Extrinsic inhomogeneity effects in magnetic, transport and magnetoresistive properties of La_{1-x}Ca_{x}MnO_{3} (x\approx 0.33) crystal prepared by the floating zone method

The paper describes a study of the magnetic, transport and magnetoresistive properties of La_{1-x}Ca_{x}MnO_{3} (x\approx 0.33) crystals prepared by the floating-zone method. We found that these properties testify to rather good crystal perfection of the sample studied. In particular, a huge magnetoresistance ([R(0)-R(H)]/R(H) in the field H = 5 T is about 2680 %) is found near the Curie temperature (216 K). At the same time, some distinct features of measured properties indicate the influence of extrinsic inhomogeneities arising due to technological factors in the sample preparation. Analysis of the data obtained shows that these are rare grain boundaries and twins. Specific influence of the grain-boundary-like inhomogeneities on the transport and magnetoresistive properties are considered.Comment: Submitted to Physica

Resonant scattering on impurities in the Quantum Hall Effect

We develop a new approach to carrier transport between the edge states via resonant scattering on impurities, which is applicable both for short and long range impurities. A detailed analysis of resonant scattering on a single impurity is performed. The results are used for study of the inter-edge transport by multiple resonant hopping via different impurities' sites. It is shown that the total conductance can be found from an effective Schroedinger equation with constant diagonal matrix elements in the Hamiltonian, where the complex non-diagonal matrix elements are the amplitudes of a carrier hopping between different impurities. It is explicitly demonstrated how the complex phase leads to Aharonov-Bohm oscillations in the total conductance. Neglecting the contribution of self-crossing resonant-percolation trajectories, one finds that the inter-edge carrier transport is similar to propagation in one-dimensional system with off-diagonal disorder. We demonstrated that each Landau band has an extended state $\bar E_N$, while all other states are localized. The localization length behaves as $L_N^{-1}(E)\sim (E-\bar E_N)^2$.Comment: RevTex 41 pages; 3 Postscript figure on request; Final version accepted for publication in Phys. Rev. B. A new section added contained a comparison with other method