Faraday effect : a field theoretical point of view

We analyze the structure of the vacuum polarization tensor in the presence of a background electromagnetic field in a medium. We use various discrete symmetries and crossing symmetry to constrain the form factors obtained for the most general case. From these symmetry arguments, we show why the vacuum polarization tensor has to be even in the background field when there is no background medium. Taking then the background field to be purely magnetic, we evaluate the vacuum polarization to linear order in it. The result shows the phenomenon of Faraday rotation, i.e., the rotation of the plane of polarization of a plane polarized light passing through this background. We find that the usual expression for Faraday rotation, which is derived for a non-degenerate plasma in the non-relativistic approximation, undergoes substantial modification if the background is degenerate and/or relativistic. We give explicit expressions for Faraday rotation in completely degenerate and ultra-relativistic media.Comment: 20 pages, Latex, uses axodraw.st

Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films

We report ferromagnetic and spin wave resonance absorption measurements on high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these films exhibit robust ferromagnetic long-range order, based on the fact that up to seven resonances are detected at low temperatures, and the resonance structure survives to temperatures close to the ferromagnetic transition. On the other hand, we observe a spin wave dispersion which is linear in mode number, in qualitative contrast with the quadratic dispersion expected for homogeneous samples. We perform a detailed numerical analysis of the experimental data and provide analytical calculations to demonstrate that such a linear dispersion is incompatible with uniform magnetic parameters. Our theoretical analysis of the ferromagnetic resonance data, combined with the knowledge that strain-induced anisotropy is definitely present in these films, suggests that a spatially dependent magnetic anisotropy is the most likely reason behind the anomalous behavior observed.Comment: 9 pages, including 6 figure

EFFECT OF PRESSURE AND ELECTRIC FIELD ON CDW INDUCED RESISTIVITY ANOMALIES IN NbSe3

Nous présentons une étude sous pression des anomalies qui apparaissent dans les mesures de résistivité à 145 K et 59 K dans le composé NbSe3. Ces anomalies sont supposées être dues à l'apparition d'ondes de densité de charge. Les températures de transition décroissent avec la pression avec la même pente dT/dp = 4 K/kbar. Nous avons aussi observé la suppression de ces deux anomalies par l'application d'un champ électrique que nous attribuons à un effet tunnel à travers des gap extrêmement petits introduits par les ondes de densité de charge.We report the effect of pressure on the two resistivity anomalies at 145 K and 59 K in NbSe3 and resulting from charge density wave formation. The rate of decrease of the critical temperature with pressure is the same at both transitions (dT/dp = 4 K/kbar). We observe also the suppression by electric fields of these two anomalies, which we attribute to Zener breakdown across extremely small gaps introduced by the CDW

mumusup+sup +SR spectroscopy: the positive muon as a magnetic probe in solids

Through its asymmetric decay, the positive muon acts as a sensitive detector of the interactions of its spin with the medium in which it comes to rest. Development of the $mu$$sup +$SR spectroscopy technique is described, and recent applications of the $mu$$sup +$ as a probe are discussed. Results for hyperfine fields in ferromagnets and impurity states in nonmetals are presented with suggestions for future studies. 6 figures. (auth

RF properties of high temperature superconductors: Cavity methods

A description of cavities used in the study of the microwave properties of the high-temperature superconductors is followed by a lumped-circuit analysis of the coupling of transmission lines and resonators. The frequency dependence of the reflected and transmitted microwave power and the character of transient cavity response are analyzed. Techniques are discussed for the introduction of samples of the high-temperature superconductors into microwave cavities. Following a discussion of sample surface impedance and sample geometry factor, the connection between surface resistance and cavity Q is examined as well as the connection between cavity frequency shift and surface reactance. Measurement techniques that utilize reflected or transmitted power or transient response are described. 35 refs., 1 fig