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Search Results: 1 - 10 of 84508 matches for " W. Huebner "
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Substrate effects in the magneto-optical second-harmonic generation from first principles: Fe/Cu(001)
Torsten Andersen,W. Huebner
Physics , 2002, DOI: 10.1103/PhysRevB.65.174409
Abstract: We compute the nonlinear optical response of an Fe monolayer placed on top of 1 to 4 monolayers of Cu(001). Our calculation is based on ab initio eigenstates of the slab, which are obtained within the full-potential linearized augmented plane-wave method. The ground-state spin-polarized electronic structure is converged self-consistently to an accuracy better than 0.1 mRy. Subsequently, we take the spin-orbit interaction into account within a second variational treatment. The new set of eigenstates allows us to calculate the magneto-optical transition matrix elements. The second-harmonic response is determined in the reflection geometry with magnetization perpendicular to the surface (the so-called polar configuration) using the surface-sheet model. Adding layers of a noble metal (Cu) to the Fe monolayer gives a new degree of freedom for the inclusion of nonmagnetic Cu d bands to the nonlinear magneto-optical response of the slab, and the energy bands show that such an addition converges essentially to an addition of d states and a small broadening of the d band with growing number of Cu layers. The screened nonlinear optical susceptibility is calculated and converges quite well with a growing number of Cu layers. Our first-principles results confirm that the magnetic tensor elements of the nonlinear optical response tensor are roughly of the same order of magnitude as the nonmagnetic ones (in contrast to linear optics, where the magnetic response is only a minor correction).
Symmetry Analysis of the Nonlinear Optical Response: New Magneto-Optical Effect in Second Harmonic Generation
A. Daehn,W. Huebner,K. H. Bennemann
Physics , 1996,
Abstract: Using symmetry arguments we show how optical second harmonic generation (SHG) can be used to detect antiferromagnetism at surfaces and in thin films. Based on the group theoretical analysis of the nonlinear electric susceptibility we propose a new nonlinear magneto-optical effect, which allows even in the presence of unit-cell doubling for the unambiguous discri- mination of antiferromagnetic spin configurations from ferro- or paramagnetic ones. As an example for this effect we discuss the polarization dependence of SHG from the fcc (001) surface of NiO in some detail.
Symmetry Analysis of Second Harmonic Generation at Surfaces of Antiferromagnets
M. Trzeciecki,A. Daehn,W. Huebner
Physics , 1999, DOI: 10.1103/PhysRevB.60.1144
Abstract: Using group theory we classify the nonlinear magneto-optical response at low-index surfaces of fcc antiferromagnets, such as NiO. Structures consisting of one atomic layer are discussed in detail. We find that optical second harmonic generation is sensitive to surface antiferromagnetism in many cases. We discuss the influence of a second type of magnetic atoms, and also of a possible oxygen sublattice distortion on the output signal. Finally, our symmetry analysis yields the possibility of antiferromagnetic surface domain imaging even in the presence of magnetic unit-cell doubling.
Theory for the giant magneto-optical Kerr rotation in CeSb
U. Pustogowa,W. Huebner,K. H. Bennemann
Physics , 1996,
Abstract: We calculate the linear magneto-optical Kerr rotation angle for CeSb in the near-infrared spectral range. Using an exact formula for large Kerr rotation angles and a simplified electronic structure of CeSb we find at $\hbar \omega = 0.46 eV$ a Kerr rotation of 90 degrees which then for decreasing $\omega $ jumps to -90 degrees in very good agreement with recent experimental observations. We identify the general origin of possible 90 degree polarization rotations from mainly optical properties and discuss its relation to the magnetic moments and magnetic dichroism of the material.
Magnetocrystalline Anisotropy Energy of Transition Metal Thin Films: A Non-perturbative Theory
A. Lessard,T. H. Moos,W. Huebner
Physics , 1996, DOI: 10.1103/PhysRevB.56.2594
Abstract: The magnetocrystalline anisotropy energy E(anis) of free-standing monolayers and thin films of Fe and Ni is determined using two different semi-empirical schemes. Within a tight-binding calculation for the 3d bands alone, we analyze in detail the relation between bandstructure and E(anis), treating spin-orbit coupling non-pertubatively. We find important contributions to E(anis) due to the lifting of band degeneracies near the Fermi level by SOC. The important role of degeneracies is supported by the calculation of the electron temperature dependence of the magnetocrystalline anisotropy energy, which decreases with the temperature increasing on a scale of several hundred K. In general, E(anis) scales with the square of the SOC constant. Including 4s bands and s-d hybridization, the combined interpolation scheme yields anisotropy energies that quantitatively agree well with experiments for Fe and Ni monolayers on Cu(001). Finally, the anisotropy energy is calculated for systems of up to 14 layers. Even after including s-bands and for multilayers, the importance of degeneracies persists. Considering a fixed fct-Fe structure, we find a reorientation of the magnetization from perpendicular to in-plane at about 4 layers. For Ni, we find the correct in-plane easy-axis for the monolayer. However, since the anisotropy energy remains nearly constant, we do not find the experimentally observed reorientation.
Spin flip from dark to bright states in InP quantum dots
D. W. Snoke,J. Huebner,W. W. Ruehle,M. Zundel
Physics , 2003, DOI: 10.1103/PhysRevB.70.115329
Abstract: We report measurements of the time for spin flip from dark (non-light emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be approximately 200 ps, independent of density and temperature below 70 K. This is much shorter than observed in other quantum dot systems. The rate of decay of the luminescence intensity, approximately 300 ps, is not simply equal to the radiative decay rate from the bright states, because the rate of decay is limited by the rate of conversion from dark excitons into bright excitons. The dependence of the luminescence decay time on the spin flip time is a general effect that applies to many experiments.
Theory for Spin-Polarized Oscillations in Nonlinear Magneto-Optics due to Quantum Well States
T. A. Luce,W. Huebner,K. H. Bennemann
Physics , 1996, DOI: 10.1103/PhysRevLett.77.2810
Abstract: Using an electronic tight-binding theory we calculate the nonlinear magneto-optical response from an x-Cu/1Fe/Cu(001) film as a function of frequency and Cu overlayer thickness (x=3 ... 25). We find very strong spin-polarized quantum well oscillations in the nonlinear magneto-optical Kerr effect (NOLIMOKE). These are enhanced by the large density of Fe $d$ states close to the Fermi level acting as intermediate states for frequency doubling. In good agreement with experiment we find two oscillation periods of 6-7 and 11 monolayers the latter being more pronounced.
Theory for the Dependence of Optical Second Harmonic Generation Intensity on Non-equilibrium Electron Temperatures at Metal Surfaces
T. A. Luce,W. Huebner,K. H. Bennemann
Physics , 1996,
Abstract: We present a theory for the electron-temperature dependence $T_{el}$ of optical second harmonic generation (SHG). Such an analysis is required to study the dynamics of metallic systems with many hot electrons not at equilibrium with the lattice. Using a tight-binding theory for the nonlinear susceptibility \cwtel and the Fresnel coefficients we present results for the SHG intensity \iwtel for a Cu monolayer. In the case of linear optical response we find that the intensity will decrease monotonously for increasing $T_{el}$. In agreement with experiment we find a frequency range where \iwtel may be enhanced or reduced depending on electron temperature. Note, \cwtel rather than the Fresnel coefficients determines essentially the temperature dependence. Our theory yields also that SHG probes effects due to hot electrons more sensitively than linear optics. We also discuss the $T_{el}$-dependence of SHG for Au and Ag.
Theory for the nonlinear optical response of a nonspherical metal cluster
J. P. Dewitz,W. Huebner,K. H. Bennemann
Physics , 1995,
Abstract: Using classical electrodynamics we determine the higher harmonic radiation by a nonspherical metal cluster in form of a matrix formalism. Extending the theory for the source of the higher harmonic radiation for spherical metal clusters as introduced by \"Ostling et al. [Z. Phys. D {\bf 28}, 169 (1993)] we calculate the sources for nonspherical particles. Employing these sources we develop the nonlinear Mie theory and determine the radiated higher harmonic fields generated by the cluster. Our theory is valid for arbitrary shape and arbitrary complex refractive index for cluster sizes much smaller and comparable to the wavelength of the incident light.
A Gendered Study of Student Ratings of Instruction  [PDF]
Lucas Huebner, Rhonda C. Magel
Open Journal of Statistics (OJS) , 2015, DOI: 10.4236/ojs.2015.56058
Abstract:

This research tests for differences in mean class averages between male and female faculty for questions on a student rating of instruction form at one university in the Midwest are considered to be in the category of “very high research activity” by the Carnegie Commission on Higher Education. Differences in variances of class averages are also examined for male and female faculty. Tests are conducted by first considering all classes across the entire university and then classes just within the College of Science and Mathematics. The proportion of classes taught by female instructors in which the average male student rating was higher than the average female student rating was compared to the proportion of classes taught by male instructors in which the average male student rating was higher than the average female student rating. Results are discussed.

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