RESUMO
To describe kinetic phenomena in disordered conductors, various acts of scattering of electrons can be often considered as independent, that is captured by the Boltzmann equation. However, in some regimes, especially, in a magnetic field, it becomes necessary to take into account the correlations between different scattering events of electrons on defects at different times in the past. Such memory effects can have a profound impact on the resistivity of 2D semiconductor systems, resulting in giant negative magnetoresistance and microwave-induced resistance oscillations phenomena. This work opens the discussion of the memory effects in 3D conducting systems featured by the presence of extended one-dimensional defects, such as screw dislocations or static charge stripes. We demonstrate that accounting for the memory effect, that is the capture of electrons on collisionless spiral trajectories winding around extended defects, leads to the strong negative magnetoresistance in case when the external magnetic field direction becomes parallel to the defects axis. This effect gives rise to a significant magnetoresistance anisotropy already for an isotropic Fermi surface and no spin-orbit effects. The proposed resistivity feature can be used to detect one-dimensional scattering defects in these systems.
RESUMO
The presence of disorder in one-dimensional crystals leads to the localization of all charge carriers and the calculation of the indirect exchange interaction (Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction) cannot be performed perturbatively on disorder. In two and three-dimensional systems it makes sense to calculate the magnitude of RKKY interaction perturbatively treating nearly free carriers scattering on the random potential, and this approach results in a rather high magnitude of the exchange interaction due to interference effects similar to weak localization. We show that in one-dimensional systems the indirect exchange interaction should be described as a random value with heavy-tail distribution function, which is calculated in this work, on scales of carriers localization length. We also demonstrate that heavy tails and the absence of a characteristic value of RKKY interaction magnitude leads to a significant change in observables for these systems. We calculate a specific heat for the one-dimensional XY model taking into account the effect of disorder and assuming that typical distance between impurities exceeds the localization length. In contrast to an ideal system, where specific heat temperature dependence has a peak at a certain temperature proportional to exchange constant describing characteristic energy scale, disorder eliminates the peak as soon as there is no characteristic excitation energy in this case anymore.
RESUMO
Intracenter resonance optical transitions between ground and first excited states of a Mn2+ ion in bulk CdMnTe crystal are theoretically and numerically studied. An algorithm is proposed to calculate the matrix elements of the optical transitions under the influence of electromagnetic waves of different polarizations from a six-fold degenerate ground state 6A1 to a twelve-fold degenerate first excited state 4T1. The calculations were carried out with the account of a strong Jahn-Teller effect, which is due to the interaction of charge carriers in the excited state of the ion with local tetragonal distortions of the CdMnTe crystal lattice. By comparison with previously obtained experimental data, it is shown that polarization properties of the photoluminescence of manganese ions in such crystals as well as its spectral features could be explained only with the account of strong Jahn-Teller coupling in the excited state. The same calculation technique based on symmetry group analysis combined with numerical methods could be applied to intracenter Mn-transitions in other II-VI materials and it could be extended to other optically addressed atoms with highly degenerate electronic states coupled to vibrational modes.
