Yu. V. Nazarov

Coherent Control of a Single Electron Spin with Electric Fields

Manipulation of single spins is essential for spin-based quantum information processing. Electrical control instead of magnetic control is particularly appealing for this purpose, because electric fields are easy to generate locally on-chip. We experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate. The electric field induced coherent transitions (Rabi oscillations) between spin-up and spin-down with 90° rotations as fast as ∼55 nanoseconds. Our analysis indicated that the electrically induced spin transitions were mediated by the spin-orbit interaction. Taken together with the recently demonstrated coherent exchange of two neighboring spins, our results establish the feasibility of fully electrical manipulation of spin qubits.

Finite-element theory of transport in ferromagnet-normal metal systems

We formulate a theory of spin dependent transport in an electronic circuit involving ferromagnetic elements with noncollinear magnetization which is based on the conservation of spin and charge current. The theory considerably simplifies the calculation of the transport properties of complicated ferromagnet-normal metal systems. We illustrate the theory by considering a novel three-terminal device.

Time-dependent resonant tunneling via two discrete states

We theoretically investigate time-dependent resonant tunneling via two discrete states in an experimentally relevant setup. Our results show that the dc transport through the system can be controlled by applying external irradiation with a frequency which matches the energy difference between the discrete states. We predict resonant phenomena which should be easily observable in experiments. Time-dependent tunneling phenomena have received in- creasing attention in recent years. In the early eighties, Butt- iker and Landauer studied the tunneling time needed for an electron to traverse a potential barrier. 1 More recent theoreti- cal work focused on the time-dependence of resonant tunnel- ing using an effective Schrodinger equation 2 and on a de- scription of the time-dependent current through mesoscopic structures in terms of nonequilibrium Greens functions. 3 In addition, the considerable improvement in nanofabrication techniques facilitated some interesting experimental studies. Kouwenhoven et al. measured the photon-assisted tunneling current through a single quantum dot with an effectively con- tinuous level spectrum, due to thermal smearing. 4 van der Vaart et al. studied the dc current through a double dot sys- tem, with well developed 0D states in each dot and clearly resolved resonances between energy levels in both dots. 5 The sharp resonance features make it very tempting to perform experiments with time-dependent fields. The dc current through such a structure in the presence of oscillating fields may be expected to display interesting phenomena, not ob- servable in a single dot. Some time-dependent aspects of resonant tunneling via two wells in layered semiconductor heterostuctures have been studied in Refs. 6,7. However, the states in such stuc- tures are not really discrete and it is plausible to disregard Coulomb blockade effects. This makes it impossible to apply the results of these works to realistic ultrasmall quantum dots. In this paper, we use the density matrix approach of Ref. 8, in which the resonant states, being true quantum- mechanical many-body states of the two dots, are described by a time-dependent tunneling Hamiltonian. Transitions be- tween nonresonant states of the system are taken into ac- count through a master equation for the density matrix ele- ments. We calculate the photoresponse of the system in several experimentally relevant limits and derive an explicit

Spin-transport in multi-terminal normal metal-ferromagnet systems with non-collinear magnetizations

Abstract:A theory of spin-transport in hybrid normal metal-ferromagnetic electronic circuits is developed, taking into account non-collinear spin-accumulation. Spin-transport through resistive elements is described by 4 conductance parameters. Microscopic expression for these conductances are derived in terms of scattering matrices and explicitly calculated for simple models. The circuit theory is applied to 2-terminal and 3-terminal devices attached to ferromagnetic reservoirs.

Interference of two electrons entering a superconductor.

The subgap conductivity of a normal-superconductor (NS) tunnel junction is thought to be due to tunneling of two electrons. There is a strong interference between these two electrons, originating from the spatial phase coherence in the normal metal at a mesoscopic length scale and the intrinsic coherence of the superconductor. We evaluated the interference effect on the transport through an NS tunnel junction. We propose the layouts to observe drastic Aharonov-Bohm and Josephson effects

Spin accumulation in small ferromagnetic double-barrier junctions

The nonequilibrium spin accumulation in ferromagnetic double barrier junctions is shown to govern the transport in small structures. Transport properties of such systems are described by a generalization of the theory of the Coulomb blockade. The spin accumulation enhances the magnetoresistance. The transient nonlinear transport properties are predicted to provide a unique experimental evidence of the spin accumulation in the form of a reversed current on time scales of the order of the spin-flip relaxation time.

Universality of the Kondo effect in a quantum dot out of equilibrium

We study the Kondo effect in a quantum dot subject to an external ac field. The Kondo effect can be probed by measuring the dc current induced by an auxiliary dc bias

Circuit Theory of Unconventional Superconductor Junctions

{V}_{\mathrm{dc}}

Subgap conductivity of a superconductor–normal-metal tunnel interface

applied across the dot. In the absence of ac perturbation, the corresponding differential conductance

Enhanced Shot Noise in Resonant Tunneling via Interacting Localized States

{G(V}_{\mathrm{dc}})