Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface

Principles of equal-channel angular pressing as a processing tool for grain refinement

Variational theorems are established and applied to the derivation of bounds for the effective magnetic permeability of macroscopically homogeneous and isotropic multiphase materials. For reasons of mathematical analogy the results are also valid for the dielectric constant, electric conductivity, heat conductivity, and diffusivity of such materials. For the case of two‐phase materials, the bounds derived are the most restrictive ones that can be given in terms of the phase permeabilities and volume fractions. Comparison of present theoretical results with existing experimental data shows good agreement.

A Variational Approach to the Theory of the Effective Magnetic Permeability of Multiphase Materials

https://www.tcd.ie/Physics/people/Graham.Cross/SF%20Poster%202011/Schuh-ActaMater07-Mechanical%20behaviour%20of%20amorphous%20alloys.pdf

Mechanical behavior of amorphous alloys

The creep rate (ė) predicted by the boundary diffusion (Db) model is ė≃150σDbWΩ/(GS)3kT, where σ is the stress, W is the boundary width, (GS) is the average grain size, and Ω is vacancy volume. The stress dependence is the same as the lattice diffusion model, given by C. Herring, while the grain size dependence and the numerical constant are greater for boundary diffusion. Discussion of the mechanism of creep in polycrystalline alumina is based on the differences between the lattice and boundary diffusion models.

A Model for Boundary Diffusion Controlled Creep in Polycrystalline Materials

According to a suggestion of Nabarro, any crystal can change its shape by self‐diffusion in such way as to yield to an applied shearing stress, and this can cause the macroscopic behavior of a polycrystalline solid to be like that of a viscous fluid. It is possible that this phenomenon is the predominant cause of creep at very high temperatures and very low stresses, though not under more usual conditions. The theory underlying it is developed quantitatively, and calculations of rate of creep, or equivalently of effective viscosity, are given for aggregates of quasi‐spherical grains and for wires composed of cylindrical grains. Allowance is made for the effect of tangential stress relaxation at the grain boundaries. It is suggested that mosaic boundaries and boundaries between grains of nearly the same orientation may be unable to serve as sources or sinks of the diffusion currents, in which case the creep rate will depend only on the configuration of grain boundaries having a sizable orientation differen...

Diffusional Viscosity of a Polycrystalline Solid

It is shown that when certain plausible assumptions are fulfilled simple scaling laws govern the times required to produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical geometrically except for a difference of scale. It is suggested that experimental studies of the effect of such a change of scale may prove valuable in identifying the predominant mechanism responsible for sintering under any particular set of conditions, and may also help to decide certain fundamental questions in fields such as creep and crystal growth.

Effect of Change of Scale on Sintering Phenomena

This paper is designed to supplement the existing extensive literature on the conductivity of a randomly inhomogeneous medium, by treating the effects of inhomogeneities on piezoelectric, galvanomagnetic, and thermoelectric measurements. The scale of the inhomogeneities is supposed small compared with the dimensions of the specimen being measured, but large compared with mean free path, Debye length, etc. Formulas for all the effects are derived which are asymptotically exact in the limit of small fractional fluctuations in the local conductivity, etc. Comparison with other approximations and application to various exactly soluble cases show that these formulas are often roughly valid for quite sizable fluctuations. For material which, if uniform, would show a high field saturation of transverse magnetoresistance, the presence of appreciable inhomogeneities in the Hall constant will cause the magnetoresistance to increase indefinitely with field. This effect is due to the current distortions arising from ...

Effect of Random Inhomogeneities on Electrical and Galvanomagnetic Measurements

In a disordered solid solution the interatomic spacings will vary because of fluctuations in the local environment. This paper extends previous theories of these variations for a system in which A and B atoms are distributed at random over the sites of a simple lattice, with atomic fractions x and (1−x), respectively, and for which the differences between A and B in regard to size and interaction characteristics are small enough to be treated in first order only. To this order exact results are derived regarding the statistics of relative positions of AA, AB, and BB near‐neighbor pairs: expressions are given especially for the departures for the average spacing of each of these three types of pairs from the corresponding spacing of the mean lattice, and for the mean‐square fluctuations of these relative positions about their means. The expressions can be evaluated from the elastic spectrum of the crystal and da/dx, the variation of the lattice constant with composition. For the common cubic lattices, grap...

Distribution of interatomic spacings in random alloys

This paper gives a brief critique, in elementary language, of the principal types of theoretical pictures which have been advanced concerning the electronic states of transition metals, especially those of the iron group. It also calls attention to the possibility that some of the properties of these metals can be correlated by the use of concepts which have an exact, not just approximate, meaning for a many-electron system. The Fermi surface is probably a concept of this type. Major conclusions are that in the iron group metals the 3d electrons ought not to differ radically from those in the free atoms either in number or in spatial distribution, and that in most, though perhaps not all, of these metals the 3d electrons, magnetic or nonmagnetic, have an itinerant behavior.

Conyers Herring

Papers

Diffusional Viscosity of a Polycrystalline Solid

Effect of Change of Scale on Sintering Phenomena

Effect of Random Inhomogeneities on Electrical and Galvanomagnetic Measurements

Distribution of interatomic spacings in random alloys

The State of d Electrons in Transition Metals