Heisenberg exchange in magnetic monoxides

The superexchange intertacion in transition-metal oxides, proposed initially by Anderson in 1950, is treated using contemporary tight-binding theory and existing parameters. We find also a direct exchange for nearest-neighbor metal ions, larger by a factor of order five than the superexchange. This direct exchange arises from Vddm coupling, rather than overlap of atomic charge densities, a small overlap exchange contribution which we also estimate. For FeO and CoO there is also an important negative contribution, related to Stoner ferromagnetism, from the partially filled minority-spin band which broadens when ionic spins are aligned. The corresponding J1 and J2 parameters are calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and the Curie-Weiss temperatures, show appropriate trends, and give a reasonable account of their volume dependences. For MnO the predicted value for the magnetic susceptibility at the Neel temperature and the crystal distortion arising from the antiferromagnetic transition were reasonably well given. Application to CuO2 planes in the cuprates gives J=1220oK, compared to an experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental 230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory (1/28 revision

Confinement-induced Berry phase and helicity-dependent photocurrents

The photocurrent in an optically active metal is known to contain a component that switches sign with the helicity of the incident radiation. At low frequencies, this current depends on the orbital Berry phase of the Bloch electrons via the "anomalous velocity" of Karplus and Luttinger. We consider quantum wells in which the parent material, such as GaAs, is not optically active and the relevant Berry phase only arises as a result of quantum confinement. Using an envelope approximation that is supported by numerical tight-binding results, it is shown that the Berry phase contribution is determined for realistic wells by a cubic Berry phase intrinsic to the bulk material, the well width, and the well direction. These results for the magnitude of the Berry-phase effect suggest that it may already have been observed in quantum well experiments.Comment: 4 pages, 2 figure

Tight-Binding Theory of Manganese and Iron Oxides

The electronic structure is found to be understandable in terms of free-atom term values and universal interorbital coupling parameters, since self-consistent tight-binding calculations indicate that Coulomb shifts of the d-state energies are small. Special-point averages over the bands are seen to be equivalent to treatment of local octahedral clusters. The cohesive energy per manganese for MnO, Mn2O3, and MnO2, in which manganese exists in valence states Mn2+, Mn3+, and Mn4+, is very nearly the same and dominated by the transfer of manganese s electrons to oxygen p states. There are small corrections, one eV per Mn in all cases, from couplings of minority-spin states. Transferring one majority-spin electron from an upper cluster state to a nonbonding oxygen state adds 1.67 eV to the cohesion for Mn2O3, and two transfers adds twice that for MnO2 . The electronic and magnetic properties are consistent with this description and appear to be understandable in terms of the same parameters.Comment: Submitting to Phys. Rev.

Networks of Power? Rethinking class, gender and entrepreneurship in English electrification, 1880-1924

Traditional energy histories have treated electrification as an inevitability: the assumption has been that making cheap energy supply readily available for the masses required the energy efficiency uniquely attainable by large-scale networked electricity grids. While our account does not question that assumption, such a rationale can only explain the onset of electrification for contexts in which large scale electricity grids are already accessible to all. It cannot explain the earliest phase of electrification: what motivated the take up of electricity before such grids and their attendant economics actually existed to make it affordable and indeed competitive? We focus on the case of England before its National Grid was launched in 1926, a time when such alternatives as coal or its by-product coal-gas offered energy in a form that was cheaper or more convenient than stand-alone electrical installations and highly localised electricity infrastructures. Our initial aim is to survey a range of cultural rather than technocratic reasons for the early take-up of electricity in the 1880s to 1890s, treating it then as a luxury rather than a commonplace utility. In doing so, we return to Thomas Hughes’ seminal Networks of Power (1983) to examine how far the growth of electrical power supply was shaped not just by engineers and politicians that predominate in his account, but by old-money inherited aristocracy that Hughes touches upon only briefly. Specifically we investigate how the nascent electrical industry looked to these powerful wealthy aristocratic technophiles, male and female, to serve as ‘influencers’ to help broaden the appeal of domestic electricity as essential to a desirable life-style of glamorous modernity

Causes of breakage and disruption in a homogeniser

Many authors have written in the past regarding the exact causes of breakage and disruption in a high pressure homogeniser, but there has been little agreement. This paper investigates some of the most likely causes of the rupture of the walls of unicellular organisms and offers suggestions obtained from various papers and work carried out