Neutron study of fracton excitations in percolating antiferromagnets Page: 2 of 9
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In recent years, considerable attention has been directed towards the
dynamical properties of highly ramified percolating networks that exhibit a
fractal geometry [1,2]. A random site-diluted Heisenberg antiferromagnet is an
ideal system for probing the existence of these excitations. In such a system and
at concentrations close to, but just above, the percolation threshold there should
be a crossover from long-wavelength spin-wave excitations to short-wavelength
fracton excitations. The origin of this crossover is the fact that the fractal
geometry is realized only at length scales shorter than the geometrical
correlation length (4o) . The geometrical correlation length of this system is
defined as G = (c-cp)-vG ao (vG=0.88 for a 3D system, and a0 is the atomic
spacing) and its corresponding wave vector is qc = a-1. Magnetic excitations in
diluted magnetic systems have been extensively studied using neutron inelastic
scattering techniques , and in order to look for fracton excitations neutron
scattering studies have been performed on a three-dimensional (3D) diluted
near-Heisenberg antiferromagnet (Mno.5Zno.5F2) .
Recently, a renewed experimental effort has been initiated to characterize
the fractal component of the dynamics in the diluted magnetic systems. For this,
it was important to distinguish fracton excitations from other localized
excitations, such as [sing cluster excitations.
We have recently performed an experiment aimed to give the first
quantitative measurement of fracton excitations in the near-percolating
Heisenberg systems . For that experiment we chose a diluted pure-
Heisenberg antiferromagnet (RbMn0.39Mg0.61F3), in which the Mn
concentration (0.39) is very close to the percolation threshold (cp =0.312). The
corresponding cross-over wave vector is qc =4G-1=0.024 reciprocal lattice units
(rlu). The excitations with wave vectors smaller than qc are expected to be spin
waves, while the excitations with wave vectors larger than qc are expected to be
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Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M. Neutron study of fracton excitations in percolating antiferromagnets, article, June 27, 1997; Tennessee. (digital.library.unt.edu/ark:/67531/metadc690509/m1/2/: accessed June 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.