Superlattices In Magnetic Field Research Articles

Interminiband absorption in a quantum ring superlattice in magnetic field with periodic vector potential

The miniband Aharonov-Bohm oscillations and the interminiband absorption coefficient have been considered theoretically for one-layer superlattices of square and rectangular symmetries, composed of cylindrical quantum rings in the external transverse magnetic field with a periodic vector potential by the lattice constants. The crossings and anticrossings of the energies corresponding to different values of quasimomentum are observed. It is shown that the energy gap between the minibands and the sequence of the energies in each miniband can be tuned by the magnetic field. The interminiband absorption coefficient qualitatively depends on the symmetry of the superlattice, magnetic field induction and the incident light polarization. The obtained results indicate on the possibility to control the electronic and optical characteristics of the devices based on quantum ring superlattices.

Electromagnetic Waves Reflectance of Graphene—Magnetic Semiconductor Superlattice in Magnetic Field

Electrodynamic properties of the graphene-magnetic semiconductor-graphene superlattice placed in magnetic field have been investigated theoretically in Faraday geometry, while taking into account dissipation processes. Frequency and field dependences of the reflectance, transmittance, and absorbtance of electromagnetic waves by such superlattice have been calculated for different numbers of periods of the structure and different sizes of the periods with using a transfer matrix method. The possibility of efficient control of electrodynamic properties of graphene-magnetic semiconductor-graphene superlattice has been shown.

Electron gas high-frequency conductivity on the surface of a nanotube with superlattice in magnetic field

Kubo formula was obtained for conductivity tensor of electron gas on the surface of nanotube with superlattice in magnetic field. The high-frequency conductivity tensor components were calculated for quantum and quasiclassical cases. Electromagnetic wave Landau damping areas in the tube were determined. The conductivity tensor components show Aharonov–Bohm type oscillations and de Haas–van Alphen ones. When Fermi energy exceeds the miniband width, beatings are observed in the plot of conductivity vs. the tube parameters. Otherwise, the beatings are absent.

Pseudo-Band-Structure of Disordered Lateral Superlattice in Magnetic Field

The density of states and the conductivity tensor of a quantum-wire array with disorder in the period, direction, and height of the corrugation are calculated in magnetic fields within a self-consistent Born approximation. A diffuse Bragg scattering causes pseudo-Landau-levels associated with the second band and pseudo-open-orbits in the gap region of a corresponding system without disorder. This fact manifests itself as various characteristic structures in the density of states, the conductivity in the direction parallel and perpendicular to the wire, and the Hall conductivity.

Optical emission and Rayleigh scattering in semiconductor superlattices in magnetic fields

We studied the Rayleigh light scattering and the optical emission in GaAs–(Al,Ga)As superlattices with ordered and intentionally disordered potential profiles, correlated and uncorrelated, in external magnetic fields. We find that the intentional disorder along z affects the resonant Rayleigh scattering and the optical emission. The external magnetic field in the same direction allows to modify the exciton localization and to study the relative modification of the light scattering and of the emission spectra.

Conductivity of a semiconductor superlattice in a transverse magnetic field

The conductivity of a semiconductor superlattice in a magnetic field perpendicular to the superlattice axis is calculated. The dependence of the conductivity on the magnetic field strength is studied. It is found that there is a magnetic-field range where the resistivity is proportional to the magnetic field strength.

Interband Light Absorption in Size-Confined Systems in Uniform Electric Fields

A simple approach to calculation of the interband absorption coefficient in a uniform electric field is developed. This approach provides a means for studying the special features of electroabsorption in a wide class of semiconductor systems on the basis of the most general relationships. The approach is used to study the electroabsorption in two-dimensional systems with different profiles of their one-dimensional potential, quantum wells, and superlattices in magnetic fields.

Studies of longitudinal and transverse resistivities of YBa2Cu3Oy/R0.7M0.3MnO3 superlattices, in which R = La or Nd and M = Ca or Sr

We measured the longitudinal and transverse Hall resistivities of YBa2Cu3Oy/R1−xMxMnO3 superlattices in magnetic fields to study the transport properties, in which R = La or Nd and M = Ca or Sr. The resistive transition in the magnetic field shows a thermally activated flux flow behaviour. The flux pinning ability of the superlattice is reduced when the coupling strength between YBa2Cu3Oy layers is decreased. The transverse Hall resistivity shows sign reversal for samples with strong pinning ability. The sign reversal diminishes as the pinning force is reduced. The magnetoresistance (MR) ratio, |Δρ(H) − Δρ(H = 0)|/Δρ(H), of the YBa2Cu3Oy/R1−xMxMnO3 superlattice is affected by the coupling strength of R1−xMxMnO3 layers. The peak values of the MR ratio were shifted to lower temperatures in superlattices due to the lowering of the magnetic ordering in ferromagnetic layers.

Transport studies of YBa 2Cu 3O y/Y xPr 1− xBa 2Cu 3O y and YBa 2Cu 3O y/ R0.7M0–3MnO 3 superlattices, in which R=La or Nd, and M=Ca or Sr

We report the transport studies of YBa 2Cu 3O y /Y x Pr 1− x Ba 2Cu 3O y and YBa 2Cu 3Oy/ R 1− x M x MnO 3 superlattices in magnetic fields in which R=La or Nd, and M=Ca or Sr. The X-ray diffraction of samples shows superlattice structure. The resistive transition in a magnetic field shows thermal activated behavior. The flux pinning is reduced when the coupling strength between YBCO layers is decreased. The angular dependence of the critical current of YBa 2Cu 3O y /PrBa 2Cu 3O y superlattices reveals the dimensionality of superlattices. The magnetoresistance ratio (MR), |Δ ρ( H=7 T)−Δ ρ( H=0)|/Δ ρ( H=7 T), of YBa 2Cu 3O y / R 1− x M x MnO 3 superlattices is affected by the layer coupling of R 1− x M x MnO 3 layers. The enhancement of the MR ratio in the tri-layer YBa 2Cu 3O y /La 0.7 M 0.3MnO 3/YBa 2Cu 3O y film in the low temperature regime is significant and has a value of 33650% at T=75 K. We attribute this enhancement of the MR to the ordering of magnetic moment in ferromagnetic layers in magnetic fields. The results are discussed in terms of existing theories.

Optical properties and collective excitations of plasmons in semiconductor superlattices in magnetic fields

Using the random-phase approximation the density-density correlation function including intra- and inter-Landau level transitions in a finite type-I superlattice is obtained under magnetic fields parallel to interfaces on the basis of a simple model of parabolic confining potentials. The dispersion relation for surface and bulk states is investigated according to the strength of magnetic field, the momentum wave vector, and the ratio of the confining potential parameter to cyclotron resonance frequency. The Raman intensities and their shifts for the bulk and surface plasmons are also examined according to the strength of magnetic field and the momentum wave vector in the presence and the absence of magnetic field.

Butterfly-like spectra and collective modes of antidot superlattices in magnetic fields

Department of Theoretical Physics, University of Lund, S¨olvegatan 14 A, S-223 62 Lund, Sweden(June 24, 2011)We calculate the energy band structure for electrons in an external periodic potential combinedwith a perpendicular magnetic field. Electron-electron interactions are included within a Hartreeapproximation. The calculated energy spectra display a considerable degree of self-similarity, justas the “Hofstadter butterfly.” However, screening affects the butterfly, most importantly the band-widths oscillate with magnetic field in a characteristic way. We also investigate the dynamic responseof the electron system in the far-infrared (FIR) regime. Some of the peaks in the FIR absorptionspectra can be interpreted mainly in semiclassical terms, while others originate from inter(sub)bandtransitions.73.20.Dx, 73.20.Mf

Magnetooscillations in the electroluminescence intensity of excited subbands in resonantly coupled superlattices

We have measured the electroluminescence (EL) from excited conduction subband populations in resonantly coupled GaAs/AlAs superlattices in magnetic fields up to 23 T perpendicular to the layers. Strong oscillations are observed in the EL intensity of the second electronic subband with increasing magnetic field, exhibiting two different periods in 1/ B for a subband spacing below the optical phonon energy. The EL intensity of the third electronic subband with a separation above the optical phonon energy only shows a distinct minimum with increasing magnetic field, which coincides with a maximum in the measured current.

Magneto-optical studies of the type I/type II crossover and band offset in [formula omitted] superlattices in magnetic fields up to 45 T

We report measurements of magneto-reflectivity in ZnTe Zn 1−xMn xTe superlattices in magnetic fields up to 45 Tesla. From an analysis of the Zeeman splitting, we investigate the change of band alignment with field and the band offset ratio. A crossing of the 1 s exciton transitions from the ZnTe buffer layer and the 1 s heavy hole σ + exciton of the superlattice is observed, providing unambiguous evidence of a band alignment change from type I to type II. The excitonic energy levels for both type I and type II band structure are calculated using a variational method. This model fits the experimental data very well at high field for both σ + and σ − transitions. A conduction band offset ratio of ΔE c ΔE s =0.72±0.04 is deduced.

Thermally activated flux motion in a-axis YBa2Cu3O7/PrBa2Cu3O7 superlattices.

We have investigated resistive properties of a-axis YBa₂Cu₃O₇/a,b-axis PrBa₂Cu₃O₇ superlattices in magnetic fields to study thermally activated flux-flow resistance. We find that in magnetic fields perpendicular to the YBa₂Cu₃O₇/PrBa₂Cu₃O₇ layering the activation energies of these superlattices deduced from the tail of the resistive transitions have a logarithmic magnetic-field dependence for small YBa₂Cu₃O₇ thicknesses (<300 A), in agreement with recent theories of thermally activated resistance in strongly pinned two-dimensional superconductors.

Electron transport through one-dimensional lateral surface superlattices in magnetic fields.

A scattering-matrix method for the calculations of electron transport through lateral quantum systems in the presence of a perpendicular magnetic field is developed and is used to investigate the effects of an applied magnetic field on electron transport through a quantum channel modulated by a smooth periodic potential along the direction of the current flow. At zero magnetic field, the calculated conductance displays regular dips due to the formation of minigaps (or the Bragg reflections) and the rapid oscillations due to electron transmission through the coupled quasi-zero-dimensional states in the cavity regions between the potential barriers. Both are shown to be suppressed when a magnetic field is applied to the quantum channel. This is interpreted as the formation of propagating edge states. However, other irregular dips are shown to appear in the conductance of the modulated channel in the presence of the magnetic field. These dips reflect the coupling between the electron states propagating along the opposite edges of the channel and may appear so densely in a wide quantum channel with a strong modulation that the conductance exhibits fluctuations. In the high-field regime where the magnetic length ${\mathit{l}}_{\mathit{B}}$ is much smaller than the channel width w, these irregular dips are seen to be also suppressed, leading to a nearly perfect recovery of the conductance quantization.

Magneto-optical studies of the type-I/type-II crossover and band offset in ZnTe/Zn1-xMnxTe superlattices in magnetic fields up to 45 T.

We report measurements of magnetoreflectivity in ZnTe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te superlattices with varying well width in magnetic fields up to 45 T. From an analysis of the Zeeman splitting, we investigate the change of band alignment with field and the band offset ratio. A crossing of the 1s exciton transitions from the ZnTe buffer layer and the 1s heavy hole ${\mathrm{\ensuremath{\sigma}}}_{+}$ exciton of the superlattice is observed, providing unambiguous evidence of a band alignment change from type I to type II. The excitonic energy levels for both type-I and type-II band structure are calculated using a variational method. This model fits the experimental data very well at high fields for both ${\mathrm{\ensuremath{\sigma}}}_{+}$ and ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{\ensuremath{-}}}$ transitions and predicts that the type-I/type-II crossover occurs at 2--3 T. Also by fitting the ${\mathrm{\ensuremath{\sigma}}}_{+}$ transition energy as a function of field a conduction band offset ratio of \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$/\ensuremath{\Delta}${\mathit{E}}_{\mathit{g}}$=0.72\ifmmode\pm\else\textpm\fi{}0.04 is deduced.

On the conductivity of lateral-surface superlattices in magnetic fields

The authors present results for the band conductivity and the density of states of electrons in realistic two-dimensional periodic potentials and magnetic fields. They show the effects of the number of flux quanta per unit cell in producing Hofstader's-butterfly-like splittings in the limit of small field, but outside the simple tight-binding limit, and for high fields where the dominant effect is the local splitting of Landau levels to form weakly coupled edge states round the minima and maxima of the potential. The broadening and splitting of these states to form Hofstader's butterfly then occurs on a much finer energy scale.

Magnetic field effects on non-periodic superlattice structures

A simple numerical method for studying the effect of an applied magnetic field on the energy spectrum of non-periodic superlattice structures is presented. The magnetic field could be either parallel or perpendicular to the growth direction. The authors method is based on the transfer matrix technique and on the effective-mass approximation. They discuss the advantages and disadvantages of the proposed approach using several examples. In particular, they study the perturbation to the energy spectrum of a periodic superlattice induced by the introduction of an enlarged well. They found that these perturbations are negligible to B//z but significant for B perpendicular to z. Preliminary results for Fibonacci superlattices in magnetic fields are also presented. In these quasi-periodic structures the energy levels become strongly dispersive in the presence of a perpendicular magnetic field.

Polarized luminescence of excitons under resonance excitation of GaAs/AlAs type II superlattices in a normal magnetic field

We have studied, theoretically, the exciton luminescence of type II GaAs/AlAs superlattices in a magnetic field. If such superlattices are grown along z‖[001], the states, allowed in luminescence polarized along x'‖[110] or y'‖[1-0], are split due to the reduced symmetry from D 2d to C 2v. We have shown that in the magnetic field, parallel to x' or y', crossing of the energy levels of these states occurs. Near the level crossings the exciton luminescence becomes partially polarized, if the excitation is polarized along x‖[100] or y‖[010]. Far from the crossings, the luminescence is completely nonpolarized.

Hydrogenic impurities in GaAs-Ga1-xAlxAs superlattices in an axial magnetic field

The binding energies of the ground state and the low-lying excited states of the hydrogenic impurity in a GaAs-Ga1-xAlxAs superlattice in a magnetic field have been calculated by applying the method of one-dimensionalization and the effective-mass approximation. In the calculations, the effects due to the different effective masses of electrons in different materials are included. The results agree better with the experimental results than those of the calculation of a single quantum well.