Magneto-coulomb oscillations in GaAs-AlGaAs quantum dot structures

We report on experiments of the magnetotransport properties of GaAs-AlGaAs lateral quantum dots. At high magnetic fields for a 1 μm square dot structure, current flow occurred via edge states and, with the point contacts adjusted to allow transmission of one or more edge states, a strong backscattering resonance followed by short period oscillations were observed in the magnetoresistance, as B increased. At higher fields for a 2 μm dot, we observe a rapid rise in the magnetoresistance associated with the depopulation of the point contacts and the isolation of the dot from the leads. At still higher fields there occur periodic oscillations whose period was two orders of magnitude larger than would result from interference, or Aharonov-Bohm type effects.We analyze these phenomena using self-consistent electronic structure calculations for our devices. In particular, we show that the evolution of the terrace like structure of the potential profile profoundly affects the single particle spectrum within the dot when several Landau levels are occupied. For the large dot device, we expect that in the high field regime with the dot isolated from the leads, only a single Landau level is occupied in both the dot and the 2DEG region. In this regime, tunneling into and out of the dot is regulated by charging effects. We have introduced a "magneto-Coulomb oscillations" explanation of the periodic resonances that are observed.     

Shell structure of a circular quantum dot in weak magnetic fields

The conductance of a circular quantum dot in a two-dimensional electron gas of a GaAlAs/GaAs heterostructure has been measured. Conductance oscillations as functions both of the magnetic field B and of the size of a dot confining about 1000 electrons are related to the formation of electronic shell structure. Modeling the dot by a circular billiard, we interpret the results semiclassically in terms of periodic orbit theory, providing a simple explanation of the B-periodic oscillations. A comparison to a harmonic confinement suitable for smaller quantum dots is given.     

GaAsδ-doped quantum wire superlattice characterization by quantum Hall effect and Shubnikov–de Haas oscillations

Quantum wire superlattices (1D) realized by controlled dislocation slipping in quantum well superlattices (2D) (atomic saw method) have already shown Magneto-phonon oscillations. This effect has been used to investigate the electronic properties of such systems and prove the quantum character of the physical properties of the wires. By cooling the temperature and using pulsed magnetic field up to 35 T, we have observed both quantum Hall effect (QHE) and Shubnikov–de Haas (SdH) oscillations for various configurations of the magnetic field. The effective masses deduced from the values of the fundamental fields are coherent with those obtained with Magneto-phonon effect. The field rotation induces a change in the resonance frequencies due to the modification of the mass tensor as in a (3D) electron gas. In view the QHE, the plateaus observed in ρ_YZare dephased relatively toρ_ZZ minima which seems to be linked to the dephasing of the minima of the density of states of the broadened Landau levels.     

Clear experimental evidence for boundary and impurity scattering related crossover field scales in GaAs/AlGaAs split-gate wires

We study the magnetic field dependence of the correlation field ΔB_cand amplitude δgof the conductance fluctuations, observed in the low temperature magnetoresistance of GaAs/AlGaAs split-gate wires. Near zero field, universality of quantum interference is retained and the magnetoresistance shows universal conductance fluctuations. At high magnetic fields, although the discrete Landau level quantization becomes resolved. ΔB_cand δgare found to increase linearly with magnetic field, with a slope which depends upon the nature of electron scatterings in the wire.     

Zener tunneling between the landau levels in quasi-two-dimensional electronic Corbino disks at large filling factors

The magnetotransport characteristics of quasi-two-dimensional electronic Corbino disks based on a wide GaAs quantum well with two filled subbands of size quantization are studied. At low temperatures and high magnetic fields, the Corbino disks exhibit the oscillations of differential conductivity, which are periodic with respect to dc electric field E _dc. It is shown that the observed oscillations are related to the Zener tunneling induced by E _dc. The tunneling occurs between the filled and empty Landau levels under conditions corresponding to the absence of the Hall field.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Shallow impurity binding energy in lateral parabolic confinement under an external magnetic field

We have described the calculation of hydrogenic impurity binding energies in cylindrical GaAs–Ga_1−xAl_xAs quantum well wires (QWWs) with lateral parabolic confinement in the presence of an axial magnetic field. The numerical calculations of this system have been performed with the use of a variational procedure in the effective mass approximation. We observed sharp changes in binding energy for critical spatial confinement radius and B$B$ magnetic field values.     

The self-consistent electronic structure of rectangular free-standing quantum wires: Fourier expansion approach

A method for the self-consistent calculation of rectangular free-standing quantum wires is presented. The method is also used for the electronic structure calculation of cladded quantum wires. It relies on the wavefunction's Fourier expansion, and uses the structure symmetry to save computation time. Calculations are performed for these two types of quantum wire at a number of dopant and surface state densities, and the influence of various parameters is analyzed. At low temperatures, the occurrence of Friedel oscillations is noticed, arising from a very limited number of well separated electronic states.     

Microwave photoresistance of a double quantum well at high filling factors

The effect of millimeter wave radiation on the electronic transport in a GaAs double quantum well at a temperature of 4.2 K in a magnetic field of up to 2 T has been studied. Resistance (conductance) oscillations have been shown to appear in the two-dimensional electronic system under investigation at high filling factors. The magnetic field positions of the oscillation maxima are determined by the condition Δ_SAS/? = lω_c, where Δ_SAS = (E ₂ ? E ₁) is the size quantization sublevel splitting in the quantum well, ω_c is the cyclotron frequency, and l is a positive integer. It has been found that the microwave field substantially modifies the oscillations in the double quantum well, which results in alternating two-frequency oscillations of photoresistance with the inverse magnetic field.     

Photoelectron injection at metal-semiconductor interfaces

A modelling of the photoinjection process is developed which permits fitting of the spectral photoresponse of Schottky barriers including the electric field dependence of barrier height and photoresponse by means of two adjustable parameters: the zero field barrier qφ_BO and λ₀ the zero temperature mean free path for optical phonon scattering of high energy electrons. The model assumes an image force potential barrier with Thomas-Fermi screening in the metal. Effects of optical phonon scattering and quantum mechanical transmission are convoluted on the Fowler photoelectron supply function. The effects of phonon scattering are frequently large because the ranges in energies associated with the transverse momentum and normal momentum are approximately the amount by which the quantum energy hv exceeds the barrier energy qφ_B. At high fields, quantum mechanical tunneling dominates the response when hv < qφ_B. At low fields, phonon assisted transmission is appreciable for the same quantum energy range. The calculation of the collection probability includes effects of multiple scattering even for electrons that do not lie initially within the cone of acceptance at the barrier maximum. An approach that considers the probability of collection the same as that of reaching the potential maximum without scattering is found to be acceptable only at high fields. Experimental results are reported from oxide-passivated epitaxial Pt_xSi-〈111〉 n-type Si Schottky barrier diodes with annular Schottky barrier guard rings measured at temperatures of 90 and 298 K for an electric field range from 5 × 10³ to 9 × 10⁴$\text{V}\text{cm}$. The field, spectral and temperature dependences of the photoresponse data are in excellent agreement with theoretical predictions with λ₀ = 110 Å at both 90 and 298 K. The zero field barrier height obtained from fitting photoresponse curves at a number of electric fields is also in excellent agreement with I-V and C-V measurements.     

Prediction of strong magnetic quantum oscillations of the nuclear spin relaxation in a quasi-two-dimensional metal

The nuclear spin lattice relaxation rate in a quasi-two-dimensional (2-D) metal under strong magnetic fields is studied in a special case where the electronic cyclotron mass is small compared with the free electron mass. In the pure limit (ω_cτ ? 1) and for sufficiently low temperatures ($\text{h}\text{?}\text{ω}{}_{\mathrm{c}}\text{> 2π}{}^2\text{k}{}_{\mathrm{B}}\text{T}$) we find remarkable quantum oscillations of the relaxation rate as a function of the magnetic field. The period of these oscillations is identical to that of the de Haas-van Alphen oscillations and their amplitude grows linearly with the magnetic field. The possibility of observing such oscillations experimentally in the quasi-2-D mercury chain compound Hg_3?δAsF₆ is discussed.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

Thermoelectric power of RAgSb(2) (R?=?Y, La, Ce, and Dy) in zero and applied magnetic fields

We report the experimental results of measurements of the thermoelectric power on the ternary intermetallic compounds RAgSb(2) (R = Y, La, Ce, and Dy) over the temperature range from 2 to 300 K and in magnetic fields up to 140 kOe. In this work, we present the thermoelectric transport properties of four materials from the same family with different ground states: a non-moment bearing paramagnetic metallic system (YAgSb(2)), a non-moment bearing charge density wave system (LaAgSb(2)), a local moment bearing compound with XY-like antiferromagnetic order in the tetragonal basal plane as well as readily accessible metamagnetism (DyAgSb(2)), and a Kondo lattice system with ferromagnetic order below T(C) = 9.7 K (CeAgSb(2)). The thermoelectric power data from these materials exhibit complex temperature and magnetic field dependences, which are associated with modification of the electronic density of states and changes in magnetic scattering. At low temperatures, quantum oscillations in the thermoelectric power are also observed. These oscillations are associated with the Landau quantization of electronic energy in an applied magnetic field.     

Galvanomagnetic properties of n-type Hg0.8Cd0.2Te

Galvanomagnetic properties of low and high mobility n-Hg_0.8Cd_0.2Te are reported. The experiments were carried out in magnetic fields up to 60 kG and between 1.8 and 77 K. The Hall coefficient does not show thermal and magnetic freeze-out of carriers. At 77 K the transversal magnetoresistance shows a proportionality ?⊥ ∝ B as was predicted by Gurevich and Firsov for the case of polar optical scattering in non-degenerated semiconductors. At 4 K where the mobility is governed by impurity scattering ?⊥ ∝ B^2.4 was observed in the extreme quantum limit. A negative longitudinal magnetoresistance was found at 77 K. The experimental results of high and low mobility samples show significant differences.     

Magnetoresistance oscillations due to Zener tunneling and microwave radiation in a 2D electron gas in GaAs quantum well with AlAs/GaAs superlattices barriers

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces magnetoresistance oscillations periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the covered frequency range depends on the microwave radiation power.     

Giant quantum oscillations of the magnetothermoelectric coefficient in semimetallic Sb-Bi and Sb-As alloys

Quantum oscillations of the magnetothermoelectric coefficient α_ii(B) are investigated in semimetallic Sb-Bi and Sb-As alloys in stationary magnetic fields up to 15 T and at temperatures from 1.9 to 30 K. Quantum oscillations of α_ii(B) of a giant amplitude are observed when the longitudinal or transverse magnetic field is oriented along a binary C ₂ axis or a bisectory C ₁ axis and also when rotating the transverse magnetic field in angle ranges up to [+55°, ?55°] around them.     

Rashba effect in Ga<Subscript>x</Subscript>In<Subscript>1-x</Subscript>As/InP quantum wire structures

An overview is given on the Rashba effect in Ga_xIn_1-xAs/InP quantum wires. First, the effect of Rashba spin–orbit coupling on the energy level spectrum of quantum wires with different shapes of the confining potential is theoretically investigated. The wave functions as well as the spin densities in the quantum wire are analyzed for different magnetic fields. It is found that, owing to the additional geometrical confinement, a modification of the characteristic beating pattern in the magnetoresistance can be expected. The theoretical findings are compared to measurements on two different types of wires: First, single wires and, second, sets of parallel wires. A characteristic beating pattern in the Shubnikov–de Haas oscillations is observed for wires with an effective width down to approximately 400 nm. The beating pattern is significantly better resolved for the samples with sets of parallel wires, owing to the effective suppression of conductance fluctuations. A comparison with theoretical simulations confirms that the strength of the Rashba effect is basically not affected by the geometrical confinement of the wires. However, for wires with a very small effective width the strong carrier confinement leads to a suppression of the characteristic beating pattern in the Shubnikov–de Haas oscillations. PACS 71.70.Ej; 73.63.-b; 71.70.Di     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

Quantum limit behavior of the magnetoresistance in Hg1-x-yCdxMnyTe alloys subjected to a high pulsed magnetic field

The transverse magnetoresistance and the Hall effect of a quaternary alloy semimagnetic semiconductor Hg_1-x-yCd_xMn_yTe were measured in high pulsed magnetic fields up to 30T. The investigation of the asymptotic power law (‘_p(B) ∝ B^α) in a quantum limit regime revealed that the short range type scattering is enhanced with increasing Mn composition, suggesting the scattering associated with the localized magnetic moment of Mn.     

Ballistic electron wave functions and negative magnetoresistance in a small ring interferometer

By two-dimensional ballistic magnetotransport calculations, it is demonstrated that large-scale resistance peaks, typical of small ring interferometers in zero magnetic field, are suppressed at B～1 T. This result is explained by the peculiarities of the interference pattern at the confluence sites of quantum wires and is in qualitative agreement with experimental data.