Electric-field controlled spin reversal in a quantum dot with ferromagnetic contacts

Manipulation of the spin-states of a quantum dot by purely electrical means is a highly desirable property of fundamental importance for the development of spintronic devices such as spin-filters, spin-transistors and single-spin memory as well as for solid-state qubits. An electrically gated quantum dot in the Coulomb blockade regime can be tuned to hold a single unpaired spin-1/2, which is routinely spin-polarized by an applied magnetic field. Using ferromagnetic electrodes, however, the properties of the quantum dot become directly spin-dependent and it has been demonstrated that the ferromagnetic electrodes induce a local exchange-field which polarizes the localized spin in the absence of any external fields. Here we report on the experimental realization of this tunneling-induced spin-splitting in a carbon nanotube quantum dot coupled to ferromagnetic nickel-electrodes. We study the intermediate coupling regime in which single-electron states remain well defined, but with sufficiently good tunnel-contacts to give rise to a sizable exchange-field. Since charge transport in this regime is dominated by the Kondo-effect, we can utilize this sharp many-body resonance to read off the local spin-polarization from the measured bias-spectroscopy. We show that the exchange-field can be compensated by an external magnetic field, thus restoring a zero-bias Kondo-resonance, and we demonstrate that the exchange-field itself, and hence the local spin-polarization, can be tuned and reversed merely by tuning the gate-voltage. This demonstrates a very direct electrical control over the spin-state of a quantum dot which, in contrast to an applied magnetic field, allows for rapid spin-reversal with a very localized addressing.Comment: 19 pages, 11 figure

Spin-polarized electron tunneling across a Si delta-doped GaMnAs/n-GaAs interface

We study the spin-polarized tunneling of electrons from the valence band of GaMnAs into the conduction band of n-type GaAs with Si delta-doping at the interface. The injection of spin-polarized electrons is detected as circular polarized emission from a GaInAs/GaAs quantum well light emitting diode, corresponding to magneto-optical Kerr effect loops. The angular momentum selection rules are simplified by the strain-induced heavy-hole/light-hole splitting, allowing a direct relation between circular polarization and spin-polarization. Comparison with the influence of Zeeman splitting allow us to conclude a spin-injection from the majority spin-band

Magnetization of ultrathin (Ga,Mn)As layers

Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin (Ga0.95Mn0.05)As layers. The thinner layers (below 250 A) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 A) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers

Symmetry of magnetoconductance fluctuations of quantum dots in the nonlinear response regime

We investigate the symmetry of magnetoconductance fluctuations of phase-coherent, two-terminal quantum dots in the nonlinear regime of transport. Specifically, we consider open, ballistic quantum dots (electron billiards) with and without symmetry axes parallel and perpendicular to the current direction and formulate a set of novel symmetry relations not observed in devices with lower symmetry. We experimentally confirm these relations, demonstrating that high-quality materials and modern semiconductor technology allow the fabrication of devices with almost perfect symmetry. Small deviations from the intended symmetry, presumably due to impurities and fabrication limitations, do exist and can be detected. We also take into account circuit-induced asymmetries of the measured conductance due to bias-dependent depletion and demonstrate that this effect can be experimentally distinguished from rectification effects that are due to a lack of device symmetry. Some open questions regarding the role of a magnetic field in the nonlinear regime of transport are highlighted

Quantum Ratchets and quantum heat pumps

Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between three contributions to the non-linear current-voltage characteristics that lead to the ratchet effect: thermal excitation over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of adiabatically rocked tunneling ratchets as heat pumps

Impact of Small-Angle Scattering on Ballistic Transport in Quantum Dots

Disorder increasingly affects performance as electronic devices are reduced in size. The ionized dopants used to populate a device with electrons are particularly problematic, leading to unpredictable changes in the behavior of devices such as quantum dots each time they are cooled for use. We show that a quantum dot can be used as a highly sensitive probe of changes in disorder potential and that, by removing the ionized dopants and populating the dot electrostatically, its electronic properties become reproducible with high fidelity after thermal cycling to room temperature. Our work demonstrates that the disorder potential has a significant, perhaps even dominant, influence on the electron dynamics, with important implications for ‘‘ballistic’’ transport in quantum dots