Geometry-Assisted Topological Transitions in Spin Interferometry.

We identify a series of topological transitions occurring in electronic spin transport when manipulating spin-guiding fields controlled by the geometric shape of mesoscopic interferometers. They manifest as distinct inversions of the interference pattern in quantum conductance experiments. We establish that Rashba square loops develop weak-(anti)localization transitions (absent in other geometries as Rashba ring loops) as an in-plane Zeeman field is applied. These transitions, boosted by nonadiabatic spin scattering, prove to have a topological interpretation in terms of winding numbers characterizing the structure of spin modes in the Bloch sphere.

Spin-transistor action via tunable Landau-Zener transitions.

Spin-transistor designs relying on spin-orbit interaction suffer from low signal levels resulting from low spin-injection efficiency and fast spin decay. Here, we present an alternative approach in which spin information is protected by propagating this information adiabatically. We demonstrate the validity of our approach in a cadmium manganese telluride diluted magnetic semiconductor quantum well structure in which efficient spin transport is observed over device distances of 50 micrometers. The device is turned "off" by introducing diabatic Landau-Zener transitions that lead to a backscattering of spins, which are controlled by a combination of a helical and a homogeneous magnetic field. In contrast to other spin-transistor designs, we find that our concept is tolerant against disorder.

Half-integer filling-factor states in quantum dots.

The emergence of half-integer filling-factor states, such as upsilon=5/2 and 7/2, is found in quantum dots by using numerical many-electron methods. These states have interesting similarities and differences with their counterstates found in the two-dimensional electron gas. The upsilon=1/2 states in quantum dots are shown to have high overlaps with the composite fermion states. The lower overlap of the Pfaffian state indicates that electrons might not be paired in quantum dot geometry. The predicted upsilon=5/2 state has a high spin polarization, which may have an impact on the spin transport through quantum dot devices.

Vortex clusters in quantum dots.

We study electronic structures of two-dimensional quantum dots in strong magnetic fields using mean-field density-functional theory and exact diagonalization. Our numerically accurate mean-field solutions show a reconstruction of the uniform-density electron droplet when the magnetic field flux quanta enter one by one the dot in stronger fields. These quanta correspond to repelling vortices forming polygonal clusters inside the dot. We find similar structures in the exact treatment of the problem by constructing a conditional operator for the analysis. We discuss important differences and limitations of the methods used.

Variation in referring newborns to special care in Finland.

We studied the frequency of referring newborns to special care in Finland and its variation between hospitals. Data was obtained from the nationwide Finnish Birth Register for 1991 and 1994. Newborns who during their first week of life had been in special care, in another hospital, in a respirator, or who had received a blood exchange were defined as having been in special care. This applied to 7% of newborns in 1991 and to 8% in 1994. The proportions of newborns in special care varied notably between hospitals of the same level. The differences could be explained by varying patient mixtures and different thresholds for referral to care. The indications for referring newborns into special care, especially in hospitals with high rates of referral, requires scrutiny.

Three-dimensionally modulated graded-index diffractive elements by thermal ion exchange in glass.

Thermal ion exchange was introduced recently [Appl. Phys. Lett. 66, 917 (1995)] as a method to generate graded refractive-index distributions that permit continuous one-dimensional phase modulation in diffractive optics. We apply enhanced diffusion-modeling techniques and a novel three-step optimization procedure to design, for the first time to our knowledge, efficient three-dimensional graded-index profiles with two-dimensional phase-modulation capability. A discrete, nonseparable inversion-symmetric signal pattern is demonstrated experimentally in which the uniformity error in the intensities of the seven signal spots is less than 5%.