Epitaxy: the motion picture

The engineering of many modern electronic devices demands control over a crystal down to the thickness of a single layer of atoms—and future demands will be even more challenging. Such control is achieved by the method of crystal growth known as epitaxy, and that makes this method the subject of intense study. More than that, recent advances are revolutionizing our knowledge of how surfaces grow. In fact, growing surfaces show a beautifully rich variety of phenomena, many of which are only now beginning to be uncovered. In the past few years many surface imaging techniques have been used to give us a close look at how crystals grow—while they are growing. The purpose of this article will be to illustrate some of the ways real surfaces grow and change as revealed by some of the latest in situ microscopic imaging technologies.

It is often said that crystal growth is more of an art than a science. Here we will show that it is emphatically both.     

Tunneling Radiation of a Torus-Like Black Hole in Anti-de Sitter Space-Time

An extension of the Parikh-Wilczek's semi-classical quantum tunneling method, the tunneling radiation of the charged particle from a torus-like black hole is investigated. Difference from the uncharged mass-less particle, the geodesics of the charged massive particle tunneling from the black hole is not light-like, but determined by the phase velocity. The derived result shows that the tunneling rate depends on the emitted particle's energy and electric charge, and takes the same functional form as uncharged particle. It proves also that the exact emission spectrum is not strictly pure thermal, but is consistent with the underlying unitary theory. PACS Numbers: 04.70.Dy, 97.60.Lf, 05.30.Ch.     

InAs/GaSb/AlSb resonant tunneling spin device concepts

We discuss device concepts for creating spin-polarized current sources without external magnetic fields, using non-magnetic 6.1 Å semiconductor resonant tunneling structures. Spin filters, spin pumps, and spin transistors that exploit structural and bulk inversion asymmetries will be examined.     

Low energy properties of identical and strongly correlated particles

Interesting qualitative consequences can arise from the quantum mechanical identity among strongly correlated particles that carry spin. This is demonstrated for properties connected with the low energy excitations in molecular and electronic systems. Spatial permutations among the identical particles are used as the key features. The particular behaviour of rotational tunneling molecules or molecular parts under the influence of dissipation are discussed together with the consequences arising for conversion transitions. The relationship between the thermal shifting of the tunneling line and the conversion rate at low and at elevated temperatures is explicated. The valuable information, that can be extracted from the conversion behaviour after isotopical substitution, is explained in detail. At low temperatures qualitative changes are predicted for the conversion rate by deuteration. Weakly hindered rotors show, also experimentally, drastic isotopic effects. The second part is devoted to finite systems of strongly interacting electrons that appear in semi-conductor nano-structures. The lowest excitation energies are strongly influenced by the interaction. They can be understood and determined starting from the limit of crystallized electrons by introducing localized many particle ‘pocket states’. The energy levels show multiplet structure, in agreement with numerical results. The total electron spin, associated with the low energy excitations, is crucially important for the nonlinear transport properties through quantum dots. It allows for instance to explain the appearance of negative differential conductances.     

H andH NMR Relaxation in Hydrogen-Bonded Solids Due to a Complex Motion: Classical Jumps over a Barrier and Incoherent Tunneling

Equations for the temperature dependence of proton and deuteron spin–lattice relaxation rates and second moments due to a complex motion consisting of classical jumps over a potential barrier and quantum mechanical tunneling through the barrier have been derived. Asymmetric double and triple potential wells are considered. These equations have been employed to analyze proton spin–lattice relaxation data for solid naphthazarin in the laboratory and rotating frames as a function of temperature. It is shown that tunneling plays an important role in the proton transfer dynamics of this compound.     

Random tunneling by means of acceptance-rejection sampling for global optimization

Any global minimization algorithm is made by several local searches performed sequentially. In the classical multistart algorithm, the starting point for each new local search is selected at random uniformly in the region of interest. In the tunneling algorithm, such a starting point is required to have the same function value obtained by the last local minimization. We introduce the class of acceptance-rejection based algorithms in order to investigate intermediate procedures. A particular instance is to choose at random the new point approximately according to a Boltzmann distribution, whose temperatureT is updated during the algorithm. AsT 0, such distribution peaks around the global minima of the cost function, producing a kind of random tunneling effect. The motivation for such an approach comes from recent works on the simulated annealing approach in global optimization. The resulting algorithm has been tested on several examples proposed in the literature.     

The Büttiker-Landauer model generalized

Büttiker and Landauer studied scattering off an oscillating rectangular barrier in order to shed light on the time aspects of tunneling. The expression for the traversal time resulting from this study is controversial. In addition, doubts have recently been expressed on technical aspects of their work. In an attempt to clarify these issues, we investigate a generalization of their model to arbitrary oscillating barriers,V(x, t)=V ₀(x)+V ₁(x)cos t. In the process, we confirm that Büttiker and Landauer's work is technically sound. However, we show, by several examples, that no direct general relation exists between the characteristic frequency of an oscillating barrier and the duration of the tunneling process. For a wide range of realistic parameters this characteristic frequency does not even exist.This paper is dedicated to E. G. D. Cohen.     

Polychronous kinetics with nonstationary rate constants. Effect of a medium

Characteristic features of the kinetics of solid-state cage reactions with distributed parameters of the relaxing matrix were considered. Depending on the ratio of the constants of the reaction rate and relaxation of environment, the kinetics of chemical conversions can be either exponential or nonexponential. Plausible reasons for the unsteady-state character of the kinetics of the processes of two types,viz., the reactions of alkyl radicals in amorphous alcohol matrices and conversions in biological systems, were discussed. The main reason for the unsteady-state character of the reactions of the first type is a dispersion of the equilibrium distances between the reagents. Kinetics of the reactions of the second type, such as rebinding of the ligands in the heme-containing proteins (e.g., in myoglobin), is determined by the distances in the pairs of reagents and the relaxation transitions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 469–476, March, 1997.     

Broadband and band-selective IMPRESS-gHMBC: compensation of refocusing inefficiency with synchronized inversion sweep

Compensation of refocusing inefficiency in a gHMBC experiment by replacing the rectangular pi pulse with a pair of adiabatic pulses with synchronized inversion sweep (CRISIS) significantly improves the performance of the gHMBC experiment. The CRISIS-gHMBC experiment retains the pure absorptive shapes in F1 and hence results in better lineshape and higher resolution than the current versions of magnitude mode gHMBC spectra. When used as a broadband experiment, CRISIS-gHMBC, owing to better refocusing efficiency of the adiabatic pulse pairs, gives improved performance across the 13C spectral width. Moreover, it is shown that CRISIS-gHMBC is a robust and improved alternative and when used along with the IMPRESS (Improved Resolution using Symmetrically Shifted pulses) technique further increases the sensitivity and resolution without additional experimental time. The IMPRESS-CRISIS combination is demonstrated for broadband gHMBC and band-selective gHMBC experiments. The ICbs-gHMBC [IMPRESS-CRISIS-band-selective gHMBC] experiment is an attractive and better alternative to individual band-selective gHMBC.     

Atomic structure of InAs quantum dots on GaAs

In recent years, the self-assembled growth of semiconductor nanostructures, that show quantum size effects, has been of considerable interest. Laser devices operating with self-assembled InAs quantum dots (QDs) embedded in GaAs have been demonstrated. Here, we report on the InAs/GaAs system and raise the question of how the shape of the QDs changes with the orientation of the GaAs substrate. The growth of the InAs QDs is understood in terms of the Stranski–Krastanow growth mode. For modeling the growth process, the shape and atomic structure of the QDs have to be known. This is a difficult task for such embedded entities.

In our approach, InAs is grown by molecular beam epitaxy on GaAs until self-assembled QDs are formed. At this point the growth is interrupted and atomically resolved scanning tunneling microscopy (STM) images are acquired. We used preparation parameters known from the numerous publications on InAs/GaAs. In order to learn more about the self-assemblage process we studied QD formation on different GaAs(0 0 1), (1 1 3)A, and ( )B substrates. From the atomically resolved STM images we could determine the shape of the QDs. The quantum “dots” are generally rather flat entities better characterized as “lenses”. In order to achieve this flatness, the QDs are terminated by high-index bounding facets on low-index substrates and vice versa. Our results will be summarized in comparison with the existing literature.