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ABSTRACT: Observation of a low-temperature transition from metallic (∂ρ/∂T > 0) to insulator (∂ρ/∂T < 0) behavior of resistivity ρ(T) induced by a perpendicular magnetic field B is reported for a two-dimensional (2D) hole system confined within Ge layers of a p-Ge1−x
Si
x
/Ge/Ge1−x
Si
x
heterostructure. The essential feature of this system is that it is described by the Luttinger Hamilton with the g-factor highly anisotropic for orientations of magnetic field perpendicular and parallel to the 2D plane (g
⊺ ≫ g
‖). The positive magnetoresistance revealed scales as a function of B/T. We attribute this finding to suppression of the triplet channel of electron-electron (hole-hole) interaction due to Zeeman
splitting in the hole spectrum.
Semiconductors 10/2007; 41(11):1315-1322. · 0.63 Impact Factor
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Studies in Mycology - STUD MYCOL. 01/2007;
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ABSTRACT: The structure of the quantum-well valence band in a Ge(111) two-dimensional layer is calculated by the self-consistent method.
It is shown that the effective mass characterizing the motion of holes along the germanium layer is almost one order of magnitude
smaller than the mass for the motion of heavy holes along the [111] direction in a bulk material (this mass is responsible
for the formation of quantum-well levels). This creates a unique situation in which a large number of subbands appear to be
populated at moderate values of the layer thickness d
w
and the hole concentration p
s
. The depopulation of two or more upper subbands in a 38-nm-thick germanium layer at a hole concentration p
s
= 5 × 1015 m−2 is revealed from the results of measuring the magnetoresistance in a strong magnetic field aligned parallel to the germanium
layers. The destruction of the quantum Hall state at a filling factor ν = 1 indicates that the two lower subbands merge together
in a self-formed potential profile of the double quantum well. It is demonstrated that, in a quasi-two-dimensional hole gas,
the latter effect should be sensitive to the layer strain.
Physics of the Solid State 12/2004; 47(1):49-53. · 0.71 Impact Factor
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ABSTRACT: For a two-dimensional (2D) hole system (confined within Ge layers of a multilayered p-Ge/Ge_{1-x}Si_x heterostructure) described by Luttinger Hamiltonian with the g-factor highly anisotropic for orientations of magnetic field perpendicular and parallel to the 2D plane (g_perp >> g_par), reported is an observation of low-temperature transition from metallic (dR/dT > 0) to insulator (dR/dT < 0) behavior of resistivity R(T) induced by a perpendicular magnetic field B. The revealed positive magnetoresistance scales as a function of B/T. We attribute this finding to a suppression of the triplet channel of electron-electron (hole-hole) interaction due to Zeeman splitting in the hole spectrum.
05/2004;
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M. V. Yakunin,
G. A. Alshanskii,
Yu. G. Arapov,
G. I. Harus,
V. N. Neverov,
N. G. Shelushinina, O. A. Kuznetsov,
B. N. Zvonkov,
E. A. Uskova,
L. Ponomarenko,
A. de Visser
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ABSTRACT: In In_xGa_{1-x}As/n-GaAs double quantum wells (DQWs) containing an electron gas, the magnetoresistance (MR) peculiarities under parallel magnetic fields caused by the passing of the tunnel gap edges through the Fermi level are revealed. Peculiarities positioned in high fields (~30 T) can only be explained if the spin-splitting of the In_xGa_{1-x}As conduction band is considered, that was neglected in the GaAs/AlGaAs heterostructures, for which solely the effects of this nature have been observed so far. In Ge/p-Ge_{1-x}Si_x DQWs containing a hole gas, local MR peculiarities under parallel fields are discovered as well. But the tunnel gap in these DQWs is too narrow to be responsible for these observations. We suppose, they are due to a complicated shape of the hole confinement subbands. Comment: To be presented at EP2DS-15, Nara, Japan, June 2003
06/2003;
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ABSTRACT: The magnetic field (0≤B≤32 T) and temperature (0.1≤T≤15 K) dependences of longitudinal and Hall resistivities have been investigated for p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 12≤d
w
≤20 nm and hole densities p
s
=(1–5)×1015 m−2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the
inter-Landau-level gaps deduced from the activation magnetotransport, etc.) to the quantum well profile is revealed in the
cases where the Fermi level reaches the second confinement subband. An unusually high density of localized states between
the Landau levels is deduced from the data. Two models for the long-range random impurity potential (the model with randomly
distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used to evaluate
the impurity potential fluctuation characteristics: the random potential amplitude, the nonlinear screening length in the
vicinity of integer filling factors v=1 and v=2, and the background density of states (DOS). The described models are suitable for explanation of the observed DOS values,
while the short-range impurity potential models fail. For half-integer filling factors, a linear temperature dependence of
the effective quantum Hall effect plateau-plateau (PP) transition widths v
0(T) is observed, contrary to the expected scaling behavior of the systems with short-range disorder. The finite T→0 width of the PP transitions may be due to an effective low-temperature screening of a smooth random potential due to the
Coulomb repulsion of electrons.
Journal of Experimental and Theoretical Physics 12/2002; 96(1):118-128. · 1.03 Impact Factor
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ABSTRACT: The temperature (0.1 K≲T≲20 K) and magnetic field (0 T≲B≲12 T) dependences of the longitudinal (ρxx) and Hall (ρxy) resistivities have been studied in detail for p-Ge/Ge1−x
Six (x=0.07) multilayer heterostructures with hole density p=(2.4–2.6)×1011 cm−2 and mobility μ=(1.1–1.7)×104 cm2 V−1 s−1. The energy spectrum parameters of two-dimensional (2D) hole gas in the quantum Hall effect mode have been determined. The
mobility gap W=(2–2.5) meV and the background density of localized states g
c
=(5–7)×1010 cm−2 meV−1 for the filling factors ν=1 and 2. The results are discussed in terms of long-range impurity potential models for selectively
doped 2D systems.
Semiconductors 04/2002; 36(5):519-526. · 0.63 Impact Factor
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ABSTRACT: Quantum corrections to the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge1-xSix heterostructures at T = (0.1 - 20.0)K in magnetic field B up to 1.5T. Negative magnetoresistance with logarithmic dependence on T and linear in B^2 is observed for B >= 0.1T. Such a behavior is attributed to the interplay of the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (F_/sigma = 0.44) and the WL and EEI contributions to the total quantum correction /Delta /sigma at B = 0 are separated (/Delta /sigma_{WL} ~ 0.3/Delta /sigma; /Delta /sigma_{EEI} ~ 0.7/Delta /sigma).
04/2002;
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ABSTRACT: We have measured the temperature (0.1≤T≤15 K) and magnetic field (0≤B≤12 T) dependences of longitudinal and Hall resistivities for p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 10≤dw≤38 nm and hole densities ps = (1-5)×1015 m-2. Two models for the long-range random impurity potential (the model with randomly distributed charged centres located outside the conducting layer and the model of the system with a spacer) are used for evaluation of the impurity potential fluctuation characteristics: the random potential amplitude, nonlinear screening length in the vicinity of integer filling factors (FFs) ν = 1 and 2 and the background density of states (DOS). The described models are suitable for an explanation of the unusually high value of DOS at ν = 1 and 2, in contrast to the short-range impurity potential models. For half-integer FFs the linear temperature dependence of the effective quantum Hall effect (QHE) plateau-to-plateau transition width ν0(T) is observed in contrast to scaling behaviour for systems with short-range disorder. The finite T→0 width of QHE transitions may be due to an effective low-temperature screening of smooth random potential owing to Coulomb repulsion of electrons.
Nanotechnology 01/2002; 13(1):86. · 3.98 Impact Factor
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ABSTRACT: We have measured the temperature (0.1 <= T <= 15 K) and magnetic field (0 <= B <= 12 T) dependences of longitudinal and Hall resistivities for the p-Ge_0.93Si_0.07/Ge multilayers with different Ge layer widths 10 <= d_w <= 38 nm and hole densities p_s = (1-5)10^11 cm^-2. Two models for the long-range random impurity potential (the model with randomly distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used for evaluation of the impurity potential fluctuation characteristics: the random potential amplitude, nonlinear screening length in vicinity of integer filling factors nu = 1 and nu = 2 and the background density of state (DOS). The described models are suitable for explanation of the unusually high value of DOS at nu = 1 and nu = 2, in contrast to the short-range impurity potential models. For half-integer filling factors the linear temperature dependence of the effective QHE plateau-to-plateau transition width nu_0(T) is observed in contrast to scaling behavior for systems with short-range disorder. The finite T -> 0 width of QHE transitions may be due to an effective low temperature screening of smooth random potential owing to Coulomb repulsion of electrons. Comment: Accepted for publication in Nanotechnology
11/2001;
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[hide abstract]
ABSTRACT: Two models for the long-range random impurity potential (the model with randomly distributed charged centers located within a layer and the model of the system with a spacer) are used for evaluation of the impurity potential fluctuation characteristics: the random potential amplitude, nonlinear screening length in vicinity of integer filling factors nu = 1 and nu = 2 and the background density of state (DOS). The described models are suitable for explanation of the unusually high value of DOS at nu = 1 and nu = 2, in contrast to the short-range impurity potential models.
06/2001;
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ABSTRACT: A negative magnetoresistance under the in-plane magnetic field, reaching maximum 30-40% of its zero-field value in fields higher than ~12 T, has been found in wide Ge_{1-x}Si_x/Ge/p-Ge_{1-x}Si_x quantum wells (QW) containing the quasi-two-dimensional hole gas. In the QWs of intermediate widths and hole densities, this negative magnetoresistance may be explained as being caused by suppression of the intersubband scattering due to the upper subband depopulation. For the widest QWs with the highest hole densities, in which the hole gas is divided into two sublayers, similar negative magnetoresistance was observed and tentatively interpreted as also been due to suppression of the intersubband scattering, but subbands are the lowest symmetric and antisymmetric states of the double quantum well structure. These subbands shift under the in-plane magnetic field not vertically in energy, but horizontally along the wave vector.
04/2001;
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ABSTRACT: We have measured the temperature (0.1 < T < 15 K) and magnetic field (0 < B < 32 T) dependences of longitudinal and Hall resistivities for the p-Ge_{1-x}Si_x/Ge, x=~0.07, multilayers with different Ge layer widths 10 < d_w < 38 nm and hole densities p_s = (1-5)x10^{15} m^{-2}. An extremely high sensitivity of the experimental data [the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activated magnetotransport etc] to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reaches the second confinement subband. The background density of states (5-10)x10^{14} m^{-2}meV^{-1} deduced from the activation behavior of the magnetoresistance is too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW has been found to separate into two sublayers for d_w > ~35 nm and p_s = ~5x10^{15} m^{-2}. A dramatic indication to this separation is the disappearance of the quantum Hall (QH) plateau for the filling factor nu = 1 as calculated for the whole Ge layer. Concomitantly a positive magnetoresistance emerges in the weakest fields, from which about a factor of two different mobilities in the sublayers have been deduced. A model is suggested to explain the existence of the QH plateaux close to the fundamental values in a system of two parallel layers with different mobilities. A comparison of the simulated structure of the QH magnetoresistivity with the experimental one indicates that the hole densities in the sublayers are not much different. Thus, the different mobilities are due to different quality of the normal and inverted interfaces of the Ge QW. Comment: pdf, 9 pages, 12 figures (included)
03/2001;
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ABSTRACT: We have measured the temperature (0.1≤T≤15 K) and magnetic field (0≤B≤32 T) dependences of longitudinal and Hall resistivities for the p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 10≤dw≤38 nm and hole densities ps = (1-5)×1015 m-2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level (LL) gaps deduced from the activation magnetotransport etc) to the quantum well (QW) characteristics has been revealed in the cases when the Fermi level reached the second confinement subband. The background density of states (5-10)×1014 m-2 meV-1 deduced from the activation behaviour of the magnetoresistance was too high to be attributed to the LL tails, but may be accounted for within a smooth random potential model. The hole gas in the Ge QW was found to separate into two sublayers for dw>~35 nm and ps≈5×1015 m-2. Concomitantly the positive magnetoresistance emerged in the weakest fields, from which different mobilities in the sublayers were deduced. A model is suggested to explain the existence of the plateaux close to the fundamental values in a system of two parallel layers with different mobilities.
Nanotechnology 11/2000; 11(4):351. · 3.98 Impact Factor
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ABSTRACT: As revealed from the high field low temperature magnetotransport measurements the joint quasi two dimensional hole gas with the density Ps (3-5) x 10(exp15)/m2 exists in a p-GeSi/Ge/p-GeSi quantum well up to the Ge layer widths dw < 25 nm, but it separates in two sublayers located at the well's sidewalls at Ps approx. = 5 x 10(exp15)/m2 and d(w) somewhat > 35 nm. The sublayers have approximately equal resistances indicating similarity of the normal and inverted interfaces in ths heterosystem. The recendy discovered quantized Hall insulator phase was detected here only in the intermediate range of sublayer separations yielding evidence that the intersublayer correlations are important to stabilize this state.
06/2000;
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ABSTRACT: Two metastable states of a multilayer Ge/p-Ge1−x
Six heterosystem with wide (∼ 35 nm) potential wells (Ge) are observed in strong magnetic fields B at low temperatures. In the first state, the Hall resistivity exhibits an inflection near the value ρxy=h/e
2 scaled to one Ge layer. The longitudinal magnetoresistivity ρxx(B) possesses a minimum in the range of fields where this inflection occurs. The temperature evolution of the inflection in
ρxy(B), the minimum of ρ xx(B), and the value of ρxy at the inflection indicates a weakly expressed state of the quantum Hall effect with a uniform current distribution over
the layers. In the second metastable state, an unusually wide plateau near h/2e
2 with a very weak field dependence is observed in ρxy(B). Estimates show that in these samples the Fermi level lies below but close to the top of the inflection in the bottom of
the well. For this reason, the second state can be explained by separation of a hole gas in the Ge layers into two sublayers,
and the saturation of ρxy(B) near h/2e
2 can be explained by the formation of a quantum Hall insulator state.
JETP Letters 07/1999; 70(4):301-308. · 1.35 Impact Factor
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ABSTRACT: An unusually wide plateau of the integer quantum Hall effect (for the filling factor nu = 1) has been revealed in a wide p-GeSi/Ge/p-GeSi quantum well. This plateau exists in one of two metastable states of the sample, for which a symmetric quasi-double-quantum-well system is formed inside the Ge layer due to the hole-hole repulsion. The plateau exists not only within a magnetic field range corresponding to the quantum-Hall liquid, but extends beyond it into a so-called quantized Hall insulator phase. According to the existing theories. this extra wide plateau may be indicative of a kind of disorder. characterized by a certain distribution of fluctuations in their size and carrier density.
06/1999;
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ABSTRACT: An unusually wide plateau in the quantized Hall resistance has been revealed for a MQW heterostructure of wide p-GeSi / Ge / p-GeSi quantum wells with the Fermi energy comparable to the well bottom bending amplitude. This plateau exists in one of two metastable states of the sample, for which a symmetric quasi-double-quantum-well system is formed inside the Ge layer, and corresponds to the filling factor nu = 1 for each of two sublayers in the Ge layer. The plateau exists not only within a magnetic field range related to the quantum-Hall liquid, but extends beyond it into a so-called quantized Hall insulator phase. For the other metastable state only a weak plateau is observed in the Hall resistivity, corresponding to nu = 1 per Ge layer as a whole. According to the existing theories, the extra wide plateau may be indicative of a kind of disorder in a conducting layer, characterized by a uniform distribution of puddles in their carrier density and by their small average size. We attribute the differences between these two states to the existence of two metastable modes in the self consistent potential profile that settle spontaneously in the multilayer system, which are characterized by different distributions of holes in the quantum well cross-section.
03/1999;
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ABSTRACT: Quantum magnetotransport is investigated in a series of selectively doped p-type (Ge1−x
Six/Ge)×N multilayered structures with Ge layer widths from 100 to 250 Å in fields up to 35 T at 1.5–4.2 K. The plots of the magnetic-field
dependence of the longitudinal (ρ
xx) and Hall (ρ
xy) magnetoresistance, as well as the ratio between the oscillation periods in strong and weak fields, vary significantly in
samples with wide Ge layers and (or) with a high density of the two-dimensional gas. These features can be attributed to the
participation of an additional subband in carrier transport. It follows from calculations of the structure of the Ge valence
band under the conditions of size quantization and quantization by a magnetic field (performed in the approximation of an
infinite rectangular potential well) that the additional subband can be the second heavy-hole quantum-well subband. Estimates
of its population correlate with the experimental manifestations of the participation of the additional subband in galvanomagnetic
phenomena.
Semiconductors 05/1998; 32(6):649-656. · 0.63 Impact Factor
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ABSTRACT: The quantum corrections to the conductivity of the high-mobility multilayer p-Ge/Ge(1-x)Si(x) heterostructures are investigated at T (0.1 : 20)K in magnefic fields B up to 1.5 T. The observed negative magnetoresistance with logarithmic dependencies both on temperature and magnetic field for B > 0. 1 T is interpreted as the consequence of the particle-particle (hole-hole) attractive interaction in the Cooper channel.
05/1998;
