Anomalous magnetoresistance peak in (110) GaAs two-dimensional holes: Evidence for Landau-level spin-index anticrossings

We measure an anomalous magnetoresistance peak within the lowest Landau level (nu = 1) minimum of a two-dimensional hole system on (110) GaAs. Self-consistent calculations of the valence band mixing show that the two lowest spin-index Landau levels anticross in a perpendicular magnetic field B consistent with where the experimental peak is measured, Bp. The temperature dependence of the anomalous peak height is interpreted as an activated behavior across this anticrossing gap. Calculations of the spin polarization in the lowest Landau levels predict a rapid switch from about -3/2 to +3/2 spin at the anticrossing. The peak position Bp is shown to be affected by the confinement electrostatics, and the utility of a tunable anticrossing position for spintronics applications is discussed.Comment: 4 pages, 4 figure

A program for calculating optimum dimensions of alpha radioisotope capsules exposed to varying stress and temperature

Method and computer program for calculating creep and optimizing dimensions of capsules filled with alpha-emitting radioisotopes and exposed to varying stress and temperatur

Electron spin orientation under in-plane optical excitation in GaAs quantum wells

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time- and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.Comment: 10 pages, 4 figures, final versio

Spin Density Matrix of Spin-3/2 Hole Systems

For hole systems with an effective spin j=3/2, we present an invariant decomposition of the spin density matrix that can be interpreted as a multipole expansion. The charge density corresponds to the monopole moment and the spin polarization due to a magnetic field corresponds to a dipole moment while heavy hole-light hole splitting can be interpreted as a quadrupole moment. For quasi two-dimensional hole systems in the presence of an in-plane magnetic field B the spin polarization is a higher-order effect that is typically much smaller than one even if the minority spin subband is completely depopulated. On the other hand, the field B can induce a substantial octupole moment which is a unique feature of j=3/2 hole systems.Comment: 8 pages, 1 figure, 3 table

Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems

We report measurements and calculations of the spin-subband depopulation, induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole systems. The results reveal that the shape of the confining potential dramatically affects the values of in-plane magnetic field at which the upper spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the carrier-carrier interaction in 2D hole systems does not significantly enhance the spin susceptibility. We interpret our findings using a multipole expansion of the spin density matrix, and suggest that the suppression of the enhancement is related to the holes' band structure and effective spin j=3/2.Comment: 6 pages, 4 figures, substantially extended discussion of result

Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells

We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on k and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.Comment: Modified version, now published. 10 pages, 3 figures, 2 table

Reentrant nu = 1 quantum Hall state in a two-dimensional hole system

We report the observation of a reentrant quantum Hall state at the Landau level filling factor nu = 1 in a two-dimensional hole system confined to a 35-nm-wide (001) GaAs quantum well. The reentrant behavior is characterized by a weakening and eventual collapse of the nu = 1 quantum Hall state in the presence of a parallel magnetic field component B||, followed by a strengthening and reemergence as B|| is further increased. The robustness of the nu = 1 quantum Hall state during the transition depends strongly on the charge distribution symmetry of the quantum well, while the magnitude of B|| needed to invoke the transition increases with the total density of the system

How red is a quantum black hole?

Radiating black holes pose a number of puzzles for semiclassical and quantum gravity. These include the transplanckian problem -- the nearly infinite energies of Hawking particles created near the horizon, and the final state of evaporation. A definitive resolution of these questions likely requires robust inputs from quantum gravity. We argue that one such input is a quantum bound on curvature. We show how this leads to an upper limit on the redshift of a Hawking emitted particle, to a maximum temperature for a black hole, and to the prediction of a Planck scale remnant.Comment: 3 pages, essay for the Gravity Research Foundatio

Spin precession and alternating spin polarization in spin-3/2 hole systems

The spin density matrix for spin-3/2 hole systems can be decomposed into a sequence of multipoles which has important higher-order contributions beyond the ones known for electron systems [R. Winkler, Phys. Rev. B \textbf{70}, 125301 (2004)]. We show here that the hole spin polarization and the higher-order multipoles can precess due to the spin-orbit coupling in the valence band, yet in the absence of external or effective magnetic fields. Hole spin precession is important in the context of spin relaxation and offers the possibility of new device applications. We discuss this precession in the context of recent experiments and suggest a related experimental setup in which hole spin precession gives rise to an alternating spin polarization.Comment: 4 pages, 2 figures, to appear in Physical Review Letter