A5060038211- High-pressure geophysics and materials
- Advanced Chemical Physics Studies
- Quantum, superfluid, helium dynamics
- Thermal properties of materials
- Topological Materials and Phenomena
- Advanced Thermoelectric Materials and Devices
- Semiconductor Quantum Structures and Devices
- Photorefractive and Nonlinear Optics
- Phase-change materials and chalcogenides
- Thermography and Photoacoustic Techniques
- Spectroscopy and Quantum Chemical Studies
- Astrophysics and Cosmic Phenomena
- Solid-state spectroscopy and crystallography
- Electronic and Structural Properties of Oxides
- Ferroelectric and Piezoelectric Materials
- Force Microscopy Techniques and Applications
- Graphene research and applications
- Cold Atom Physics and Bose-Einstein Condensates
- Photonic and Optical Devices
- Advanced Materials Characterization Techniques
- Graphene and Nanomaterials Applications
- Terahertz technology and applications
- Quantum Information and Cryptography
- Laser-Matter Interactions and Applications
- Geophysics and Gravity Measurements
National University of Ireland
2006-2023
Imperial College London
2016-2022
University of Cambridge
2015-2017
University of Wollongong
2014-2015
Nanyang Technological University
2014-2015
University College Cork
2004-2015
Toshiba (United Kingdom)
2015
ARC Centre of Excellence for Electromaterials Science
2014
University of California, Davis
2012-2013
Rutgers, The State University of New Jersey
2009-2010
We propose a second version of the van der Waals density functional Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing more accurate semilocal exchange and use large-$N$ asymptote gradient correction in determining vdW kernel. The predicted binding energy, equilibrium separation, potential-energy curve shape are close to those quantum chemical calculations on 22 duplexes. anticipate enabling chemically sparse materials importance for condensed matter, surface, chemical, biological physics.
Sparse matter is abundant and has both strong local bonds weak nonbonding forces, in particular nonlocal van der Waals (vdW) forces between atoms separated by empty space. It encompasses a broad spectrum of systems, like soft matter, adsorption systems biostructures. Density-functional theory (DFT), long since proven successful for dense seems now to have come point, where useful extensions sparse are available. In particular, functional form, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92...
Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high densities, the interatomic forces bind solids and determine many their properties substantially altered. Here, we present detailed mapping carrier densityâdependent potential bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques use high-brightness linear electron acceleratorâbased x-ray source...
We present a comparison of exchange-only interaction energies obtained using several standard exchange functionals in the generalized gradient approximation to Hartree-Fock results for interacting molecules. observe that with an enhancement factor 2/5 power dependence on density large gradients offer consistently better agreement calculations than alternative functionals. revisit functional offering closest and recalculate it include its exact dependence.
Composites of graphene in a chitosan-lactic acid matrix were prepared to create conductive hydrogels that are processable, exhibit tunable swelling properties and show excellent biocompatibility. The addition the polymer also resulted significant improvements mechanical strength hydrogels, with just 3 wt% resulting tensile strengths increasing by over 200%. composites could be easily processed into three-dimensional scaffolds finely controlled dimensions using additive fabrication techniques...
Squeezed states are quantum of a harmonic oscillator in which the variance two conjugate variables each oscillate out phase. Ultrafast optical excitation crystals can create squeezed phonon states, where atomic displacements oscillates due to sudden change interatomic bonding strength. With femtosecond x-ray diffraction we measure squeezing oscillations bismuth and conclude that they consistent with model electronic softens all modes by constant scaling factor.
Separate theoretical and experimental investigations of the effect lattice anharmonicity on ${A}_{1g}$ phonon dynamics in photoexcited bismuth are presented. First-principles density functional calculations show that anharmonic contribution to period is negligible for an excitation 1.25% or less valence electrons, corresponding electronic frequency softening from $2.9\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$. Experiments using...
Evidence has been mounting that many of the transverse jet B fields observed in BL Lac objects on parsec scales represent dominant toroidal component intrinsic fields. Such could come about, for example, as a result ‘winding up’ an initial ‘seed’ field with significant longitudinal by rotation central accreting object. If this is case, should give rise to gradients measure across jets, due systematic change line-of-sight field. We present evidence four objects, strengthening arguments jets...
Abstract The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV–VI, V related materials, these strong, exist near electronic structural phase transitions. prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity thermoelectricity. Here we show that it primarily electron-phonon coupling involving electron states band edges leads instability in PbTe. Using...
We present a first principles theoretical framework that accurately accounts for several properties of ice, over wide pressure range. In particular, we show that, by using recently developed nonlocal van der Waals functional and taking into account hydrogen zero point motion, one can properly describe the temperature equation state, vibrational spectra, dielectric ice at low VIII, stable phase between 2 60 GPa. While semilocal density functionals yield transition from XI to VIII is...
The phonon dispersion relations for equilibrium and photoexcited bismuth are calculated from first-principles density-functional perturbation theory, with constrained occupation of excited electronic states. dependence frequency on electron-hole plasma density is found modes throughout the Brillouin zone. resulting curves in good agreement available neutron-scattering data bands. We find effect softening by to be substantially larger optical than acoustic
Fundamental to integrated photonic quantum computing is an on-chip method for routing and modulating light emission. We demonstrate a hybrid integration platform consisting of arbitrarily designed waveguide circuits single-photon sources. InAs dots (QD) embedded in GaAs are bonded SiON chip such that the QD emission coupled mode. The waveguides core SiO2 cladding. A tuneable Mach Zehnder interferometer (MZI) modulates between two output ports can act as path-encoded qubit preparation device....
Processable graphene/polycaprolactone composites for tissue engineering were produced in a simultaneous one-step polymerisation/reduction process without toxic reducing agents.
The OH stretching vibrational spectrum of water was measured in a wide range temperatures across the triple point, 269 K <T < 296 K, using Inelastic Neutron Scattering (INS). hydrogen projected density states and proton mean kinetic energy, <EK>OH, were determined for first time within framework harmonic description dynamics. We found that liquid value <EK>OH is nearly constant as function T, indicating quantum effects on frequency are weakly dependent temperature. In case ice, ab initio...
We show how tuning the proximity to soft optical mode phase transition via chemical composition affects lattice thermal conductivity $\ensuremath{\kappa}$ of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}\mathrm{Te}$ alloys. Using first-principles virtual-crystal simulations, we find that anharmonic contribution is minimized at due maximized acoustic-optical interaction. Mass disorder significantly lowers and flattens dip in over a wide range, thus shifting minimum away from transition....
A study of highly symmetric site-controlled pyramidal In0.25Ga0.75As quantum dots (QDs) is presented. It discussed that polarization-entangled photons can be also obtained from QDs different designs the one already reported in Juska et al. [Nat. Photonics 7, 527 (2013)]. Moreover, some limitations for a higher density entangled photon emitters are addressed. Among these issues (1) remaining small fine-structure splitting and (2) an effective QD charging under non-resonant excitation...
We report channel-resolved measurements of the anharmonic coupling coherent ${A}_{1g}$ phonon in photoexcited bismuth to pairs high wave vector acoustic phonons. The decay a can be understood as parametric resonance process whereby atomic displacement periodically modulates frequency broad continuum modes. This drives temporal oscillations mean-square displacements at that are observed across Brillouin zone by femtosecond x-ray diffuse scattering. extract constants between and several...
The timescales for structural changes in a single crystal of bismuth after excitation with an intense near-infrared laser pulse are studied femtosecond pump-probe X-ray diffraction. Changes the intensity and reciprocal-lattice vector several reflections give quantitative information on structure factor lattice strain as function time, resolution 200 fs. results indicate that majority excess carrier energy remains near surface is transferred to vibrational modes timescale about 10 ps,...
The major obstacle in the design of materials with low lattice thermal conductivity is difficulty efficiently scattering phonons across entire frequency spectrum. Using first principles calculations, we show that driving PbTe to brink ferroelectric phase transition could be a powerful strategy solve this problem. We illustrate concept by applying tensile [001] strain and its alloys another rock-salt IV-VI material, PbSe; alloying rhombohedral GeTe. This induces extremely soft optical modes...
GeTe is a well-known ferroelectric and thermoelectric material that undergoes structural phase transition from rhombohedral to the rocksalt structure at $\sim 600-700$ K. We present first principles approach calculate thermal expansion of in up Curie temperature. find minimum Helmholtz free energy with respect each parameter manner similar traditional Gr{\"u}neisen theory, account for temperature dependence elastic constants. obtain variation parameters very good agreement experiments. In...
We investigate coupled electron-lattice dynamics in the topological insulator Bi2Te3 with time-resolved photoemission and x-ray diffraction. It is well established that coherent phonons can be launched by optical excitation, but selection rules generally restrict these modes to zone-center wavevectors Raman-active branches. find surface state couples additional modes, including a continuum of surface-projected bulk from both Raman- infrared-branches, possible contributions surface-localized...
Quantifying electron-phonon interactions for the surface states of topological materials can provide key insights into surface-state transport, superconductivity, and potentially how to manipulate state using a structural degree freedom. We perform time-resolved x-ray diffraction (XRD) angle-resolved photoemission (ARPES) measurements on Bi2Te3 Bi2Se3, following excitation coherent A1g optical phonons. extract compare deformation potentials coupling electronic local A1g-like displacements in...
Ferroelectric domain walls are boundaries between regions with different polarization orientations in a ferroelectric material. Using first-principles calculations, we characterize all types of forming on ($11\overline{1}$), (111), and ($1\overline{1}0$) crystallographic planes thermoelectric GeTe. We find large structural distortions the vicinity most these walls, which driven by variations. show that such strong strain-order parameter coupling will considerably reduce lattice thermal...
Recent work suggested that head-to-head and tail-to-tail domain walls could be induced to form in ferroelectric superlattices by introducing compensating ``delta doping'' layers via chemical substitution specified atomic planes [Phys. Rev. B 73, 020103(R) (2006)]. Here we investigate a variation this approach which are formed of alternately stacked groups II-IV I-V perovskite layers, the ``polar discontinuity'' at II-IV/I-V interface effectively provides delta-doping layer. Using...
Compact and electrically controllable on-chip sources of indistinguishable photons are desirable for the development integrated quantum technologies. We demonstrate that two dot light emitting diodes (LEDs) in close proximity on a single chip can function as tunable, all-electric source. Light emitted by an excited driving LED is used to excite dots neighbouring diode. The wavelength emission from driven diode tuned via confined Stark effect. also show we tune fine structure splitting.