Anomalous spin-resolved point-contact transmission of holes due to cubic Rashba spin-orbit coupling.

Evidence is presented for the finite wave vector crossing of the two lowest one-dimensional spin-split subbands in quantum point contacts fabricated from two-dimensional hole gases with strong spin-orbit interaction. This phenomenon offers an elegant explanation for the anomalous sign of the spin polarization filtered by a point contact, as observed in magnetic focusing experiments. Anticrossing is introduced by a magnetic field parallel to the channel or an asymmetric potential transverse to it. Controlling the magnitude of the spin splitting affords a novel mechanism for inverting the sign of the spin polarization.

Anisotropic modification of the effective hole g factor by electrostatic confinement.

We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexpectedly, the effective g factor does not depend on the 1D energy level number N for B parallel[01[over ]1] while it has strong N dependence for B parallel[2[over ]33]. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions, we also report qualitative differences in the appearance of spin-split plateaus, with nonquantized plateaus observed only for the confinement in the [01[over ]1] direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.

Spontaneous spin polarization in quantum point contacts.

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in the low-density regime, when the conductance of a point contact is tuned to < 2e2/h. Polarization is stronger in samples with a well-defined "0.7 structure."

Spin separation in cyclotron motion.

Charged carriers with different spin states are spatially separated in a two-dimensional hole gas. Because of strong spin-orbit interaction, holes at the Fermi energy in GaAs have different momenta for two possible spin states traveling in the same direction, and, correspondingly, different cyclotron orbits in a weak magnetic field. Two point contacts, acting as a monochromatic source of ballistic holes and a narrow detector arranged in the magnetic focusing geometry are demonstrated to work as a tunable spin filter.

Coherent electron transport in a Si quantum dot dimer.

We show that the coherence of charge transfer through a weakly coupled double-dot dimer can be determined by analyzing the statistics of the conductance pattern, and does not require a large phase coherence length in the host material. We present an experimental study of the charge transport through a small Si nanostructure, which contains two quantum dots. The transport through the dimer is shown to be coherent. At the same time, one of the dots is strongly coupled to the leads, and the overall transport is dominated by inelastic cotunneling processes.

Magnetically induced reconstruction of the ground state in a few-electron Si quantum dot.

We report unexpected fluctuations in the positions of Coulomb blockade peaks at high magnetic fields in a small Si quantum dot. The fluctuations have a distinctive sawtooth pattern: as a function of magnetic field, linear shifts of peak positions are compensated by abrupt jumps in the opposite direction. The linear shifts have large slopes, suggesting formation of the ground state with a nonzero angular momentum. The value of the momentum is found to be well defined, despite the absence of the rotational symmetry in the dot.