Large Anomalous Hall effect in a silicon-based magnetic semiconductor

Magnetic semiconductors are attracting high interest because of their potential use for spintronics, a new technology which merges electronics and manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently emerged as the most popular materials for this new technology. While Curie temperatures are rising towards room temperature, these materials can only be fabricated in thin film form, are heavily defective, and are not obviously compatible with Si. We show here that it is productive to consider transition metal monosilicides as potential alternatives. In particular, we report the discovery that the bulk metallic magnets derived from doping the narrow gap insulator FeSi with Co share the very high anomalous Hall conductance of (GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens up a new arena for spintronics, involving a bulk material based only on transition metals and Si, and which we have proven to display a variety of large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure

f(R) theories

Over the past decade, f(R) theories have been extensively studied as one of the simplest modifications to General Relativity. In this article we review various applications of f(R) theories to cosmology and gravity - such as inflation, dark energy, local gravity constraints, cosmological perturbations, and spherically symmetric solutions in weak and strong gravitational backgrounds. We present a number of ways to distinguish those theories from General Relativity observationally and experimentally. We also discuss the extension to other modified gravity theories such as Brans-Dicke theory and Gauss-Bonnet gravity, and address models that can satisfy both cosmological and local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in Relativity, Published version, Comments are welcom

Redistribution of the c-axis spectral weight and conductivity sum rules in LSCO as revealed by optical transmission

Optical transmission measurements provide a detailed picture of redistribution of the c-axis spectral weight of La2-xSrxCuO4 (x = 0.12, 0.15) and allows to examine the Ferrell-Glover-Tinkham (FGT) sum rule up to 1.8 eV. A new conductivity peak is observed below T-c at similar to120 meV, which indicates a possible new collective mode and strongly increases the FGT sum rule violation. The absolute value of the violation is larger for the optimally doped sample. The sum rule starts to recover in the range of the low-lying interband transitions (similar to1 eV). In the normal state no clear energy scale of the pseudogap can be seen. (C) 2004 Elsevier B.V. All rights reserved