Magnetic quantum dots and rings in two dimensions

This is the final version of the article. Available from the publisher via the DOI in this record.We consider the motion of electrons confined to a two-dimensional plane with an externally applied perpendicular inhomogeneous magnetic field, both with and without a Coulomb potential. We find that as long as the magnetic field is slowly decaying, bound states in magnetic quantum dots are indeed possible. Several example cases of such magnetic quantum dots are considered in which one can find the eigenvalues and eigenfunctions in closed form, including two hitherto unknown quasi-exactly-solvable models treated with confluent and biconfluent Heun polynomials. It is shown how a modulation of the strength of the magnetic field can exclude magnetic vortexlike states, rotating with a certain angular momenta and possessing a definite spin orientation, from forming. This indicates one may induce localization-delocalization transitions and suggests a mechanism for spin separation

Radiative frequency shifts in nanoplasmonic dimers

This is the author accepted manuscript. The final version is available from APS via the DOI in this record.We study the effect of the electromagnetic environment on the resonance frequency of plasmonic excitations in dimers of interacting metallic nanoparticles. The coupling between plasmons and vacuum electromagnetic fluctuations induces a shift in the resonance frequencies, analogous to the Lamb shift in atomic physics, which is usually not measurable in an isolated nanoparticle. In contrast, we show that this shift leads to sizeable corrections to the level splitting induced by dipolar interactions in nanoparticle dimers. The ratio between the level splitting for the longitudinal and transverse hybridized modes takes a universal form dependent only on the interparticle distance and thus is highly insensitive to the precise fabrication details of the two nanoparticles. We discuss the possibility to successfully perform the proposed measurement using state-of-the-art nanoplasmonic architectures.This work was partially funded by the Agence Nationale de la Recherche (Project ANR-14-CE26-0005 Q-MetaMat), the Centre National de la Recherche Scientifique through the Projet International de Cooperation Scientifique program (Contract Nr. 6384 APAG), the Leverhulme Trust (Research Project Grant RPG-2015-101), and the Royal Society (International Exchange Grant Nr. IE140367, Newton Mobility Grants 2016/R1 UK-Brazil, and Theo Murphy Award TM160190)

Magnetic coupling constants for MnO as calculated using hybrid density functional theory

The properties of MnO have been calculated using generalised gradient approximation (GGA-) and hybrid (h-) density functional theory (DFT), specifically variants of the popular PBE and PBESol exchange–correlation functionals. The GGA approaches are shown to be poor at reproducing experimental magnetic coupling constants and rhombohedral structural distortions, with the PBESol functional performing worse than PBE. In contrast, h-DFT results are in reasonable agreement with experiment. Calculation of the Néel temperatures using the mean-field approximation gives overestimates relative to experiment, but the discrepancies are as low as 15 K for the PBE0 approach and, generally, the h-DFT results are significant improvements over previous theoretical studies. For the Curie–Weiss temperature, larger disparities are observed between the theoretical results and previous experimental results

Investigating catalytic activity at oxide surfaces using a QM/MM methodology

A set of complementary studies has been undertaken to investigate the interaction of CO2 with metal oxide surfaces. Beginning with the simple and well-studied magnesium oxide surface, work progressed to include a manganese dopant near the MgO active site before shifting to manganese oxide. All work made use of the Quantum Mechanical/Molecular Mechanical (QM/MM) methodology implemented within the ChemShell code, which combines information from an electronic structure calculation on atoms in the vicinity of the adsorption site with relaxation effects from a large component of the surrounding catalyst system. Initial findings showed that CO2 interacts favourably with the MgO (100) terrace, and that the presence of trapped electrons at surface oxygen vacancies opens up the possibility for catalytic chemical processes to occur. Particular attention was paid to the CO2 radical anion species formed when the adsorbate binds to a vacancy containing a single electron, and the addition of hydrogen to the surface-adsorbate complex allowed for a number of catalytic cycles to be identi- fied. Manganese doping was used to investigate the effect of a transition metal on the interaction between the adsorbate and the vacancy, before moving on to the transition metal oxide where more complex effects such as lattice distortion and antiferromagnetic ordering were included in the model. Finally, work was performed on the related system Li-doped MgO in order to investigate an open question regarding the activation barrier for methyl radical formation as part of the oxidative coupling of methane reaction

Trapping Charge Carriers in Low-Dimensional Dirac Materials

This is the author accepted manuscript. The final version is available from World Scientific Publishing via the DOI in this recordWe consider the problem of confining the famously elusive Dirac-like quasiparticles, as found in some recently discovered low-dimensional systems. After briefly surveying the existing theoretical proposals for creating bound states in Dirac materials, we study relativistic excitations with a position-dependent mass term. With the aid of an exactly-solvable model, we show how bound states begin to emerge after a critical condition on the size of the mass term is met. We also reveal some exotic properties of the unusual confinement discovered, including an elegant chevron structure of the bound state energies as a function of the size of the mass.Juan de la Cierva program (MINECO, Spain)EU H2020 RISE project CoExANITMO Fellowship and Professorship Progra