Atomistic Simulation Centre School of Mathematics and Physics Queen's University Belfast University Road Belfast BT7 1NN Northern Ireland

Degrees, Awards and Honours

    Interests

      Most Recent Publications

      1. Evaluation of La1−xSrxMnO3 (0 ≤ x < 0.4) synthesised via a modified sol–gel method as mediators for magnetic fluid hyperthermia, CrystEngComm, 2016, 18, No. 3, pp. 407
        doi: 10.1039/C5CE01890K Abstract Full Text

        A range of lanthanum strontium manganates (La1−xSrxMnO3–LSMO) where 0 ≤ x < 0.4 were prepared using a modified peroxide sol–gel synthesis method. The magnetic nanoparticle (MNP) clusters obtained for each of the materials were characterised using scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and infra-red (IR) spectroscopy in order to confirm the crystalline phases, crystallite size and cluster morphology. The magnetic properties of the materials were assessed using the Superconducting quantum interference device (SQUID) to evaluate the magnetic susceptibility, Curie temperature (Tc) and static hysteretic losses. Induction heating experiments also provided an insight into the magnetocaloric effect for each material. The specific absorption rate (SAR) of the materials was evaluated experimentally and via numerical simulations. The magnetic properties and heating data were linked with the crystalline structure to make predictions with respect to the best LSMO composition for mild hyperthermia (41 °C ≤ T ≤ 46 °C). La0.65Sr0.35MnO3, with crystallite diameter of 82.4 nm, (agglomerate size of ∼10 μm), Tc of 89 °C and SAR of 56 W gMn−1 at a concentration 10 mg mL−1 gave the optimal induction heating results (Tmax of 46.7 °C) and was therefore deemed as most suitable for the purposes of mild hyperthermia, vide infra.

      2. Electron-phonon thermalization in a scalable method for real-time quantum dynamicshttp://dx.doi.org/10.1039/C5CE01890K, Physical Review B, 2016, 93, No. 2
        doi: 10.1103/PhysRevB.93.024306 Abstract Full Text

        We present a quantum simulation method that follows the dynamics of out-of-equilibrium many-body systems of electrons and oscillators in real time. Its cost is linear in the number of oscillators and it can probe time scales from attoseconds to hundreds of picoseconds. Contrary to Ehrenfest dynamics, it can thermalize starting from a variety of initial conditions, including electronic population inversion. While an electronic temperature can be defined in terms of a nonequilibrium entropy, a Fermi-Dirac distribution in general emerges only after thermalization. These results can be used to construct a kinetic model of electron-phonon equilibration based on the explicit quantum dynamics.

      3. High-order-harmonic generation in benzene with linearly and circularly polarized laser pulseshttp://dx.doi.org/10.1103/PhysRevB.93.024306, Physical Review A, 2016, 93, pp. 023428
        doi: 10.1103/PhysRevA.93.023428 Abstract
        High-order-harmonic generation in benzene is studied using a mixed quantum-classical approach in which the electrons are described using time-dependent density-functional theory while the ions move classically. The interaction with both linearly and circularly polarized infrared (λ=800 nm) laser pulses of duration of ten cycles (26.7 fs) is considered. The effect of allowing the ions to move is investigated as is the effect of including self-interaction corrections to the exchange-correlation functional. Our results for circularly polarized pulses are compared with previous calculations in which the ions were kept fixed and self-interaction corrections were not included, while our results for linearly polarized pulses are compared with both previous calculations and experiment. We find that even for the short-duration pulses considered here, the ionic motion greatly influences the harmonic spectra. While ionization and ionic displacements are greatest when linearly polarized pulses are used, the response to circularly polarized pulses is almost comparable, in agreement with previous experimental results.

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      Lorenzo Stella

      Lecturer in Atomistic Simulation

      Office

      David Bates Building (DBB)
      Second floor
      Room 02.019

      Address

      Atomistic Simulation Centre (ASC)
      Department of Physics and Astronomy
      School of Mathematics and Physics
      Queen's University Belfast
      University Road
      Belfast BT7 1NN
      Northern Ireland

      Contact information

      Tel: +44 (0) 28 9097 6022
      Fax: +44 (0) 28 9097 5359
      Email: l.stella@qub.ac.uk

      1. Correlated electron-ion dynamics (CEID) and electronic decoherence

      As long with many other properties, the colour of molecules and solids can be probed with light sources, from lamps to lasers. In fact, the colour of matter can be understood and predicted from an atomistic point of view, i.e, by studying the motion of the electrons and the nuclei molecules and solids are made of. In most cases, it is fine to neglect the nuclear motion, as the atomic nuclei are much heavier than the electrons. However, this is not always the case, as neglecting the nuclear motion might lead to very wrong predictions, e.g., for the 'smart' materials organic solar cells and organic LEDs are made of! To fix this issue, here at the ASC we have developed a novel and powerful numerical techniques, called Correlated Electron-Ion Dynamics (CEID), which I like to apply to all the difficult cases in which the electronic and nuclear motion cannot be easily disentangled.

      The variant of the CEID I have devised employs a convergent expansion of the quantum fluctuations of the nuclei about their mean-field motion to provide an accurate, yet affordable, numerical solution of the nonadiabatic Schroedinger problem. This variant of CEID has been successfully tested for semi-empirical models of short molecules and conjugated polymers (in collaboration with A.P. Horsfield and A.J. Fisher) Recently, CEID has been also found useful to study the electronic decoherence in low dimensional systems from an atomistic point of view (in collaboration with I. Franco and H. Appel). An open-source CEID code based on my approach can be downloaded from this repository.

      Relevant Papers

      1. Title: Analog of Rabi oscillations in resonant electron-ion systems

        Author(s): Stella L., Miranda R.P., Horsfield A.P., Fisher A.J.,

        The Journal of Chemical Physics, 134, No. 19, Art. No. 194105 (21 May 2011)

      2. Title: Robust nonadiabatic molecular dynamics for metals and insulators

        Author(s): Stella L., Meister M., Fisher A. J., Horsfield A. P.

        Journal of Physical Chemistry, 127, No. 21, Art. No. 214104 (4 December 2007)

      Evolution of the electronic purity (a measure of the electronic coherence) for a model conjugated molecule. The electronic coherence gradually decreases with time (decoherence) through a series of repeated 'revivals', see inset. By CEID one can model this subtle electron-nuclear correlation effect.

      2. Quantum plasmonics

      We are all familiar with electronic devices, which exploit the possibility to control the electron flux in metals and semiconductors. Plasmonic devices also take advantage of the electron motion in metals. However, this is a very peculiar kind of motion which originates from the coherent oscillations of many electrons, also known as plasmons. Surface plasmons are oscillations which take place at the interface between a metal and a dielectric (i.e., air) and they can be easily generated and controlled using a laser. For this reason, plasmonic devices are good candidates for faster (i.e., terahertz) and smaller (i.e., nanoscale) components for cheap and reliable communication technologies.

      To faithfully model the plasmonic response at the nanoscale, we use a simple and robust jellium approximation along with a Time-Dependent Density-Functional Theory (TDDFT) description of the electron dynamics. Hence, we can include the contributions from the quantum delocalisation of the electron in a transferable and unbiased way (in collaboration with P. Zhang, F.J. García Vidal, and P. García Gonzalez). Our method scales favourably with the size of the system and is suited to model the plasmonic response of systems beyond the possibilities of atomistic simulations.

      Relevant Papers

      1. Title: Performance of Nonlocal Optics When Applied to Plasmonic Nanostructures

        Author(s): Stella L., Zhang P., Garcia-Vidal F.J., Rubio A., Garcia-Gonzalez P.,

        The Journal of Physical Chemistry C, 117, No. 17, pp. 8941- 8949 (2 May 2013)

      Field enhancement between a pair of sodium nanowires. At variance with classical local and semi-local approximations, the jellium/TDDFT approach yields an enhancement localised in the gap between the nanowires. This is a consequence of the quantum delocalisation of the electrons.

      3. Generalised Langevin Equation (GLE) and out-of-equilibrium molecular dynamics

      For the sake of simplicity, when we observe a physical system, e.g., a piece of solid, we quietly assume that this system is just weakly coupled to the rest of the universe. However, this assumption is just an approximation and in principle there is always an exchange of energy from the system to its environment, e.g., a heated piece of solid, however well isolated, will eventually cool down to room temperature. This unavoidable exchange of energy between the system and its environment also acts at molecular and atomic scales. The Generalised Langevin Equation formalism provides an invaluable tool to model realistic environments in an efficient, yet accurate way.

      For a rather wide class of solids, a GLE can be derived from the classical Lagrangian of the system and its environment. I have recently devised an efficient molecular dynamics algorithm to integrate such atomistic GLE to model the microscopic dynamics of systems driven out-of-equilibrium (in collaboration with L. Kantorovich and C. Lorenz). In the near future, I plan to apply this method to investigate the impact of non-trivial system-environment correlations to the system's dissipative dynamics, e.g., to accurately model heat generation and transfer in nanostructures.

      Relevant Papers

      1. Title: Generalized Langevin equation: An efficient approach to nonequilibrium molecular dynamics of open systems

        Author(s): Stella L., Lorenz C.D., Kantorovich L.,

        Physical Review B, 89, pp. 134303- (7 April 2014)

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