Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits

A quantum dot spin LED provides a test of carrier spin injection into a qubit, as well as a means of analyzing carrier spin injection in general and local spin polarization. The polarization of the observed light is, however, significantly influenced by the dot geometry so the spin may be more polarized than the emitted light would naively suggest. We have calculated carrier polarization-dependent optical matrix elements using 8-band strain-dependent k.p theory for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and hole spin injection into a range of quantum dot sizes and shapes, and for arbitrary emission directions. The observed circular polarization does not depend on whether the injected spin-polarized carriers are electrons or holes, but is strongly influenced by the SAQD geometry and emission direction. Calculations for typical SAQD geometries with emission along [110] show light that is only ~5% circularly polarized for spin states that are 100% polarized along [110]. Therefore observed polarizations [Chye et al. PRB 66, 201301(R)] of ~1% imply a spin polarization within the dot of ~20%. We also find that measuring along the growth direction gives near unity conversion of spin to photon polarization, and is the least sensitive to uncertainties in SAQD geometry.Comment: 4 pages, 6 figure

Nuclear spin pumping and electron spin susceptibilities

In this work we present a new formalism to evaluate the nuclear spin dynamics driven by hyperfine interaction with non-equilibrium electron spins. To describe the dynamics up to second order in the hyperfine coupling, it suffices to evaluate the susceptibility and fluctuations of the electron spin. Our approach does not rely on a separation of electronic energy scales or the specific choice of electronic basis states, thereby overcoming practical problems which may arise in certain limits when using a more traditional formalism based on rate equations.Comment: 9 pages, 2 figure

Interaction Effects in Conductivity of Si Inversion Layers at Intermediate Temperatures

We compare the temperature dependence of resistivity \rho(T) of Si MOSFETs with the recent theory by Zala et al. This comparison does not involve any fitting parameters: the effective mass m* and g*-factor for mobile electrons have been found independently. An anomalous increase of \rho with temperature, which has been considered a signature of the "metallic" state, can be described quantitatively by the interaction effects in the ballistic regime. The in-plane magnetoresistance \rho(B) is qualitatively consistent with the theory; however, the lack of quantitative agreement indicates that the magnetoresistance is more susceptible to the sample-specific effects than \rho(T).Comment: 4 pages, 5 figures. References update

Reply to Comment on "Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high mobility (100) Si-inversion layers"

Reply to the Comment by V.T. Dolgopolov and A.V. Gold [cond-mat/0203276].Comment: 2 page

The peer model advantage in infants’ imitation of familiar gestures performed by differently aged models

Infants’ imitation of differently aged models has been predominately investigated with object-related actions and so far has lead to mixed evidence. Whereas some studies reported an increased likelihood of imitating peer models in contrast to adult models, other studies reported the opposite pattern of results. In the present study, 14-month-old infants were presented with four familiar gestures (e.g., clapping) that were demonstrated by differently aged televised models (peer, older child, adult). Results revealed that infants were more likely to imitate the peer model than the older child or the adult. This result is discussed with respect to a social function of imitation and the mechanism of imitating familiar behavior

Quantum Dot as Spin Filter and Spin Memory

We consider a quantum dot in the Coulomb blockade regime weakly coupled to current leads and show that in the presence of a magnetic field the dot acts as an efficient spin-filter (at the single-spin level) which produces a spin-polarized current. Conversely, if the leads are fully spin-polarized the up or down state of the spin on the dot results in a large sequential or small cotunneling current, and thus, together with ESR techniques, the setup can be operated as a single-spin memory.Comment: 4 pages, 3 figures, REVTe