T. L. Mitran

Magnetic behavior and clustering effects in Mn-doped boron nitride sheets

Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS.

Effects of graded distribution of scattering centers on ballistic transport

The transmission coefficient of a two dimensional scattering region connected to ideal leads was calculated for the case of electrons interacting with an inhomogeneous distribution of repulsive or attractive scattering centers. The scattering centers with Gaussian profiles were positioned at regular intervals perpendicular to the transport direction, but were spaced according to a power law along this direction. The transmission function was obtained using a scattering formalism based on the R-matrix method. The simulations revealed that although, overall, the transmission coefficient decreases and becomes almost monotonously dependent on energy as the inhomogeneity of both attractive and repulsive scattering centers increases, the redistribution of transmission between open channels depends on the type of scattering centers.

Background analysis of field-induced electron emission from nanometer-scale heterostructured emitters

Theoretical approaches to electron field emission from nanostructured cathodes often need to predict stationary vacuum currents by means of the time-dependent decay theory of metastable states. This rigorous (but practically untractable) treatment is usually mitigated by various procedures. In this work the authors present a new method based on the hypothesis of the continuity of the electron localization probability at the vacuum interface of the heterostructure. The method is compared to other conventional approaches, in terms of both the obtained vacuum probability current and field-emission current. The computed probability current is very close to that obtained from conventional approaches for the same energy spectra. However, conventional methods fail to predict the field-emission behavior from shallow-well heterostructures.

Ballistic transport in graphene Y-junctions in transverse electric field

We investigate the prospects for current modulation in single layer graphene Y-junctions in the ballistic regime, under an external electric field. Overcoming the inability of inducing field effect in graphene nanoribbons by a stacked gate, the proposed in-plane electric field setup enables a controlled current transfer between the branches of the Y-junction. This behavior is further confirmed by changing the angular incidence of the electric field. The ballistic transmission functions are calculated for the three terminal system using the non-equilibrium Greens function formalism, in the framework of density functional theory, under finite bias conditions. The edge currents dominating the transport in zigzag nanoribbons are strongly influenced by the induced dipole charge, facilitating the current modulation even for the metallic-like character of the Y-junctions. Spin polarization effects indicate the possibility of achieving spin filtering even in the absence of the external field provided the antiferromagnetic couplings between the edges are asymptotically set. Overall, our results indicate a robust behavior regarding the tunability of the charge current in the two outlet ports, showing the possibility of inducing field effect control in a single layer graphene system.

Helical graphite metamaterials for intense and locally controllable magnetic fields

We propose a novel class of bulk metamaterials, termed helical graphite (HG), which is able to produce intense magnetic fields under an external electrical bias. The nanometer-sized helical structures that make up such a material are formed by introducing periodic SDs in graphite or its associated boron nitride hybrids. The system behaves as a collection of individual, closely packed nano-solenoids, which generate magnetic field when a current flows through them. Systems based solely on carbon or hybrid carbon–boron nitride are analyzed comparatively, assessing their stability and potential in generating strong magnetic fields. Our results show that the magnetic field produced by HG structures is not only tunable but may also surpass the typical values obtained in rare-earth magnets.

Ballistic electron transport in wrinkled superlattices

Abstract Inspired by the problem of elastic wave scattering on wrinkled interfaces, we studied the scattering of ballistic electrons on a wrinkled potential energy region. The electron transmission coefficient depends on both wrinkle amplitude and periodicity, having different behaviors for positive and negative scattering potential energies. For scattering on potential barriers, minibands appear in the electron transmission, as in superlattices, whereas for scattering on periodic potential wells the transmission coefficient has a more complex form. Besides suggesting that tuning of electron transmission is possible by modifying the scattering potential via voltages on wrinkled gate electrodes, our results emphasize the analogies between ballistic electrons and elastic waves even in scattering problems on non-typical configurations.

Ballistic transport in disordered nano-ribbons

The ballistic transport properties are investigated for disordered two-dimensional nano-ribbons which contain scattering centers (impurities) with attractive and repulsive interactions. The disordered systems are modelled by arrays of Gaussian potentials which are distributed according to a given order parameter. In the framework of coherent transport, we obtain the transmission functions using a scattering formalism based on the R-matrix method. Two different qualitative behaviors are obtained for the transmission functions for several degrees of order, depending on the sign of interacting potentials.

Ab-initio investigation of point-like defects in AlN nanowires

We have studied the relaxed atomic configurations and formation energies of intrinsic and some extrinsic point-like defects in atomic-sized w?rtzite AlN wires, by means of density-functional calculations, using local basis sets. The results indicate that the most likely native defects in small AlN nanowires are interstitial N atoms, while in the case of doping with Si, the impurity is most likely to substitute an Al atom.