Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion

With a variational three--body calculation we study the role of the interplay between the onsite Coulomb, Hund's rule, and superexchange interactions on the spinwave excitation spectrum of itinerant ferromagnets. We show that correlations between a Fermi sea electron--hole pair and a magnon result in a very pronounced zone boundary softening and strong deviations from the Heisenberg spinwave dispersion. We show that this spin dynamics depends sensitively on the Coulomb and exchange interactions and discuss its possible relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid communication

Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors

We develop a theory of the magnetization dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. We describe the effects of the strong carrier spin relaxation on the nonlinear optical response by using the Lindblad semigroup method. We demonstrate magnetization control during femtosecond timescales via the interplay between circularly polarized optical excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin interactions. Our results show a light-induced magnetization precession and relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure

Faraday-rotation fluctuation spectroscopy with static and oscillating magnetic fields

By Faraday-rotation fluctuation spectroscopy one measures the spin noise via Faraday-induced fluctuations of the polarization plane of a laser transmitting the sample. In the fist part of this paper, we present a theoretical model of recent experiments on alkali gas vapors and semiconductors, done in the presence of a {\em static} magnetic field. In a static field, the spin noise shows a resonance line, revealing the Larmor frequency and the spin coherence time $T_2$ of the electrons. Second, we discuss the possibility to use an {\em oscillating} magnetic field in the Faraday setup. With an oscillating field applied, one can observe multi-photon absorption processes in the spin noise. Furthermore an oscillating field could also help to avoid line broadening due to structural or chemical inhomogeneities in the sample, and thereby increase the precision of the spin-coherence time measurement.Comment: 5 pages, 7 figure

Topological insulator and the Dirac equation

We present a general description of topological insulators from the point of view of Dirac equations. The Z_{2} index for the Dirac equation is always zero, and thus the Dirac equation is topologically trivial. After the quadratic B term in momentum is introduced to correct the mass term m or the band gap of the Dirac equation, the Z_{2} index is modified as 1 for mB>0 and 0 for mB<0. For a fixed B there exists a topological quantum phase transition from a topologically trivial system to a non-trivial one system when the sign of mass m changes. A series of solutions near the boundary in the modified Dirac equation are obtained, which is characteristic of topological insulator. From the solutions of the bound states and the Z_{2} index we establish a relation between the Dirac equation and topological insulators.Comment: 9 pages, published versio

Contribution of Scalar Loops to the Three-Photon Decay of the Z

I corrected 3 mistakes from the first version: that were an omitted Feynman integration in the function f^3_{ij}, a factor of 2 in front of log f^3_{ij} in eq.2 and an overall factor of 2 in Fig.1 c). The final result is changed drastically. Doing an expansion in the Higgs mass I show that the matrix element is identically 0 in the order (MZ/MH)^2, which is due to gauge invariance. Left with an amplitude of the order (MZ/MH)^4 the final result is that the scalar contribution to this decay rate is several orders of magnitude smaller than those of the W boson and fermions.Comment: 6 pages, plain Tex, 1 figure available under request via fax or mail, OCIP/C-93-5, UQAM-PHE-93/0

Signatures of few-body resonances in finite volume

We study systems of bosons and fermions in finite periodic boxes and show how the existence and properties of few-body resonances can be extracted from studying the volume dependence of the calculated energy spectra. Using a plane-wave-based discrete variable representation to conveniently implement periodic boundary conditions, we establish that avoided level crossings occur in the spectra of up to four particles and can be linked to the existence of multi-body resonances. To benchmark our method we use two-body calculations, where resonance properties can be determined with other methods, as well as a three-boson model interaction known to generate a three-boson resonance state. Finding good agreement for these cases, we then predict three-body and four-body resonances for models using a shifted Gaussian potential. Our results establish few-body finite-volume calculations as a new tool to study few-body resonances. In particular, the approach can be used to study few-neutron systems, where such states have been conjectured to exist.Comment: 13 pages, 10 figures, 2 tables, published versio

The relation of phase noise and luminance contrast to overt attention in complex visual stimuli

Models of attention are typically based on difference maps in low-level features but neglect higher order stimulus structure. To what extent does higher order statistics affect human attention in natural stimuli? We recorded eye movements while observers viewed unmodified and modified images of natural scenes. Modifications included contrast modulations (resulting in changes to first- and second-order statistics), as well as the addition of noise to the Fourier phase (resulting in changes to higher order statistics). We have the following findings: (1) Subjects' interpretation of a stimulus as a “natural” depiction of an outdoor scene depends on higher order statistics in a highly nonlinear, categorical fashion. (2) Confirming previous findings, contrast is elevated at fixated locations for a variety of stimulus categories. In addition, we find that the size of this elevation depends on higher order statistics and reduces with increasing phase noise. (3) Global modulations of contrast bias eye position toward high contrasts, consistent with a linear effect of contrast on fixation probability. This bias is independent of phase noise. (4) Small patches of locally decreased contrast repel eye position less than large patches of the same aggregate area, irrespective of phase noise. Our findings provide evidence that deviations from surrounding statistics, rather than contrast per se, underlie the well-established relation of contrast to fixation