36 research outputs found
Magnetotransport in wide parabolic PbTe quantum wells
The 3D- and 2D- behaviour of wide parabolic PbTe single quantum wells, which
consist of PbTe p-n-p-structures, are studied theoretically and experimentally.
A simple model combines the 2D- subband levels and the 3D-Landau levels in
order to calculate the density of states in a magnetic field perpendicular to
the 2D plane. It is shown that at a channel width of about 300nm on can expect
to observe 3D- and 2D-behaviour at the same time. Magnetotransport experiments
in selectively contacted Hall bar samples are performed at temperatures down to
T = 50 mK and at magnetic fields up to B = 17 T.Comment: postscript file including 2 figs, 4 pages, Paper presented at
EP2DS-XI, Nottingham 199
Anomalous magnetotransport in wide quantum wells
We present magneto transport experiments of quasi 3D PbTe wide quantum wells.
A plateau-like structure in the Hall resistance is observed, which corresponds
to the Shubnikov de Haas oscillations in the same manner as known from the
quantum Hall effect. The onsets of plateaux in Rxy do not correspond to 2D
filling factors but coincide with the occupation of 3D (bulk-) Landau levels.
At the same time a non-local signal is observed which corresponds to the
structure in Rxx and Rxy and fulfils exactly the Onsager-Casimir relation
(Rij,kl(B) = Rkl,ij(-B)). We explain the behaviour in terms of edge channel
transport which is controlled by a permanent backscattering across a system of
"percolative EC - loops" in the bulk region. Long range potential fluctuations
with an amplitude of the order of the subband splitting are explained to play
an essential role in this electron system.Comment: postscript file including 3 figs, 5 page
The microwave induced resistance response of a high mobility 2DEG from the quasi-classical limit to the quantum Hall regime
Microwave induced resistance oscillations (MIROs) were studied experimentally
over a very wide range of frequencies ranging from ~20 GHz up to ~4 THz, and
from the quasi-classical regime to the quantum Hall effect regime. At low
frequencies regular MIROs were observed, with a periodicity determined by the
ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz
the magnetic field dependence of MIROs waveform is well described by a
simplified version of an existing theoretical model, where the damping is
controlled by the width of the Landau levels. In the THz frequency range MIROs
vanish and only pronounced resistance changes are observed at the cyclotron
resonance. The evolution of MIROs with frequency are presented and discussed.Comment: 4 pages, presented at EP2DS, to be published in Physica
Conductance Fluctuations in PbTe Wide Parabolic Quantum Wells
We report on conductance fluctuations which are observed in local and
non-local magnetotransport experiments. Although the Hall bar samples are of
macroscopic size, the amplitude of the fluctuations from the local measurements
is close to e^2/h. It is shown that the fluctuations have to be attributed to
edge channel effects.Comment: postscript file including 3 figs, 3 pages, Paper presented at 3rd
Int. Symposium on "New Phenomena in Mesoscopic Structures" in Maui, Hawaii
199
Landau levels in two and three-dimensional electron gases in a wide parabolic quantum well
Interaction effects at the magnetic-field induced metal-insulator transition in Si/SiGe superlattices
A metal-insulator transition was induced by in-plane magnetic fields up to 27
T in homogeneously Sb-doped Si/SiGe superlattice structures. The localisation
is not observed for perpendicular magnetic fields. A comparison with
magnetoconductivity investigations in the weakly localised regime shows that
the delocalising effect originates from the interaction-induced spin-triplet
term in the particle-hole diffusion channel. It is expected that this term,
possibly together with the singlet particle-particle contribution, is of
general importance in disordered n-type Si bulk and heterostructures.Comment: 5 pages, 3 figures, Solid State Communications, in prin
Fock-Darwin-like quantum dot states formed by charged Mn interstitial ions
We report a method of creating electrostatically induced quantum dots by thermal diffusion of interstitial Mn ions out of a p-type (GaMn)As layer into the vicinity of a GaAs quantum well. This approach creates deep, approximately circular, and strongly confined dotlike potential minima in a large (200 μm) mesa diode structure without need for advanced lithography or electrostatic gating. Magnetotunneling spectroscopy of an individual dot reveals the symmetry of its electronic eigenfunctions and a rich energy level spectrum of Fock-Darwin-like states with an orbital angular momentum component |lz| from 0 to 11
Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties
We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices
with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML)
(23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA).
While previous reports state that GaMnAs layers thinner than 50 \AA are
paramagnetic in the whole Mn composition range achievable using MBE growth (up
to 8% Mn), we have found that short period superlattices exhibit a
paramagnetic-to-ferromagnetic phase transition with a transition temperature
which depends on both the thickness of the magnetic GaMnAs layer and the
nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the
magnetic layers in superlattices are ferromagnetically coupled for both thin
(below 50 \AA) and thick (above 50 \AA) GaMnAs layers.Comment: Proceedings of 4th International Workshop on Molecular Beam Epitaxy
and Vapour Phase Epitaxy Growth Physics and Technology, September 23 - 28
(2001), Warszawa, Poland, to appear in Thin Solid Films. 24 pages, 8 figure
Activation Energy in a Quantum Hall Ferromagnet and Non-Hartree-Fock Skyrmions
The energy of Skyrmions is calculated with the help of a technique based on
the excitonic representation: the basic set of one-exciton states is used for
the perturbation-theory formalism instead of the basic set of one-particle
states. We use the approach, at which a skyrmion-type excitation (at zero Lande
factor) is considered as a smooth non-uniform rotation in the 3D spin space.
The result within the framework of an excitonically diagonalized part of the
Coulomb Hamiltonian can be obtained by any ratio [where is the typical Coulomb
energy ( being the magnetic length); is the cyclotron
frequency], and the Landau-level mixing is thereby taken into account. In
parallel with this, the result is also found exactly, to second order in terms
of the (if supposing to be small) with use of the
total Hamiltonian. When extrapolated to the region , our
calculations show that the skyrmion gap becomes substantially reduced in
comparison with the Hartree-Fock calculations. This fact brings the theory
essentially closer to the available experimental data.Comment: 14 pages, 1 figure. to appear in Phys. Rev. B, Vol. 65 (Numbers ~
19-22), 200
The Effects of Disorder on the Quantum Hall State
A disorder-averaged Hartree-Fock treatment is used to compute the density of
single particle states for quantum Hall systems at filling factor . It
is found that transport and spin polarization experiments can be simultaneously
explained by a model of mostly short-range effective disorder. The slope of the
transport gap (due to quasiparticles) in parallel field emerges as a result of
the interplay between disorder-induced broadening and exchange, and has
implications for skyrmion localization.Comment: 4 pages, 3 eps figure