Time-dependent density functional theory study of charge transfer in collisions

We study the charge transfer between colliding ions, atoms, or molecules, within time-dependent density functional theory. Two particular cases are presented, the collision between a proton and a Helium atom, and between a gold atom and a butane molecule. In the first case, proton kinetic energies between 16 keV and 1.2 MeV are considered, with impact parameters between 0.31 and 1.9 angstrom. The partial transfer of charge is monitored with time. The total cross-section is obtained as a function of the proton kinetic energy. In the second case, we analyze one trajectory and discuss spin-dependent charge transfer between the different fragments.

Implementation and testing of Lanczos-based algorithms for Random-Phase Approximation eigenproblems

The treatment of the Random-Phase Approximation Hamiltonians, encountered in different frameworks, like time-dependent density functional theory or Bethe-Salpeter equation, is complicated by their non-Hermicity. Compared to their Hermitian Hamiltonian counterparts, computational methods for the treatment of non-Hermitian Hamiltonians are often less efficient and less stable, sometimes leading to the breakdown of the method. Recently [Gruning et al. Nano Lett. 8 (2009) 28201, we have identified that such Hamiltonians are usually pseudo-Hermitian. Exploiting this property, we have implemented an algorithm of the Lanczos type for Random-Phase Approximation Hamiltonians that benefits from the same stability and computational load as its Hermitian counterpart, and applied it to the study of the optical response of carbon nanotubes. We present here the related theoretical grounds and technical details, and study the performance of the algorithm for the calculation of the optical absorption of a molecule within the Bethe-Salpeter equation framework. (C) 2011 Elsevier B.V. All rights reserved.

Electron paramagnetic resonance studies of the neutral nitrogen vacancy in diamond

Despite the numerous experimental and theoretical studies on the negatively charged nitrogen vacancy center (NV-) in diamond and the predictions that the neutral nitrogen vacancy center (NV0) should have an S=1/2 ground state, NV0 has not previously been detected by electron paramagnetic resonance (EPR). We report new EPR data on a trigonal nitrogen-containing defect in diamond with an S=3/2 excited state populated via optical excitation. Analysis of the spin Hamiltonian parameters and the wavelength dependence of the optical excitation leads to assignment of this S=3/2 state to the (4)A(2) excited state of NV0. This identification, together with an examination of the electronic structure of the NV centers in diamond, provides a plausible explanation for the lack of observation (to date) of an EPR signal from the NV0 ground state.

Delay times and rates for Type Ia supernovae and thermonuclear explosions from double-detonation sub-Chandrasekhar mass models

We present theoretical delay times and rates of thermonuclear explosions that are thought to produce Type Ia supernovae (SNe Ia), including the double-detonation sub-Chandrasekhar mass model, using the population synthesis binary evolution code startrack. If detonations of sub-Chandrasekhar mass carbon-oxygen white dwarfs following a detonation in an accumulated layer of helium on the white dwarf's surface ('double-detonation' models) are able to produce thermonuclear explosions which are characteristically similar to those of SNe Ia, then these sub-Chandrasekhar mass explosions may account for at least some substantial fraction of the observed SN Ia rate. Regardless of whether all double-detonations look like 'normal' SNe Ia, in any case the explosions are expected to be bright and thus potentially detectable. Additionally, we find that the delay time distribution of double-detonation sub-Chandrasekhar mass SNe Ia can be divided into two distinct formation channels: the 'prompt' helium-star channel with delay times

2D simulations of the double-detonation model for thermonuclear transients from low-mass carbon-oxygen white dwarfs

Thermonuclear explosions may arise in binary star systems in which a carbon-oxygen (CO) white dwarf (WD) accretes helium-rich material from a companion star. If the accretion rate allows a sufficiently large mass of helium to accumulate prior to ignition of nuclear burning, the helium surface layer may detonate, giving rise to an astrophysical transient. Detonation of the accreted helium layer generates shock waves that propagate into the underlying CO WD. This might directly ignite a detonation of the CO WD at its surface (an edge-lit secondary detonation) or compress the core of the WD sufficiently to trigger a CO detonation near the centre. If either of these ignition mechanisms works, the two detonations (helium and CO) can then release sufficient energy to completely unbind the WD. These 'double-detonation' scenarios for thermonuclear explosion of WDs have previously been investigated as a potential channel for the production of Type Ia supernovae from WDs of ~ 1 M . Here we extend our 2D studies of the double-detonation model to significantly less massive CO WDs, the explosion of which could produce fainter, more rapidly evolving transients. We investigate the feasibility of triggering a secondary core detonation by shock convergence in low-mass CO WDs and the observable consequences of such a detonation. Our results suggest that core detonation is probable, even for the lowest CO core masses that are likely to be realized in nature. To quantify the observable signatures of core detonation, we compute spectra and light curves for models in which either an edge-lit or compression-triggered CO detonation is assumed to occur. We compare these to synthetic observables for models in which no CO detonation was allowed to occur. If significant shock compression of the CO WD occurs prior to detonation, explosion of the CO WD can produce a sufficiently large mass of radioactive iron-group nuclei to significantly affect the light curves. In particular, this can lead to relatively slow post-maximum decline. If the secondary detonation is edge-lit, however, the CO WD explosion primarily yields intermediate-mass elements that affect the observables more subtly. In this case, near-infrared observations and detailed spectroscopic analysis would be needed to determine whether a core detonation occurred. We comment on the implications of our results for understanding peculiar astrophysical transients including SN 2002bj, SN 2010X and SN 2005E. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

On the brightness distribution of type ia supernovae from violent white dwarf mergers

We investigate the brightness distribution expected for thermonuclear explosions that might result from the ignition of a detonation during the violent merger of white dwarf (WD) binaries. Violent WD mergers are a subclass of the canonical double degenerate scenario where two carbon-oxygen (CO) WDs merge when the larger WD fills its Roche lobe. Determining their brightness distribution is critical for evaluating whether such an explosion model could be responsible for a significant fraction of the observed population of Type Ia supernovae (SNe Ia). We argue that the brightness of an explosion realized via the violent merger model is mainly determined by the mass of Ni produced in the detonation of the primary COWD. To quantify this link, we use a set of sub-Chandrasekhar mass WD detonation models to derive a relationship between primary WD mass (m) and expected peak bolometric brightness (M). We use this m-M relationship to convert the masses of merging primary WDs from binary population models to a predicted distribution of explosion brightness. We also investigate the sensitivity of our results to assumptions about the conditions required to realize a detonation during violent mergers ofWDs. We find a striking similarity between the shape of our theoretical peak-magnitude distribution and that observed for SNe Ia: our model produces a M distribution that roughly covers the range and matches the shape of the one observed for SNe Ia. However, this agreement hinges on a particular phase of mass accretion during binary evolution: the primary WD gains ~0.15-0.35M? from a slightly evolved helium star companion. In our standard binary evolution model, such an accretion phase is predicted to occur for about 43 per cent of all binary systems that ultimately give rise to binary CO WD mergers. We also find that with high probability, violent WD mergers involving the most massive primaries (?1.3M?, which should produce bright SNe) have delay times ?500 Myr. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Global quantum correlations in finite-size spin chains

We perform an extensive study of the properties of global quantum correlations in finite-size one-dimensional quantum spin models at finite temperature. By adopting a recently proposed measure for global quantum correlations (Rulli and Sarandy 2011 Phys. Rev. A 84 042109), called global discord, we show that critical points can be neatly detected even for many-body systems that are not in their ground state. We consider the transverse Ising model, the cluster-Ising model where three-body couplings compete with an Ising-like interaction, and the nearest-neighbor XX Hamiltonian in transverse magnetic field. These models embody our canonical examples showing the sensitivity of global quantum discord close to criticality. For the Ising model, we find a universal scaling of global discord with the critical exponents pertaining to the Ising universality class.

Measuring the Characteristic Function of the Work Distribution

We propose an interferometric setting for the ancilla-assisted measurement of the characteristic function of the work distribution following a time-dependent process experienced by a quantum system. We identify how the configuration of the effective interferometer is linked to the symmetries enjoyed by the Hamiltonian ruling the process and provide the explicit form of the operations to implement in order to accomplish our task. We finally discuss two physical settings, based on hybrid optomechanical-electromechanical devices, where the theoretical proposals discussed in our work could find an experimental demonstration.

Atmospheric pressure, nonthermal plasma inactivation of MS2 bacteriophage: effect of oxygen concentration on virucidal activity

AimsThe main aim of this study was to determine the virucidal inactivation efficacy of an in-house-designed atmospheric pressure, nonthermal plasma jet operated at varying helium/oxygen feed gas concentrations against MS2 bacteriophage, widely employed as a convenient surrogate for human norovirus.

Methods and ResultsThe effect of variation of percentage oxygen concentration in the helium (He) carrier gas was studied and found to positively correlate with MS2 inactivation rate, indicating a role for reactive oxygen species (ROS) in viral inactivation. The inactivation rate constant increased with increasing oxygen concentrations up to 075% O-2. 3 log(10) (999%) reductions in MS2 viability were achieved after 3min of exposure to the plasma source operated in a helium/oxygen (9925%:075%) gas mixture, with >7 log(10) reduction after 9min exposure.

ConclusionsAtmospheric pressure, nonthermal plasmas may have utility in the rapid disinfection of virally contaminated surfaces for infection control applications.

Significance and Impact of StudyThe atmospheric pressure, nonthermal plasma jet employed in this study exhibits rapid virucidal activity against a norovirus surrogate virus, the MS2 bacteriophage, which is superior to previously published inactivation rates for chemical disinfectants.

Super-luminous Type Ic Supernovae: Catching a Magnetar by the Tail

We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely, PTF10hgi, SN2011ke, PTF11rks, SN2011kf, and SN2012il. Photometric imaging of the transients at +50 to +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase for four of these SL-SNe. A high-resolution, optical, and near-infrared spectrum from xshooter provides detection of a broad He I ?10830 emission line in the spectrum (+50 days) of SN2012il, revealing that at least some SL-SNe Ic are not completely helium-free. At first sight, the tail luminosity decline rates that we measure are consistent with the radioactive decay of 56Co, and would require 1-4 M ? of 56Ni to produce the luminosity. These 56Ni masses cannot be made consistent with the short diffusion times at peak, and indeed are insufficient to power the peak luminosity. We instead favor energy deposition by newborn magnetars as the power source for these objects. A semi-analytical diffusion model with energy input from the spin-down of a magnetar reproduces the extensive light curve data well. The model predictions of ejecta velocities and temperatures which are required are in reasonable agreement with those determined from our observations. We derive magnetar energies of 0.4 <~ E(1051 erg) lsim 6.9 and ejecta masses of 2.3 <~ M ej(M ?) lsim 8.6. The sample of five SL-SNe Ic presented here, combined with SN 2010gx—the best sampled SL-SNe Ic so far—points toward an explosion driven by a magnetar as a viable explanation for all SL-SNe Ic.

Understanding the synthesis of directly spinnable carbon nanotube forests

Carbon nanotube forests that can be spun directly from the growth substrate into pure, highly aligned webs, ribbons or yarn promise novel applications that capture the strength and other characteristics of this material. The precise conditions for high spinnability over a maximum proportion of the reactor space are extremely sensitive. The roles of catalyst, substrate, temperature, gas flow rates, reaction time with acetylene etc. were studied to identify and understand the key parameters and develop a robust, scalable process. Using a 44 mm (id) reactor, the optimum values for these variables were determined as comprising a 2.3 nm thick iron catalyst layer on a silicon substrate with 50 nm of thermal oxide; 670 °C running temperature; 650 sccm helium and 34 sccm acetylene for 20 min. The effects of deviating from these optima were explored and the role of amorphous carbon deposition clarified. Crown Copyright © 2009.

ESTIMATION OF PURITY FOR A QUANTUM HARMONIC OSCILLATOR INITIALLY PREPARED IN A DISPLACED THERMAL STATE

We address the estimation of purity for a quantum oscillator initially prepared in a displaced thermal state and probed by a suitably prepared qubit interacting with the oscillator via Jaynes-Cummings Hamiltonian without the rotating-wave approximation. We evaluate the quantum Fisher information (QFI) and show that optimal estimation of purity can be achieved by measuring the population of the qubit after a properly chosen interaction time. We also address the estimation of purity at fixed total energy and show that the corresponding precision is independent of the presence of a coherent amplitude.

The Type IIb Supernova 2011dh from a Supergiant Progenitor

A set of hydrodynamical models based on stellar evolutionary progenitors is used to study the nature of SN 2011dh. Our modeling suggests that a large progenitor star ---with R ~200 Rsun---, is needed to reproduce the early light curve of SN 2011dh. This is consistent with the suggestion that the yellow super-giant star detected at the location of the SN in deep pre-explosion images is the progenitor star. From the main peak of the bolometric light curve and expansion velocities we constrain the mass of the ejecta to be ~2 Msun, the explosion energy to be E= 6-10 x 10^50 erg, and the 56Ni mass to be approximately 0.06 Msun. The progenitor star was composed of a helium core of 3 to 4 Msun and a thin hydrogen-rich envelope of ~0.1 M_sun with a main sequence mass estimated to be in the range of 12--15 Msun. Our models rule out progenitors with helium-core masses larger than 8 Msun, which correspond to M_ZAMS > 25 Msun. This suggests that a single star evolutionary scenario for SN 2011dh is unlikely.

Nonlinear optics from an ab initio approach by means of the dynamical Berry phase: Application to second- and third-harmonic generation in semiconductors

We present an ab initio real-time-based computational approach to study nonlinear optical properties in condensed matter systems that is especially suitable for crystalline solids and periodic nanostructures. The equations of motion and the coupling of the electrons with the external electric field are derived from the Berry-phase formulation of the dynamical polarization [Souza et al., Phys. Rev. B 69, 085106 (2004)]. Many-body effects are introduced by adding single-particle operators to the independent-particle Hamiltonian. We add a Hartree operator to account for crystal local effects and a scissor operator to correct the independent particle band structure for quasiparticle effects. We also discuss the possibility of accurately treating excitonic effects by adding a screened Hartree-Fock self-energy operator. The approach is validated by calculating the second-harmonic generation of SiC and AlAs bulk semiconductors: an excellent agreement is obtained with existing ab initio calculations from response theory in frequency domain [Luppi et al., Phys. Rev. B 82, 235201 (2010)]. We finally show applications to the second-harmonic generation of CdTe and the third-harmonic generation of Si. 

Liouville-space R-matrix-Floquet description of atomic radiative processes involving autoionizing states in the presence of intense electromagnetic fields

A reduced-density-operator description is developed for coherent optical phenomena in many-electron atomic systems, utilizing a Liouville-space, multiple-mode Floquet–Fourier representation. The Liouville-space formulation provides a natural generalization of the ordinary Hilbert-space (Hamiltonian) R-matrix-Floquet method, which has been developed for multi-photon transitions and laser-assisted electron–atom collision processes. In these applications, the R-matrix-Floquet method has been demonstrated to be capable of providing an accurate representation of the complex, multi-level structure of many-electron atomic systems in bound, continuum, and autoionizing states. The ordinary Hilbert-space (Hamiltonian) formulation of the R-matrix-Floquet method has been implemented in highly developed computer programs, which can provide a non-perturbative treatment of the interaction of a classical, multiple-mode electromagnetic field with a quantum system. This quantum system may correspond to a many-electron, bound atomic system and a single continuum electron. However, including pseudo-states in the expansion of the many-electron atomic wave function can provide a representation of multiple continuum electrons. The 'dressed' many-electron atomic states thereby obtained can be used in a realistic non-perturbative evaluation of the transition probabilities for an extensive class of atomic collision and radiation processes in the presence of intense electromagnetic fields. In order to incorporate environmental relaxation and decoherence phenomena, we propose to utilize the ordinary Hilbert-space (Hamiltonian) R-matrix-Floquet method as a starting-point for a Liouville-space (reduced-density-operator) formulation. To illustrate how the Liouville-space R-matrix-Floquet formulation can be implemented for coherent atomic radiative processes, we discuss applications to electromagnetically induced transparency, as well as to related pump–probe optical phenomena, and also to the unified description of radiative and dielectronic recombination in electron–ion beam interactions and high-temperature plasmas.