate is limited by hydrogen desorption and therefore increases with increasing surface temperature. In the flux-limited or adsorption-limited growth regime, the growth rate is predicted to decrease with increasing surface temperature due to a drop in the adsorption probability, resulting in a maximum in the growth rate for a given set of deposition conditions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Disilane adsorption probabilities have been measured on Si(100)-2×1 over a wide range of incident kinetic energies, incident angles, and surface temperatures using supersonic molecular beam techniques. The trapping-mediated chemisorption mechanism is shown to be the dominant adsorption pathway under the conditions investigated. The first step in such a mechanism, namely trapping into the physical adsorption well, has been studied directly via measurements at a surface temperature of 77 K. As expected, the trapping probability drops with increasing kinetic energy, but nearly 50% of incident molecules trap at 1 eV incident energy, indicating that trapping is quite efficient over a wide range of translational energies. Chemisorption probability values measured at higher surface temperatures are fit to a simple trapping-mediated chemisorption model that can be used to predict adsorption probabilities over a wide range of conditions. Measurements of the chemisorption probability at 500 K are independent of incident angle at kinetic energies of 0.75 eV and below. However, trapping probabilities measured at 77 K are shown to decrease with increasing angle of incidence at kinetic energies of 0.6 eV and above. This unusual effect is discussed in terms of molecular scattering during parallel momentum accommodation. In order to investigate the effect of surface hydrogen formed as a result of disilane decomposition, adsorption probabilities were measured as a function of monohydride coverage as well. On a monohydride-saturated surface the trapping probability is found to be lower than on a bare surface, most likely due to a decreased disilane physical adsorption binding energy compared to the bare surface. Also, the trapping probability varies linearly with hydrogen coverage between bare-surface and monohydride-saturated values. On the other hand, the hydrogen coverage dependence of the chemisorption probability is found to follow a simple second-order kinetic s... [ABSTRACT FROM AUTHOR]

We have used a suite of diffraction, thermodynamic, and transport measurements to study the localized rattling of Al guest atoms in VAl10,1. The mean-square displacement of the rattling atom shows a concave-down temperature dependence. This is characteristic of an anharmonic vibration, the frequency of which increases with amplitude, akin to a particle in a box. We find that the rattling is best described in terms of a sixth-order interatomic potential, with negligible contributions from harmonic and quartic terms. The rattler has a characteristic temperature of θRM = 21 K and couples strongly to both the acoustic phonons and conduction electrons. The coupling to the phonons is evident from the large value of the Gruneisen parameter, which increases with decreasing temperature to &Ggr; = 43 at 5 K. Below 6 K, the electrical resistivity varies as T3, which can be explained in terms of electron scattering from damped sixth-order vibrations. VAl10,1 is a superconductor below Tc = 1.53 K, with an upper critical field of ∼ 1 kG. [ABSTRACT FROM AUTHOR]

We have studied the Fe-based superconductor Ba0.55K045Fe1.95Co0.5As2 in order to identify the structure of the order parameter in this system by performing extensive low-temperature specific heat measurements in zero and in applied magnetic field. The pronounced superconducting jump at Tc = 32.5 K and &Dgr;0/(kBTc) = 2.57 indicate strong-coupling superconductivity in this material. Here we provide evidence for fully gapped superconductivity in Ba0.55K0.45Fe1.95Co0.05As2. A linear magnetic field dependence of the low-temperature specific heat points to s-wave symmetry of the order parameter. Below Tc the superconducting electronic part of the specific heat can be described by the BCS α model with &Dgr;0 = 7.2 meV. [ABSTRACT FROM AUTHOR]

We have measured the composition and temperature dependence of the shear moduli C' and C(44) for two-phase (alpha+beta)- and single-phase beta-PdH(x). In the two-phase region, the alpha- and beta-phases are coherent. Here, the composition dependence of C(44) and C' deviate negatively from a Vegard-type volume average. We attribute the deviations to two effects: (1) the partly in-series arrangement of the precipitate and matrix phases, relative to the externally applied stress, and (2) thermally activated anelastic relaxations involving the rapid motion of H interstitial atoms, leading to slight changes in the shape of coherent precipitates. The first effect is present for both C' and C(44) and is temperature-independent, whereas the second is present only for C' and is strongly temperature-dependent.

A low-temperature peak in C(p)/T(3) vs is ubiquitous to glasses. It arises from an abundance of low-frequency vibrations, the origin of which remains unclear. A comparable C(p)/T(3) vs peak is observed in crystals due to the dispersion of acoustic phonons and/or the excitation of optical phonons. We compared the C(p)/T(3) vs peaks in metallic and oxide glasses to elemental crystals by analyzing specific heat, phonon density of states, and elastic constant data. We observe no clear distinction in the peak temperature or amplitude between metallic glasses and crystals. Surprisingly, the peak is larger in single crystal Pd(40)Cu(40)P(20) than in glassy Pd(40)Cu(40)P(20).