Zener Tunneling between the Landau Levels in a Two-Dimensional Electron System with One-Dimensional Periodic Modulation

Nonlinear magnetotransport in a two-dimensional electron gas in one-dimensional lateral lattices fabricated from a selectively doped GaAs/AlAs heterostructure is investigated. One-dimensional potential modulation is imposed on the two-dimensional electron gas by means of a set of metal strips formed on the planar surface of Hall bars. The dependences of the differential resistance r_xx on the magnetic field B < 0.5 T are studied at a temperature T = 1.6 K in lattices with a period of a ≈ 200nm. It is shown that periodic oscillations in r_xx(1/B) occur in such lattices under the action of a current-induced Hall field due to Zener tunneling between Landau levels. Interference is found between Zener oscillations and commensurability oscillations of r_xx in two-dimensional electron systems with one-dimensional periodic modulation. The experimental results are qualitatively explained by the role of Landau bands in nonlinear transport at large filling factors.     

Electronic transport through a Majorana bound state coupled to a T-shaped quantum-dot system with Coulomb interaction

Using the Green’s function technique, we respectively investigate the electron transport properties of two spin components through the system of a T-shaped double quantum dot structure coupled to a Majorana bound state, in which only one quantum dot is connected with two metallic leads. We explore the interplay between the Fano effect and the MBSs for different dot-MBS coupling strength λ, dot-dot coupling strength t, and MBS-MBS coupling strength ε_M in the noninteracting case. Then the Coulomb interaction and magnetic field effect on the conductance spectra are investigated. Our results indicate that G_↓(ω) is not affected by the Majorana bound states, but a “0.5” conductance signature occurs in the vicinities of Fermi level of G_↑(ω). This robust property persists for a wide range of dot-dot coupling strength and dot-MBS coupling strength, but it can be destroyed by Coulomb interaction in quantum dots. By adjusting the size and direction of magnetic field around the quantum dots, the “0.5” conductance signature damaged by U can be restored. At last, the spin magnetic moments of two dots by applying external magnetic field are also predicted.     

Microwave photoresistance of a two-dimensional electron gas in a ballistic microbar

The effect of millimeter microwave radiation on the electron transport of two-dimensional (2D) ballistic microbars formed on the basis of individual GaAs quantum wells at a temperature of T = 4.2 K in magnetic fields B < 0.6 T has been investigated. Differences have been revealed in the magnetic field dependences of the microwave photoresistance of a 2D electron gas in Hall bars with a length L and a width W for the cases L, W > l _p and L, W < l _p , where l _p is the electron mean free path for momentum. The microwave photoresistance in macroscopic bars (L, W > l _p ) is a periodic alternating function of the inverse magnetic field; in microbars (L, W < l _p ), it is a periodic positive function of 1/B. The experimental results indicate that the mechanisms of the microwave photoresistance of a 2D electron gas are different for macroscopic and microscopic bars.     

Disorder effect on the quantum Hall effect in thin films of three-dimensional topological insulators

We numerically study the quantum Hall effect (QHE) in three-dimensional topological insulator (3DTI) thin film in the presence of the finite Zeeman energy g and the hybridization gap Δ under a strong magnetic field and disorder. For Δ = 0 but g ≠ 0, the Hall conductivity remains to be odd-integer quanti-zed σ _xy = ν(e ²/h) , where ν = 2? + 1 with ? being an integer. In the presence of disorder, the Hall plateaus can be destroyed through the float-up of extended levels toward the band center and the higher plateaus disappear first. The two central plateaus with ν = ± 1 around the band center are strongest against disorder scattering. With the increasing of the disorder strength, Hall plateaus are destroyed faster for the system with a weaker magnetic field. If g = 0 but Δ ≠ 0, there is a splitting of the central (n = 0) Landau level, yielding a new plateau with ν = 0, in addition to the original odd-integer plateaus. In the strong-disorder regime, the QHE plateaus can be destroyed due to the float-up of extended levels toward the band center. The ν = 0 plateau around the band center is strongest against disorder scattering, which eventually disappears. For both g ≠ 0 and Δ ≠ 0, the simultaneous presence of nonzero g and Δ causes the splitting of the degenerating Landau levels, so that all integer Hall plateaus ν = ? appear. The ν = 0,1 plateaus are the most stable ones. In the strong-disorder regime, all QHE states are destroyed by disorder, and the system transits into an insulating phase.     

Spin-dependent transport properties and Seebeck effects for a crossed graphene superlattice <Emphasis Type="Italic">p-n</Emphasis> junction with armchair edge

Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W₁ of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W₁ = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W₁ = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 µV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.     

Zero differential resistance of a two-dimensional electron gas in a one-dimensional periodic potential at high filling factors

The nonlinear magnetotransport of a two-dimensional (2D) electron gas in one-dimensional lateral superlattices based on a selectively doped GaAs/AlAs heterostructure is studied. The one-dimensional potential modulation of the 2D electron gas is performed by means of a series of metallic strips formed on the surface of a heterostructure with the use of electron beam lithography and a lift-off process. The dependence of the differential resistance r_xx on the magnetic field B < 1.5T in superlattices with the period a = 400 nm at a temperature of T = 4.2 K is investigated. It is found that electronic states with r_xx ≈ 0 appear in one-dimensional lateral superlattices in crossed electric and magnetic fields. It is shown that states with r_xx ≈ 0 in 2D electronic systems with one-dimensional periodic modulation arise at the minima of commensurability oscillations of the magnetoresistance.     

Transport phenomena in nondegenerate semiconductors considering electron-electron scattering

Hall mobility and other transport properties of the electrons inn-type of semiconductors have been investigated when a magnetic field is applied, taking into account electron-electron scattering. Results have been presented for different impurity densities.     

Reconstruction of the 2D hole gas spectrum for selectively doped <Emphasis Type="Italic">p</Emphasis>-Ge/Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>ix</Subscript> heterostructures

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-Ge_0.93Si_0.07/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 v₀(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.     

Conductance of a single-wall carbon nanotube in a one-parameter tight-binding model

The dependence of current-voltage characteristics of single-wall nanotubes on their radius and chirality is studied theoretically. It is shown that the conductance of a single-wall nanotube at low voltages can assume discrete values equal to zero for a dielectric tube and 4(e²/h) for a conducting tube (e is the electron charge, h is the Planck constant). The current-voltage characteristic of a nanotube exhibits kinks related to the discreteness of the electron spectrum. The behavior of the conductance of the nanotube at zero temperature is analyzed in a quantizing longitudinal magnetic field that changes the type of tube conduction. In a magnetic field, the conductance of a dielectric tube at low voltages can assume a value of 2(e²/h) in the region where the tube becomes conducting. In a weak magnetic field, a conducting tube becomes dielectric with an energy gap depending on the magnitude of the magnetic field. The conductance of a carbon nanotube is calculated as a function of the temperature and longitudinal magnetic field.     

Nonmonotonic temperature dependence of the Hall resistance of a 2D electron system in silicon

For a 2D electron system in silicon, the temperature dependence of the Hall resistance ρ_xy(T) is measured in a weak magnetic field in the range of temperatures (1–35 K) and carrier concentrations n where the diagonal resistance component exhibits a metallic-type behavior. The temperature dependences ρ_xy(T) obtained for different n values are nonmonotonic and have a maximum at T_max ～ 0.16T_F. At lower temperatures T < T_max, the change δρ_xy(T) in the Hall resistance noticeably exceeds the interaction quantum correction and qualitatively agrees with the semiclassical model, where only the broadening of the Fermi distribution is taken into account. At higher temperatures T > T_max, the dependence ρ_xy(T) can be qualitatively explained by both the temperature dependence of the scattering time and the thermal activation of carriers from the band of localized states.     

Galvanomagnetic study of the quantum-well valence band of germanium in the Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript>/Ge/Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript> potential well

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.     

Anomalous Hall effect and ferromagnetism in the new diluted magnetic semiconductor Sb<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>x</Subscript>Te<Subscript>3</Subscript>

The magnetic and galvanomagnetic properties of single crystals of the new diluted magnetic semiconductor p-Sb_2?xCr_xTe₃ (0 ≤ x ≤ 0.02) have been studied in the temperature range 1.7–300 K. A ferromagnetic phase with the Curie temperature T_c ≈ 5.8 K and the maximum Cr content x = 0.0215 has been revealed. The easy magnetization axis is parallel to the C₃ crystallographic axis. In the presence of strong magnetic fields, the Shubnikov-de Haas effect has been observed. Analysis of this effect shows that doping with chrome reduces the concentration of holes. Negative magnetoresistance and the anomalous Hall effect are observed at liquid helium temperature.     

Nonlinear Hall effect in a quasi-two-dimensional electron system

The nonlinear electron transport in GaAs double quantum wells with two occupied size-quantization levels has been studied at a temperature of 4.2 K in the magnetic fields B < 1 T. It has been found that a sinusoidal electric current I _ac induces the generation of higher harmonics of both longitudinal V _xx (B) and Hall V _xy (B) voltages in the quasi-two-dimensional electron system under consideration. The Hall voltage oscillating in the magnetic field has been shown to appear in the electron system with two occupied size-quantization levels in the presence of microwave radiation and dc electric current I _dc. The experimental data indicate the independent contributions of the diagonal and off-diagonal components of the conductivity tensor to the nonlinear magnetotransport at high filling factors.     

Energy spectrum of holes in Sb<Subscript>2</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> solid solution according to the data on the transfer phenomena

The temperature dependence of the Hall coefficient of a single crystal of the p-Sb₂Te_2.9Se_0.1 solid solution grown by the Czochralski technique is studied in the temperature range 77–450 K. The data on the Hall coefficient of the p-Sb₂Te_2.9Se_0.1 are analyzed in combination with the data on the Seebeck and Nernst–Ettingshausen effects and the electrical conductivity with allowance for interband scattering. From an analysis of the temperature dependences of the four kinetic coefficients, it follows that, at T < 200 K, the experimental data are qualitatively and quantitatively described in terms of the one-band model. At higher temperatures, a complex structure of the valence band and the participation of the second-kind additional carriers (heavy holes) in the kinetic phenomena should be taken into account. It is shown that the calculations of the temperature dependences of the Seebeck and Hall coefficients performed in the two-band model agree with the experimental data with inclusion of the interband scattering when using the following parameters: effective masses of the density of states of light holes m_d1^*≈ 0.5m₀ (m₀ is the free electron mass) and heavy holes m_d2^*≈ 1.4m₀, the energy gap between the main and the additional extremes of the valence band ΔE_v ≈ 0.14 eV that is weakly dependent on temperature.     

Quantum oscillations of Hall resistance in bismuth bicrystals with twist low-angle internal boundaries

Quantum oscillations of the Hall resistance ρ_ij(B) of bismuth bicrystals are investigated in magnetic fields up to 35 T. It is found that the twist low-angle internal boundary possesses n-type conductivity and comprises a central part and two adjacent layers, which are characterized by the specific features of the Fermi surface of electrons.     

Period of photoconductivity oscillations and charge dynamics of quantum dots in <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">i</Emphasis>–<Emphasis Type="Italic">n</Emphasis> GaAs/InAs/AlAs heterojunctions

Quantum oscillations of photoconductivity in p–i–n GaAs/InAs/AlAs quantum-dot heterojunctions have been studied. The dominating effect of the dynamics of charge accumulation of optically excited holes at quantum dots on the oscillation period and on the general evolution of the holes with a change in the illumination power has been shown within a simple electrostatic model. Investigation of the temperature dependence of the oscillating structure of the current–voltage characteristics has confirmed our interpretation.     

Genesis of the anomalous Hall effect in CeAl<Subscript>2</Subscript>

The Hall coefficient R _H , resistivity ρ, and Seebeck coefficient S of the CeAl₂ compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences R _H (? T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic R _H ^am and main R _H ^a components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component R _H ^am were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution R _H ^a (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl₂ intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl₂ matrix were estimated to be (55–90)m₀ and 6–10 Å, respectively. The behaviors of the parameters R _H , S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl₂.     

Galvanomagnetic properties of Heusler alloy Co<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Al (<Emphasis Type="Italic">Y</Emphasis> = Ti,V, Cr,Mn, Fe,and Ni)

The Hall effect and the magnetoresistance of ferromagnetic Heusler alloys Co₂ YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. Normal R ₀ and anomalous R _S Hall coefficients are shown to be maximal in magnitudes in the middle of the 3d period of the periodic table of elements. Coefficient R ₀ changes the negative sign to positive sign in going from weak (Y = Ti, V) to strong (Y = Cr, Mn, Fe, and Ni) ferromagnetic alloys. Constant R _S is positive and proportional to ρ^2.9 in all the alloys. The magnetoresistance of the alloys is not higher than several percent and its magnitude is changed fairly significantly in the dependence on the number of valence electrons z; the magnetoresistance signs vary arbitrarily.