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  • 51.
    Angelopoulos, A.
    et al.
    -.
    Apostolakis, A.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    K 0–K̄0 mass and decay-width differences: CPLEAR evaluation1999Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 471, nr 2, s. 332-338Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The CPT-violation parameters Re(δ) and Im(δ) determined recently by CPLEAR are used to evaluate the K0 mass and decay-width differences, as given by the difference between the diagonal elements of the neutral-kaon mixing matrix (M−iΓ/2). The results – GeV and GeV – are consistent with CPT invariance. The CPT invariance is also shown to hold within a few times 10−3–10−4 for many of the amplitudes describing neutral-kaon decays to different final states.

  • 52.
    Angelopoulos, Angelos
    et al.
    -.
    Apostolakis, A.
    -.
    Aslanides, E.
    -.
    Backenstoss, Gerhard
    -.
    Bargassa, P.
    -.
    Bee, C P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    Physics at CPLEAR2003Ingår i: Physics Reports, Vol. 374, nr 3, s. 165-270Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LEAR offered unique opportunities to study the symmetries which exist between matter and antimatter. At variance with other approaches at this facility, CPLEAR was an experiment devoted to the study of CP, T and CPT symmetries in the neutral-kaon system. A variety of measurements allowed us to determine with high precision the parameters which describe the time evolution of the neutral kaons and their antiparticles, including decay amplitudes, and the related symmetry properties. Limits concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been obtained. An account of the main features of the experiment and its performances is given here, together with the results achieved.

  • 53.
    Angelopoulos, Angelos
    et al.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    K0⇋ K̄0 transitions monitored by strong interactions: a new determination of the K L–K S mass difference2001Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 503, nr 1, s. 49-57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The CPLEAR set-up (modified) has been used to determine the KL–KS mass difference by a method where neutral-kaon strangeness oscillations are monitored through kaon strong interactions, rather than semileptonic decays, thus requiring no assumptions on CPT invariance for the decay amplitudes. The result, Δm=(0.5343±0.0063stat±0.0025syst)×1010ℏ/s, provides a valuable input for CPT tests.

  • 54.
    Angelopoulos, Angelos
    et al.
    -.
    Locher, M P
    -.
    Markushin, V E
    -.
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    Dispersion relation analysis of the neutral kaon regeneration amplitude in carbon1999Ingår i: The European Physical Journal C-Particles and Fields, ISSN 434-6044, Vol. 10, nr 1, s. 19-25Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We apply a forward dispersion relation to the regeneration amplitude for kaon scattering on 12" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">12C using all available data. The CPLEAR data at low energies allow the determination of the net contribution from the subthreshold region which turns out to be much smaller than earlier evaluations, solving a long standing puzzle.

  • 55.
    Anglart, Henryk
    KTH, Tidigare Institutioner, Fysik.
    MODELING OF ENTRAINMENT RATES FOR ANNULAR FLOW IN VERTICAL ANNULI2004Ingår i: Proceeding of the Sixth International Conference on Nuclear Thermal-Hydraulics, Operation and Safety, 2004Konferensbidrag (Refereegranskat)
    Abstract [en]

    One of the challenges in thermal-hydraulic analyses of BWRs is a correct prediction of dryout occurrence in a fuel assembly. Traditionally the critical powers in fuel assemblies have been found from correlations that are based on experimental data. The drawback of this approach is that correlations are valid only for these fuel assemblies on which the experiments have been conducted. Other limiting factors are the limited ranges of experimental working conditions including pressure, mass flux and axial power distributions. To overcome the above-mentioned limitations, several different models have been proposed to predict the dryout occurrence. One of the most popular approaches is to employ a phenomenological model of annular flow, in which the mass transfer between the liquid film and the gas core is based on entrainment and deposition correlations. Most of these correlations are derived from water-air flows in vertical tubes and their applicability to other geometries in general, and rod-bundles in particular, should be analyzed. This paper presents an analysis of the entrainment rate in vertical annuli. Using the standard approach to calculate the entrainment rate, one can show that the results deviate from measurements.

  • 56.
    Anglart, Henryk
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Podowski, Michael
    Rensselaer.
    Theoretical and Computational Investigation of Internal Structure of Slug Flows2004Ingår i: Proceedings of the Fifth International Conference on Multiphase Flow, 2004Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper is concerned with the modeling and analysis of detailed distributions of flow field and phase concentration in slug flows. The computational approach was based on first-principle simulations of transient three-dimensional combined gas/liquid flows. The tracking of moving gas/liquid interfaces was accomplished using a modified Volume-of-Fluid method. The results of such ‘virtual experiments’ have been ensemble-averaged and compared against experimental data. The main focus of the analysis was on a consistent derivation and proper physical interpretation of the various local terms arising from the averaging, which appear in the average multifield conservation equations. It has been demonstrated that for flow conditions in which the geometry of gas/liquid interface (such as the shape of Taylor bubbles in slug flows) is a part of the solution, the process of ensemble-averaging introduces two groups of terms: the ‘bulk terms’ that are determined with respect to phasic average variables, and the interface excess terms that normally combine the average variables and those determined strictly at the interface. In particular, the above applies to the total shear stress, as well as to the interfacial forces in slug flows. A detailed analytical derivation of all such terms has been performed. The consistency of the proposed approach has been demonstrated by comparing the results of model predictions against experimental data for various flow regimes. It has been shown that the distributions of average (but still local, radially dependent) flow parameters such as the: velocity, void fraction, shear-induced turbulent stress, and bubble-induced turbulent stress, are in good agreement with the measurements.

  • 57. Apostolakis, A
    et al.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    KTH, Skolan för teknikvetenskap (SCI).
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    A determination of the CP violation parameter η+- from the decay of strangeness-tagged neutral kaons1999Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 458, nr 4, s. 545-552Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    LEAR offered unique opportunities to study the symmetries which exist between matter and antimatter. At variance with other approaches at this facility, CPLEAR was an experiment devoted to the study of CP, T and CPT symmetries in the neutral-kaon system. A variety of measurements allowed us to determine with high precision the parameters which describe the time evolution of the neutral kaons and their antiparticles, including decay amplitudes, and the related symmetry properties. Limits concerning quantum-mechanical predictions (EPR, coherence of the wave function) or the equivalence principle of general relativity have been obtained. An account of the main features of the experiment and its performances is given here, together with the results achieved.

  • 58.
    Apostolakis, A.
    et al.
    -.
    Aslanides, E.
    -.
    Backenstoss, G.
    -.
    Bargassa, P.
    -.
    Behnke, O.
    -.
    Benelli, A.
    -.
    Bertin, V.
    -.
    Blanc, F.
    -.
    Bloch, P.
    -.
    Carlson, P.
    Danielsson, Mats
    KTH, Tidigare Institutioner, Fysik.
    Measurement of the energy dependence of the form factor f+ in K 0 e3 decay2000Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 473, nr 1, s. 186-192Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Neutral-kaon decays to πeν were analysed to determine the q2 dependence of the K0e3 electroweak form factor f+. Based on 365612 events, this form factor was found to have a linear dependence on q2 with a slope λ+=0.0245±0.0012stat±0.0022syst.

  • 59.
    Armiento, Richard
    KTH, Tidigare Institutioner                               , Fysik.
    Subsystem functionals in density functional theory2002Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 60.
    Armiento, Rickard
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Matsson, A. E.
    Interface Sci. Department MS 1415, Sandia National Laboratories, Albuquerque.
    Subsystem functionals: investigating the exchange energy per particle2002Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, nr 16, s. 1651171-16511717Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A viable way of extending the successful use of density-functional theory into studies of even more complex systems than are addressed today has been suggested by Kohn and Mattsson [W. Kohn and A. E. Mattsson, Phys. Rev. Lett. 81, 3487 (1998); A. E. Mattsson and W. Kohn, J. Chem. Phys. 115, 3441 (2001)], and is further developed in this work. The scheme consists of dividing a system into subsystems and applying different approximations for the unknown (but general) exchange-correlation energy functional to the different subsystems. We discuss a basic requirement on approximative functionals used in this scheme; they must all adhere to a single explicit choice of the exchange-correlation energy per particle. From a numerical study of a model system with a cosine effective potential, the Mathieu gas, and one of its limiting cases, the harmonic oscillator model, we show that the conventional definition of the exchange energy per particle cannot be described by an analytical series expansion in the limit of slowly varying densities. This indicates that the conventional definition is not suitable in the context of subsystem functionals. We suggest alternative definitions and approaches to subsystem functionals for slowly varying densities and discuss the implications of our findings on the future of functional development.

  • 61.
    Armiento, Rickard
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Mattsson, A. E.
    Compl. Mat. and Molecular Biology, Sandia National Laboratories, Albuquerque.
    Alternative separation of exchange and correlation in density-functional theory2003Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, nr 24, s. 2451201-2451205Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    It has recently been shown that local values of the conventional exchange energy per particle cannot be described by an analytic expansion in the density variation. Yet, it is known that the total exchange-correlation (XC) energy per particle does not show any corresponding nonanalyticity. Indeed, the nonanalyticity is here shown to be an effect of the separation into conventional exchange and correlation. We construct an alternative separation in which the exchange part is made well behaved by screening its long-ranged contributions, and the correlation part is adjusted accordingly. This alternative separation is as valid as the conventional one, and introduces no new approximations to the total XC energy. We demonstrate functional development based on this approach by creating and deploying a local-density-approximation-type XC functional. Hence, this work includes both the theory and the practical calculations needed to provide a starting point for an alternative approach towards improved approximations of the total XC energy.

  • 62. Arvidsson, G.
    et al.
    Laurell, Fredrik
    KTH, Tidigare Institutioner, Fysik.
    Anordning i form av koherent ljuskälla baserad på frekvenskonvertering av ljuset från lasrar: [Controllable laser light source uses two laser sources to enable frequency mixing such that three or four new wavelengths may be generated]1990Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    The present invention concerns a coherent light source based on frequency conversion of the radiation from two lasers (20, 21) by frequency mixing in optical waveguides (10; 90), which are provided in a substrate (1). The wavelengths for the two lasers (20, 21) should be such, that the phase matching condition for the optically nonlinear frequency conversion is fulfilled in the waveguide structure (10; 90). The frequency conversion is accomplished in the form of frequency mixing, frequency doubling or down conversion in frequency by parametric oscillation so that in total three or four new wavelengths can be generated. By using integrated optics technique the output radiation can be controlled in various ways, for example be switched to different output waveguides, varied in intensity or colour balance.

  • 63. Arvidsson, G.
    et al.
    Laurell, Fredrik
    KTH, Tidigare Institutioner, Fysik.
    Webjörn, J.
    Vågledare samt förfarande för framställning av sådan: [Quasi-phase-matched optical waveguides and method of making same]1988Patent (Övrig (populärvetenskap, debatt, mm))
    Abstract [en]

    A waveguide (and a method for fabrication of a waveguide) for electromagnetic waves, arranged in an optically non-linear crystal substrate (1), such as LiNbO3, LiTaO3 or KTiOPO4, whereby a periodically domain-inverted structure, with periodically reversed ferroelectric polarity, is provided in a channel waveguide (6), having a higher refractive index than other parts of the substrate. The regions of reversed ferroelectric polarity are achieved by means of a heat treatment yielding a domain-inversion governed by a periodic perturbation in the crystal surface, obtained by use of a periodic mask structure on the crystal surface. The periodic structure is arranged in such a way that it constitutes a quasi-phase-matching structure. Thereby the waveguide is capable of converting light or radiation of a certain wavelength (certain wavelengths) to light or radiation of another predetermined wavelength (other predetermined wavelengths) with high efficiency.

  • 64.
    Arvidsson, Magnus
    KTH, Tidigare Institutioner                               , Fysik.
    Stability and accuracy in microchip lasers for high precision range finding2001Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 65. Ashrafi, S.
    et al.
    Lipoglavsek, M.
    Likar, A.
    Vidmar, T.
    Cederkall, J.
    Fahlander, C.
    Grawe, H.
    Johnson, Arne
    KTH, Tidigare Institutioner                               , Fysik.
    Mitarai, S.
    Norlin, L. O.
    Nyberg, J.
    Palacz, M.
    Persson, J.
    Seweryniak, D.
    Shizuma, T.
    Sletten, G.
    Monte-Carlo simulation of the charged particle detector used in the NORDBALL gamma-ray spectrometer2001Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 458, nr 3, s. 690-697Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The NORDBALL silicon detector array for detecting light charged particles emitted in fusion evaporation reactions was simulated with the Monte-Carlo method. The data from the reaction of 261 MeV Ni-58 ions with Cr-50 nuclei in a 4.8 mg/cm(2) thick target was used to adjust the simulation parameters. Relative population of residual nuclei in the reaction was determined, by comparing the intensities of gamma -rays.

  • 66.
    Asseh, Adel
    KTH, Tidigare Institutioner                               , Fysik.
    All-fibre optical sensor system for chemical sensing based on fibre Bragg grating technology1997Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This thesis deals with the realization of an all-fibresensor system for chemical sensing through the evanescentfield, based on fibre Bragg grating technology. The advantagesof all-fibre systems are most apparent for the potential userof the system. These systems are of small size, have highmobility, and require no precision alignment. Optical fibresare higly adapted to remote sensing and are very robust toexposure of hazardous chemicals, radiation, and hightemperatures. They are electrical isolators and chemicallyinert, thus offering for instance an intrinsic medicalsafety.

    Fibre Bragg grating technology can be used to produceversatile in-fibre components which may be utilized as atransducer elements for many generic sensor types or as afeedback elements in fibre lasers, which are highly suitablelightsources for fibre sensor systems.

    A new method for realizing very long-non-homogenous fibreBragg grating structures is presented. These structures offerconsiderable performance advantages, both for transducerelements and as feedback structures in fibre lasers.

    A long non-homogenous grating structure is utilized toaccomplish a Yb3+ DFB fibre laser. This laser exhibits robustsingle-mode operation and has a potential continuous tuningrange over 10 nm. A theoretical model is introduced to describesome of the characteristics of rare-earth doped fibre DFBlasers.

    A sampled grating structure has been realized to increasethe potential tuning range in linear fibre laserconfigurations. It may' ultimately result in widely tunable,narrow linewidth lightsources.

    A sensor concept based on the wavelength response of a longnarrow linewidth etched fibre Bragg grating is proposed anddemonstrated. Different system aspects as multiplexing andself-referencing are also addressed. The sensor system iseasily multiplexed in the wavelength domain by the utilizationof a continuously tunable DBR laser and the use of additionalfibre Bragg gratings with slightly different Bragg wavelengths.By including a grating that is not etched, self-referencedmeasurements are demonstrated. This makes the sensor systemimpervious to temperature fluctuations and wavelength drift ofthe lightsource.

    The sensor system is finally demonstrated in a modelexperiment of an industrial application where some of theadvantageous characteristics of optical fibres are fullyutilized.

    Keywords:fibre optical sensors, chemical sensing,evanescent field sensing, fibre Bragg gratings, fibrelasers.

  • 67. Asztalos, S. J.
    et al.
    Lee, I. Y.
    Vetter, K.
    Cederwall, Bo
    KTH, Tidigare Institutioner                               , Fysik.
    Clark, R. M.
    Deleplanque, M. A.
    Diamond, R. M.
    Fallon, P.
    Jing, K.
    Phair, L.
    Macchiavelli, A. O.
    Stephens, F. S.
    Wozniak, G. J.
    Bernstein, L. A.
    McNabb, D. P.
    Hua, P. F.
    Sarantites, D. G.
    Saladin, J. X.
    Yu, C. H.
    Cizewski, J. A.
    Isotopic yields of neutron-rich nuclei from deep-inelastic reactions2000Ingår i: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 6101, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We follow up on our earlier work involving a light projectile (Ca-48) to populate high spin states in neutron-rich nuclei with results from experiments involving a heavier projectile (Sm-154) fur the purposes of studying isotopic yields. These yields, which in some cases were mensurable down to a level of 0.1 of the total reaction cross section, are presented from three separate reactions. A trend in the isotopic yields towards N/Z equilibration is observed in one experiment having a large disparity in N/Z ratios between the projectile and target. In the two other reactions, where the N/Z driving force is less pronounced, the yields are instead clustered around the projectile and target nuclei. We present correlated projectilelike and targetlike fragment isotopic yields derived from gamma-gamma coincidences, a technique that enables one to partition the yield of an isotope according to the amount of neutron evaporation. Using this method we find that for the zero-neutron evaporation channel transfer occurs predominantly into the light fragment, consistent with the nature of the deep-inelastic mechanism. We further find that multiple-neutron evaporation contributes substantially to the yields of the isotopes.

  • 68. Aubert, B.
    et al.
    Ballansat, J.
    Bazan, A.
    Beaugiraud, B.
    Cros, P.
    Lund-Jensen, Bengt
    KTH, Tidigare Institutioner                               , Fysik.
    Lundqvist, J.
    Pearce, Mark
    KTH, Tidigare Institutioner                               , Fysik.
    Rydstrom, S.
    et al,
    Performance of the ATLAS electromagnetic calorimeter end-cap module 02003Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 500, nr 03-jan, s. 178-201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The construction and beam test results of the ATLAS electromagnetic end-cap calorimeter pre-production module 0 are presented. The stochastic term of the energy resolution is between 10% and 12.5% GeV1/2 over the full pseudorapidity range. Position and angular resolutions are found to be in agreement with simulation. A global constant term of 0.6% is obtained in the pseudorapidity range 2.5 < η < 3.2 (inner wheel).

  • 69. Aubert, B.
    et al.
    Ballansat, J.
    Bazan, A.
    Beaugiraud, B.
    Lund-Jensen, Bengt.
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Lundqvist, J.
    Pearce, Mark
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Rydstrom, S.
    et al,
    Performance of the ATLAS electromagnetic calorimeter barrel module 02003Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 500, nr 03-jan, s. 202-231Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The construction and performance of the barrel pre-series module 0 of the future ATLAS electromagnetic calorimeter at the LHC is described. The signal reconstruction and performance of ATLAS-like electronics has been studied. The signal to noise ratio for muons has been found to be 7.11 +/- 0.07. An energy resolution of better than 9.5% GeV1/2 / rootE (sampling term) has been obtained with electron beams of up to 245 GeV. The uniformity of the response to electrons in an area of Deltaeta x Deltaphi = 1.2 x 0.075 has been measured to be better than 0.8%.

  • 70.
    Axnäs, Johan
    KTH, Tidigare Institutioner                               , Fysik.
    Superconductiong fluctuations in the magnetoconductivity of YBa2Cu3O7-[delta]and other high-temperature superconductors2000Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 71. Balcerzyk, M.
    et al.
    Klamra, Wlodzimierz
    KTH, Tidigare Institutioner                               , Fysik.
    Moszynski, M.
    Kapusta, M.
    Szawlowski, M.
    Energy resolution and light yield non-proportionality of ZnSe: Te scintillator studied by large area avalanche photodiodes and photomultipliers2002Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 482, nr 3, s. 720-727Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The ZnSe: Te scintillator has been studied by means of a photomultiplier with an extended bialkali photocathode, a large area avalanche photodiode (LAAPD) and a PIN photodiode. The light output was determined to be 28 300+/-1700 photons/MeV. Results indicate good proportionality for light output versus gamma-ray energy. Measurements of the 662 keV gamma-ray energy spectrum recorded using a LAAPD resulted in an energy resolution of 5.4+/-0.3%. Based on these results, an intrinsic energy resolution of 3.3+/-0.7% has been calculated.

  • 72.
    Ban, Shufang
    et al.
    KTH, Tidigare Institutioner, Fysik. KTH, Skolan för bioteknologi (BIO), Centra, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Meng, J
    Wyss, Ramon
    KTH, Tidigare Institutioner, Fysik. KTH, Skolan för bioteknologi (BIO), Centra, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Nuclear symmetry energy for A=48 isobars in relativistic mean field theory2004Ingår i: Gaoneng wuli yu he wuli, ISSN 0254-3052, Vol. 28, s. 66-68Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recently it was found that the nuclear symmetry energy can be directly associated with the mean level density and an iso-vector potential. In this paper, the nuclear symmetry energy is studied within the relativistic mean field (RMF) theory. The potential of the RMF theory can be separated into an isovector and isoscalar components. The nuclear binding energies in A = 48 isobaric chain calculated from RMF theory with or without the isovector terms for effective interactions PK1, NLSH, NL3, and TM1 have been used to analyze the nuclear symmetry energy in detail, i.e., mean level spacing epsilon and the effective isovector potential strength K.

  • 73. Basylko, S. A.
    et al.
    Kundrotas, P. J.
    Onischouk, V. A.
    Tornau, E. E.
    Rosengren, Anders
    KTH, Tidigare Institutioner                               , Fysik.
    Coulomb gap in a model with finite charge-transfer energy2001Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6302, nr 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Coulomb gap in a donor-acceptor model with finite charge-transfer energy Delta describing the electronic system on the dielectric side of the metal-insulator transition is investigated by means of computer simulations on two- and three-dimensional finite samples with a random distribution of equal amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry of the model presented with respect to the exchange of donors and accepters are derived. In the immediate neighborhood of the Fermi energy mu. the single-particle density of states g(epsilon) is determined solely by finite size effects, and g(epsilon) further away from mu is described by an asymmetric power law with a nonuniversal exponent, depending on the parameter Delta.

  • 74. Basylko, S. A.
    et al.
    Onischouk, V. A.
    Kundrotas, P. J.
    Tornau, E. E.
    Rosengren, Anders
    KTH, Tidigare Institutioner                               , Fysik.
    Ground-state study of a donor-acceptor model with finite charge-transfer energy2000Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 271, nr 02-jan, s. 134-138Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A donor-acceptor model with randomly distributed N donor and N acceptor sites and with finite charge transfer energy Delta is investigated at zero temperature by means of computer simulations on two-dimensional samples of different sizes, We demonstrate that the density of one-electron excitations g(epsilon) for accepters in the vicinity of the Fermi energy mu depends essentially on the size of the sample investigated and that g(epsilon) further away from mu exhibits a size-independent power-law with the non-universal exponent decreasing with increasing Delta. Rigorous relations for mu and for the mapping of g(epsilon) for donors into g(epsilon) for accepters are also given.

  • 75.
    Basylko, S. A.
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Onischouk, V. A.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    ac conductivity of a Coulomb glass from computer simulations2004Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, nr 2, s. 024201-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A method for calculating the photon-induced hopping ac conductivity of a Coulomb glass by computer simulation is proposed. Results obtained by using an effective relaxation algorithm for two three-dimensional models of a Coulomb glass are reported. ac conductance data clearly demonstrate the transition from super-linear to a sub-quadratic power law. We argue that the same qualitative behavior should be expected for compensated semiconductors. It is shown that the transition is driven by the Coulomb energy of sites forming resonant pairs and not by the width of the Coulomb gap.

  • 76. Basylko, S. A.
    et al.
    Onischouk, V. A.
    Rosengren, Anders
    KTH, Tidigare Institutioner                               , Fysik.
    Coulomb glass in the random phase approximation2002Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, nr 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A three-dimensional model of the electrons localized on randomly distributed donor sites of density n and with the acceptor charge uniformly smeared on these sites, -Ke on each, is considered in the random phase approximation (RPA). For the case K = 1/2 the free energy, the density of the one-site energies (DOSE) epsilon, and the pair OSE correlators are found. In the high-temperature region (e(2) n(1/3)/T) < 1 (T is the temperature) RPA energies and DOSE are in a good agreement with the corresponding data of Monte Carlo simulations. Thermodynamics of the model in this region is similar to the one of an electrolyte in the regime of Debye screening. In the vicinity of the Fermi level μ = 0 the OSE correlations, depending on sgn(ε(1) . ε(2)) and with very slow decoupling law, have been found. The main result is that even in the temperature range where the energy of a Coulomb glass is determined by Debye screening effects, the correlations of the long-range nature between the OSE still exist.

  • 77. Basylko, S. A.
    et al.
    Onischouk, V. A.
    Rosengren, Anders
    KTH, Tidigare Institutioner                               , Fysik.
    Coulomb glass in the restricted random phase approximation2002Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 298, nr 06-maj, s. 416-421Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A three-dimensional model of N/2 electrons localized on N randomly distributed donor sites of density n and with the acceptor charge uniformly smeared on these sites, -e/2 on each. has been considered in the restricted random phase approximation (RRPA) which uses the set of 3N Fourier components of the local charge density as the independent collective variables. The energy e(T) per site and the density of one-site excitation energy (DOSE) g(epsilon) are shown to he, respectively, in excellent quantitative and in good qualitative agreement with the Monte Carlo simulation data in the whole temperature region 0.1 < T < 100, where these simulations were performed (e(2)n(1/3)/k(B) being the temperature scale). The pair correlator for the one-site energies g(epsilon,+/-epsilon, r) is found to have the slow decoupling law similar to(+/-1/r) for T less than or similar to 1 and r much greater than r(d) (r(d), Debye screening radius). The status of the commonly accepted asymptotic g(epsilon) similar to (epsilon - mu)(d-1) (d and mu being, respectively, the spatial dimension and the Fermi energy) is discussed. The deduction of this asymptotic is shown to be based on the strong and questionable conjecture about decoupling of correlations between one-site excitation energies in the vicinity of mu.

  • 78. Baxter, G. W.
    et al.
    Schlup, P.
    McKinnie, I. T.
    Hellstrom, J.
    Laurell, Fredrik
    KTH, Tidigare Institutioner, Fysik.
    Single-mode near-infrared optical parametric oscillator amplifier based on periodically poled KTiOPO42001Ingår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 40, nr 36, s. 6659-6662Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A singly resonant, single-axial-mode, optical parametric oscillator (OPO) based on periodically poled KTiOPO4 (PPKTP) is reported. Signal (1.68 mum) and idler (2.90 mum) optical bandwidths have been narrowed to < 400 MHz by use of a diffraction grating at grazing incidence. The OPO generates 370 muJ of signal radiation when pumped by 3.1 mJ of 1.064-mum radiation. We implemented a single-pass PPKTP amplifier to yield 2.15-mJ signal and 1.17-mJ idler radiation without broadening the spectral bandwidths.

  • 79.
    Becanovic, Vlatko
    KTH, Tidigare Institutioner                               , Fysik.
    Signal and image processing in engineering physics2000Doktorsavhandling, monografi (Övrigt vetenskapligt)
  • 80.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Molecular dynamics study of melting and fcc-bcc transitions in Xe2001Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 8716, nr 16, s. art. no.-165505Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have investigated the phase diagram of Xe over a wide pressure-temperature range by molecular dynamics. The calculated melting curve is in good agreement with earlier experimental data. At a pressure of around 25 GPa and a temperature of about 2700 K we find a triple fcc-bcc liquid point. The calculated fcc-bcc boundary is in nice agreement with the experimental points, which, however, were interpreted as melting. This finding suggests that the transition from close-packed to bcc structure might be more common at high pressure and high temperature than was previously anticipated.

  • 81.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Ahuja, Rajeev
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Stability of the body-centred-cubic phase of iron in the Earth's inner core2003Ingår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 424, nr 6952, s. 1032-1034Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Iron is thought to be the main constituent of the Earth's core(1), and considerable efforts(2-14) have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves(15). Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h. c. p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon(16-18), and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.

  • 82.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Gutierrez, G.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Molecular dynamics simulation of the structure of yttria Y2O3 phases using pairwise interactions2001Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6418, nr 18Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.

  • 83.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    LeBacq, O.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Molecular dynamics study of phase transitions in Xe2002Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 117, nr 15, s. 7233-7244Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A full account is given of our recent theoretical discovery [A. B. Belonoshko, R. Ahuja, and B. Johansson, Phys. Rev. Lett. 87, 165505 (2001)] of the fcc-bcc transition in Xe at high pressure and temperature. The interaction model and method for calculating phase boundaries are exhaustively tested by independent methods. The model was carefully checked against experimental data and results of ab initio molecular dynamics and it was found to perform very well. The two-phase method employed for finding the melting transition was compared with the robust thermodynamic approach and was found to provide data in exact agreement with the latter. The deviation of the calculated melting curve from the experimental one is quite tolerable at low pressures. After a reinterpretation of the experimental data, our results are also in good agreement with recent diamond anvil cell experiments. At a pressure of around 25 GPa and a temperature of about 2700 K, we find a triple fcc-bcc-liquid point. The fcc-bcc boundary is calculated without reference to the experimental data, in contrast to our previous work, and found to be in nice agreement with previous calculations as well as with the experimental data points, which, however, were interpreted as melting. Our finding concerning the fcc-bcc transition is confirmed by the direct molecular dynamics simulation of the fcc, bcc, and liquid phases in the same computational cell. In this simulation, it was observed that while the fcc phase melts, the bcc structure solidifies. Since Xe is a typical rare-gas solid, the fcc-bcc transition can now be expected for a number of other van der Waals systems, first of all in Ar and Kr. Our finding suggests, that the transition from close packed to bcc structure might be more common at high pressure and high temperature than was previously anticipated. The performed thorough test of methods and models in this study leads us to suggest that the original interpretation of experimental results is erroneous.

  • 84.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Li, S.
    Ahuja, R.
    Johansson, Börje
    KTH, Tidigare Institutioner, Materialvetenskap.
    High-pressure crystal structure studies of Fe, Ru and Os2004Ingår i: Journal of Physics and Chemistry of Solids, ISSN 0022-3697, E-ISSN 1879-2553, Vol. 65, nr 09-aug, s. 1565-1571Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In order to reveal structural trends with increasing pressure in d transition metals, we performed full potential linear muffin-tin orbital calculations for Fe, Ru, and Os in the hexagonal close packed structure. The calculations cover a wide volume range and demonstrate that all these hexagonal close-packed metals have non-ideal c/a at low pressures which, however, increases with pressure and asymptotically approaches the ideal value at very high compressions. These results are in accordance with most recent experiment for Ru and Os. The experimental data for iron is not conclusive, but it is believed that the c/a ratio decreases weakly with increasing pressure at moderate compression. Since, the experimental and calculated equations of state for iron are in increasingly good agreement with increasing pressure, it is possible that either the negative c/a trend is valid only for a restricted pressure range, or related to the experimental difficulties (e.g. non-hydrostaticity).

  • 85.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    Dong, Qian
    KTH, Tidigare Institutioner, Materialvetenskap.
    Hultquist, Gunnar
    KTH, Tidigare Institutioner, Materialvetenskap.
    Leygraf, Christofer
    KTH, Tidigare Institutioner, Materialvetenskap.
    First-principles study of hydrogen diffusion in alpha-Al2O3 and liquid alumina2004Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, nr 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the energetics and mobility of neutral hydrogen in alumina Al2O3 using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum (alpha-Al2O3) as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in alpha-Al2O3 as D(T)=(21.7x10(-8) m(2)/s)exp(-1.24 eV/kT). The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is D(T)=(8.71x10(-7) m(2)/s)exp(-0.91 eV/kT). The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.

  • 86.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    Dong, Qian
    KTH, Tidigare Institutioner, Materialvetenskap.
    Hultquist, Gunnar
    KTH, Tidigare Institutioner, Materialvetenskap.
    Leygraf, Christofer
    KTH, Tidigare Institutioner, Materialvetenskap.
    First-principles study of hydrogen diffusion in α-Al 2O3 and liquid alumina2004Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 69, nr 2, s. 243021-243026Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the energetics and mobility of neutral hydrogen in alumina Al2O3 using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum (α-Al2O 3) as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in α-Al 2O3 as D(T)=(21.7 × 10-8 m 2/s)exp(-1.24 eV/kT). The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is D(T)=(8.71 × 10-7 m2/s)exp(-0.91 eV/kT). The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.

  • 87.
    Belonoshko, Anatoly B.
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Simak, S. I.
    Kochetov, A. E.
    Johansson, Börje
    KTH, Tidigare Institutioner, Materialvetenskap.
    Burakovsky, L.
    Preston, D. L.
    High-pressure melting of molybdenum2004Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 92, nr 19Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.

  • 88.
    Belymam, Awatif
    KTH, Tidigare Institutioner                               , Fysik.
    Performance of the ATLAS electromagnetic calorimeter for low energy electrons1999Licentiatavhandling, monografi (Övrigt vetenskapligt)
  • 89.
    Bergagio, Mattia
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnenergiteknik.
    Fan, Wenyuan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnenergiteknik.
    Thiele, Roman
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnenergiteknik.
    Anglart, Henryk
    KTH, Tidigare Institutioner (före 2005), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnenergiteknik.
    Large eddy simulation of thermal mixing with conjugate heat transfer at BWR operating conditionsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Thermal fatigue occurs in most metals under cyclic heat loads and can threaten the structural integrity of metal parts. Detailed knowledge of these loads is of utter importance to prevent such issues. In this study, a large eddy simulation (LES) with wall-adapting local eddy viscosity (WALE) subgrid model is performed to better understand turbulent thermal mixing in an annulus with a pair of opposing cold inlets at a low axial level (z = 0.15 m) and with a pair of opposing hot inlets at a higher axial level (z = 0.80 m). Each inlet pair is 90° from each other in the azimuthal direction. Conjugate heat transfer between fluid and structure is accounted for. The geometry simplifies a control-rod guide tube (CRGT) in a boiling water reactor (BWR). LES results are compared with measurement data. This is one of the first times BWR conditions are met in both experiments and LES: pressure equals 7.2 MPa, while the temperature difference between hot and cold inlets reaches 216 K. LES temperatures at the fluid-structure interface are fairly correlated with their experimental equivalents, with regard to mean values, local variances, and dangerous oscillation modes in fatigue-prone areas (z = 0.65-0.67 m). An elastic analysis of the structure is performed to evaluate stress intensities there. From them, cumulative fatigue usage factors are estimated and used as screening criteria in the subsequent frequency analysis of temperature time series at the fluid-structure interface. Cracks are likely to initiate after 97 h.

  • 90. Bergenius, S.
    et al.
    Pearce, Mark
    KTH, Tidigare Institutioner, Fysik.
    Proton irradiation response of CsI(Tl) crystals for the GLAST calorimeter2004Konferensbidrag (Övrigt vetenskapligt)
  • 91.
    Bergenius, Sara
    KTH, Tidigare Institutioner, Fysik.
    GLAST CsI(Tl) Crystals2004Licentiatavhandling, monografi (Övrigt vetenskapligt)
  • 92.
    Bergkvist, Sara
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Henelius, Patrik
    KTH, Tidigare Institutioner, Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    Ground state of the random-bond spin-1 Heisenberg chain2002Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 66, s. 134407-1-134407-9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Stochastic series expansion quantum Monte Carlo is used to study the ground state of the antiferromagnetic spin-1 Heisenberg chain with bond disorder. Typical spin- and string-correlation functions behave in accordance with real-space renormalization group predictions for the random-singlet phase. The average string-correlation function decays algebraically with an exponent of -0.378(6), in very good agreement with the prediction of -(3-root5)/2similar or equal to-0.382, while the average spin-correlation function is found to decay with an exponent of about -1, quite different from the expected value of -2. By implementing the concept of directed loops for the spin-1 chain we show that autocorrelation times can be reduced by up to two orders of magnitude.

  • 93.
    Bergkvist, Sara
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Henelius, Patrik
    KTH, Tidigare Institutioner, Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    Local-density approximation for confined bosons in an optical lattice2004Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, s. 053601-1-053601-7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate local and global properties of the one-dimensional Bose-Hubbard model with an external confining potential, describing an atomic condensate in an optical lattice. Using quantum Monte Carlo techniques we demonstrate that a local-density approximation, which relates the unconfined and the confined model, yields quantitatively correct results in most of the interesting parameter range. We also examine claims of universal behavior in the confined system, and demonstrate the origin of a previously calculated fine structure in the experimentally accessible momentum distribution.

  • 94.
    Bergkvist, Sara
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Henelius, Patrik
    KTH, Tidigare Institutioner, Fysik.
    Rosengren, Anders
    KTH, Tidigare Institutioner, Fysik.
    Reduction of the sign problem using the meron-cluster approach2003Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 68, s. 016122-1-016122-8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The sign problem in quantum Monte Carlo calculations is analyzed using the meron-cluster solution. A meron is a loop that alters the sign of the configuration, and the concept of merons can be used to solve the sign problem for a limited class of models. Here we show that the method can be used to reduce the sign problem in a wider class of models. We investigate how the meron solution evolves between a point in parameter space where it eliminates the sign problem and a point where it does not affect the sign problem at all. In this intermediate regime, the merons can be used to reduce the sign problem. The average sign still decreases exponentially with system size and inverse temperature, but with a different prefactor. The sign exhibits the slowest decrease in the vicinity of points where the meron-cluster solution eliminates the sign problem. We have used stochastic series expansion quantum Monte Carlo combined with the concept of directed loops.

  • 95. Berglund, M.
    et al.
    Rymell, L.
    Peuker, M.
    Wilhein, T.
    Hertz, Hans M.
    KTH, Tidigare Institutioner                               , Fysik.
    Compact water-window transmission X-ray microscopy2000Ingår i: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 197, s. 268-273Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate sub-100 nm resolution water-window soft X-ray full-field transmission microscopy with a compact system. The microscope operates at lambda = 3.37 nm and is based on a 100 Hz table-top regenerative debris-free droplet-target laser-plasma X-ray source in combination with normal-incidence multilayer condenser optics for sample illumination. High-spatial-resolution imaging is performed with a 7.3% efficiency nickel zone plate and a 1024 x 1024 pixel CCD detector. Images of dry test samples are recorded with exposure times of a few minutes and show features smaller than 60 nm.

  • 96.
    Berglund, Magnus
    KTH, Tidigare Institutioner                               , Fysik.
    A compact soft X-ray microscope based on a laser-plasma source1999Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
  • 97. Bergsten, T.
    et al.
    Delsing, P.
    Haviland, David B.
    KTH, Tidigare Institutioner                               , Fysik.
    Hall resistance in two-dimensional arrays of Josephson junctions2000Ingår i: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 284, s. 1818-1819Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have measured the Hall resistance in two-dimensional arrays of ultrasmall aluminium Josephson junctions. We found that the Hall resistance was periodical with respect to an external magnetic field applied perpendicular to the plane of the array. We also found that the Hall resistance was affected by an applied voltage to a nearby gate electrode, but not by a gate plane silting underneath the array.

  • 98.
    Bergström, David
    KTH, Tidigare Institutioner                               , Fysik.
    Antiprotons in the Cosmic Radiation Measured by the CAPRICE98 Experiment2001Doktorsavhandling, monografi (Övrigt vetenskapligt)
  • 99.
    Bergström, David
    KTH, Tidigare Institutioner                               , Fysik.
    Performance of the CAPRICE98 gas-RICH detector1999Licentiatavhandling, monografi (Övrigt vetenskapligt)
  • 100.
    Betan, Rodolfo Id
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Liotta, Roberto J.
    KTH, Tidigare Institutioner, Fysik.
    Sandulescu, Nicolae
    Vertse, Tamas
    KTH, Tidigare Institutioner, Fysik.
    A shell model representation with antibound states2004Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 584, nr 1-2, s. 48-57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An unified shell model scheme is introduced that evaluates simultaneously the contributions of bound single-particle states, Gamow resonances and antibound states to processes occurring in the continuum part of nuclear spectra. This new scheme allows us to study the effect of the antibound pole and the remaining part of the complex continuum separately. The calculations, performed in the complex energy plane, are applied to the study of weakly bound nuclei. The influence of antibound states upon physical quantities in light as well as in heavy nuclei is assessed.

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