Browsing by Author "Hu, Xiaomin"
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Item Multinuclei coherent effects on the widths and energy shifts on low energy neutron resonance(1993) Hu, Xiaomin; Trammell, George T.The exponentially decaying semi-stationary states of an assembly of identical classical radiators can have decay rates and resonant frequencies significantly different from that of an isolated radiator. These coherent effects on classical radiators emitting classical waves apply also to the particle processes in quantum limits, leading to the superradiant and subradiant modes with widths(decay rates) greatly enhanced or suppressed and resonant energies significantly shifted. The purpose of this thesis is to investigate these coherent effects on slow neutron-heavy nuclei interaction. A simple field model is introduced to show the coherent effects on the decay rates and resonant energies of two identical decaying states and on the neutron widths and resonant energies of neutron-two nuclei scattering. The latter case is also obtained by multiple scattering theory and is extended to a more general case of neutron-N nuclei scattering. The coherent effects on neutron width near Bragg angle in a crystal are also investigated.Item Quasi-elastic resonant x-ray scattering(1997) Hu, Xiaomin; Hannon, James P.In the fast collision approximation, the scattering amplitude operator of the quasi-elastic scattering is expressed as the summation of multipole moment operators $M\sp{(k)}(l\sb{i},s\sb{i})$ of the valence shell involved in the resonance$\sp1$ with distinct polarization factors. Each multipole moment operator is expressed as the sum of an orbital moment operator and two spin-orbital moment operators with unique coefficients. The explicit form of these coefficients is obtained and the numerical values are calculated. For the transitions to continuous bands, the explicit forms of $M\sp{(k)}(l\sb{i},s\sb{i})$ are extended from electric dipole transitions to any electric multipole transitions. Within the manifolds of good total L and good total S, the $k\sp{\rm th}$ rank multipole moment operator $M\sp{(k)}(l\sb{i},s\sb{i})$ can be expressed in terms of the $k\sp{\rm th}$ rank spin-orbital moments $M\sp{(k)}({\bf L,S})$ of the total L- and total ${\bf S}$-operators of the valence shell involved in the resonance. Furthermore, within the manifolds of good total J, $M\sp{(k)}(l\sb{i},s\sb{i})$ can be further simplified in terms of the spherical tensor operators of the total J of the resonance valence shell. For Hund's rule ground states, the corresponding proportionality coefficients for both cases were obtained. For rare earths, we obtained the thermal expectation value of $M\sp{(k)}(l\sb{i},s\sb{i})$ at T = 0 for coherent elastic scattering. These results are inconsistent with Hamrick's single electron method$\sp2$ for the second half of the rare earth series. For the first half of the rare earth series, we showed that the single electron method is an approximation of our theory. In spiral antiferromagnets, such as holmium, the magnetic sensitivity results in a series of magnetic satellites distributed at each side of Bragg peak. This behavior can be understood on the basis of the XRES electric multipole transition theory we developed. As temperature increases, the higher order harmonics decrease more rapidly than the lower order harmonics, which can be qualitatively explained by mean-field theory. Just above the Neel temperature, there is weak magnetic scattering which can be understood as the short range moment-moment correlations of different spin-orbital multipole moment operators. ftn $\sp1$J. Luo, J. P. Hannon, G. T. Trammell, Phys. Rev. Lett., 71 287 (1993). $\sp2$M. Hamrick, M.A. Thesis, Physics Department, Rice University, 1991.