Features of the conductivity and magnetoresistance of doped two-dimensional structures near a metal-insulator transition

Theoretical and experimental studies of the conductivity and magnetoresistance of selectively doped structures of GaAs/AlGaAs quantum well structures near a metal-insulator phase transition have been reviewed. Special attention is focused on the role of the structure of impurity bands, which are narrow in the absence of intentional compensation and, in the case of doping of barriers, include the partially filled upper Hubbard band. It has been shown that the indicated structures exhibit (i) specific mixed conductivity, which can, in particular, include the contribution from delocalized states in the impurity band; (ii) the virtual Anderson transition, which is suppressed with an increase in disorder owing to compensation or with an increase in the concentration of a dopant; (iii) slow relaxations of the hopping magnetoresistance caused by the Coulomb glass effects, including, in particular, the states of the upper Hubbard band; and (iv) the suppression of the negative interference magnetoresistance owing to the spin effects.     

Suppression of the virtual anderson transition in the impurity band of doped quantum well structures

We have previously observed activation-type conductivity with low activation energies of heavily doped p-GaAs/AlGaAs quantum well structures at low temperatures. It has been attributed to the delocalization of the electron states near the maximum of a narrow impurity band in the sense of the Anderson transition. The possibility of this delocalization at a relatively low impurity concentration is associated with the narrowness of the impurity band in the presence of weak disorder. In this case, charge carriers were activated from the tail of the band and their presence was due to the background (weak) compensation. In this work, we study the dependence of the above virtual Anderson transition on the external compensation and impurity concentration. It has been found that an increase in the compensation does not initially affect the Anderson transition; however, at a higher compensation, it leads to the suppression of the transition owing to the growing disorder. An increase in the impurity concentration also initially leads to the suppression of the Anderson transition due to the disorder associated with the partial overlap of the Hubbard bands. However, the conductivity becomes metallic at a fairly high concentration due to the Mott transition.     

On the negative magnetoresistance puzzle at variable range hopping at low temperatures

In this work, we report the results of magnetoresistance studies of variable range hopping down to 30 mK in isotopically engineered Ge with low compensation, and in n-CdTe crystals. Experimentally we find a decrease and disappearance of the negative magnetoresistance with decreasing temperature down to 200 mK, in weak magnetic fields. Such behaviour is in disagreement with the quantum interference theory of Nguen, Spivak and Shklovskii.     

Virtual Anderson transition in a narrow impurity band of doped <Emphasis Type="Italic">p</Emphasis>-GaAs/AlGaAs layers

In highly doped uncompensated layers of p-GaAs/AlGaAs quantum wells, activation conduction with low activation energies is observed at low temperatures and this conduction is not explained by known mechanisms (ε₄ conduction). Such behavior is attributed to the delocalization of electron states near the maximum of a narrow impurity band in the sense of the Anderson transition. In this case, conduction is implemented due to the activation of minority carriers from the Fermi level to the indicated delocalized-state band.     

Effect of temperature and magnetic field on disorder in semiconductor structures

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value corresponding to the metal–insulator transition). The above factors include (i) ionization of localized states to the region of delocalized states above the mobility edge, which is presumed to exist in the impurity band; (ii) the coexistence in the upper and lower Hubbard bands (upon doping of QWs as well as barriers); in this case, in particular, the external magnetic field determines the relative contribution of the upper Hubbard band due to spin correlations at doubly filled sites; and (iii) the contribution of the exchange interaction at pairs of sites, in which the external magnetic field can affect the relation between ferromagnetic and antiferromagnetic configurations. All these factors, which affect the structure and degree of disorder, lead to specific features in the temperature dependence of resistivity and determine specific features of the magnetoresistance. Our conclusions are compared with available experimental data.     

Slow relaxation of magnetoresistance in doped p-GaAs/AlGaAs layers with partially filled upper Hubbard band

We observed slow relaxation of magnetoresistance in quantum well structures GaAs–AlGaAs with a selective doping of both wells and barrier regions which allowed partial filling of the upper Hubbard band. Such a behavior is explained as related to magnetic-field driven redistribution of the carriers between sites with different occupation numbers due to spin correlation on the doubly occupied centers. This redistribution, in its turn, leads to slow multi-particle relaxations in the Coulomb glass formed by the charged centers.     

Anomalous Hall Effect in GaAs–AlGaAs Quantum Wells Doped by Nonmagnetic Impurities near the Metal–Insulator Transition

Journal of Experimental and Theoretical Physics - We report on the results of experiments indicating the existence of the anomalous Hall effect associated with the ferromagnetic ordering of spins...     

Orbital and spin effects in the low-temperature behavior of the magnetoresistance of doped CdTe crystals

An observation of the suppression of negative magnetoresistance in samples of doped CdTe that are far from the metal-insulator transition as the temperature is lowered in the temperature range 3–0.4 K was previously reported [N. V. Agrinskaya, V. I. Kozub, and D. V. Shamshur, JETP 80, 1142 (1995)]. The results of an investigation of samples that are closer to the transition in the low-temperature region below 36 mK are presented. It is discovered that the samples investigated (which do not exhibit the suppression of negative magnetoresistance at comparatively high temperatures) display this effect at low temperatures and that, as previously, the suppression of the negative magnetoresistance correlates with the transition to conduction via Coulomb-gap states. A plateau-like magnetoresistance feature is displayed at low temperatures for the sample that is closest to the metal-insulator transition. The results obtained are analyzed within existing theoretical models that take into account the role of both the orbital and spin degrees of freedom. In particular, the low-temperature feature indicated is interpreted as a manifestation of positive magnetoresistance caused by spin effects. Nevertheless, it is shown within a detailed analysis supplemented by numerical calculations that the observed suppression of the negative magnetoresistance cannot be attributed only to the appearance of spin positive magnetoresistance. Moreover, the possibility of observing spin positive magnetoresistance is determined to a certain extent specifically by the suppression of the negative magnetoresistance competing with it. Zh. éksp. Teor. Fiz. 111, 1477–1493 (April 1997)     

Dynamic Spin Phenomena in Complex Structures Based on Ferromagnetic Metals and Semiconductors (Scientific Summary)

JETP Letters - A brief review of theoretical and experimental studies of spin phenomena in various hybrid structures based on magnetically ordered metals and semiconductors is presented. In...     

Manifestation of coulomb gap in two-dimensional <Emphasis Type="Italic">p</Emphasis>-GaAs-AlGaAs structures with filled upper Hubbard band

The transport properties of multilayer GaAs/AlGaAs structures doped modulationally with Be so as to fill, in equilibrium, the states of upper Hubbard band (A⁺ centers) with holes were studied. For the concentration of dopants on the order of 5×10¹¹cm^?2, the hopping conduction over the states in the Coulomb gap was observed in the temperature range 0.4–4 K. The characteristic temperature (T₁) was determined from the temperature dependence of conductance and found to be appreciably lower (by 30 times) than its theoretically predicted value. This discrepancy is assumed to be due to the correlated hopping effect. In the temperature dependence of magnetoresistance, the suppression of negative magnetoresistance was observed with lowering temperature. This is explained by the weakness of underbarrier scattering in the transport via the upper Hubbard band.