3,514 research outputs found
Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots
We investigate exciton spin memory in individual InAs/GaAs self-assembled
quantum dots via optical alignment and conversion of exciton polarization in a
magnetic field. Quasiresonant phonon-assisted excitation is successfully
employed to define the initial spin polarization of neutral excitons. The
conservation of the linear polarization generated along the bright exciton
eigenaxes of up to 90% and the conversion from circular- to linear polarization
of up to 47% both demonstrate a very long spin relaxation time with respect to
the radiative lifetime. Results are quantitatively compared with a model of
pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure
Optically probing the fine structure of a single Mn atom in an InAs quantum dot
We report on the optical spectroscopy of a single InAs/GaAs quantum dot (QD)
doped with a single Mn atom in a longitudinal magnetic field of a few Tesla.
Our findings show that the Mn impurity is a neutral acceptor state A^0 whose
effective spin J=1 is significantly perturbed by the QD potential and its
associated strain field. The spin interaction with photo-carriers injected in
the quantum dot is shown to be ferromagnetic for holes, with an effective
coupling constant of a few hundreds of micro-eV, but vanishingly small for
electrons.Comment: 5 pages, 3 figure
Study of the general mechanism of stress corrosion of aluminum alloys and development of techniques for its detection Quarterly report, 1 Dec. 1967 - 29 Feb. 1968
Stress corrosion of aluminum alloys and techniques for its detectio
Study of the general mechanism of stress corrosion of aluminum alloys and development of techniques for its detection Annual summary report, 2 Jun. 1967 - 1 Jun. 1968
Stress corrosion cracking of high strength aluminum alloys investigated by electrochemical, mechanical, and electron microscopic technique
Anisotropic magneto-resistance in a GaMnAs-based single impurity tunnel diode: a tight binding approach
Using an advanced tight-binding approach, we estimate the anisotropy of the
tunnel transmission associated with the rotation of the 5/2 spin of a single Mn
atom forming an acceptor state in GaAs and located near an AlGaAs tunnel
barrier. Significant anisotropies in both in-plane and out-of-plane geometries
are found, resulting from the combination of the large spin-orbit coupling
associated with the p-d exchange interaction, cubic anisotropy of heavy-hole
dispersion and the low C2v symmetry of the chemical bonds.Comment: 4 pages, 3 figure
Electronic states and optical properties of GaAs/AlAs and GaAs/vacuum superlattices by the linear combination of bulk bands method
The linear combination of bulk bands method recently introduced by Wang,
Franceschetti and Zunger [Phys. Rev. Lett.78, 2819 (1997)] is applied to a
calculation of energy bands and optical constants of (GaAs)/(AlAs) and
(GaAs)/(vacuum) (001) superlattices with n ranging from 4 to 20.
Empirical pseudopotentials are used for the calculation of the bulk energy
bands. Quantum-confined induced shifts of critical point energies are
calculated and are found to be larger for the GaAs/vacuum system. The
peak in the absorption spectra has a blue shift and splits into two peaks for
decreasing superlattice period; the transition instead is found to be
split for large-period GaAs/AlAs superlattices. The band contribution to linear
birefringence of GaAs/AlAs superlattices is calculated and compared with recent
experimental results of Sirenko et al. [Phys. Rev. B 60, 8253 (1999)]. The
frequency-dependent part reproduces the observed increase with decreasing
superlattice period, while the calculated zero-frequency birefringence does not
account for the experimental results and points to the importance of
local-field effects.Comment: 10 pages, 11 .eps figures, 1 tabl
Spin-orbit coupling and intrinsic spin mixing in quantum dots
Spin-orbit coupling effects are studied in quantum dots in InSb, a narrow-gap
material. Competition between different Rashba and Dresselhaus terms is shown
to produce wholesale changes in the spectrum. The large (and negative)
-factor and the Rashba field produce states where spin is no longer a good
quantum number and intrinsic flips occur at moderate magnetic fields. For dots
with two electrons, a singlet-triplet mixing occurs in the ground state, with
observable signatures in intraband FIR absorption, and possible importance in
quantum computation.Comment: REVTEX4 text with 3 figures (high resolution figs available by
request). Submitted to PR
Anything You Can Do, You Can Do Better: Neural Substrates of Incentive-Based Performance Enhancement
Performance-based pay schemes in many organizations share the fundamental assumption that the performance level for a given task will increase as a function of the amount of incentive provided. Consistent with this notion, psychological studies have demonstrated that expectations of reward can improve performance on a plethora of different cognitive and physical tasks, ranging from problem solving to the voluntary regulation of heart rate. However, much less is understood about the neural mechanisms of incentivized performance enhancement. In particular, it is still an open question how brain areas that encode expectations about reward are able to translate incentives into improved performance across fundamentally different cognitive and physical task requirements
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