Showing papers in "Physical Review in 1993"

Multielectron Inner-shell Photoexcitation in Absorption Spectra of Kr: Theory and Experiment

Abstract: The probability of Kr 1[ital s] photoionization alone and accompanied by 4[ital p], 3[ital d], 3[ital p], 2[ital s], and 2[ital s]4[ital p] excitations has been calculated as a function of photon energy in the vicinity of the respective thresholds; pertinent energies have been computed including relativistic, quantum-electrodynamic, and relaxation effects. Sharp features from two-electron excitations are expected in absorption spectra only if at least one of the electrons undergoes a transition to a bound state; this becomes less probable in inner shells. An absorption-spectrometry experiment on Kr has been performed with synchrotron radiation; results generally confirm the predictions but also point toward refinements that are required in the theory and the need for improvements in techniques and synchrotron-radiation sources.

Magnetic dilution in the strongly frustrated kagome antiferromagnet SrGa12-xCrxO19

Abstract: A study of the structural and magnetic characteristics of the strongly frustrated kagome system ${\\mathrm{SrGa}}_{12\\mathrm{\\ensuremath{-}}\\mathit{x}}$${\\mathrm{Cr}}_{\\mathit{x}}$${\\mathrm{O}}_{19}$ (1\\ifmmode\\times\\else\\texttimes\\fi{}9) is reported with focus on the effects of the magnetic ion dilution. High-resolution powder neutron diffraction shows no evidence of long-range magnetic ordering down to 1.5 K and a nearly random substitution of Cr by Ga atoms within the octahedral sites. An intrinsic quasibidimensional structure integrated by kagome blocks is inferred. ac and dc magnetic susceptibility measurements down to 1.6 K make evident the existence of a freezing phenomenon similar to that of spin glasses with the freezing temperature ${\\mathit{T}}_{\\mathit{f}}$, decreasing as the degree of dilution of the ${\\mathrm{Cr}}^{3+}$ ions increases. The extrapolated value of ${\\mathit{T}}_{\\mathit{f}}$ for the fully occupied lattice is 4.15 K, indicating that the nondiluted oxide could be considered as a prototype of geometrically frustrated magnetic systems. The magnetic frustration parameter \\ensuremath{\\Theta}/${\\mathit{T}}_{\\mathit{f}}$\\ensuremath{\\approxeq}135\\ifmmode\\pm\\else\\textpm\\fi{}5 is independent of the degree of dilution, giving further support to the idea of an intrinsic structural origin of the frustration in this system. Both magnetic susceptibility measurements and ESR studies show a Curie-Weiss-like behavior in the high-temperature range with magnetic correlations starting around 100 K.