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  • Erhart, Paul, 1978- (författare)
  • A first-principles study of helium storage in oxides and at oxide-iron interfaces
  • 2012
  • Ingår i: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 111:11
  • Tidskriftsartikel (refereegranskat)abstract
    • Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides, He migration is significantly slower and He-He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe-MgO, Fe-FeO-MgO). In both cases, the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.
  • Alegret, Joan, 1977- (författare)
  • Numerical Simulations of Plasmonic Nanostructures
  • 2008
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • This thesis focuses on the study of metallic nanostructures that support plasmons. Special emphasis is devoted to two specific numerical methods that allow us to predict plasmon characteristics: the discrete dipole approximation (DDA) and the Green's tensor (GT) method.DDA is an approximate method that produces fast and accurate results, but it can only be applied to systems in which the nanostructure is situated in a homogeneous background. In this thesis, DDA has been applied to predict the field enhancement and field decay around nano-rings, showing that the structure is well suited for biosensing; to obtain the spectral characteristics of silver trimers, showing that the actual plasmon modes are closely related to symmetry-adapted coordinates derived from group-theory; and to calculate the optical forces between two spherical particles illuminated by a plane wave, showing that the illumination wavelength determines the separation between the particles.The GT method, on the other hand, is an exact method, in the sense that the system can be solved to arbitrary precision depending on the size of the discretization elements. Its major drawback is the long time it takes to perform the calculations. To tis end, this thesis introduces a novel algorithm, called the top-down extended meshing algorithm (TEMA), that speeds up GT calculations by reducing the number of elements in the discretization process. This decreases the total time needed to perform the calculations, while keeping the precision of the result essentially unaltered. The GT method with TEMA meshes has successfully been used to study single holes of different sizes and shapes (circular and ellipsoidal) in the near- and far-field regime, as well as hole pairs as a function of their separation distance. The results compare very well with experiments, demonstration that the GT method is well suited for predicting the behavior of nano-holes.
  • Forssen, C., et al. (författare)
  • The Ab Initio No-core Shell Model
  • 2009
  • Ingår i: Workshop on Critical Stability of Few-Body Quantum Systems,Erice, Italy,2008-10-10. - Springer.
  • Konferensbidrag (refereegranskat)abstract
    • This contribution reviews a number of applications of the ab initio no-core shell model (NCSM) within nuclear physics and beyond. We will highlight a nuclear-structure study of the A = 12 isobar using a chiral NN + 3NF interaction. In the spirit of this workshop we will also mention the new development of the NCSM formalism to describe open channels and to approach the problem of nuclear reactions. Finally, we will illustrate the universality of the many-body problem by presenting the recent adaptation of the NCSM effective-interaction approach to study the many-boson problem in an external trapping potential with short-range interactions.
  • Jiao, Yang, 1985-, et al. (författare)
  • Signatures of van der Waals binding: A coupling-constant scaling analysis
  • 2018
  • Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:8
  • Tidskriftsartikel (refereegranskat)abstract
    • The van der Waals (vdW) density functional (vdW-DF) method [Rep. Prog. Phys. 78, 066501 (2015)RPPHAG0034-488510.1088/0034-4885/78/6/066501] describes dispersion or vdW binding by tracking the effects of an electrodynamic coupling among pairs of electrons and their associated exchange-correlation holes. This is done in a nonlocal-correlation energy term Ecnl, which permits density functional theory calculation in the Kohn-Sham scheme. However, to map the nature of vdW forces in a fully interacting materials system, it is necessary to also account for associated kinetic-correlation energy effects. Here, we present a coupling-constant scaling analysis, which permits us to compute the kinetic-correlation energy Tcnl that is specific to the vdW-DF account of nonlocal correlations. We thus provide a more complete spatially resolved analysis of the electrodynamical-coupling nature of nonlocal-correlation binding, including vdW attraction, in both covalently and noncovalently bonded systems. We find that kinetic-correlation energy effects play a significant role in the account of vdW or dispersion interactions among molecules. Furthermore, our mapping shows that the total nonlocal-correlation binding is concentrated to pockets in the sparse electron distribution located between the material fragments.
  • Sundstrom, A., et al. (författare)
  • Fast collisional electron heating and relaxation in thin foils driven by a circularly polarized ultraintense short-pulse laser
  • 2020
  • Ingår i: Journal of Plasma Physics. - 0022-3778 .- 1469-7807. ; 86:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The creation of well-thermalized, hot and dense plasmas is attractive for warm dense matter studies. We investigate collisionally induced energy absorption of an ultraintense and ultrashort laser pulse in a solid copper target using particle-in-cell simulations. We find that, upon irradiation by a 2 x 10(20) Wcm(-2) intensity, 60 fs duration, circularly polarized laser pulse, the electrons in the collisional simulation rapidly reach a well-thermalized distribution with similar to 3 :5 keV temperature, while in the collisionless simulation the absorption is several orders of magnitude weaker. Circular polarization inhibits the generation of suprathermal electrons, while ensuring efficient bulk heating through inverse bremsstrahlung, a mechanism usually overlooked at relativistic laser intensity. An additional simulation, taking account of both collisional and field ionization, yields similar results: the bulk electrons are heated to similar to 2:5 keV, but with a somewhat lower degree of thermalization than in the pre-set, fixed-ionization case. The collisional absorption mechanism is found to be robust against variations in the laser parameters. At fixed laser pulse energy, increasing the pulse duration rather than the intensity leads to a higher electron temperature.
  • Tornsö, Marcus, 1993- (författare)
  • Holographic descriptions of collective modes in strongly correlated media
  • 2019
  • Ingår i: Tillämpad strängteori - holografiska metoder för starkt kopplade system. - Gothenburg : Chalmers tekniska högskola.
  • Licentiatavhandling (övrigt vetenskapligt)abstract
    • Solving the puzzle of high temperature superconductivity may be one of the most desired scientific breakthroughs of our time, as access to room temperature superconductivity could revolutionize society as we know it. In this thesis, we strive to increase the theoretical understanding of such matter, by studying the phase above, in temperature, the superconducting phase - the "strange metal". The strange metal phase is a phase characterized by the absence of a quasi-particle description. The electrons in this phase are strongly coupled, which means that conventional methods, such as perturbation theory in quantum field theory and Monte Carlo methods fall short of being able to describe their dynamics. Perhaps surprisingly, string theory provides a different method, capable of describing precisely such systems - the holographic duality. Whereas there has been significant effort devoted to the applications of the duality since its inception in 1997, and even more so in the last decade after it was observed that it worked remarkably well for condensed matter theory, it wasn't until our project that the dynamical polarization of such strongly coupled systems where properly treated. In this thesis, we introduce the minimal constraints required for a sensible description of a polarizing medium, and convert those to boundary conditions to the equations of motion provided by the holographic dual. These boundary conditions deviate from previous holographic studies, and we contrast the quasinormal modes previously studied with the emergent collective modes we find for some different models. We find novel results, as well as confirm the predictions of less general models in their respective regions of validity and pave the way for more complex future models.
  • Johannesson, Henrik, 1953-, et al. (författare)
  • Conformal field theory approach to the two-channel Anderson model
  • 2004
  • Ingår i: Journal of Magnetism and Magnetic Materials. - 0304-8853. ; 272-276:SUPPL. 1
  • Tidskriftsartikel (refereegranskat)abstract
    • The two-channel Anderson impurity model serves as a prototype for describing heavy-fermion materials with a possible mixed-valent regime with both quadrupolar and magnetic character. We report on the low-energy physics of the model, using a conformal field theory approach with exact Bethe Ansatz results as input.
  • Dissanayake, M. A. K. L., et al. (författare)
  • Highly efficient, PbS:Hg quantum dot-sensitized, plasmonic solar cells with TiO2 triple-layer photoanode
  • 2019
  • Ingår i: Journal of Solid State Electrochemistry. - 1432-8488. ; 23:6, s. 1787-1794
  • Tidskriftsartikel (refereegranskat)abstract
    • Highly efficient, PbS:Hg quantum dot-sensitized, plasmonic solar cells with TiO2 triple-layer photoanode were fabricated by successive ionic layer adsorption and reaction (SILAR) method. These nanostructured photoanodes were characterized by optical and morphological techniques and the solar cells were characterized by optical and electrical techniques. The light absorption by the photoanode was enhanced by effective light scattering process using a triple-layer TiO2 nanostructure, fabricated with a TiO2 nanofiber layer sandwiched between two TiO2 nanoparticle layers. The best plasmon-enhanced quantum dot-sensitized solar cell showed an efficiency of 5.41% with short circuit current density of 18.02mAcm(-2) and open-circuit voltage of 679.83mV. The overall efficiency and photocurrent density of the Q-dot-sensitized solar cell are enhanced by 15.84% and 38.83% respectively due to the plasmonic effect. The enhanced efficiency appears to be due to the improved short circuit current density by increased light absorption by the triple-layered photoanode nanostructure as well as by the localized surface plasmon resonance (LSPR) effect of the plasmonic gold nanoparticles. This is the first report on plasmon-enhanced, triple-layered TiO2 photoanode sensitized with PbS:Hg Q-dots.
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