23 research outputs found
Using NMR to Measure Fractal Dimensions
A comment is made on the recent PFG NMR measurements by Stallmach, et al. on
water-saturated sands [Phys. Rev. Lett. 88, 105505 (2002)]. It is pointed out
that the usual law for the time-dependent diffusion coefficient D(t) used by
these authors is not valid for a fractal surface. It is shown that (1-D(t)/D0)
\~ t^[(3-Ds)/2] at short times for a surface of fractal dimension Ds, where D0
is the bulk diffusion coefficient. Preliminary PFG NMR data on water saturated
limestone and plastic beads are presented to illustrate this analysis.Comment: 1 page, 1 figur
Transverse Fluctuations in an Ising Spin-Glass:Fe0.4Mg0.6Cl2
ac-susceptibility and neutron-diffraction experiments show that Fe0.4Mg0.6Cl2 is a three-dimensional Ising spin-glass with a transition temperature Tsg = 3.4 K. Inelastic-scattering experiments show a sharp spin-wave peak at the zone center for all temperature T~<2Tsg. This is the first direct observation of long-wavelength excitation in a spin-glass system. We suggest that it is due to the precession of spin clusters around the easy axis
Numerical study of multilayer adsorption on fractal surfaces
We report a numerical study of van der Waals adsoprtion and capillary
condensation effects on self-similar fractal surfaces. An assembly of uncoupled
spherical pores with a power-law distributin of radii is used to model fractal
surfaces with adjustable dimensions. We find that the commonly used fractal
Frankel-Halsey-Hill equation systematically fails to give the correct dimension
due to crossover effects, consistent with the findings of recent experiments.
The effects of pore coupling and curvature dependent surface tension were also
studied.Comment: 11 pages, 3 figure
Methods of the physics of porous media
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Coexistence of Spin-Glass and Antiferromagnetic Orders in the Ising System Fe0.55Mg0.45Cl2
The low-temperature phase of the dilute Ising antiferromagnet Fe0.55Mg0.45Cl2 in zero field is studied by specific-heat, ac-susceptibility, and neutron-scattering experiments. We find that in this phase, spin-glass behavior and antiferromagnetic long-range order coexist. Such a mixed-order phase is predicted by the mean-field theory of reentrant spin-glasses, but has never been observed before.