Ab-initio transport theory for digital ferromagnetic heterostructures

MnAs/GaAs superlattices, made by $\delta$-doping GaAs with Mn, are known as digital ferromagnetic heterostructures. Here we present a theoretical density functional study of the electronic, magnetic and transport properties of such heterostructures. In the absence of intrinsic donors these systems show an half metallic density of states, with an exchange interaction much stronger than that of a random alloy with the same Mn concentration. {\it Ab initio} ballistic transport calculations show that the carriers with energies close to the Fermi energy are strongly confined within a few monolayers around the MnAs plane. This strong confinement is responsible for the large exchange coupling. Therefore the system can be described as a two dimensional half metal with large conductance in the MnAs plane and small conductance in the perpendicular direction

Exchange parameters from approximate self-interaction correction scheme

The approximate atomic self-interaction corrections (ASIC) method to density functional theory is put to the test by calculating the exchange interaction for a number of prototypical materials, critical to local exchange and correlation functionals. ASIC total energy calculations are mapped onto an Heisenberg pair-wise interaction and the exchange constants J are compared to those obtained with other methods. In general the ASIC scheme drastically improves the bandstructure, which for almost all the cases investigated resemble closely available photo-emission data. In contrast the results for the exchange parameters are less satisfactory. Although ASIC performs reasonably well for systems where the magnetism originates from half-filled bands, it suffers from similar problems than those of LDA for other situations. In particular the exchange constants are still overestimated. This reflects a subtle interplay between exchange and correlation energy, not captured by the ASIC.Comment: 10 page

A pertubative approach to the Kondo effect in magnetic atoms on nonmagnetic substrates

Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably these spectra are characterized by a competition between the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second order perturbation theory in the electron-spin coupling. We have now derived a third order analytic expression for the electron-spin self-energy, which can be readily used in combination with the non-equilibrium Green's function scheme for electron transport at finite bias. We demonstrate that our method is capable of quantitative description the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail

Magnetic properties of 3d-impurities substituted in GaAs

We have calculated the magnetic properties of substituted 3d-impurities (Cr-Ni) in a GaAs host by means of first principles electronic structure calculations. We provide a novel model explaining the ferromagnetic long rang order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism is shown to be due to delocalized spin-uncompensated As dangling bond electrons. Besides the quantitative prediction of the magnetic moments, our model provides an understanding of the halfmetallicity, and the raise of the critical temperature with the impurity concentration

Resonant and Kondo tunneling through molecular magnets

Transport through molecular magnets is studied in the regime of strong coupling to the leads. We consider a resonant-tunneling model where the electron spin in a quantum dot or molecule is coupled to an additional local, anisotropic spin via exchange interaction. The two opposite regimes dominated by resonant tunneling and by Kondo transport, respectively, are considered. In the resonant-tunneling regime, the stationary state of the impurity spin is calculated for arbitrarily strong molecule-lead coupling using a master-equation approach, which treats the exchange interaction perturbatively. We find that the characteristic fine structure in the differential conductance persists even if the hybridization energy exceeds thermal energies. Transport in the Kondo regime is studied within a diagrammatic approach. We show that magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias voltages.Comment: 13 pages, 5 figures, version as publishe

Urban mobility demand profiles: Time series for cars and bike-sharing use as a resource for transport and energy modeling

The transport sector is currently facing a significant transition, with strong drivers including decarbonization and digitalization trends, especially in urban passenger transport. The availability of monitoring data is at the basis of the development of optimization models supporting an enhanced urban mobility, with multiple benefits including lower pollutants and CO2 emissions, lower energy consumption, better transport management and land space use. This paper presents two datasets that represent time series with a high temporal resolution (five-minute time step) both for vehicles and bike sharing use in the city of Turin, located in Northern Italy. These high-resolution profiles have been obtained by the collection and elaboration of available online resources providing live information on traffic monitoring and bike sharing docking stations. The data are provided for the entire year 2018, and they represent an interesting basis for the evaluation of seasonal and daily variability patterns in urban mobility. These data may be used for different applications, ranging from the chronological distribution of mobility demand, to the estimation of passenger transport flows for the development of transport models in urban contexts. Moreover, traffic profiles are at the basis for the modeling of electric vehicles charging strategies and their interaction with the power grid

Inelastic scattering and heating in a molecular spin pump

We consider a model for a spin field-effect molecular transistor, where a directed pure spin current is controlled by an external electric field. Inelastic scattering effects of such molecular device are discussed within a framework of full counting statistics for a multi-level molecular system. We propose that the heating of the molecular junction can be controlled by external electric and magnetic fields. Characteristic features of the model are demonstrated by numerical calculations.Comment: 9 pages, 5 figure

Spin blockade at semiconductor/ferromagnet junctions

We study theoretically extraction of spin-polarized electrons at nonmagnetic semiconductor/ferromagnet junctions. The outflow of majority spin electrons from the semiconductor into the ferromagnet leaves a cloud of minority spin electrons in the semiconductor region near the junction, forming a local spin-dipole configuration at the semiconductor/ferromagnet interface. This minority spin cloud can limit the majority spin current through the junction creating a pronounced spin-blockade at a critical current. We calculate the critical spin-blockade current in both planar and cylindrical geometries and discuss possible experimental tests of our predictions.Comment: to be published in PR