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

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

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

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)

Two-electron atom with a screened interaction

This is the author accepted manuscript. The final version is available from the American Physical Society via the DOI in this recordWe present analytical solutions to a quantum-mechanical three-body problem in three dimensions, which describes a heliumlike two-electron atom. Similarly to Hooke's atom, the Coulombic electron-nucleus interaction potentials are replaced by harmonic potentials. The electron-electron interaction potential is taken to be both screened (decaying faster than the inverse of the interparticle separation) and regularized (in the limit of zero separation). We reveal the exactly solvable few-electron ground state, which explicitly includes electron correlation, for certain values of the harmonic containment.Centre national de la recherche scientifique (CNRS

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

Topological collective plasmons in bipartite chains of metallic nanoparticles

This is the final version. Available from the American Physical Society via the DOI in this record. We study a bipartite linear chain constituted by spherical metallic nanoparticles, where each nanoparticle supports a localized surface plasmon. The near-field dipolar interaction between the localized surface plasmons gives rise to collective plasmons, which are extended over the whole nanoparticle array. We derive analytically the spectrum and the eigenstates of the collective plasmonic excitations. At the edge of the Brillouin zone, the spectrum is of a pseudorelativistic nature similar to that present in the electronic band structure of polyacetylene. We find the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent one-dimensional Dirac-like massive bosonic excitations. Therefore, the plasmonic lattice exhibits similar effects to those found for electrons in one-dimensional Dirac materials, such as the ability for transmission with highly suppressed backscattering due to Klein tunneling. We also show that the system is governed by a nontrivial Zak phase, which predicts the manifestation of edge states in the chain. When two dimerized chains with different topological phases are connected, we find the appearance of the bosonic version of a Jackiw-Rebbi midgap state. We further investigate the radiative and nonradiative lifetimes of the collective plasmonic excitations and comment on the challenges for experimental realization of the topological effects found theoretically.CNRSAN

Topological plasmons in dimerized chains of nanoparticles: robustness against long-range quasistatic interactions and retardation effects

This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this recordWe present a simple model of collective plasmons in a dimerized chain of spherical metallic nanoparticles, an elementary example of a topologically nontrivial nanoplasmonic system. Taking into account long-range quasistatic dipolar interactions throughout the chain, we provide an exact analytical expression for the full quasistatic bandstructure of the collective plasmons. An explicit calculation of the Zak phase proves the robustness of the topological physics of the system against the inclusion of long-range Coulomb interactions, despite the broken chiral symmetry. Using an open quantum systems approach, which includes retardation through the plasmon–photon coupling, we go on to analytically evaluate the resulting radiative frequency shifts of the plasmonic spectrum. The bright plasmonic bands experience size-dependent radiative shifts, while the dark bands are essentially unaffected by the light-matter coupling. Notably, the upper transverse-polarized band presents a logarithmic singularity where the quasistatic spectrum intersects the light cone. At wavevectors away from this intersection and for subwavelength nanoparticles, the plasmon–photon coupling only leads to a quantitative reconstruction of the bandstructure and the topologically-protected states at the edge of the first Brillouin zone are essentially unaffected.Agence Nationale de la Recherche (ANR)Ministerio de Economıa y Competitivida

Unbalanced gain and loss in a quantum photonic system

This is the final version. Available from IoP Publishing via the DOI in this record. Data and materials availability: All data is available within the manuscript and the Supplementary Material.Theories in physics can provide a kind of map of the physical system under investigation, showing all of the possible types of behavior which may occur. Certain points on the map are of greater significance than others, because they describe how the system responds in a useful or interesting manner. For example, the point of resonance is of particular importance when timing the pushes onto a person sat on a swing. More sophisticatedly, so-called exceptional points have been shown to be significant in optical systems harbouring both gain and loss, as typically described by non-Hermitian Hamiltonians. However, expressly quantum points of interest—be they exceptional points or otherwise—arising in quantum photonic systems have been far less studied. Here we consider a paradigmatic model: a pair of coupled qubits subjected to an unbalanced ratio of gain and loss. We mark on its map several flavours of both exceptional and critical points, each of which are associated with unconventional physical responses. In particular, we uncover the points responsible for characteristic spectral features and for the sudden loss of quantum entanglement in the steady state. Our results provide perspectives for characterizing quantum photonic systems beyond effective non-Hermitian Hamiltonians, and suggest a hierarchy of intrinsically quantum points of interest.Royal SocietyEngineering and Physical Sciences Research Council (EPSRC

Quantum confinement in Dirac-like nanostructures

This is the author accepted manuscript. The final version is available from Academic Press via the DOI in this record In Westminster Abbey, in a nave near to Newton’s monument, lies a memorial stone to Paul Dirac. The inscription on the stone includes the relativistic wave equation for an electron: the Dirac equation. At the turn of the 21st century, it was discovered that this eponymous equation was not simply the preserve of particle physics. The isolation of graphene by Andre Geim and Konstantin Novoselov in Manchester led to the exploration of a novel class of materials – Dirac materials - whose electrons behave like Dirac particles. While the mobility of these quasi-relativistic electrons is attractive from the perspective of potential ultrafast devices, it also presents a distinct challenge: how to confine Dirac particles so as to avoid making inherently leaky devices? Here we discuss the unconventional quantum tunnelling of Dirac particles, we explain a strategy to create bound states electrostatically, and we briefly review some pioneering experiments seeking to trap Dirac electrons