arXiv subjects

M. V. Yakunin

Publications and source records attributed to M. V. Yakunin.

15 recordsLinked to original sources

arXiv2018

Unconventional Reentrant Quantum Hall Effect in a HgTe/CdHgTe Double Quantum Well

We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a $ p$-type HgTe/Cd$_x$Hg$_{1-x}$Te double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two $i=2$ states (plateaus at $ρ_{xy}=h/ie^2$) separated by an intermediate $i=1$ state, which looks like some anomalous peak on the extra-long $i=2$ plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes $p_s$ and a high-field QH structure with a $2-1$ plateau--plateau transition corresponding to much larger $p_s$. We show that only a part of holes, residing in an additional light hole subband in the DQW, participate in QHE at weak fields while the rest of holes is excluded into the reservoir formed in the lateral maximum of the valence subband. All the holes come into play at high fields due to a peculiar behavior of the zero-mode levels.

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arXiv2018

Effect of exchange electron-electron interaction on conductivity of InGaAs single and double quantum wells in ballistic regime

We report an experimental study of quantum conductivity corrections for two-dimensional electron gas in a GaAs/InGaAs/GaAs single and double quantum wells in a wide temperature range (1.8-100) K. We perform a comparison of our experimental data for the longitudinal conductivity at zero magnetic field to the theory of interaction-induced corrections to th transport coefficients. In the temperature range from 10 K up to (45-60) K, wich covers the ballistic interaction regimes for our samples, a rather good agreement between the theory and our experimental results has been found.

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arXiv2017

Large-scale impurity potential in the quantum Hall effect for the HgTe quantum well with inverted band structure

We report on the longitudinal and Hall resistivities of a HgTe quantum well with inverted energy spectrum (dQW = 20.3 nm) measured in the quantum Hall (QH) regime at magnetic fields up to 9 T and temperatures 2-50 K. The temperature dependence of the QH plateau-plateau transition (PPT) widths and of variable range hopping (VRH) conduction on the Hall plateaus are analyzed. The data are presented in a genuine scale form both for PPT regions and for VRH regime. Estimations for the degree of the carrier localization length divergence reveal a decisive role of the long-range random potential (the potential of remote ionized impurities) in the localization - delocalization processes in the QH regime for the system under study.

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arXiv2015

Magnetotransport in Double Quantum Well with Inverted Energy Spectrum: HgTe/CdHgTe

We present the first experimental study of the double-quantum-well (DQW) system made of 2D layers with inverted energy band spectrum: HgTe. The magnetotransport reveals a considerably larger overlap of the conduction and valence subbands than in known HgTe single quantum wells (QW), which may be regulated by an applied gate voltage $V_g$. This large overlap manifests itself in a much higher critical field $B_c$ separating the range above it where the quantum peculiarities shift linearly with $V_g$ and the range below with a complicated behavior. In the latter case the $N$-shaped and double-$N$-shaped structures in the Hall magnetoresistance $ρ_{xy}(B)$ are observed with their scale in field pronouncedly enlarged as compared to the pictures observed in an analogous single QW. The coexisting electrons and holes were found in the whole investigated range of positive and negative $V_g$ as revealed from fits to the low-field $N$-shaped $ρ_{xy}(B)$ and from the Fourier analysis of oscillations in $ρ_{xx}(B)$. A peculiar feature here is that the found electron density $n$ remains almost constant in the whole range of investigated $V_g$ while the hole density $p$ drops down from the value a factor of 6 larger than $n$ at extreme negative $V_g$ to almost zero at extreme positive $V_g$ passing through the charge neutrality point. We show that this difference between $n$ and $p$ stems from an order of magnitude larger density of states for holes in the lateral valence band maxima than for electrons in the conduction band minimum. We interpret the observed reentrant sign-alternating $ρ_{xy}(B)$ between electronic and hole conductivities and its zero resistivity state in the quantum Hall range of fields on the basis of a calculated picture of magnetic levels in a DQW.

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arXiv2012

Effects of Spin Polarization in the HgTe Quantum Well

Magnetoresistivity features connected with the spin level coincidences under tilted fields in a $Γ_8$ conduction band of the HgTe quantum well were found to align along straight trajectories in a $(B_\bot,B_{||})$ plane between the field components perpendicular and parallel to the layer meaning a linear spin polarization dependence on magnetic field. Among the trajectories is a noticeable set of lines descending from a single point on the $B_{||}$ axis, which is shown to yield a field of the full spin polarization of the electronic system, in agreement with the data on the electron redistribution between spin subbands obtained from Fourier transforms of oscillations along circle trajectories in the $(B_\bot,B_{||})$ plane and with the point on the magnetoresistivity under pure $B_{||}$ separating a complicated weak field dependence from the monotonous one. The whole picture of coincidences is well described by the isotropic $g$-factor although its value is twice as small as that obtained from oscillations under pure perpendicular fields. The discrepancy is attributed to different manifestations of spin polarization phenomena in the coincidences and within the exchange enhanced spin gaps. In the quantum Hall range of $B_\bot$, the spin polarization manifests in anticrossings of magnetic levels, which were found to depend dramatically nonmonotonously on $B_\bot$.

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arXiv2011

Spin Polarization Phenomena and Pseudospin Quantum Hall Ferromagnetism in the HgTe Quantum Well

The parallel field of a full spin polarization of the electron gas in a \Gamma8 conduction band of the HgTe quantum well was obtained from the magnetoresistance by three different ways in a zero and quasi-classical range of perpendicular field component Bper. In the quantum Hall range of Bper the spin polarization manifests in anticrossings of magnetic levels, which were found to strongly nonmonotonously depend on Bper.

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arXiv2005

Transport properties of 2D-electron gas in a n-InGaAs/GaAs DQW in a vicinity of low magnetic-field-induced Hall insulator--quantum Hall liquid transition

The resistivity (R) of low mobility dilute 2D-electron gas in a n-InGaAs/GaAs double quantum well (DQW) exhibits the monotonic 'insulating-like' temperature dependence (dR/dT < 0) at T = 1.8 -- 70K in zero magnetic field. This temperature interval corresponds to a ballistic regime (kTtau/hbar > 0.1 -- 3.5) for our samples, and the electron density is on a 'insulating' side of the so-called B = 0 2D metal--insulator transition. We show that the observed localization and Landau quantization is due to the Sigma_xy(T)anomalous T-dependence.

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arXiv2004

Effect of Zeeman splitting on magnetoresistivity of 2D hole gas in a Ge_{1-x}Si_x/Ge/Ge_{1-x}Si_x quantum well

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.

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arXiv2003

Parallel magnetic field induced magnetoresistance peculiarities of the double quantum well filled with electrons or holes

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.

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arXiv2002

Localization and electron-electron interaction effects in magnetoresistance of p-type Ge/Ge_{1-x}Si_x heterostructures

We report on the results of investigation the conductivity and magnetoresistance (MR) temperature dependencies for the two strained multilayer p-type Ge/Ge_{1-x}Si_x heterostructures. The usual logarithmic temperature dependencies for zero magnetic field conductivity due to the weak localization (WL) and electron- electron interaction (EEI) effects take place in both samples. For one of the samples the negative MR is observed in a whole range of magnetic fields up to ~1T at T <=12K, but for the other sample the MR transforms from the negative to positive at B >= 0.2T and T >=1.3K. We attribute such a behavior to the interplay of two types of holes due to partial filling of the second subband. Extrapolation of the observed high-field parabolic MR to B = 0 allows to separate WL and EEI contributions to the total quantum corrections to conductivity at B = 0 resulting for both of our structures in that EEI part is ~2/3 and the WL part is ~1/3.

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arXiv2001

The key role of smooth impurity potential in formation of hole spectrum for p-Ge/Ge_{1-x}Si_x heterostructures in the quantum Hall regime

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.

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arXiv2001

Impurity potential fluctuations for selectively doped p-Ge/Ge_{1-x}Si_x heterostructures in the quantum Hall regime

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.

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arXiv2001

Probing the p-Ge_{1-x}Si_x/Ge/p-Ge_{1-x}Si_x quantum well by means of the quantum Hall effect

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.

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arXiv2001

Parallel magnetic field induced strong negative magnetoresistance in a wide p-Ge_{1-x}Si_x/Ge/p-Ge_{1-x}Si_x quantum well

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.

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arXiv1999

Unusually wide plateau of quantized Hall resistance in a quasi bilayer hole system inside the p-GeSi / Ge / p-GeSi quantum well

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.

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