Motion Of Many Atoms Research Articles

Experimental Protein Molecular Dynamics: Broadband Dielectric Spectroscopy coupled with nanoconfinement

Protein dynamics covers multiple spatiotemporal scale processes, among which slow motions, not much understood even though they are underlying protein folding and protein functions. Protein slow motions are associated with structural heterogeneity, short-lived and poorly populated conformations, hard to detect individually. In addition, they involve collective motions of many atoms, not easily tracked by simulation and experimental devices. Here we propose a biophysical approach, coupling geometrical nanoconfinement and broadband dielectric spectroscopy (BDS), which distinguishes protein conformations by their respective molecular dynamics. In particular, protein-unfolding intermediates, usually poorly populated in macroscopic solutions are detected. The protein dynamics is observed under unusual conditions (sample nanoconfinement and dehydration) highlighting the robustness of protein structure and protein dynamics to a variety of conditions consistent with protein sustainability. The protein dielectric signals evolve with the temperature of thermal treatments indicating sensitivity to atomic and molecular interaction changes triggered by the protein thermal unfolding. As dipole fluctuations depend on both collective large-scale motions and local motions, the approach offers a prospect to track in-depth unfolding events.

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids.

We study the conversion of bulk Se and Te, consisting of intertwined a helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our ab initio calculations reveal that previously unknown and unusually stable δ and η 2D allotropes may form in an intriguing multistep process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers ≲0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting.

Observation of coherent delocalized phonon-like modes in DNA under physiological conditions.

Underdamped terahertz-frequency delocalized phonon-like modes have long been suggested to play a role in the biological function of DNA. Such phonon modes involve the collective motion of many atoms and are prerequisite to understanding the molecular nature of macroscopic conformational changes and related biochemical phenomena. Initial predictions were based on simple theoretical models of DNA. However, such models do not take into account strong interactions with the surrounding water, which is likely to cause phonon modes to be heavily damped and localized. Here we apply state-of-the-art femtosecond optical Kerr effect spectroscopy, which is currently the only technique capable of taking low-frequency (GHz to THz) vibrational spectra in solution. We are able to demonstrate that phonon modes involving the hydrogen bond network between the strands exist in DNA at physiologically relevant conditions. In addition, the dynamics of the solvating water molecules is slowed down by about a factor of 20 compared with the bulk.

Vibrational Control of Bimolecular Reactions with Methane by Mode, Bond, and Stereo Selectivity.

Vibrational motions of a polyatomic molecule are multifold and can be as simple as stretches or bends or as complex as concerted motions of many atoms. Different modes of excitation often possess different capacities in driving a bimolecular chemical reaction, with distinct dynamic outcomes. Reactions with vibrationally excited methane and its isotopologs serve as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. Here, some recent progress in this area is briefly reviewed. Particular emphasis is placed on the key concepts developed from those studies. The interconnections among mode and bond selectivity, Polanyi's rules, and newly introduced vibrational-induced steric phenomena are highlighted.

Ligand- and oxygen-isotope-exchange pathways of geochemical interest

Environmental context Most chemical processes in water are either ligand- or electron-exchange reactions. Here the general reactivity trends for ligand-exchange reactions in aqueous solutions are reviewed and it is shown that simple rules dominate the chemistry. These simple rules shed light on most molecular processes in water, including the uptake and degradation of pesticides, the sequestration of toxic metals and the corrosion of minerals. Abstract It is through ligand-exchange kinetics that environmental geochemists establish an understanding of molecular processes, particularly for insulating oxides where there are not explicit electron exchanges. The substitution of ligands for terminal functional groups is relatively insensitive to small changes in structure but are sensitive to bond strengths and acid–base chemistry. Ligand exchanges involving chelating organic molecules are separable into two classes: (i) ligand substitutions that are enhanced by the presence of the chelating ligand, called a ‘spectator’ ligand and (ii) chelation reactions themselves, which are controlled by the Lewis basicity of the attacking functional group and the rates of ring closure. In contrast to this relatively simple chemistry at terminal functional groups, substitutions at bridging oxygens are exquisitely sensitive to details of structure. Included in this class are oxygen-isotope exchange and mineral-dissolution reactions. In large nanometer-sized ions, metastable structures form as intermediates by detachment of a surface metal atom, often from a underlying, highly coordinated oxygen, such as μ4-oxo, by solvation forces. A metastable equilibrium is then established by concerted motion of many atoms in the structure. The newly undercoordinated metal in the intermediate adds a water or ligand from solution, and protons transfer to other oxygens in the metastable structure, giving rise to a characteristic broad amphoteric chemistry. These metastable structures have an appreciable lifetime and require charge separation, which is why counterions affect the rates. The number and character of these intermediate structures reflect the symmetry of the starting structure.

Adding reactivity to structure--reaction dynamics in a nanometer-size oxide ion in water

We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates ot dissociation and molecular products. This molecule, the decaniobate ion ([H x Nb 10 O 28 ] (6-x) , is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity. The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.

Molecular structure of H-bonded complexes of N,N-diphenylformamidine studied by IR and NMR spectroscopy and quantum chemical calculations

The results of experimental studies and theoretical calculations of vibrational frequencies and structure of H-bonded associates of N,N-diphenylformamidine and its complexes with carboxylic acids and hydrogen chloride in solution are discussed. The IR and low temperature NMR spectra show the existence of equilibrium between s-trans-monomers and cyclic dimers, such as cyclic H-bonded complexes with carboxylic acids in solution. The complexes with weak acids have molecular structure. The interaction with strong acids and hydrogen chloride results in formation of H-bonded ionic pairs with proton transfer to the N atom of the base. The results of quantum chemical calculations confirm the formation of cyclic molecular complexes with two H-bonds NH…O C and OH…N at the interaction with weak carboxylic acids; the proton transfer along the OH…N bridge was found for the complexes with stronger proton donors. The vibrational frequencies and intensities were obtained also by a version of variational multidimensional anharmonic calculations of vibrational electrooptic parameters in space of normal coordinates. It was shown that this approach is more preferable for calculating the high-frequency XH stretch in systems where the corresponding normal mode is less characteristic and involves motions of many atoms.

Atomic-level dynamic behavior of a diffuse interphase boundary in an Au–Cu alloy

This investigation uses in situ high-resolution transmission electron microscopy to quantify the atomic-level dynamic behavior of a diffuse interphase boundary between ordered and disordered phases in an Au–Cu alloy at elevated temperature. It is found that both the interphase boundary position and thickness fluctuate with time and that the behavior of the disordered side of the interphase boundary differs from that of the ordered side. Analysis of the interphase boundary shape shows that it fluctuates by a mechanism involving the collective motion of many atoms rather than by single-atom jumps. These features are explained in terms of the physical properties of the different phases at the interphase boundary.

Elasticity study on collective motion in metallic glasses

The frequency dependence and the strain amplitude dependence of the dynamic Young's modulus in amorphous (a-) Cu 50Ti 50, a-Zr 50Cu 50 and a-Zr 55Cu 30Al 10Ni 5 were investigated at room temperature by means of the vibrating reed method. In the following, f and ɛ denote the frequency and the strain amplitude, respectively. The frequency dependence of the dynamic Young's modulus was measured at ɛ ∼ 10 −6 in the frequency range between 10 1 and 10 4 Hz, where a strong decrease in the dynamic Young's modulus between 10 1 and 10 4 Hz was found. The strain amplitude dependence of the dynamic Young's modulus was measured at f ∼ 10 2 Hz in the strain amplitude range between 10 −5 and 10 −3, where a strong increase in the dynamic Young's modulus with increasing strain amplitude was found. The effect of passing an electric direct-current on the dynamic Young's modulus at ɛ ∼ 10 −6 and f ∼ 10 2 Hz was also investigated, where a net increase in the dynamic Young's modulus with increasing current density in the range of 10 7 A/m 2 was found. These results were discussed from the point of view of the resonant collective motions of many atoms in metallic glasses.

Anelasticity of nanocrystalline metals

Abstract Nanocrystalline (n-) materials posses a characteristic ultrafine structure where small crystallites less than ∼100 nm in diameter are connected by highly disordered grain boundaries (GBs). Although the recent mechanical tests on the high-density n-metals suggest that the dynamic modulus is close to the polycrystalline metals, we have found a large anelastic strain comparable with the elastic strain in the quasi-static tests. The estimated activation energy as low as 0.2 eV suggests that certain cooperative motions of many atoms in the GBs are responsible for this large anelastic strain. The strain amplitude dependence (SAMD) in the resonant frequency, f , was measured by the vibrating reed technique, where f decreased by about 1% with increasing strain, e , up to 10 −4 and then turned to increase showing the saturation at e =10 −4 –5×10 −3 . This strange SAMD in f showed no change by the low-temperature irradiation, in contrast, f showed a large increase due to the accumulation of the point defects probably in the GBs during the irradiation. We surmise that the (an)elastic properties of n-metals are governed by the several processes not only in the GBs but also in the crystallites.

Anomalous elastic response of amorphous alloys suggesting collective motions of many atoms

The dynamic Young’s modulus, E , of amorphous (a-) Zr 60 Cu 30 Al 10 (numbers indicate at.%) alloy was measured as a function of frequency, f , with a strain amplitude, ε t , of 10 −6 , E (10 −6 , f ), and also as a function of ε t for f near 10 2 Hz, E ( ε t ,10 2 Hz), by means of the vibrating reed methods. The elasticity study under the passing of electric current (PEC) was carried out too. E (10 −6 , f ) is lower than E 0 for f between 10 and 10 4 Hz showing local minima near 5×10, 5×10 2 and 5×10 3 Hz, which are indicative of the resonant collective motion of many atoms, where E 0 is the static Young’s modulus. E ( ε t ,10 2 Hz) increases showing saturation with increasing ε t . Qualitatively, the outlines of E (10 −6 , f ) and E ( ε t ,10 2 Hz) observed for a-Zr 60 Cu 30 Al 10 are similar to those reported for various a-alloys. Quantitatively, a change in E ( ε t ,10 2 Hz) for a-Zr 60 Cu 30 Al 10 is smallest among that reported for various a-alloys, presumably reflecting that the crystallization volume, (Δ V / V ) x , is smallest for a-Zr 60 Cu 30 Al 10 . The effective charge number, Z ∗ , estimated from the change in E (10 −6 ,10 2 Hz) due to PEC is 3.0×10 5 , which is comparable with Z ∗ reported for various a-alloys. We surmise that the number of atoms in the collective motions excited near 10 2 Hz is similar among various a-alloys. The E (10 −6 , f ) data suggest that the spatial sizes of the density fluctuations may show a distribution.

Characteristic creep behavior of nanocrystalline metals found for high-density gold

Nanocrystalline (n) Au specimens with a density of $19.4\ifmmode\pm\else\textpm\fi{}0.2{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ and a mean grain size of about 20 nm were prepared below 300 K by the gas deposition method, where two types of $n\ensuremath{-}\mathrm{Au}$ specimens were obtained as a function of a deposition rate, the type-H specimens above 800 nm/s and the type-L specimens below 800 nm/s. The anelastic and the plastic creep responses are similar qualitatively but different quantitatively between the type-H and type-L specimens. The anelastic strain ${\ensuremath{\varepsilon}}_{an,\mathrm{GB}},$ associated with the grain boundary (GB) regions, increases linearly with $(T\ensuremath{-}{T}_{\mathrm{an}1})({\ensuremath{\sigma}}_{\mathrm{ap}}\ensuremath{-}{\ensuremath{\sigma}}_{\mathrm{an}1}),$ when the temperature T is higher than a threshold temperature ${T}_{\mathrm{an}1}$ of 200 K and the applied stress ${\ensuremath{\sigma}}_{\mathrm{ap}}$ is higher than a threshold stress, ${\ensuremath{\sigma}}_{\mathrm{an}1},$ of a few MPa. The ratio of ${\ensuremath{\varepsilon}}_{an,\mathrm{GB}}$ to the elastic strain is as large as 1.1 for the type-H specimens and 0.2 for the type-L specimens at 320 K for ${\ensuremath{\sigma}}_{\mathrm{ap}}\ensuremath{\gg}{\ensuremath{\sigma}}_{\mathrm{an}1}.$ The activation energy for the GB anelastic process is 0.2 eV. We surmise that cooperative motions of many atoms in the GB regions are responsible for ${\ensuremath{\varepsilon}}_{an,\mathrm{GB}},$ and both ${T}_{\mathrm{an}1}$ and ${\ensuremath{\sigma}}_{\mathrm{an}1}$ show a distribution depending on the number of atoms associated. The plastic creep rate ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ vs ${\ensuremath{\sigma}}_{\mathrm{ap}}$ data show a letter S-like curve. We classified the creep response into three categories, region I for the linear creep rate region for ${\ensuremath{\sigma}}_{\mathrm{ap}}$ between ${\ensuremath{\sigma}}_{\mathrm{pc}1}$ and ${\ensuremath{\sigma}}_{\mathrm{pc}2},$ region II for the transient creep rate region for ${\ensuremath{\sigma}}_{\mathrm{ap}}$ between ${\ensuremath{\sigma}}_{\mathrm{pc}2}$ and ${\ensuremath{\sigma}}_{\mathrm{pc}3},$ and region III for the saturation creep rate region for ${\ensuremath{\sigma}}_{\mathrm{ap}}$ between ${\ensuremath{\sigma}}_{\mathrm{pc}3}$ and ${\ensuremath{\sigma}}_{y}.$ The threshold stresses ${\ensuremath{\sigma}}_{\mathrm{pc}1}$ and ${\ensuremath{\sigma}}_{\mathrm{pc}2}$ and the yield stress ${\ensuremath{\sigma}}_{y}$ are about 30, 150, and 360 MPa for the type-H specimens, and about 60, 300, and 500 MPa for the type-L specimens, respectively. ${\ensuremath{\sigma}}_{\mathrm{pc}3}$ is slightly lower than ${\ensuremath{\sigma}}_{y}.$ From scanning tunneling microscopy images, we surmise that the localized GB slip takes place in region I, and the mean separation between the localized GB slips decreases with increasing ${\ensuremath{\sigma}}_{\mathrm{ap}}$ in region II and becomes comparable with the mean grain size in region III. The plastic creep in region III may be explained by the Ashby creep. The present view for the creep behavior explains the low-temperature creep behavior of fcc n metals.

Anomalous elastic properties of amorphous alloys suggesting a collective motion of many atoms

The Young's modulus, E( ε t, f), was measured as a function of the strain amplitude, ε t and the measurement frequency, f, at room temperature for melt-spun amorphous (a-) Pd 80Si 20, a-Pd 80Ge 20, a-Zr 60Cu 30Al 10, a-Zr 60Cu 40 and a-Cu 50Ti 50 alloys. E(10 −6, f) found in the vibrating reed measurements commonly shows a decrease from E 0 in the frequency range of 10 1–10 4 Hz, and local minima near 10 2 Hz and between 10 3 and 10 4 Hz, where E 0 is the Young's modulus found in quasi-static tensile tests. It is suggested that density fluctuations exist in a-alloys, where a high density region undergoes a resonant collective motion producing the resonant anelastic strains, and the size of the high density regions shows a distribution in dimensions. E(ε t ,10 2 Hz) increases with increasing ε t and finally saturates at ε t of 2–4×10 −3. It is suggested that anelastic strains resulted from the resonant collective motions of the high density regions saturate at the high value of ε t.

Crystallization under electropulsing suggesting a resonant collective motion of many atoms and modification of thermodynamic parameters in amorphous alloys

For melt-spun amorphous $(a\ensuremath{-}){\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ crystallization under electropulsing was studied by means of the discharge of a condenser with initial current density ${i}_{d0}$ of the order of ${10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ and decay time \ensuremath{\tau} in between 2 and 0.1 ms, where a specimen was sandwiched by AlN/BN substrates to minimize an effect of joule heating. The crystallization proceeds during electropulsing when ${i}_{d0}$ is higher than the threshold ${i}_{d0,c}.$ Here ${i}_{d0,c}$ is a function of \ensuremath{\tau} and shows a minimum value of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ at $\ensuremath{\tau}\ensuremath{\sim}2\mathrm{ms}$ for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $2.6\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{A}/\mathrm{m}}^{2}$ at $\ensuremath{\tau}\ensuremath{\sim}0.9\mathrm{ms}$ for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ where the maximum increase in temperature during electropulsing is about 120 K for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and 50 K for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ respectively. One-half of the specimen volume crystallizes after a few repetitions of electropulsing with ${i}_{d0}$ beyond ${i}_{d0,c}$ for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and after several repetitions for $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20}.$ We surmise that for the density fluctuations existing in amorphous alloys, under electropulsing a high-density region undergoes a resonant collective motion as a whole, which induces migrational motions of atoms in the low-density matrix around it. For $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20},$ it is observed that an unknown phase was formed in the early stage of the crystallization under electropulsing and disappeared after further electropulsing. It is also found for $a\ensuremath{-}{\mathrm{Cu}}_{50}{\mathrm{Ti}}_{50}$ and $a\ensuremath{-}{\mathrm{Pd}}_{80}{\mathrm{Si}}_{20}$ that for electropulsing with high ${i}_{d0},$ the electrical resistivity of a specimen decreased at the early stage of the crystallization and then turned to increase for further electropulsing. These phenomena may be associated with changes in the thermodynamic free energy of phases under an electric current predicted by the theoretical works. We surmise that present electropulsing excites a resonant collective motion of many atoms and modifies the thermodynamic free energy of phases too.

Low-Temperature Homogeneous Deformation in Amorphous Alloys Induced under Electropulsing

Very recently it has been found that the passing electric current (PEC) induces a considerable internal stress in amorphous alloys, suggesting an excitation of a collective motion of many atoms under the PEC. In the present work it has been further found that the combination of external stress and the excitation of the collective motion can cause the low-temperature homogeneous plastic deformation of amorphous alloys.

Dynamic anelastic response of amorphous alloys suggesting collective motions of many atoms

For amorphous (a-) Pd80Si20, we measured the Young's modulus E(εt,f) as a function of the strain amplitude εt and the measurement frequency f in the temperature range below 380 K. Linear elasticity is found in the static tensile and bending tests where E(εt,0) remains constant for εt < 0.3%. For the vibrating reed tests at εt = 10−6, with increasing f, E(10−6,f) shows a deviatory decrease from E(εt,0) for f below (103∼104) Hz showing a minimum at around 102 Hz ( = fC hereafter) and increases beyond E(εt,0) for f above (103∼104) Hz. For the vibrating reed tests at around fC with increasing εt, E(εt,fC) shows an increase and saturation in the strain range below 0.3%, where E(0.3%,fC) is considerably higher than E(εt,0), and nearly equal to the Young's modulus estimated for the crystalline state. The characteristic elastic responses diminish after annealing for the crystallization. We surmise that both the localized shear deformation in the elementary structures and the probable collective motions of many atoms are responsible for the characteristic elastic response.

An in-situ high-resolution transmission electron microscopy study of the growth and dissolution of θ{111}precipitate plates in an Al-Cu-Mg-Ag alloy

Abstract In-situ hot-stage high-resolution transmission electron microscopy investigations were performed to determine the atomic mechanisms and growth kinetics of θ-Al2Cu precipitate plates with a {111} habit plane in an Al-3.9 wt% Cu-0.5 wt % Ag-0.5 wt % Ag alloy. In order to obtain a three-dimensional description of precipitate growth mechanisms, the studies were performed along two principal directions. In the first direction, along [001]0\\[121]α the θ{111} plate faces were parallel to the direction of observation. This allowed the motion of ledges moving across the faces and at the edges of the precipitates to be recorded. In the second direction, along [110]θ\\[111]α the θ{111} plate faces were perpendicular to the direction of observation and data on the nucleation and propagation of kinks on the growth ledges were obtained. The results from these studies allowed the behaviour of single-plane and multiple-plane ledges, as well as multiple-plane ledge interactions, to be described in terms of the nucleation and propagation behaviour of kinks on the ledges. A nucleation rate and a free energy for kink nucleation were determined by analysing the velocities of ledges and kinks. The lengthening rate of plates contained within the foil was consistent with diffusion control limited by the nucleation of kinks at the plate edge although kinetic analyses of ledge motion indicate that surface diffusion may dominate the growth kinetics of θ{111} plates which intersect the transmission electron microscope foil surface in thin foils. These studies also show that ledge motion and interface migration involve the cooperative motion of many atoms and that these local processes at the interface can occur at rates up to six orders of magnitude faster than those predicted by long-range volume diffusion control.

Collective motion of many atoms in amorphous alloys induced under passing electric current

The effects of passing a direct electric current (PEC) on the dynamic Young's modulus E of a-Cu 50Zr 50, Cu 50Ti 50 and Pd 50Si 50 were investigated for the current density i d below 4 × 10 3 A/cm 2 using a vibrating reed technique. We observed the increases in E due to PEC, and further found that the effects of PEC on E are identical to those of applied stress on E after the scaling between i d and stress. The scaling gives the effective charge number Z* which measures the electromigration force under PEC as being around the order of 10 5 for all the a-alloys, suggesting that the collective motion of many atoms is induced under PEC. From the changes in Z* and in the specific volume due to annealing, we evaluated the number of atoms associated with the collective motion as being several hundred.

Effects of passing electric current on the elastic property of amorphous Cu50Zr50 and Cu50Ti50

For both amorphous Cu50Zr50 and Cu50Ti50, the dynamic Young's modulus M was found to increase under passing electric d.c.-current (PEC) with the current density id below 5 × 103 A/cm2, suggesting that PEC induces internal stress. The id dependence of M observed at low strain amplitude εt was found to be identical to the εt dependence of M without PEC after scaling between id and εt, giving Z* in the order of 105, where Z* is the apparent charge number measuring the electromigration force (EM-force) under PEC. We surmise that such a large Z* reflects the concentration of EM-force through the collective motion of many atoms, Z* = z*ξ, for ξ atoms with the charge number z* per atom. Application of this view to the observed effect of annealing on Z* gives ξ as several hundreds in the as-quenched state. Based on the present results, the effects of PEC on the structural relaxation and crystallization processes reported are also discussed.

Diffusion in thin-film amorphous metallic alloys

Due to their potential technological value and the interest from a basic scientific point of view, amorphous metallic alloys have been studied extensively in recent years. This includes investigations of atomic transport processes which play a key role for understanding, for example, crystallization processes and the solid-state amorphization reaction. The first measurements of diffusion in amorphous alloys were carried out about two decades ago, being difficult because of crystallization yielding very narrow temperature and time intervals in which the measurements can be carried out. Such measurements have been made possible only with the development of ion beam analysis and sputter sectioning techniques. A short summary of the experimental diffusion studies, mainly based on ion beam analysis, is given together with various theoretical models. These are based on defect-mediated diffusion and collective motion of many atoms. Nonconventional diffusion behavior caused, e.g., by buildup of stress during thermal annealing, relevant for solid-state amorphization reactions, will be discussed, and studies of the thermodynamic factor will be addressed.