Recent Publications

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  1. Title: Evaluation of La1−xSrxMnO3 (0 ≤ x < 0.4) synthesised via a modified sol–gel method as mediators for magnetic fluid hyperthermia

    Author(s): McBride K., Cook J., Gray S., Felton S., Stella L., Poulidi D.

    CrystEngComm, 18, No. 3, pp. 407-416 (21 January 2016)

    doi: 10.1039/C5CE01890K

    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. Title: Electron-phonon thermalization in a scalable method for real-time quantum dynamics

    Author(s): Rizzi V., Todorov T.N., Kohanoff J.J., Correa A.A.,

    Physical Review B, 93, No. 2 (27 January 2016)

    doi: 10.1103/PhysRevB.93.024306

    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. Title: High-order-harmonic generation in benzene with linearly and circularly polarized laser pulses

    Author(s): Wardlow A., Dundas D.

    Physical Review A, 93, pp. 023428- (25 February 2016)

    doi: 10.1103/PhysRevA.93.023428

    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.

  4. Title: Efficient simulations with electronic open boundaries

    Author(s): Horsfield A.P., Boleininger M., D'Agosta R., Iyer V., Thong A., Todorov T.N., White C.

    Physical Review B, 94, pp. 075118- (10 August 2016)

    doi: 10.1103/PhysRevB.94.075118

    We present a reformulation of the hairy-probe method for introducing electronic open boundaries that is appropriate for steady-state calculations involving nonorthogonal atomic basis sets. As a check on the correctness of the method we investigate a perfect atomic wire of Cu atoms and a perfect nonorthogonal chain of H atoms. For both atom chains we find that the conductance has a value of exactly one quantum unit and that this is rather insensitive to the strength of coupling of the probes to the system, provided values of the coupling are of the same order as the mean interlevel spacing of the system without probes. For the Cu atom chain we find in addition that away from the regions with probes attached, the potential in the wire is uniform, while within them it follows a predicted exponential variation with position. We then apply the method to an initial investigation of the suitability of graphene as a contact material for molecular electronics. We perform calculations on a carbon nanoribbon to determine the correct coupling strength of the probes to the graphene and obtain a conductance of about two quantum units corresponding to two bands crossing the Fermi surface. We then compute the current through a benzene molecule attached to two graphene contacts and find only a very weak current because of the disruption of the π conjugation by the covalent bond between the benzene and the graphene. In all cases we find that very strong or weak probe couplings suppress the current.

  5. Title: Origin of scale-free intermittency in structural first-order phase transitions

    Author(s): Perez-Reche F.J., Triguero C., Zanzotto G., Truskinovsky L.

    Physical Review B, 94, No. 14, pp. 144102- (1 October 2016)

    doi: 10.1103/PhysRevB.94.144102

    Full Text

    A salient feature of cyclically driven first-order phase transformations in crystals is their scale-free avalanche dynamics. This behavior has been linked to the presence of a classical critical point but the mechanism leading to criticality without extrinsic tuning remains unexplained. Here we show that the source of scaling in such systems is an annealed disorder associated with transformation-induced slip which coevolves with the phase transformation, thus ensuring the crossing of a critical manifold. Our conclusions are based on a model where annealed disorder emerges in the form of a random field induced by the phase transition. Such a disorder exhibits supertransient chaotic behavior under thermal loading, obeys a heavy-tailed distribution, and exhibits long-range spatial correlations. We show that the universality class is affected by the long-range character of elastic interactions. In contrast, it is not influenced by the heavy-tailed distribution and spatial correlations of disorder.

  6. Title: Efficient Blue Electroluminescence Using Quantum-Confined Two-Dimensional Perovskites

    Author(s): Kumar S., Jagielski J., Yakunin S., Rice P., Chiu Y., Wang M., Nedelcu G., Kim Y., Lin S., Santos E.J.G., Kovalenko M.V., Shih C.,

    ACS Nano (29 September 2016)

    doi: 10.1021/acsnano.6b05775

    Full Text

    Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.

  7. Title: Multiscale Analysis for Field-Effect Penetration through Two-Dimensional Materials

    Author(s): Tian T., Rice P. Santos E.J.G., Shih C.,

    ACS Nano Letters, 16, No. 8, pp. 5044-5052 (10 August 2016)

    doi: 10.1021/acs.nanolett.6b01876

    Full Text

    Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, whereas the penetration of the field effect through the one-molecule-thick material is often ignored or oversimplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson–Boltzmann equation methods to model penetration of the field effect through graphene in a metal–oxide–graphene–semiconductor (MOGS) QC, including quantifying the degree of “transparency” for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversion layer at the semiconductor/graphene interface. The degree of transparency is determined by the combined effect of graphene quantum capacitance and the semiconductor capacitance, which allows us to predict the ranking for a variety of monolayer 2D materials according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > phosphorene > MoSe2 > MoTe2, when the majority carrier is electron. Our findings reveal a general picture of operation modes and design rules for the 2D-materials-based QCs.

  8. Title: Length Matters: Keeping Atomic Wires in Check

    Author(s): Cunningham B., Todorov T.N., Dundas D.

    MRS Proceedings, 1753 (2015)

    doi: 10.1557/opl.2015.197

    Dynamical effects of non-conservative forces in long, defect free atomic wires are investigated. Current flow through these wires is simulated and we find that during the initial transient, the kinetic energies of the ions are contained in a small number of phonon modes, closely clustered in frequency. These phonon modes correspond to the waterwheel modes determined from preliminary static calculations. The static calculations allow one to predict the appearance of non-conservative effects in advance of the more expensive real-time simulations. The ion kinetic energy redistributes across the band as non-conservative forces reach a steady state with electronic frictional forces. The typical ion kinetic energy is found to decrease with system length, increase with atomic mass, and its dependence on bias, mass and length is supported with a pen and paper model. This paper highlights the importance of non-conservative forces in current carrying devices and provides criteria for the design of stable atomic wires.

  9. Title: Strong second harmonic generation in SiC, ZnO, GaN two-dimensional hexagonal crystals from first-principles many-body calculations

    Author(s): Attaccalite C., Nguer A., Cannuccia E., Gruning M.

    Physical Chemistry Chemical Physics (25 February 2015)

    doi: 10.1039/C5CP00601E

    The second harmonic generation (SHG) intensity spectrum of SiC, ZnO, GaN two-dimensional hexagonal crystals is calculated by using a real-time first-principles approach based on Green's function theory [Attaccalite \textit{et al.,Phys Rev B} 2013 \textbf{88}, 235113 ]. This approach allows one to go beyond the independent particle description used in standard first-principles nonlinear optics calculations by including quasiparticle corrections (by means of the $GW$ approximations), crystal local field effects and excitonic effects. Our results show that the SHG spectra obtained by the latter approach differ significantly from their independent particle counterparts. In particular they show strong excitonic resonances at which the SHG intensity is about two times stronger than within the independent particle approximation. All the systems studied (whose stability have been predicted theoretically) are transparent and at the same time exhibit a remarkable SHG intensity in the range of frequencies at which Ti:Sapphire and Nd:YAG lasers operates thus they can be of interest for nanoscale nonlinear frequency conversion devices. Specifically the SHG intensity at 800 nm (1.55 eV) ranges from about 40-80 pm/V in ZnO and GaN to 0.6 nm/V in SiC. The latter value in particular is 1 order of magnitude larger than values in standard nonlinear crystals.

  10. Title: Applications of the generalized Langevin equation: Towards a realistic description of the baths

    Author(s): Ness H., Stella L., Lorenz c.D., Kantorovich L.

    Physical Review B, 91, pp. 014301- (5 January 2015)

    doi: 10.1103/PhysRevB.91.014301

    The generalized Langevin equation (GLE) method, as developed previously [L. Stella et al., Phys. Rev. B 89, 134303 (2014)], is used to calculate the dissipative dynamics of systems described at the atomic level. The GLE scheme goes beyond the commonly used bilinear coupling between the central system and the bath, and permits us to have a realistic description of both the dissipative central system and its surrounding bath. We show how to obtain the vibrational properties of a realistic bath and how to convey such properties into an extended Langevin dynamics by the use of the mapping of the bath vibrational properties onto a set of auxiliary variables. Our calculations for a model of a Lennard-Jones solid show that our GLE scheme provides a stable dynamics, with the dissipative/relaxation processes properly described. The total kinetic energy of the central system always thermalizes toward the expected bath temperature, with appropriate fluctuation around the mean value. More importantly, we obtain a velocity distribution for the individual atoms in the central system which follows the expected canonical distribution at the corresponding temperature. This confirms that both our GLE scheme and our mapping procedure onto an extended Langevin dynamics provide the correct thermostat. We also examined the velocity autocorrelation functions and compare our results with more conventional Langevin dynamics.