Beats of quantum oscillations of the hall coefficient and resistivity in semiconductor alloys n-Bi0.93Sb0.07 at a small deviation of the magnetic field direction from the trigonal axis

Quantum oscillations of the resistivity ??₂₂ and Hall coefficient R _12.3 in the semiconductor alloy n-Bi_0.93Sb_0.07 have been studied at H ?? C ₃ and j ?? C ₁ in magnetic fields to 14 T and at temperatures of 1.5, 4.5, 10, and 20 K. At temperatures of 1.5 and 4.5 K, beats of quantum oscillations of ??₂₂ and R _12.3 due to a small deviation of the magnetic field H from the crystallographic C ₃ axis have been observed. To determine the oscillation period ?? _i , cyclotron mass m _ci , cyclotron frequency ?? _ci , and extreme section S _extri, experimentally measured quantum oscillation beats have been compared with the model beats of oscillations of three harmonic functions, two of which have close frequencies. The deviation of the parameters ?? _i , m _ci , and S _extri from the same parameters when the magnetic field H exactly coincides with the trigonal C ₃ axis has made it possible to estimate the magnetic field H deflection angle from the trigonal C ₃ axis, which is ??1°.     

Redistribution of electrons between ellipsoids in a magnetic field in the quantum limit range in n-Bi-Sb alloys

The magnetoresistivities ρ₂₂(H) and ρ₃₂(H) and the Hall coefficient R _32.1 for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconducting alloy have been measured at low temperatures in magnetic fields up to H=14 T at H∥C ₂. The samples with three electron concentrations n ₁=1.25 × 10¹⁶ cm^s-3, n ₂=3.5×10¹⁶ cm^s-3, and n ₃=1.6×10¹⁷ cm^s-3 have been studied. The strong anisotropy of the electron spectrum of the alloys has made it possible to observe quantum oscillations of the magnetoresistivity ρ₂₂ (H) at H∥C ₂ for electrons of the secondary ellipsoids with the transition to the quantum limit in high magnetic fields. However, in the same magnetic fields, the quantization condition for electrons of the main ellipsoid is not satisfied. An increase in the energy of electrons of the secondary ellipsoids in the magnetic fields of the quantum limit leads to their migration to the main ellipsoid. After the complete migration, the Fermi energy for the alloy samples with the electron concentrations n ₁, n ₂, and n ₃ increases from 7.0 to 11.3 meV, from 11.0 to 17.1 meV, and from 20.2 to 30.6 meV, respectively. After the migration, the magnetoresistivity for electrons of the main ellipsoid increases with an increase in the magnetic field and the specific features in the behavior of the kinetic coefficients are observed in the vicinity of the magnetic field H=10 T. Therefore, the electronic topological transition from the three-valley electron spectrum to the single-valley electron spectrum occurs in the Bi_0.93Sb_0.07 single crystals for H∥C ₂ at low temperatures in the range of magnetic fields of the quantum limit.     

Exciton spectra of semiconductor superlattices in a parallel magnetic field

Low-temperature photoluminescence and photoluminescence excitation spectra of GaAs/AlGaAs semiconductor superlattices having different potential barrier widths (b=20, 30, 50, and 200 Å), i.e., degrees of tunnel coupling between quantum wells, are studied in magnetic fields up to 5 T oriented parallel and perpendicular to the layers of the structure. The changes in the qualitative character of the photoluminescence excitation spectra observed in a parallel magnetic field with increasing tunnel transparency of the barrier correspond to a transition from a quasi-two-dimensional to a quasi-three-dimensional electronic spectrum as a miniband develops in the superlattice. In the photoluminescence excitation spectra of the superlattice with b=50 Å, as the parallel magnetic field is increased, a new line appears in the violet wing of the spatially indirect exciton excitation line, which is absent in a perpendicular field. A similar line was also observed to arise in the photoluminescence spectra. It is shown that the indirect exciton luminescence line can be suppressed by both parallel and perpendicular magnetic fields.     

System of quantum wells in a parallel magnetic field

The energy spectrum and quantum states of electrons in a system of quantum wells in a strong magnetic field parallel to the heterogeneous boundaries are studied. The combined effect of the quantizing magnetic field and the potential of the system of quantum wells leads to a radical change in the electron dispersion relation owing to the appearance of one-dimensional Landau bands. The neighborhoods of the anticrossing points of the different bands correspond to an effective redistribution of the electron envelope functions, which becomes stronger as the magnetic field is raised. The character of the electron-state density in the size-quantization subbands is examined qualitatively in connection with the change in the system of isoenergy contours when a magnetic field is applied. Fiz. Tverd. Tela (St. Petersburg) 40, 1719–1723 (September 1998)     

Cancellation of fine-structure splitting in quantum dots by a magnetic field

We demonstrate control of the fine-structure splitting of the exciton emission lines in single InAs quantum dots by the application of an in-plane magnetic field. The composition of the barrier material and the size and symmetry of the quantum dot are found to determine decrease or increase in the linear polarization splitting of the dominant exciton emission lines with increasing magnetic field. This enables the selection of dots for which the splitting can to be tuned to zero, within the resolution of our experiments. General differences in the g-factors and exchange splittings are found for different types of dot.     

Experimental observation of quantum corrections to electrical resistivity in nanocrystalline soft magnetic alloys

X-ray diffraction patterns of nanocrystalline Fe-Cu-Nb-Si-B (FINEMET) alloys reveal that bcc α-Fe/α-FeSi crystallites with the average grain size of 20(5) nm are dispersed in amorphous matrix. Enhanced electron—electron interaction (EEI) and quantum interference (QI) effects as well as electron-magnon (and/or electron-spin fluctuation) scattering turn out to be the main mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron-phonon scattering is the most effective mechanism to destroy phase coherence of electron wave functions. The diffusion constant, density of states at the Fermi level and the inelastic scattering time have been estimated, for the first time, for the alloys in question Article presented at the International Symposium on Advances in Superconductivity and Magnetism: Materials, Mechanisms and Devices, ASMM2D-2001, 25–28 September 2001, Mangalore, India.     

Remanent zero field spin splitting of self-assembled quantum dots in a paramagnetic host

We report on the observation of a finite spin splitting at zero magnetic field in resonant tunneling experiments on CdSe self-assembled quantum dots in a (Zn,Be,Mn)Se barrier. This is remarkable since bulk II-VI dilute magnetic semiconductors are paramagnets. Our experiment may be viewed as tunneling through a single magnetic polaron, where the carriers contained inside the dot act to mediate an effective ferromagnetic interaction between Mn ions in their vicinity. The effect is observable up to relatively high temperatures, which we tentatively ascribe to a feedback mechanism with the electrical current, previously predicted theoretically.     

Plasmons in double quantum wells in a parallel magnetic field

Collective intraband charge-density excitations in the quasi-two-dimensional electron system of double GaAs/AlGaAs quantum wells in an external parallel magnetic field B_∥ are studied by inelastic light scattering. It has been found that the energy of the excitations under study (acoustic and optical plasmons) exhibits anisotropy depending on the mutual orientation of B_∥ and the excitation quasi-momentum k. It is shown theoretically that, in a strong parallel magnetic field, the effects associated with the finite width of the quantum wells dominate over the effects associated with interlayer tunneling and determine the anisotropy of plasmons. The experimental data are compared with a theoretical calculation.     

Electrodynamic response of a quantum nanotube in a parallel magnetic field

This paper reports on a theoretical study of intraband resonances arising in the absorption of electromagnetic radiation by a quantum nanotube both with and without involvement of optical phonons. Explicit relations are derived for the absorption coefficients. The relative intensities of the resonance peaks are analyzed.     

Spin–spin interaction in magnetic semiconductor quantum dots

Self-assembled Cd(Mn)Se/Zn(Mn)Se quantum dots have been investigated by means of spatially and time-resolved magneto-optical spectroscopy. In such quasi zero-dimensional diluted magnetic semiconductors, the exchange interaction couples the spins of optically generated charge carriers with localized magnetic ion spins. We demonstrate that this can be used on the one hand to monitor nanoscale magnetization with a resolution of <100 μ_B by a purely optical technique and on the other hand to optically manipulate the magnetization in a semiconductor quantum dot.     

Magnetic quantum oscillations in a monolayer graphene under a perpendicular magnetic field

The de Haas-van Alphen(dHvA) oscillations of electronic magnetization in a monolayer graphene with structureinduced spin-orbit interaction(SOI) are studied.The results show that the dHvA oscillating centre in this system deviates from the well known(zero) value in a conventional two-dimensional electron gas.The inclusion of SOI will change the well-defined sawtooth pattern of magnetic quantum oscillations and result in a beating pattern.In addition,the SOI effects on Hall conductance and magnetic susceptibility are also discussed.     

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.     

Spin-orbit effects in a GaAs quantum dot in a parallel magnetic field

We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. Counterintuitively we argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B( parallel), even if they are negligible for B( parallel) = 0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B( parallel). Our picture is consistent with the experimental observations of Folk et al.     

Photo-induced magnetic-solitons and spin dynamics in diluted magnetic semiconductor quantum wells

The localization mechanism of transport property in the randomly distributed system of the hole-induced magnetic solitons with the alloy potential fluctuations in diluted magnetic semiconductors has been proposed, by using the effective Lagrangian of diffusion modes. The mechanism of the long relaxation of the spin dynamics below Curie temperature in diluted magnetic semiconductor wells and the bulk system has been discussed.     

Excitation hierarchy of the quantum sine-gordon spin chain in a strong magnetic field

The magnetic excitation spectrum of copper pyrimidine dinitrate, a material containing S = 1 / 2 antiferromagnetic chains with alternating g tensor and the Dzyaloshinskii-Moriya interaction and exhibiting a field-induced spin gap, is probed using submillimeter wave electron spin resonance spectroscopy. Ten excitation modes are resolved in the low-temperature spectrum, and their frequency-field diagram is systematically studied in magnetic fields up to 25 T. The experimental data are sufficiently detailed to make a very accurate comparison with predictions based on the quantum sine-Gordon field theory. Signatures of three breather branches and a soliton, as well as those of several multiparticle excitation modes, are identified.     

Calculation of tunnel splitting in a biaxial spin particle with an applied magnetic field

The level splitting formulae of excited states as well as ground state for a biaxial spin particle in the presence of an applied magnetic field are obtained in a simple way from Schrödinger theory. Considering the boundary condition of the wave function, we obtain the tunneling splitting of the energy levels for half-integral spins as well as for the integral spins. The results obtained are compared with those previously derived by complicated pseudoparticle methods and numerical calculation values.Received: 12 May 2004, Published online: 3 August 2004PACS: 75.45. + j Macroscopic quantum phenomena in magnetic systems - 75.50.Xx Molecular magnets - 73.40.Gk Tunneling     

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).     

Control of spin state in CdSe quantum dots by coupling with magnetic semiconductor quantum dots

We studied spin states of CdSe quantum dots (QDs) coupled with CdMnSe QDs by probing circular polarization of photoluminescence spectrum under external magnetic fields. The bandgap energies of CdSe and CdMnSe QDs are close to each other and photoluminescence mainly originates from CdSe QDs due to relatively low radiation efficiency of CdMnSe QDs. The photoluminescence lifetime as well as its intensity was decreased with increasing magnetic field, which was ascribed to the increase in the ground state wavefunctions in CdMnSe QDs. The decrease was more pronounced for spin down electrons, which was explained by the difference in spin up and down wave functions under magnetic fields. Our results show that the spin state of CdSe QDs can be manipulated by coupling with CdMnSe QDs.     

Electron spin dynamics in a self-assembled semiconductor quantum dot: the limit of low magnetic fields

Using the trion as an optical probe, we uncover novel electron spin dynamics in CdSe/ZnSe Stranski-Krastanov quantum dots. The longitudinal spin lifetime obeys an inverse power law associated with recharging processes in the dot ensemble. No hint at spin-orbit mediated spin relaxation is found. At very weak magnetic fields (< 50 mT), electron spin dynamics related to the hyperfine interaction with the lattice nuclei is uncovered. A strong Knight field gives rise to nuclear ordering and formation of dynamical polarization on a 100-micros time scale under continuous electron spin pumping. The associated spin transients are temperature robust and can be observed up to 100 K.