Measurements are reported of the magnetoresistance (MR) for fields up to 23 T in LazCu04 single crystals, which order antiferromagnetically at T&-240 K, and in which the conductivity at low temperature is characterized by hopping in localized states. Using the MR, the phase diagram of the spin-Aop transition, observed when the magnetic field is applied parallel to the zero-field staggered magnetization, is mapped out. Two transitions of the background Cu + spins are observed, which are governed by the symmetric and antisymmetric anisotropic components of the superexchange tensor. The antiferromagnetic propagation vector changes from r~~a at zero field to r~~c at the highest fields. This subtle change in the ordering of the Cu' spins is accompanied by a large enhancement of the interlayer hopping conductivity up to a factor 2. We show that the magnetoconductance is proportional to the three-dimensional staggered moment with r~~c direction. In an appendix we discuss the possible relevance of these results to the behavior of superconducting La2, (Sr,Ba)"Cu04.
Fiber-optic probes are widely used in optical spectroscopy of biological tissues and other turbid media. Only limited information exists, however, on the ways in which the illumination-collection geometry and the overall probe design influence the interrogation of media. We have investigated both experimentally and computationally the effect of probe-to-target distance (PTD) on the diffuse reflectance collected from an isotropically (Lambertian) scattering target and an agar-based tissue phantom. Studies were conducted with three probes characterized by either common (single-fiber) or separate (two bifurcated multifiber probes) illumination and collection channels. This study demonstrates that PTD, probe design, and tissue scattering anisotropy influence the extent of the transport of light into the medium, the light-collection efficiency, and the sampling volume of collected light. The findings can be applied toward optimization of fiber-optic probe designs for quantitative optical spectroscopy of turbid media including biological tissues.
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