Competing conduction mechanisms of two-dimensional electrons and bulk-like electrons in the n-type surface of the naturally oxidized p-type HgCdTe thin film

The surface transport properties of naturally oxidized p-type Hg_0.776Cd_0.224Te thin film were investigated in the magnetic-field region 0–14 T and in the temperature region 8–300 K. The Hall electron concentration increases with temperature, while the surface concentration of the two-dimensional electrons in the naturally oxidized surface, calculated by Shubnikov–de Haas oscillations, decreases as temperature increases at temperatures below 20 K. The contradiction and the extraordinary quantum Hall filling factors are accounted for by assuming extra bulk-like electrons in the surface region, which dominate the surface transport properties at temperatures over 8 K.     

HgTe/HgCdTe量子阱中巨大电子Rashba自旋分裂

主要研究具有倒置能带结构的n-HgTe/HgCdTe第三类量子阱Shubnikov-de Haas（SdH）振荡中的拍频现象.发现在量子阱中电子存在强烈的Rashba自旋分裂，通过对SdH振荡进行三种不同方法的分析：SdH振荡对1/B关系的快速傅里叶变换、SdH振荡中拍频节点分析和对SdH振荡拍频数值拟合，得到了完全一致的电子Rashba自旋分裂能量（28—36 meV）. 关键词： n-HgTe/HgCdTe Shubnikov-de Haas振荡 Rashba自旋分裂     

Anisotropy of the effective mass of 2D electrons in a heterojunction with a lateral superlattice on vicinal faces

The anomalous shift in the maxima of the Shubnikov—de Haas transverse magnetoresistance oscillations in AlGaAs(Si)/GaAs with 2D electrons oriented in the [110] direction with respect to [{ie468-1}10] in large magnetic fields is explained by the formation of a lateral superlattice with an increase of the effective mass (m_[110]* > m_[{ie468-2}]*). Ryazan State Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 5, pp. 79–82, May, 1998.     

Influence of the illumination on the subband structure and?occupation in AlxGa1?xN/GaN heterostructures

The subband structure and occupation in the triangular quantum well at Al _x Ga_1−x N/GaN heterointerfaces have been investigated by means of temperature dependent Shubnikov–de Haas (SdH) measurements at low temperatures and high magnetic fields under illumination. After the illumination of the heterostructures, the total two-dimensional electron gas concentration increases, and the SdH oscillation amplitudes are enhanced when there is no additional subband occupation. It is also found that the energy separation between the subbands decreases after the illumination. We suggest that the illumination decreases the electric field and thus weakens the quantum confinement of the triangular quantum well at Al _x Ga_1−x N/GaN heterointerfaces. The GaN layer is thought to be the primary contributor of the excited electrons by the illumination.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

Suppression of ferroelectricity and quantum fluctuations in Ru-doped TbMnO<Subscript>3</Subscript>

We investigate the effects of Ru-doping in polycrystalline TbMn_1−x Ru _x O₃ (x≤0.10) on the multiferroicity. It is observed that the Ru substitution gradually melts away the dielectric anomaly at the ferroelectric transition point and the ferroelectricity by suppressing the polarization, accompanied with a surprising low-temperature dielectric plateau. While it is reasonable to observe the significant suppression of ferroelectricity, owing to the fact that the Ru-doping disrupts the Mn spiral spin ordering and reduces the Mn–Mn spin angle, quantum fluctuations associated with the Ru substitution, responsible for the low-temperature dielectric plateau, seems to be significant.     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

Nature of the low-energy excitations of a charge-ordered phase of La<Subscript>0.25</Subscript>Ca<Subscript>0.75</Subscript>MnO<Subscript>3</Subscript> manganites

The nature of the low-energy excitations of polycrystalline and nanostructured La_0.25Ca_0.75MnO₃ samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that the electrodynamic response spectra of La_0.25Ca_0.75MnO₃ in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies of 20–40 and 80–100 cm^–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem.     

Small mass enhancement near the metal–insulator transition in gated silicon inversion layers

The strong resistivity changes in the metallic state of two-dimensional electron systems have recently been assigned to quantum interaction corrections in the ballistic regime. We have performed analysis of Shubnikov–de Haas oscillations on high-mobility silicon inversion layers where we have explicitly taken into account that the back scattering angle has different influence on momentum relaxation and quantum life time. The consistent analysis under the assumption of the ballistic interaction corrections leads to smaller increase of the effective mass with decreasing electron density as usually reported.     

a-Si/SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> multilayered light absorber for solar cell

40 alternate a-Si/SiN _x multilayer are incorporated as an absorber layer in a p–i–n solar cell. The device is fabricated using hot-wire chemical vapor deposition (HWCVD) technique. The structure of the multilayer film is examined by high resolution transmission electron microscopy (HR-TEM) which shows distinct formation of alternate a-Si and SiN _x layers. The a-Si and SiN _x layers have thickness of ~3.5 and 4 nm, respectively. The photoluminescence (PL) of multilayer film shows bandgap energy of ~2.52 eV, is larger than that of the c-Si and a-Si. Dark and illuminated current–voltage (I–V) characterization of the ML films shows that these ML are photosensitive. In the present work, it is seen that the p–i–n structure with i-layer as ML quantum well (QW) structures show photovoltaic effect with relatively high open-circuit voltage (V _OC). The increment of bandgap energy in PL and high V _OC of the device is attributed to the quantum confinement effect (QCE).     

Magnetotransport properties of a magnetically modulated two-dimensional electron gas with the spin-orbit interaction

We study the electrical transport properties of a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction in the presence of a constant perpendicular magnetic field (B(0)( ?z) which is weakly modulated by B1 = B1 cos(qx) ?z, where B(1) ? B(0) and q = 2π/a with a the modulation period. We obtain the analytical expressions of the diffusive conductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and produce beating patterns in the amplitude of the Weiss and Shubnikov-de Haas oscillations. We show that the Rashba strength can be determined by analyzing the beating pattern in the Weiss oscillation. We find a simple equation which determines the Rashba spin-orbit interaction strength if the number of Weiss oscillations between any two successive nodes is known from the experiment. We compare our results with the electrically modulated 2DEG with the Rashba interaction. For completeness, we also study the beating pattern formation in the collisional and the Hall conductivities.     

Photomagnetic effect in bilayer two-dimensional electron–hole systems

In tilted magnetic fields a bilayer electron–hole system is found to generate a photocurrent under terahertz radiation as the system is tuned to electron cyclotron resonance conditions. The photoinduced current amplitude oscillates with the magnetic field in correlation with Shubnikov–de Haas oscillations for electrons. The phenomenon is accounted for by a photomagnetic effect in electron–hole systems in the quantum Hall regime and has potentialities for terahertz detection and spectroscopy.     

赝形InGaAs/InAlAs渐变异质结中的零磁场自旋分裂

在赝形渐变InGaAs/In_0.52Al_0.48As异质结的二维电子气中,发现了自旋方向向上的电子和自旋向下的电子在零磁场下存在着自旋分裂.利用Shubnikov-de Haas振荡研究了异质结中的自旋分裂行为,通过振荡中的拍频现象,发现了零磁场下的自旋分裂量为8.76meV. 关键词：     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Magnetic moment of a quasi-one-dimensional nanostructure in an inclined magnetic field

Shubnikov-de Haas oscillations in quasi-one-dimensional nanostructures (carbon nanotubes and quantum channels) are investigated. It is shown that two types of aperiodic oscillations arise in such systems: oscillations involving a change in the strength of the magnetic field, and oscillations involving a change in the angle of inclination of the field with respect to the symmetry axis of the system. It is found that the monotonic part of the magnetic moment lies in the plane of size confinement of the system and that the oscillating part has both longitudinal and transverse components. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 549–552 (10 April 1996)     

Preparation of MgB<Subscript>2</Subscript>/Fe superconductor wire by electrical self-heating

MgB₂/Fe wires were prepared by electrical self-heating of in situ powder-in-tube wires for the first time at ambient conditions. Characterization of the wires processed at 750 °C, 800 °C and 850 °C for 15 min by XRD, SEM, ϱ–T, susceptibility and J_C measurements shows that the MgB₂ formed is of high quality particularly with respect to phase purity and transport J_C. The method considerably reduces the overall energy consumption vis-à-vis the production cost, simplifies the complexity of the fabrication procedure and is promising for manufacture of high-quality MgB₂ superconducting wires. PACS 74.70.Ad; 74.62.Bf; 74.25.Fy; 74.25.Ha; 81.20.Hy     

Magnetotransport in thin epitaxial Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> films

The magnetoconductivity of thin Bi₂Se₃ films covered by a protective Se layer and grown at (111) BaF₂ substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov?de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.     

Tunable quantum capacitance and magnetic oscillation in bilayer graphene device

We address the quantum capacitance of a bilayer graphene device in the presence of Rashba spin–orbit interaction (SOI) by applying external magnetic fields and interlayer biases. Quantum capacitance reflects the mixing of the spin-up and spin-down states of Landau levels and can be effectively modulated by the interlayer bias. The interplay between interlayer bias and Rashba SOI strongly affects magnetic oscillations. The typical beating pattern changes tuned by Rashba SOI strength, interlayer bias energy, and temperature are examined as well.     

The Optical Gain of a Si-Based Lattice-Matched Si<Subscript>0.15</Subscript>Ge<Subscript>0.621</Subscript>Sn<Subscript>0.229</Subscript>/Si<Subscript>0.637</Subscript>Ge<Subscript>0.018</Subscript>Sn<Subscript>0.345</Subscript> MQW Laser

We study the optical-gain characteristics of a Si-based MQW laser, in which the active region has 20 Si_0.15Ge_0.621Sn_0.229 quantum wells separated by 20 Si_0.637Ge_0.018Sn_0.345 barriers. We reach a maximum optical gain of 2300 cm^?1 with an estimated carrier concentration of 5·10¹⁸ cm^?3, which is equivalent to the transparent current density equal to 0.5 kA/cm². Furthermore, we discuss the optical confinement factor and modal gain. The modal gain depends sensitively on the number of the quantum wells (QWs), and this fact restricts the optical confinement factor. The modal gain of the model we proposed can reach 1500 cm^?1 at the injection current density equal to 3 kA/cm². We hope that our results show the possibility to obtain a Si-based near-infrared laser.     

De Haas-van Alphen oscillations in the organic quasi-two-dimensional metal (ET)<Subscript>8</Subscript>[Hg<Subscript>4</Subscript>Cl<Subscript>12</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl)<Subscript>2</Subscript>]

The behavior of de Haas-van Alphen oscillations in the quasi-2D organic metal (ET)₈[Hg₄Cl₁₂(C₆H₅Cl)₂] was studied in detail. The section of the Fermi surface of this metal is a two-dimensional network of magnetic breakdown orbits. Only two frequencies, which corresponded to allowed closed orbits, F_A and F_MB, were detected. This is in agreement with the earlier studies of Shubnikov-de Haas oscillations in this metal. The reason for the absence of other allowed frequencies remains unclear. The angular dependences of the amplitudes of F_A and F_MB oscillations contain a series of “spin zeros.” An analysis of their positions led us to suggest that many-particle interactions were weakened in (ET)₈[Hg₄Cl₁₂(C₆H₅Cl)₂].