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ABSTRACT: Transport properties of multigraphene layers on 6H-SiC substrates fabricated
by thermal graphitization of SiC were studied. The principal result is that
these structures were shown to contain a nearly perfect graphene layer situated
between the SiC substrate and multgraphene layer. It was found that the curves
of magnetoresistance and Shubnikov- de Haas oscillations shown the features,
typical for single-layered graphene. The low temperature resistance
demonstrated an increase with temperature increase, which also corresponds to a
behavior typical for single-layered graphene (antilocalization). However at
higher temperatures the resistance decreased with an increase of temperature,
which corresponds to a weak localization. We believe that the observed behavior
can be explained by a parallel combination of contributions to the conductivity
of single-layered graphene and of multigraphene, the latter allowing to escape
damages of the graphene by atmosphere effect.
12/2012;
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ABSTRACT: The superconductor-insulator transition that occurs at liquid helium temperatures in the (PbzSn1−z
)0.84In0.16Te semiconductor system with varying lead concentration z = 0.5–0.9 is experimentally investigated. The transition is attributed to the change in the energy characteristics of In
impurity centers due to the variation in the amount of lead. The insulator state appears with the transition from the mixed
band-impurity conduction, which is characterized by resonant scattering of carriers into the quasilocal indium impurity states,
to the hopping conduction between indium impurity states. The sample with z = 0.8 is found to exhibit a variable range hopping conduction described by Mott’s law. Factors that lead to the hopping conduction
via impurity states are considered.
JETP Letters 04/2012; 84(1):35-40. · 1.35 Impact Factor
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ABSTRACT: Mixed conduction due to simultaneous contributions from allowed states in the valence band and extended impurity (acceptor)
states, which occur in the impurity band at high impurity concentrations because of the Anderson transition, is observed in
a series of GaAs/AlGaAs structures. Mixed conduction manifests itself in the existence of a minimum in the temperature dependence
of the carrier concentration and a noticeable bend in the temperature dependences of the conductivity. Expressions for the
mixed conductivity on which the calculations are based were derived taking into account the spectrum of impurity states in
the quantum wells (the upper and lower Hubbard impurity bands), their occupancies, and the sign of charge carriers in the
valence and the impurity bands; the important assumption was made that the width of impurity bands is much smaller than the
spacing from the valence band. The calculation results agree well with the experiment and were used to determine the binding
energies for the upper and lower Hubbard bands, the acceptor concentration, and the degree of compensation. It is shown that
formulas commonly used for the calculation of mixed conductivity need significant corrections in the case of narrow impurity
bands.
Semiconductors 04/2012; 44(4):472-477. · 0.63 Impact Factor
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ABSTRACT: In this study, we carried out for the first time a galvanomagnetic investigation of 3C–SiC/6H–SiC heterostructures at liquid-helium
temperatures and observed in n-3C–SiC low resistance of the samples and the appearance of a negative magnetoresistance in
weak fields (~1 T). Analysis of the results we obtained shows that the low resistance is in all probability due to a metal—insulator
transition in 3C–SiC epitaxial films. It was also found that the negative magnetoresistance magnitude decreases as the density
of intertwine boundaries in a 3C–SiC epitaxial film becomes lower.
Journal of Materials Science Materials in Electronics 04/2012; 19(8):793-796. · 1.08 Impact Factor
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ABSTRACT: Despite the fact that the problem of interference mechanism of
magnetoresistance in semiconductors with hopping conductivity was widely
discussed, most of existing studies were focused on the model of spinless
electrons. This model can be justified only when all electron spins are frozen.
However there is always an admixture of free spins in the semiconductor.
This study presents the theory of interference contribution to
magnetoresistance that explicitly includes effects of both frozen and free
electron spins. We consider the cases of small and large number of scatterers
in the hopping event. For the case of large number of scatterers the approach
is used that takes into account the dispersion of the scatterer energies. We
compare our results with existing experimental data.
01/2012;
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ABSTRACT: We consider Joule heat release in a quantum wire joining two classical reservoirs under the action of a nonstationary periodic electric field. The rate of heat generation and its spatial distribution is discussed. The heat is spread over the lengths of electron mean free paths in the reservoirs. We find that the total rates of heat generation in both reservoirs that are joined by the nanostructure are the same.
Journal of Physics Condensed Matter 10/2011; 23(40):405302. · 2.55 Impact Factor
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ABSTRACT: Earlier we reported an observation at low temperatures of activation
conductivity with small activation energies in strongly doped uncompensated
layers of p-GaAs/AlGaAs quantum wells. We attributed it to Anderson
delocalization of electronic states in the vicinity of the maximum of the
narrow impurity band. A possibility of such delocalization at relatively small
impurity concentration is related to the small width of the impurity band
characterized by weak disorder. In this case the carriers were activated from
the "bandtail" while its presence was related to weak background compensation.
Here we study an effect of the extrinsic compensation and of the impurity
concentration on this "virtual" Anderson transition. It was shown that an
increase of compensation initially does not affect the Anderson transition,
however at strong compensations the transition is suppressed due to increase of
disorder. In its turn, an increase of the dopant concentration initially leads
to a suppression of the transition due an increase of disorder, the latter
resulting from a partial overlap of the Hubbard bands. However at larger
concentration the conductivity becomes to be metallic due to Mott transition.
08/2011;
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ABSTRACT: We consider a residual resistance and Joule heat release in 2D nanostructures as well as in ordinary 3D conductors. We assume that elastic scattering of conduction electrons by lattice defects is predominant. Within a rather intricate situation in such systems we discuss in detail two cases. (1) The elastic scattering alone (i.e. without regard of inelastic mechanisms of scattering) leads to a transition of the mechanical energy (stored by the electrons under the action of an electric field) into heat in a traditional way. This process can be described by the Boltzmann equation where it is possible to do the configuration averaging over defect positions in the electron-impurity collision term. The corresponding conditions are usually met in metals. (2) The elastic scattering can be considered with the help of the standard electron-impurity collision integral only in combination with some additional averaging procedure (possibly including inelastic scattering or some mechanisms of electron wavefunction phase destruction). This situation is typical for degenerate semiconductors with a high concentration of dopants and conduction electrons. Quite often, heat release can be observed via transfer of heat to the lattice, i.e. via inelastic processes of electron-phonon collisions and can take place at distances much larger than the size of the device. However, a direct heating of the electron system can be registered too by, for instance, local measurements of the current noise or direct measurement of an electron distribution function.
Journal of Physics Condensed Matter 06/2011; 23(24):245303. · 2.55 Impact Factor
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ABSTRACT: We observed a slow relaxation of the magnetoresistance in response to an applied magnetic field in selectively doped p-GaAs-AlGaAs structures with a partially filled upper Hubbard band. We have paid special attention to excluding the effects related to temperature fluctuations. Although these effects are important, we have found that the general features of slow relaxation persist. This behavior is interpreted as related to the properties of the Coulomb glass formed by charged centers with account taken of spin correlations, which are sensitive to an external magnetic field. Variation of the magnetic field changes the numbers of the impurity complexes of different types. As a result, it affects the shape and depth of the polaron gap formed at the states belonging to the percolation cluster responsible for the conductance. The suggested model explains both the qualitative behavior and the order of magnitude of the slowly relaxing magnetoresistance.
Journal of Physics Condensed Matter 10/2010; 22(40):405301. · 2.55 Impact Factor
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ABSTRACT: We reconsider the theory of magnetoresistance in hopping semiconductors. First, we have shown that the random potential of the background impurities affects significantly preexponential factor of the tunneling amplitude which becomes to be a short-range one in contrast to the long-range one for purely Coulomb hopping centers. This factor to some extent suppresses the negative interference magnetoresistance and can lead to its decrease with temperature decrease which is in agreement with earlier experimental observations. We have also extended the theoretical models of positive spin magnetoresistance, in particular, related to a presence of doubly occupied states (corresponding to the upper Hubbard band) to the case of acceptor states in 2D structures. We have shown that this mechanism can dominate over classical wave-shrinkage magnetoresistance at low temperatures. Our results are in semi-quantitative agreement with experimental data. Comment: 19 pages, 3 figures
02/2010;
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ABSTRACT: The nonlocal dynamical response of a ballistic nanobridge to an applied potential oscillating with frequency ω is considered. It is shown that, in addition to the active conductance, there is also a reactive contribution. This contribution turns out to be inductive for relatively small frequencies ω. For bigger frequencies the current response is either inductive or capacitive, depending on the ratio of ωL/v(F), where L is the length of the bridge and v(F) is the Fermi velocity.
Journal of Physics Condensed Matter 01/2010; 22(2):025304. · 2.55 Impact Factor
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ABSTRACT: We suggest that negative magnetoresistance in small magnetic fields at temperatures lower than 3 K reported in the paper under discussion may be related to superconducting transition in In leads (with Tc = 3.4 K). Comment: 4 pages
12/2009;
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ABSTRACT: The paper reports a study of galvanomagnetic properties of n-3C -SiC /n-6H -SiC heterostructures at liquid-helium temperatures. 3C -SiC epitaxial layers were grown by sublimation epitaxy in a vacuum on the (0001)C face of 6H -SiC substrates produced by the Lely method and 4H -SiC substrates grown by modified Lely method. The x-ray topography demonstrated the high quality structure of the epitaxial layers and the absence of any transition regions between 3C -SiC epitaxial layer and substrate. The low-temperature conductivity and magnetoresistance of the films have been studied as functions of their doping level and structural quality. It was found that the metal-insulator transition occurs in the n-3C -SiC layer at concentrations N<sub>d</sub>-N<sub>a</sub>≤3×10<sup>17</sup> cm <sup>-3</sup> .
Journal of Applied Physics 02/2009; · 2.17 Impact Factor
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ABSTRACT: 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. Comment: 6 pages, 3 figures
10/2008;
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ABSTRACT: For highly doped uncompensated p-type layers located within the central part of GaAs/AlGaAs quantum wells, we observed the activated low-temperature behavior of conductivity. The low values of the activation energy, ε4 = (1–3) meV, cannot apparently be ascribed to standard mechanisms. We attribute this behavior to the existence of a narrow band of extended states near the maximum of the density of states in the impurity band. The Hubbard repulsion prevents metallic transport of holes over these states. However, the minority carriers—electrons—supplied by background defects and situated at low temperatures within the tail of the impurity band can be activated to the above mentioned band of extended states. We refer to this behavior as the virtual Anderson transition since the conductance is maintained by the extended states formed within the impurity band though the conductivity is not metallic. The low-temperature () conductance is strongly non-Ohmic: the I–V curves are S-shaped that leads to a breakdown behavior. We explain the observed low threshold fields () by the fact that we are dealing with the impact ionization of the electrons from the states below the chemical potential to the band of extended impurity states situated close to the chemical potential, the ionization energy being small.
Journal of Physics Condensed Matter 08/2008; 20(39):395216. · 2.55 Impact Factor
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ABSTRACT: We discuss memory effects in the conductance of hopping insulators due to slow rearrangements of many-electron clusters leading to formation of polarons close to the electron hopping sites. An abrupt change in the gate voltage and corresponding shift of the chemical potential change populations of the hopping sites, which then slowly relax due to rearrangements of the clusters. As a result, the density of hopping states becomes time dependent on a scale relevant to rearrangement of the structural defects leading to the excess time dependent conductivity.
05/2008;
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ABSTRACT: In highly doped uncompensated p-type layers within the central part of GaAs/AlGaAs quantum wells at low temperatures we observed an activated behavior of the conductivity with low activation energies (1-3) meV which cannot be ascribed to standard mechanisms. We attribute this behavior to the delocalization of hole states near the maximum of the narrow impurity band (characteristic for 2D) in the sense of the Anderson transition. Low temperature conduction ε4 is supported by an activation of minority carriers (resulting from a weak compensation by back-ground defects) from the Fermi level to the band of delocalized states mentioned above. The corresponding behavior can be specified as virtual Anderson transition. Low temperature transport (<4 K) exhibits also strong nonlinearity of a breakdown type characterized in particular by S-shaped I-V curve. The nonlinearity is observed in unexpectedly low fields (<10 V/cm). Such a behavior can be explained by a simple model implying an impact ionization of the localized states of the minority carriers mentioned above to the band of Anderson-delocalized states. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2008; 5(3):853 - 857.
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ABSTRACT: In highly doped uncompensated p-type layers within the central part of GaAs/AlGaAs quantum wells at low temperatures we observed an activated behavior of the conductivity with low activation energies (1-3) meV which can not be ascribed to standard mechanisms. We attribute this behavior to the delocalization of hole states near the maximum of the narrow impurity band in the sense of the Anderson transition. Low temperature conduction $\epsilon_4$ is supported by an activation of minority carriers - electrons (resulting from a weak compensation by back-ground defects) - from the Fermi level to the band of delocalized states mentioned above. The corresponding behavior can be specified as virtual Anderson transition. Low temperature transport ($<4$ K) exhibits also strong nonlinearity of a breakdown type characterized in particular by S-shaped I-V curve. The nonlinearity is observed in unexpectedly low fields ($<10$ V/cm). Such a behavior can be explained by a simple model implying an impact ionization of the localized states of the minority carriers mentioned above to the band of Anderson-delocalized states.
11/2007;
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ABSTRACT: A theory of disorder-induced low-frequency noise produced by soft atomic potentials in normal conductors and superconductors is developed. Our theoretical predictions are in reasonable agreement with experimental data by Buhrman and co-workers.
EPL (Europhysics Letters) 07/2007; 10(8):753. · 2.17 Impact Factor
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ABSTRACT: We discuss memory effects in the conductance of hopping insulators due to slow rearrangements of structural defects leading to formation of polarons close to the electron hopping states. An abrupt change in the gate voltage and corresponding shift of the chemical potential change populations of the hopping sites, which then slowly relax due to rearrangements of structural defects. As a result, the density of hopping states becomes time dependent on a scale relevant to rearrangement of the structural defects leading to the excess time dependent conductivity. Comment: 6 pages, 1 figure
05/2007;
