Sj Lee

Band structure of a superlattice with a two well period

Abstract We present calculations for a superlattice with wells of alternating widths, w 1 and w 2 , and barriers of alternating widths, b 1 and b 2 , to investigate the effect of such a pattern in modulating superlattice minibands. Enormous changes in the electronic states of a superlattice can be made by adjusting width parameters in this way. The band structure of such a superlattice generally displays characteristics of both constituent superlattices, i.e. one with w 1 and b 1 and the other with w 2 and b 2 . Whenever there is an overlap of minibands between the two constituent superlattices, a band splitting is observed in the composite structure. Enhancement of the minibands widths and the creation of new Bloch states can also be acheived by suitable arangements of the width parameters. These results are compared with recent calculations for a superlattice interleaved with a periodic array of δ-doped defects in the barrier or well layers.

Electronic structure of the triangular quantum well in a tilted magnetic field

Abstract The quantized electron energies in the triangular well of a Si-MOSFET subjected to strong magnetic fields at tilt angles B=(Bx, 0, Bz) are calculated. Approximate but analytic solutions are also provided with numerical solutions. It is shown that the role of the coupling Hamiltonian (∼Bx·Bz) is crucial in obtaining the correct magnetization, although its contribution to the total energy is minor. This coupling term is responsible for the oscillation of the magnetizations with respect to the chemical potential or the two-dimensional electron density. The latter is also confirmed in the case of a parabolic quantum well.

Investigation of localization in an infinite superlattice

Abstract An exact method of determining localized electronic states within the framework of a Kronig-Penney-like model is presented for an infinite superlattice in which there exist slight fluctuations in layer thickness. It is shown that fluctuations in well layer thickness, regardless of degree, always produce localized states residing in the minigaps of a superlattice. Outside of the perturbed well, these localized wavefunctions decay exponentially in an oscillating pattern. The energies of these localized states are closely related to eigenvalues of a corresponding single quantum well. However, the localized well energy lies further from the minibands than the single quantum well energy. The present observations regarding an infinite superlattice are also compared with previous observations regarding a finite superlattice.

Anisotropy effects on the orbital magnetism of electrons in a spheroidal quantum dot

Anisotropy effects in the orbital magnetic susceptibility of electrons in a spheroidal quantum dot are investigated by applying tilted magnetic field with parabolic confinement potential. The results show dramatic phase change; magnetic susceptibility as a function of the electron concentration changes from para- or diamagnetic in the case of anisotropic quantum dot to diamagnetic only in the isotropic case. Temperature, field and tilted angle dependences are also extensively studied.

Edge states in a magnetic quantum dot

Abstract We have considered two dimensional electrons further confined in an inhomogeneous magnetic field which is B =0 for r r 0 and B ≠0 elsewhere. We have noticed, through the exact evaluation of the single particle energy spectra, the existence of current-carrying edge states near the dot boundary circulating either clockwise or counterclockwise. These magnetic edge states are shown to have quite different properties from those originated from electrostatic confinements.

Heat capacities of multi-quantum well and wire structures in high magnetic fields

Abstract The electronic structure and the heat capacity of multi-quantum well and wire structures in strong magnetic fields are calculated. The specific heat as a function of magnetic field shows strongly oscillating behavior. For a well structure in the presence of tilted fields, the hybrid-quantization of electronic energies reveals the variation of width and the non-uniform spacing of energy levels in each branch of the Landau fan and sometimes causes the intralevel contribution to have asymmetric shapes. For a wire structure, the specific heat consists of sets of asymmetric double peaks and shows a non-linear dependence on the temperature in contrast to the well case.

Electron density dependence of the effective g-factor in a two-dimensional electron gas at strong magnetic fields

The dependence of the effective g-factor (g * ) on the electron density and the Landau level filling factor ν are studied for a two-dimensional electron gas in strong magnetic fields. The trial many-body wavefunctions are constructed by multiplying the Jastrow correlation factor to the Laughlin-like wavefunctions, then, the ground state energy are calculated using a variational Monte Carlo method for 1 < ν < 2. For a dimensionless parameter r S = 1.33, g * is estimated to be about 11 at ν = 1.0, while the measured value is 7.3. Considering the level broadening due to impurity and phonon scatterings, a better agreement of g * = 9 is found.

Anomalous magnetotransport in narrow quantum Hall conductors

Abstract We present the anomalous magnetotransport observed in a narrow high mobility Hall device; both the peaks and minima in the Shubnikov de Haas oscillations of the longitudinal resistance appear in the quantized plateau region of the Hall resistance. We interpret this feature as a consequence of an inhomogeneous distribution of two dimensional electron gas and provide a quantitative view with a model based on the edge current picture.

Electronic structure of quantum dots with two electrons in tilted magnetic fields

Abstract The energy levels of a spheroidal quantum dot in tilted magnetic fields are calculated and compared with those for a disk-like dot. The magneto-optical spectrum and the ground state show considerable difference between the two quantum dots. We find that this feature can be understood by considering the magneto-electric hybridization and electron-electron interaction effects on the energy spectrum.