Theory of spin injection

Diffusive theory of spin injection is reviewed and a number of new results is presented for the dc and ac regimes. They were derived by means of the "gamma-technique" allowing to simplify the calculations by choosing the spin injection coefficients through different interfaces as the basic variables. The prospects for increasing spin injection by using resistive spin-selective contacts are emphasized and spin non-conserving contacts are introduced. Finding the basic parameters of a junction from the ac data is discussed.Comment: 4 pages, 2 column REVTeX, to be published in Proceedings of Intern. Symposium on Mesoscopic Superconductivity and Spintronics (Atsugi, March 2002

Complex impedance of a spin injecting junction

Theory of the ac spin injection from a ferromagnetic electrode into a normal conductor through a tunnel or Schottky contact is developed. Diffusion and relaxation of non-equilibrium spins results in a frequency dependent complex impedance controlled by the spin relaxation rates and the resistances involved. Explicit expression for the impedance is presented. Experimental investigation of the frequency dependence of the impedance should allow measuring spin relaxation times in both conductors, their effective resistances, and also the parameters of the contact controlling the spin injection.Comment: 3 pages, 2 column REVTeX, to appear in Appl. Phys. Let

Diffusion theory of spin injection through resistive contacts

Insertion of a resistive contact between a ferromagnetic metal and a semiconductor microstructure is of critical importance for achieving efficient spin injection into a semiconductor. However, the equations of the diffusion theory are rather cumbersome for the junctions including such contacts. A technique based on deriving a system of self-consistent equations for the coefficients of spin injection, "gamma", through different contacts are developed. These equations are concise when written in the proper notations. Moreover, the resistance of a two-contact junction can be expressed in terms of "gamma"'s of both contacts. This equation makes calculating the spin valve effect straightforward, allows to find an explicit expression for the junction resistance and to prove that its nonequilibrium part is positive. Relation of these parameters to different phenomena like spin-e.m.f. and the junction transients is established. Comparative effect of the Coulomb screening on different parameters is clarified. It is also shown that the spin non-conservation in a contact can have a dramatic effect on the non-equilibrium resistance of the junction.Comment: 16 pages, 2 column REVTeX format, minor editorial changes, references to recent papers added, to appear in Euro. Phys. Journal

Driven nonlinear dynamics of two coupled exchange-only qubits

Inspired by creation of a fast exchange-only qubit (Medford et al., Phys. Rev. Lett., 111, 050501 (2013)), we develop a theory describing the nonlinear dynamics of two such qubits that are capacitively coupled, when one of them is driven resonantly at a frequency equal to its level splitting. We include conditions of strong driving, where the Rabi frequency is a significant fraction of the level splitting, and we consider situations where the splitting for the second qubit may be the same or different than the first. We demonstrate that coupling between qubits can be detected by reading the response of the second qubit, even when the coupling between them is only of about $1\%$ of their level splittings, and calculate entanglement between qubits. Patterns of nonlinear dynamics of coupled qubits and their entanglement are strongly dependent on the geometry of the system, and the specific mechanism of inter-qubit coupling deeply influences dynamics of both qubits. In particular, we describe the development of irregular dynamics in a two-qubit system, explore approaches for inhibiting it, and demonstrate existence of an optimal range of coupling strength maintaining stability during the operational time.Comment: 11 pages, 6 figures; One additional figure with changes to the text about the results. Additional references include

Super-Resolution of Positive Sources: the Discrete Setup

In single-molecule microscopy it is necessary to locate with high precision point sources from noisy observations of the spectrum of the signal at frequencies capped by $f_c$, which is just about the frequency of natural light. This paper rigorously establishes that this super-resolution problem can be solved via linear programming in a stable manner. We prove that the quality of the reconstruction crucially depends on the Rayleigh regularity of the support of the signal; that is, on the maximum number of sources that can occur within a square of side length about $1/f_c$. The theoretical performance guarantee is complemented with a converse result showing that our simple convex program convex is nearly optimal. Finally, numerical experiments illustrate our methods.Comment: 31 page, 7 figure

Rainwater Harvesting Potential for Domestic Water Supply in Edo State

In the face of increasing scarcity of water resources, there is a need for communities to undertake audits of their current rainwater harvesting potential as a practical and promising alternative solution for water shortage. Despite the importance of rainwater harvest in socio-economic development of communities, very little information exists in the literature concerning it. This paper is an attempt to bridge this gap by examining the techniques and materials used for rainwater harvest with a focus on the geographical spread of its use and an analysis to support its wide acceptance by considering a case study from Edo State. Investigations also relate to health implications of rainwater harvest and impact on food production. Also, examined are institutional arrangements and policies guiding water supply and distribution in the state as opposed to rainwater harvest. The total volume of water supplied by the rain (in gallons), and the volume of conserved were evaluated from hydro-meteorological data collection system and through a survey in different senatorial districts of the state. The results of the analysis show that majority of the people empty their tanks mid-way into the dry season, suggesting that the current volume of the tanks is not enough to sustain the people with water during the dry season period. New constructions of bigger tanks are therefore recommended, particularly for families who use harvested rainwater for cassava processing.Rainwater, Harvesting, Edo State, Stream, Tanks