Low-temperature hopping conduction over the upper Hubbard band in p-GaAs/AlGaAs multilayered structures

The low-temperature 2D variable range hopping conduction over the states of the upper Hubbard band is investigated in detail for the first time in multilayered Be-doped p-type GaAs/AlGaAs structures with quantum wells of 15-nm width. This situation was realized by doping the layer in the well and a barrier layer close to the well for the upper Hubbard band (A ⁺ centers) in the equilibrium state filled with holes. The conduction was of the Mott hopping type in the entire temperature range (4?0.4 K). The positive and negative magnetoresistance branches as well as of non-Ohmic hopping conduction at low temperature are analyzed. The density of states and the localization radius, the scattering amplitude, and the number of scatterers in the upper Hubbard band are estimated. It is found that the interference pattern of phenomena associated with hopping conduction over the A ⁺ band is qualitatively similar to the corresponding pattern for an ordinary impurity band, but the tunnel scattering is relatively weak.     

Transition from strong to weak localization in the split-off impurity band in two-dimensional <Emphasis Type="Italic">p</Emphasis>-GaAs/AlGaAs structures

A crossover from strongly localized behavior to weak localization (SL-WL) was observed in two-dimensional modulation-doped GaAs/Al_0.3Ga_0.7 As structures as the impurity concentration increased. In this case, it was observed that the low-temperature dependence of the conductivity changed its character (from exponential to logarithmic) and the magnetoresistance changed its sign (from linear negative to root positive). For 2D structures, it is shown that this transition proceeds in the impurity band separated from the valence band by the mobility gap, whereas the effective mass in the impurity band is larger than in the valence band.     

Nature of the nonlinear second-order optical susceptibility in molecular materials with C 2v molecular symmetry

The method of second-harmonic generation at the wavelength 1.06 μm is used to study the nonlinear second-order optical susceptibility of polycrystalline molecular materials with C _2v molecular symmetry belonging to the group of 2,6-dibenzadiene substituted cyclohexanes. The magnitude of the second harmonic signal is comparable to or greater than its value for polycrystalline urea and depends on the type of substituent (the stronger the donor properties of the substituent, the stronger the signal). It is shown that in the majority of the compounds investigated the large nonlinear optical polarizability observed is associated with the octupole moment of the molecule. Fiz. Tverd. Tela (St. Petersburg) 39, 1687–1689 (September 1997)     

Positive magnetoresistance peaked at a ferromagnetic transition in Mn-doped GaAs/AlGaAs quantum wells

A large positive magnetoresistance peaked at the Curie temperature has been observed in quantum well structures GaAs/AlGaAs doped by Mn. We suggest a new mechanism of magnetoresistance within low T _c ferromagnets resulting from a pronounced dependence of spin polarization at the vicinity of T _c on the external magnetic field. As a result, any contribution to resistance dependent on the Zeeman splitting of the spin subbands is amplified with respect to the direct effect of the external field. In our case we believe that the corresponding contribution is related to the upper Hubbard band. We propose that the mechanism considered here can be exploited as the mark of ferromagnetic transition.     

Spin Glass and Isotropic Negative Magnetoresistance in GaAs–AlGaAs Quantum Wells with a Virtual Anderson Transition

Journal of Experimental and Theoretical Physics - We report on the results of experiments indicating the existence of spin glass in beryllium-doped GaAs–AlGaAs quantum wells with a virtual...     

Second-order nonlinear optical susceptibility in chalcone-group crystalline molecular compounds

A study is reported of the second-order nonlinear optical susceptibility in polycrystalline molecular materials of the chalcone group made by second-harmonic generation at a wavelength of 1.06 μm. Some of the compounds studied exhibit a high nonlinear activity and are transparent in the blue region of the spectrum. The measured macroscopic nonlinear polarizability χ ² was compared with the calculated values of β, the second-order molecular polarizability tensor. The high nonlinear optical polarizability observed in some chalcone compounds is shown to be related to crystal symmetry type. Fiz. Tverd. Tela (St. Petersburg) 41, 2084–2087 (November 1999)     

Spin-controlled negative magnetoresistance resulting from exchange interactions

We studied conductivity of AlGaAs–GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than 4 K in magnetic fields up 10 T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields 0.1 T < H < 1 T, we observed negative isotropic magnetoresistance, which was linear in magnetic field while for magnetic field normal with respect to the plane of the wells the magnetoresistance was positive at H > 2T. To the best of our knowledge, it was the first observation of linear negative magnetoresistance, which would be isotropic with respect to the direction of magnetic field. While the isotropic character of magnetoresistance apparently evidences role of spins, the existing theoretical considerations concerning spin effects in conductance fail to explain our experimental results. We believe that such a behavior can be attributed to spin effects supported by exchange interactions between localized states.