31 research outputs found
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. We argue
that spin-orbit effects may become important in the presence of a large
parallel magnetic field B_{||}, even if they are negligble for B_{||}=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_{||}. Our picture is consistent with the experimental
observations of Folk et al.Comment: 5 page
Spin splitting and precession in quantum dots with spin-orbit coupling: the role of spatial deformation
Extending a previous work on spin precession in GaAs/AlGaAs quantum dots with
spin-orbit coupling, we study the role of deformation in the external
confinement. Small elliptical deformations are enough to alter the precessional
characteristics at low magnetic fields. We obtain approximate expressions for
the modified factor including weak Rashba and Dresselhaus spin-orbit terms.
For more intense couplings numerical calculations are performed. We also study
the influence of the magnetic field orientation on the spin splitting and the
related anisotropy of the factor. Using realistic spin-orbit strengths our
model calculations can reproduce the experimental spin-splittings reported by
Hanson et al. (cond-mat/0303139) for a one-electron dot. For dots containing
more electrons, Coulomb interaction effects are estimated within the
local-spin-density approximation, showing that many features of the
non-iteracting system are qualitatively preserved.Comment: 7 pages, 7 figure
Radiation-induced oscillatory magnetoresistance as a sensitive probe of the zero-field spin splitting in high mobility GaAs/AlGaAs devices
We suggest an approach for characterizing the zero-field spin splitting of
high mobility two-dimensional electron systems, when beats are not readily
observable in the Shubnikov-de Haas effect. The zero-field spin splitting and
the effective magnetic field seen in the reference frame of the electron is
evaluated from a quantitative study of beats observed in radiation-induced
magnetoresistance oscillations.Comment: 4 pages, 4 color figure
Many-body spin related phenomena in ultra-low-disorder quantum wires
Zero length quantum wires (or point contacts) exhibit unexplained conductance
structure close to 0.7 X 2e^2/h in the absence of an applied magnetic field. We
have studied the density- and temperature-dependent conductance of
ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2 mu
m, fabricated from structures free of the disorder associated with modulation
doping. In a direct comparison we observe structure near 0.7 X 2e^2/h for l=0
whereas the l=2 mu m wires show structure evolving with increasing electron
density to 0.5 X 2e^2/h in zero magnetic field, the value expected for an ideal
spin-split sub-band. Our results suggest the dominant mechanism through which
electrons interact can be strongly affected by the length of the 1D region.Comment: 5 Pages, 4 figure
Growth variations and scattering mechanisms in metamorphic In0.75Ga0.25As/In-0.75 Al0.25As quantum wells grown by molecular beam epitaxy
Modulation doped metamorphic In0.75Ga0.25As/In0.75Al0.25As quantum wells (QW) were grown on GaAs substrates by molecular beam epitaxy (MBE) with step-graded buffer layers. The electron mobility of the QWs has been improved by varying the MBE growth conditions, including substrate temperature, arsenic over pressure and modulation doping level. By applying a bias voltage to SiO2 insulated gates, the electron density in the QW can be tuned from 1Ă1011 to 5.3Ă1011 cmâ2. A peak mobility of 4.3Ă105 cm2Vâ1sâ1 is obtained at 3.7Ă1011 cmâ2 at 1.5 K before the onset of second subband population. To understand the evolution of mobility, transport data is fitted to a model that takes into account scattering from background impurities, modulation doping, alloy disorder and interface roughness. According to the fits, scattering from background impurities is dominant while that from alloy disorder becomes more significant at high carrier density
Interaction Effects in a One-Dimensional Constriction
We have investigated the transport properties of one-dimensional (1D)
constrictions defined by split-gates in high quality GaAs/AlGaAs
heterostructures. In addition to the usual quantized conductance plateaus, the
equilibrium conductance shows a structure close to , and in
consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135
(1996)] this 0.7 structure has been investigated in a wide range of samples as
a function of temperature, carrier density, in-plane magnetic field
and source-drain voltage . We show that the 0.7
structure is not due to transmission or resonance effects, nor does it arise
from the asymmetry of the heterojunction in the growth direction. All the 1D
subbands show Zeeman splitting at high , and in the wide channel
limit the -factor is , close to that of bulk GaAs.
As the channel is progressively narrowed we measure an exchange-enhanced
-factor. The measurements establish that the 0.7 structure is related to
spin, and that electron-electron interactions become important for the last few
conducting 1D subbands.Comment: 8 pages, 7 figures (accepted in Phys. Rev. B
Interference of Spin Splittings in Magneto-Oscillation Phenomena in Two-Dimensional Systems
The spin splitting caused by the terms linear in wavevector in the effective
Hamiltonian containing can give rise to the new magneto-oscillation phenomena
in two-dimensional systems. It is shown that the joint action of the
spin-dependent contributions due to the heterostructure asymmetry and to the
lack of inversion center in the bulk material suppresses beats that arise in
the magneto-oscillation phenomena in the presence of the terms of only one of
these types.Comment: pdf file, 4 pages, 2 figure
Conductance anomalies and the extended Anderson model for nearly perfect quantum wires
Anomalies near the conductance threshold of nearly perfect semiconductor
quantum wires are explained in terms of singlet and triplet resonances of
conduction electrons with a single weakly-bound electron in the wire. This is
shown to be a universal effect for a wide range of situations in which the
effective single-electron confinement is weak. The robustness of this generic
behavior is investigated numerically for a wide range of shapes and sizes of
cylindrical wires with a bulge. The dependence on gate voltage, source-drain
voltage and magnetic field is discussed within the framework of an extended
Hubbard model. This model is mapped onto an extended Anderson model, which in
the limit of low temperatures is expected to lead to Kondo resonance physics
and pronounced many-body effects
Binding Energy of Hydrogen-Like Impurities in Quantum Well Wires of InSb/GaAs in a Magnetic Field
The binding energy of a hydrogen-like impurity in a thin size-quantized wire of the InSb/GaAs semiconductors with Kaneâs dispersion law in a magnetic fieldBparallel to the wire axis has been calculated as a function of the radius of the wire and magnitude ofB, using a variational approach. It is shown that when wire radius is less than the Bohr radius of the impurity, the nonparabolicity of dispersion law of charge carriers leads to a considerable increase of the binding energy in the magnetic field, as well as to a more rapid growth of binding energy with growth ofB
Ultrafast single photon emitting quantum photonic structures based on a nano-obelisk
A key issue in a single photon source is fast and efficient generation of a single photon flux with high light extraction efficiency. Significant progress toward high-efficiency single photon sources has been demonstrated by semiconductor quantum dots, especially using narrow bandgap materials. Meanwhile, there are many obstacles, which restrict the use of wide bandgap semiconductor quantum dots as practical single photon sources in ultraviolet-visible region, despite offering free space communication and miniaturized quantum information circuits. Here we demonstrate a single InGaN quantum dot embedded in an obelisk-shaped GaN nanostructure. The nano-obelisk plays an important role in eliminating dislocations, increasing light extraction, and minimizing a built-in electric field. Based on the nano-obelisks, we observed nonconventional narrow quantum dot emission and positive biexciton binding energy, which are signatures of negligible built-in field in single InGaN quantum dots. This results in efficient and ultrafast single photon generation in the violet color region