Transformation of interfaces in nanoamorphous solids

A model is proposed for the diffusive smearing of interfaces of different geometries in metal nanoglasses. It is shown that the smearing of planar interfaces occurs faster than that of triple junctions, which, in turn, takes place faster than that of the nodes. This accounts for the variation of the mean positron lifetime in metal nanoglasses induced by thermal treatment. Fiz. Tverd. Tela (St. Petersburg) 41, 1627–1629 (September 1999)     

Superfluid 4He quantum interference grating

We report the first observation of quantum interference from a grating structure consisting of four weak link junctions in superfluid 4He. We find that an interference grating can be implemented successfully in a superfluid matter wave interferometer to enhance its sensitivity while trading away some of its dynamic range. We also show that this type of device can be used to measure absolute quantum mechanical phase differences. The results demonstrate the robust nature of superfluid phase coherence arising from quantum mechanics on a macroscopic scale.     

Molecular dynamics study of interfaces in solids

The interfaces of heterostructures were studied in the framework of molecular dynamics to reveal a periodic interface stress distribution on the micro scale. The calculations were performed for copper and silver and for two different types of heterostructures: a heterostructure consisting of perfect Cu and Ag crystals and a heterostructure formed by Ag vapor deposition on a perfect Cu substrate. The calculation results allowed several conclusions about the origin of a chessboard interface structure and about certain mechanisms of its modification depending on the nanostructure size, boundary conditions, and method of heterostructure formation.     

Structural dynamics in quantum solids

Structural dynamics in semi-quantum and quantum solids (Ne, H₂ and D₂) is reported, and compared to results in classical solids such as Ar. The structural dynamics is driven by excitation of the lowest Rydberg state of the NO impurity. The resulting charge redistribution induces a local radial deformation of the medium (‘bubble’ formation) around the impurity. The steady-state spectroscopic signatures of this process are presented and analysed in the configuration coordinate model and harmonic approximation. Intermolecular potentials describing the impurity-medium interaction are obtained. The dynamics of ‘bubble’ formation and the ensuing medium response are probed in real-time by femtosecond pump-probe spectroscopy. In the very soft H₂ and D₂ environments, ‘bubble’ formation is a one-way process without recurrence of the cage motion and is complete in ∼1–2 ps. In the case of solid Ne, the dynamics are characterized by an initial expansion of the matrix cage around the impurity, followed by a low frequency recurrence. The results in solid Ne are complemented by preliminary molecular dynamics simulations. Overall the trend observed is that the expansion becomes slower in the sequence Ar–Ne–hydrogens, which is counterintuitive. This is discussed in terms of the quantum (delocalised) character of the lighter media, which introduces an additional microscopic friction. These results establish our experimental procedure based on the use of low-n Rydberg states as a novel method for probing structural and electronic solvation dynamics in non-polar media.     

Response functions for surface polaritons at interfaces in solids

Surface excitations of phonon and plasmon character are reviewed and discussed using the electric field response function. Calculations of the surface mode dispersion curves are exhibited for InSb for carrier concentrations in the 1 to 6 × 10¹⁷ cm^?3 range. Some comparisons are made with the reflectivity data of Anderson et al. which confirm the presence of a damaged layer on their InSb samples.     

Inhomogeneous quantum diffusion of muons in solids

This article deals with the problems raised when a muon(muonium) quantum diffusion in a crystal is highly inhomogeneous. It is shown how static disorder arising from the crystal doping influence the diffusion process and drastically changes both the time decay of the polarization function and the temperature dependence of the depolarization rate. The spin depolarization of muons moving in a spatially inhomogeneous defect potential and trapping of particles by the long-ranged traps is studied in detail. Most attention is given to the particle localization and delocalization phenomena resulting in the two-component behavior of muon polarization at low temperature. Finally, the experimental data on muon depolarization in insulators KCl, GaAs, N₂ and superconducting metals Al, V are analyzed.     

Two-level systems and mass deficit in quantum solids

We study a disordered quantum solid incorporating two-level systems in which a group of atoms (or a single atom) can experience coherent tunnelling between two different positions and demonstrate that an effective mass deficit induced by the presence of such objects can manifest itself only at relatively high frequencies and should vanish in the low-frequency limit. The crossover to the regime which can be associated with the appearance of an effective mass deficit has been observed in recent torsional oscillator experiments.     

Asymmetric double quantum wells with smoothed interfaces

We have derived and analyzed the wavefunctions and energy states for an asymmetric double quantum well (ADQW), broadened due to interdiffusion or other static interface disorder effects, within a known discreet variable representative approach for solving the one-dimensional Schrodinger equation. The main advantage of this approach is that it yields the energy eigenvalues, and the eigenvectors, in semiconductor nanostructures of different shapes as well as the strengths of the optical transitions between them. The behaviour of ADQW states for the different mutual widths of coupled wells, for the different degree of broadening, and under increasing external electric field is investigated. We have found that interface broadening effects change and shift energy levels, not monotonously, but the resonant conditions near an energy of sub-band coupling regions do not strongly distort. Also, it is shown that an external electric field may help to achieve resonant conditions for inter-sub-band inverse population by intrawell emission of LO-phonons in diffuse ADQW.     

Line shapes of multiple quantum NMR coherences in one-dimensional quantum spin chains in solids

General formulae for intensities of multiple quantum (MQ) NMR coherences in systems of nuclear spins coupled by the dipole-dipole interactions are derived. The second moments of the MQ coherences of zero- and second orders are calculated for infinite linear chains in the approximation of the nearest neighbor interactions. Supercomputer simulations of intensities of MQ coherences of linear chains are performed at different times of preparation and evolution periods of MQ NMR experiments. The second moments obtained from the developed theory are compared with the results of the supercomputer analysis of MQ NMR dynamics. The linewidth information in MQ NMR experiments is discussed.     

Superfluid to Mott insulator quantum phase transition in a 2D permanent magnetic lattice

The accessibility of the critical parameters for the superfluid to Mott insulator quantum phase transition in a 2D permanent magnetic lattice is investigated. We determine the hopping matrix element J, the on-site interaction U, and hence the ratio J/U, in the harmonic oscillator wave function approximation. We show that for a range of realistic parameters the critical values of J/U, predicted by different methods for the Bose-Hubbard model in 2D, such as mean field theory and Monte Carlo simulations, are accessible in a 2D permanent magnetic lattice. The calculations are performed for a 2D permanent magnetic lattice created by two crossed arrays of parallel rectangular magnets plus a bias magnetic field.     

Silicon–germanium interdiffusion and interfaces in self-assembled quantum dots

A scanning transmission electron microscope (STEM) study of silicon–germanium alloying using annular dark field (ADF) or Z-contrast imaging and electron energy loss spectroscopy (EELS) is presented. Results and techniques are discussed. Growth of 11 equivalent monolayers of germanium on silicon at 650 °C results in dome-shaped islands or quantum dots that contain up to ∼40% silicon. The interface between the as-grown island and substrate shows a highly disordered or amorphous zone ∼1.5-nm wide directly under the island. Annealing for 60 min at 650 °C gives larger pyramidal islands with diffuse crystalline interfaces and an equilibrium distribution of up to ∼70% silicon in the islands. PACS 61.16.Bg; 68.35.Dv; 68.35.Rh     

Thermodynamics of stressed solids: Slow deformation and roughening of material interfaces

At every turn in nature we are confronted with complex patterns. Patterns often formed in multiphase systems by an intricate dynamics of mass transport, e.g. diffusion and/or advection, and mass exchange between individual phases. Here we consider instabilities of phase boundaries in idealized stressed multiphase systems. Specifically, we study the growth of small perturbations of surfaces by considering mass transport from regions, where the stress and chemical potential is high, to surrounding regions where the stress and chemical potential is low. We present a linear stability analysis for various stress configurations and their corresponding stability diagrams.     

Broadly tunable quantum cascade laser in cantilever-enhanced photoacoustic infrared spectroscopy of solids

An external cavity quantum cascade laser (EC-QCL) is applied in the photoacoustic detection of solid samples. The EC-QCL used has a broad tuning range of 676 cm^?1 (970–1,646 cm^?1) in the mid-infrared region, which enables accurate broadband spectroscopy of large molecules. The high spectral power density of the EC-QCL is combined with an extremely sensitive optical cantilever microphone of the photoacoustic detector to achieve an ultimate sensitivity. The carbon black, polyethylene, and hair fiber samples were measured with the EC-QCL photoacoustic detection using electrical amplitude modulation to demonstrate the possibilities of the setup. The same measurements were repeated with a Fourier transform infrared (FTIR) spectrometer combined with a photoacoustic detector for a comparison. The EC-QCL photoacoustic setup yielded roughly a decade better signal-to-noise ratios than the FTIR setup with the same measurement time.     

The use of the repulsive potential in the quantum theory of solids

A new method is proposed for calculating the energy in certain special points of the Brillouin zone. The wave functions of valence and conduction electrons are given in the form of the linear combination of plane waves and the orthogonality condition of these functions to the wave functions of lower states is replaced by the repulsive potential. The practical application of this very simple method is illustrated on the energy spectrum of silicon in the centre of the Brillouin zone. It is proved that the results are comparable with some other methods, e. g. the orthogonalized plane-wave method.

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The basic thesis of this paper, together with concrete calculation of the energy spectrum of diamond, was delivered at the Czechoslovak-Polish conference in Sopoty in November 1956.

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In conclusion the autor would like to thank K. Trnková for carrying out the numerical calculations.     

Lattice response of quantum solids to an impulsive local perturbation

The lattice response of solid para-H2 to an impulsive electronic excitation was studied using femtosecond pump-probe spectroscopy. The evolution of an electronic bubble in the crystal, created upon excitation of the A(3ssigma) Rydberg state of an NO impurity, was followed in real time, with a resolution of 100 fs. The experimental results, interpreted in connection with molecular dynamics simulations with quantum corrections, indicate the presence of three stages in the dynamics: a sub-100 fs "adiabatic" phase, a 0.5-1 ps phase, corresponding to the interaction of the first with the next shells driven by the bubble expansion, and a 5 ps phase, corresponding to a slow rearrangement of the environment surrounding the impurity. These findings indicate that the lattice response in solid para-H2 resembles that of a liquid.     

Giant strain-sensitivity of acoustic energy dissipation in solids containing dry and saturated cracks with wavy interfaces

Mechanisms of acoustic energy dissipation in heterogeneous solids attract much attention in view of their importance for material characterization, nondestructive testing, and geophysics. Due to the progress in measurement techniques in recent years, it has been revealed that rocks can demonstrate extremely high strain sensitivity of seismoacoustic loss. In particular, it has been found that strains of order 10(-8) produced by lunar and solar tides are capable of causing variations in the seismoacoustic decrement on the order of several percent. Some laboratory data (although obtained for higher frequencies) also indicate the presence of very high dissipative nonlinearity. Conventionally discussed dissipation mechanisms (thermoelastic loss in dry solids, Biot and squirt-type loss in fluid-saturated ones) do not suffice to interpret such data. Here the dissipation at individual cracks is revised taking into account the influence of wavy asperities of their surfaces quite typical of real cracks, which can drastically change the values of the relaxation frequencies and can result in giant strain sensitivity of the dissipation without the necessity of assuming the presence of unrealistically thin (and, therefore, unrealistically soft) cracks. In particular, these mechanisms suggest interpretation for observations of pronounced amplitude modulation of seismo-acoustic waves by tidal strains.