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  • 1.
    Andersson, Britt
    et al.
    Umeå university.
    Sundqvist, Bertil
    Umeå university.
    Niska, John
    Luleå University of Technology.
    Loberg, Bengt
    Thermal conductivity of polycrystalline YBa2Cu4O81994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 49, no 6, p. 4189-4198Article in journal (Refereed)
    Abstract [en]

    We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30-300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.

  • 2.
    Andersson, O.
    et al.
    Department of Experimental Physics, Umeå University, S-90187 Umeå, Sweden.
    Soldatov, Alexander
    Department of Experimental Physics, Umeå University, S-90187 Umeå, Sweden.
    Sundqvist, B.
    Department of Experimental Physics, Umeå University, S-90187 Umeå, Sweden.
    Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the 50-300 K range1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, no 5, p. 3093-3100Article in journal (Refereed)
  • 3.
    Bergqvist, Lars
    et al.
    KTH, Royal Institute of Technology, Department of Materials and Nano Physics.
    Taroni, Andrea
    Department of Physics and Astronomy, Uppsala University.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University.
    Etz, Corina
    Department of Physics and Astronomy, Uppsala University.
    Eriksson, Olle
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Atomistic spin dynamics of low-dimensional magnets2013In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 87, article id 144401Article in journal (Refereed)
    Abstract [en]

    We investigate the magnetic properties of a range of low-dimensional ferromagnets using a combination of first-principles calculations and atomistic spin dynamics simulations. This approach allows us to evaluate the ground state and finite temperature properties of experimentally well characterized systems such as Co/Cu(111), Co/Cu(001), Fe/Cu(001) and Fe/W(110), for different thicknesses of the magnetic layer. We compare our calculated spin wave spectra with experimental data available in the literature, and find a good quantitative agreement. We also predict magnon spectra for systems for which no experimental data exist at the moment, and estimate the role of temperature effects

  • 4.
    Bhandary, Sumanta
    et al.
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Brena, Barbara
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Panchmatia, P. M.
    School of Applied Sciences, University of Huddersfield, Queensgate.
    Brumboiu, Iulia
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Bernien, M.
    Institut für Experimentalphysik, Freie Universität Berlin.
    Weis, Claudia
    Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen.
    Krumme, B.
    Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen.
    Etz, Corina
    Department of Physics and Astronomy, Uppsala University.
    Kuch, W.
    Institut für Experimentalphysik, Freie Universität Berlin.
    Wende, H.
    Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen.
    Eriksson, Olle
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Sanyal, Biplab
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates2013In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 88, no 2, article id 24401Article in journal (Refereed)
  • 5.
    Breuer, S. J.
    et al.
    Department of Physics, The University of Exeter, Exeter, United Kingdom.
    Jones, R.
    Department of Physics, The University of Exeter, Exeter, United Kingdom.
    Briddon, P. R.
    Department of Physics, The University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Interstitial hydrogen and enhanced dissociation of C-H complexes in GaAs1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 53, no 24, p. 16289-16296Article in journal (Refereed)
    Abstract [en]

    Local-density-functional-based ab initio calculations are used to investigate hydrogen and carbon-hydrogen defects in GaAs. The equilibrium structure for both the C-H and C-H- complexes are shown to be similar, with the hydrogen located at a C-Ga bond-centered site. The dissociation of these complexes is investigated and it is found that the energy barrier of 1.84 eV for the process C-H → C- + H+ is substantially lowered to 0.88 eV in the presence of an electron resonantly bound to the defect. This is in good quantitative agreement with recent experiments. Isolated interstitial hydrogen is found to lie at a Ga-As bond-centered site for both H+ and H0 and at an antibonding site relative to a Ga atom for H-. It is also found that the stable form of the hydrogen dimer is a H2 molecule, the dissociation energy of which is 1.64 eV, and that interstitial hydrogen is a negative-U defect. Finally, a mechanism for minority-carrier-induced device degradation is proposed.

  • 6.
    Budde, M.
    et al.
    Institute of Physics and Astronomy, University of Aarhus, Aarhus, Denmark.
    Bech Nielsen, B.
    Institute of Physics and Astronomy, University of Aarhus, Aarhus, Denmark.
    Jones, R.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Goss, J.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Local modes of the H2* dimer in germanium1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, no 8, p. 5485-5494Article in journal (Refereed)
    Abstract [en]

    Local vibrational modes of the H2* defect in crystalline germanium are identified by a combination of infrared-absorption spectroscopy, uniaxial stress measurements, and ab initio theory. Germanium crystals are implanted with protons and/or deuterons at 30 K, and subsequently annealed at room temperature. A number of local vibrational modes of hydrogen are revealed by infrared-absorption spectroscopy. In particular, modes at 765, 1499, 1774, and 1989 cm-1 originate from the same defect which has trigonal symmetry according to the uniaxial stress measurements. The 765-cm-1 mode is two dimensional, while the 1774- and 1989-cm-1 modes are one dimensional. Measurements on samples coimplanted with protons and deuterons show that the defect contains a pair of weakly coupled and inequivalent hydrogen atoms. The 765-, 1499-, 1774-, and 1989-cm-1 modes are ascribed to the H2* defect. The 765-cm-1 mode is a Ge-H bend mode with an overtone at 1499 cm-1 and the modes at 1774 and 1989 cm-1 are Ge-H stretch modes. An excellent fit to the stretch frequencies is obtained with a simple model based on two coupled Morse-potential oscillators. In addition, the model gives intensity ratios in fair agreement with those observed. The structure, the local-mode frequencies, and the isotope shifts of H2* are calculated with ab initio local-density-functional cluster theory. The theoretical frequencies are consistently 5-10 % too high, as expected from the theory which often leads to overbinding. The isotope shifts, however, are in fair agreement with observations. These results provide additional support for our assignments, and show that the 765- and 1774-cm-1 modes primarily involve the hydrogen at the antibonding site, while the 1989-cm-1 mode is related mainly to vibration of the hydrogen near the bond-center site.

  • 7.
    Chen, Wenhua
    et al.
    Physics III, Royal Institute of Technology, 100 44 Stockholm, Sweden.
    Severin, Lukas
    Department of Physics, Uppsala University, Uppsala, Sweden.
    Göthelid, Mats
    Physics III, Royal Institute of Technology, 100 44 Stockholm, Sweden.
    Hammar, Mattias
    Physics III, Royal Institute of Technology, 100 44 Stockholm, Sweden.
    Cameron, Steve
    Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, Route 22E, United States.
    Paul, Jan
    Physics III, Royal Institute of Technology, 100 44 Stockholm, Sweden.
    Electronic and geometric structure of clean Pt3Ti(111)1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 8, p. 5620-5627Article in journal (Refereed)
  • 8.
    Chico, Jonathan
    et al.
    Uppsala universitet, Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Etz, Corina
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Bergqvist, Lars
    KTH, Royal Institute of Technology, Department of Materials and Nano Physics.
    Eriksson, Olle
    Uppsala universitet, Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Fransson, Jonas
    Uppsala universitet.
    Delin, Anna
    Uppsala universitet, Kungliga tekniska högskolan, KTH, Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Bergman, Anders
    Department of Physics and Astronomy, Uppsala University, Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Thermally driven domain-wall motion in Fe on W(110)2014In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 90, no 1, article id 14434Article in journal (Refereed)
    Abstract [en]

    It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange

  • 9.
    Etz, Corina
    et al.
    Department of Physics and Astronomy, Uppsala University.
    Costa, Marcio
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Eriksson, Olle
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Bergman, Anders
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Accelerating the switching of magnetic nanoclusters by anisotropy-driven magnetization dynamics2012In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 86, no 22, article id 224401Article in journal (Refereed)
    Abstract [en]

    In this work, the magnetization dynamics of clusters supported on nonmagnetic substrates is shown to exhibit a complex response when subjected to external magnetic fields. The field-driven magnetization reversal of small Co clusters deposited on a Cu(111) surface has been studied by means of first-principles calculations and atomistic spin dynamics simulations. For applied fields ranging from 1 to 10 Tesla, we observe a coherent magnetization reversal with switching times in the range of several tenths of picoseconds to several nanoseconds, depending on the field strength. We find a nonmonotonous dependence of the switching times with respect to the strength of the applied field, which we prove has its origin in the complex magnetic anisotropy landscape of these low-dimensional systems. This effect is shown to be stable for temperatures around 10 K, and is possible to realize over a range of exchange interactions and anisotropy landscapes. Possible experimental routes to achieve this unique switching behavior are discussed

  • 10.
    Etz, Corina
    et al.
    Department of Physics and Astronomy, Uppsala University.
    Maznichenko, Igor
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Böttcher, D.
    Max-Planck-Institut für Mikrostrukturphysik.
    Henk, J.
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Yaresko, A.N.
    Max-Planck-Institut für Festkörperforschung.
    Hergert, W.
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Mazin, I.I.
    Naval Research Laboratory, Washington, DC.
    Mertig, Ingrid
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Ernst, A.
    Max-Planck-Institut für Mikrostrukturphysik.
    Indications of weak electronic correlations in SrRuO3 from first-principles calculations2012In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 86, no 6, article id 64441Article in journal (Refereed)
    Abstract [en]

    We provide, by a detailed first-principles investigation, evidence for weak electronic correlations in SrRuO 3. The magnetism in SrRuO 3, in terms of the equilibrium magnetization and critical temperature, is well described by the generalized gradient approximation. Including Hubbard-type correlations results in worse agreement with experiment

  • 11.
    Etz, Corina
    et al.
    Vienna University of Technology.
    Vernes, Andras
    Vienna University of Technology.
    Szunyogh, Laszlo
    Budapest University of Technology and Economics.
    Weinberger, Peter
    Vienna University of Technology.
    Ab initio magneto-optical properties of bcc Ni/Ni(100)2008In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 77, no 6, article id 64420Article in journal (Refereed)
    Abstract [en]

    The inter- and intralayer contributions to the layer-resolved complex optical conductivity tensor for semi-infinite layered systems are calculated in terms of the Luttinger formula within the spin-polarized relativistic screened Korringa-Kohn-Rostoker method. Ab initio Kerr angles are then obtained for arbitrary geometry and incidence via a 2x2 matrix technique including all multiple reflections and all optical interferences. Applied to in-plane single-domain magnetized bcc Ni/Ni(100), it is proven that the assumed appropriate formula of Kerr angles widely used to explain magneto-optical Kerr effect with rotating magnetic field measurements fully agrees with our ab initio Kerr data. From the experimental Kerr data of Tian [Phys. Rev. Lett. 94, 137210 (2005)], however, it cannot be concluded that the deduced magnetic properties apply for bulk Ni, since about 75% of the contributions to the Kerr rotation angle arise from the surface.

  • 12.
    Goss, J.P.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    (CAs)2-hydrogen defects in GaAs: a first-principles study1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 23, p. 15576-15580Article in journal (Refereed)
    Abstract [en]

    Hydrogen-related local vibrational modes at 2643, 2651, 2688, 2725, 2729, and 2775 cm-1 are thought to arise from C-H stretch modes from defects similar to the hydrogen-passivated carbon acceptor complex CAsH. These lines appear in samples that are grown using trimethylgallium metalorganic precursors, and it has been suggested that the 2688-, 2725-, 2729-, and 2775-cm-1 bands may be due to CAs dimers decorated with one or more H atoms. We present here the structures, energies, and vibrational modes of (CAs)2, (CAs)2H, and (CAs)2H2 complexes obtained from ab initio local-density-functional cluster calculations to investigate these assignments.

  • 13.
    Grånäs, Oscar
    et al.
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    di Marco, Igor
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Eriksson, Olle
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Nordström, Lars
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Etz, Corina
    Department of Physics and Astronomy, Uppsala University.
    Electronic structure, cohesive properties, and magnetism of SrRuO32014In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 90, article id 165130Article in journal (Refereed)
    Abstract [en]

    We have performed an extensive test of the ability of density functional theory within several approximations for the exchange-correlation functional, local density approximation + Hubbard U, and local density approximation + dynamic mean field theory to describe magnetic and electronic properties of SrRuO3. We focus on the ferromagnetic phase, illustrating differences between the orthorhombic low-temperature structure versus the cubic high-temperature structure. We assess how magnetism, spectral function, and cohesive properties are affected by methodology, onsite Hubbard U, and double-counting corrections. Further, we compare the impact of the impurity solver on the quasiparticle weight Z, which is in turn compared to experimental results. The spectral functions resulting from the different treatments are also compared to experimental data. Finally, the impact of spin-orbit coupling is studied, allowing us to determine the orbital moments. In the orthorhombic phase, the orbital moments are found to be tilted with respect to the spin moments, emphasizing the importance of taking into account the distortion of the oxygen octahedra

  • 14.
    Hoffmann, L.
    et al.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Bach, J. C.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Bech Nielsen, B.
    Institute of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
    Leary, P.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Jones, R.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Substitutional carbon in germanium1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 17, p. 11167-11173Article in journal (Refereed)
    Abstract [en]

    Carbon impurities implanted into single-crystalline germanium are studied with infrared absorption spectroscopy and ion channeling. After implantation of 12C+ at room temperature and subsequent annealing at 350 °C, a sharp infrared absorption line is observed at 531 cm-1. When 12C+ is substituted by 13C+, the line shifts down in frequency to 512 cm-1 and co-implantation of 12C+ and 13C+ does not give rise to additional lines. Therefore, the 531-cm-1 line represents a local vibrational mode of a defect containing a single carbon atom. Channeling measurements are carried out around the 〈100〉, 〈110〉, and 〈111〉 axes in 12C+-implanted samples annealed at 450 °C. The analysis of the data shows that 31±3 % of the carbon atoms are located at substitutional sites, while the remaining carbon atoms appear to be located randomly. The population of the substitutional site and the intensity of the 531-cm-1 mode have identical temperature dependencies. It is concluded that the 531-cm-1 mode is the three-dimensional T2 stretch mode of substitutional carbon. The effective charge of the mode is determined to be (3.4±0.5)e.mAb initio local density functional cluster theory is applied to calculate the structure and the local vibrational modes of substitutional carbon in germanium. The calculated frequencies and isotope shifts for the T2 stretch mode are in good agreement with the observations.

  • 15.
    Jensen, Henrik Jeldtoft
    et al.
    The Niels Bohr Institute, Copenhagen, Denmark.
    Weber, Hans
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Nordic Institute of Theoretical Physics (NORDITA), Copenhagen, Denmark.
    Phenomenological study of vortices in a two-dimensional XY model in a magnetic field1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 45, no 18, p. 10468-10472Article in journal (Refereed)
    Abstract [en]

    We study the two-dimensional XY model in a magnetic field from a phenomenological point of view by means of Monte Carlo simulations and a simple analytic approach. We consider the system as consisting of an ideal gas of vortex-antivortex pairs. Within this model we can calculate the density of vortex pairs, the average size of the pairs, the specific heat, and the depletion of the magnetization by the vortices. These quantities are compared with Monte Carlo results in which we have extracted the pure vortex contribution by means of a Lagrange-multiplier technique.

  • 16.
    Jones, R
    et al.
    University of Exeter.
    Goss, J
    University of Exeter.
    Ewels, C
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ab initio calculations of anharmonicity of the C-H stretch mode in HCN and GaAs1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 12, p. 8378-8388Article in journal (Refereed)
    Abstract [en]

    The anharmonicities of the C-H stretch modes in HCN and the passivated C acceptor in GaAs are investigated using ab initio local-density-functional cluster theory. The effective-mass parameter χ for the C-H stretch mode is shown to be less than unity in HCN, and greater than unity for the GaAs case. The calculated anharmonic parameter for the first defect is found to be 106 cm-1 and is in very good agreement with experiment. For the second defect, the anharmonicity is about 50% larger in agreement with empirical estimates. The frequencies of the fundamental transitions in both systems are shown to be very sensitive to the C-H length. This limits the accuracy of theoretical investigations of these high frequency H modes. Finally, the effects of electrical anharmonicity are considered and it is shown that they reduce the intensity of the overtone in the C-H complex in GaAs by about 70%.

  • 17.
    Jones, R.
    et al.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Goss, J.P.
    Department of Physics, University of Exeter, Exeter, United Kingdom.
    Briddon, P.R.
    Department of Physics, University of Newcastle upon Tyne, Newcastle, United Kingdom.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    N2 and N4 optical transitions in diamond: A breakdown of the vacancy model1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 56, no 4, p. 1654-1656Article in journal (Refereed)
    Abstract [en]

    The vacancy model for impurity vacancy defects in semiconductors assumes that the ground and low-energy excited states are derivable from the four sp3 hybrid orbitals on atoms bordering the vacancy. There are many cases where this model works but we describe here a counterexample concerning the lowest excited state of the [V-N3] defect in diamond. It is shown that a shallow electron trap, localized outside the vacancy, is involved in the first excited state and responsible for the N2 and N4 optical bands associated with the defect.

  • 18.
    Jones, R
    et al.
    University of Exeter.
    Umerski, A
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ab initio calculation of the local vibratory modes of interstitial oxygen in silicon1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 45, no 19, p. 11321-11323Article in journal (Refereed)
    Abstract [en]

    A local-density-functional cluster method is used to calculate the structure and vibrational modes of interstitial oxygen in silicon. We find Si-O lengths and the Si-O-Si bond angle to be 1.59 Å and 172°, respectively. The asymmetric and symmetric stretch frequencies are 1104 and 554 cm-1 and are close to observed modes at 1136 and 518 cm-1. The effective charge of the upper mode is 3.5e. Isotopic shifts of the modes are also reported. We find that the symmetric stretch mode at 554 cm-1 is independent of the O-isotopic mass in agreement with observation.

  • 19.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Structure and dynamics of the DX center in GaAs:Si1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 7, p. 3407-3408Article in journal (Refereed)
    Abstract [en]

    We have carried out local-density functional cluster calculations on the SiGa- defect in GaAs. We find that the distortion proposed by Chadi and Chang involving a large Si movement along 〈111〉 breaking an Si-As bond has the same energy as a simple breathing distortion. The calculated local vibratory modes of one Chadi-Chang structure do not agree with those recently assigned to DX from an infrared-absorption experiment. On the other hand, the calculated triplet mode due to the other structure above is in reasonable agreement with these observations. The effective charge of the second type of defect is about three times that of the first. It is proposed that both defects coexist with the Chadi-Chang one being almost infrared inactive.

  • 20.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of carbon complexes in aluminum arsenide1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 49, no 8, p. 5306-5312Article in journal (Refereed)
    Abstract [en]

    The structural and dynamic properties of carbon defects in aluminum arsenide are investigated using first-principles local-density-functional cluster theory. The method accounts satisfactorily for the structure and phonon modes of AlAs. The carbon acceptor and donor possess triplet modes in the band gap between the acoustic and optic branches as well as localized triplet modes. The local mode of the acceptor lies within 40 cm-1 of the observed mode. Four modes of a close-by acceptor pair lie within 40 cm-1 of the local mode of the isolated acceptor and this supports a previous assignment of four satellite lines seen in heavily doped material to this defect. The modes of a donor-acceptor pair are investigated but there is no evidence of their existence.

  • 21.
    Jones, R
    et al.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of the structure and dynamics of the C impurity and C-H complex in GaAs1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 8, p. 3673-3677Article in journal (Refereed)
    Abstract [en]

    Carbon is an important impurity in metalorganic molecular-beam-epitaxy-grown GaAs. CAs is a single acceptor that can be passivated by H. We describe local-density-functional cluster calculations on the structure and dynamics of the impurities and passivated complexes. The lowest-energy structure of the passivated acceptor is a H atom located 1.1 Å from C in a bond-centered orientation. The H-Ga distance is 2.1 Å. The other three C-Ga lengths are 2.18 Å. The H stretch frequency is found to be 2605 cm-1 and is observed at 2635 cm-1. We have also calculated C-H bend modes that should be visible in Raman but not in infrared experiments. The activation energy for the reorientation of the complex is 0.67 eV. Also described are the local modes of the two substitutional C defects in addition to a C-C pair. Modes of the latter are found around 553 and 425 cm-1, respectively, and have effective charges of about 0.5.

  • 22.
    Leary, P
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Torres, V J B
    Universidade de Aveiro.
    Dynamic properties of interstitial carbon and carbon-carbon pair defects in silicon1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 4, p. 2188-2194Article in journal (Refereed)
    Abstract [en]

    Interstitial carbon, Ci, defects in Si exhibit a number of unexplained features. The Ci defect in the neutral charge state gives rise to two almost degenerate vibrational modes at 920 and 931 cm-1 whose 2:1 absorption intensity ratio naturally suggests a trigonal defect in conflict with uniaxial stress measurements. The dicarbon, Cs-Ci, defect is bistable, and the energy difference between its A and B forms is surprisingly small even though the bonding is very different. In the B form appropriate to the neutral charge state, a silicon interstitial is believed to be located near a bond-centered site between two Cs atoms. This must give rise to vibrational modes which involve the motion of both C atoms in apparent conflict with the results of photoluminescence experiments. We use an ab initio local density functional cluster method, AIMPRO, to calculate the structure and vibrational modes of these defects and find that the ratio of the absorption intensities of the local modes of Ci is in reasonable agreement with experiment even though the structure of the defect is not trigonal. We also show that modes in the vicinity of those detected by photoluminescence for the B form of the dicarbon center involve independent movements of the two C atoms. Finally, the trends in the relative energies of the A and B forms in three charge states are investigated.

  • 23.
    Lehto, Niklas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Shallow electron states of bounded intrinsic stacking faults in silicon1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 55, no 23, p. 15601-15607Article in journal (Refereed)
    Abstract [en]

    The electronic structure of bounded intrinsic stacking faults in silicon is studied. Especially the influence of the stacking fault width on the electronic states in the band gap is investigated. The extended defect studied comprises an intrinsic stacking fault with two reconstructed 90° partials as boundaries. The atomic structure is determined by different valence force fields. These are the Keating potential, the bond-charge model, and an anharmonic version of the bond-charge model. The electronic structure is calculated by linear combinations of atomic orbitals. Ten Gaussian-type atomic orbitals of s, p, and d-type are used, and up to fourth nearest neighbor interactions are taken into account. The levels in the band gap are evaluated by the recursion method for nonorthogonal basis functions, and by a continued fraction representation of the local density of states

  • 24.
    Lehto, Niklas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Interaction of vacancies with partial dislocations in silicon1997In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 56, no 20, p. R12706-R12709Article in journal (Refereed)
    Abstract [en]

    The interaction of vacancies with 30° and 90° partial dislocations in silicon is examined. In particular, the structures and binding energies are calculated using hydrogen-terminated clusters and local density-functional theory. Moreover the electronic structure is determined using supercells containing dislocation dipoles. Vacancies are found to have binding energies of approximately 2.0 eV and 0.9 eV to 90° and 30° partials, respectively. The elastic strain field of the partials makes the fourfold vacancy reconstruct, which essentially clears the fundamental gap

  • 25.
    Minnhagen, Petter
    et al.
    Department of Theoretical Physics, University of Umeå, S-901 87 Umeå, Sweden.
    Weber, Hans
    Department of Theoretical Physics, University of Umeå, S-901 87 Umeå, Sweden.
    Resistance scaling function for two-dimensional superconductors and Monte Carlo vortex-fluctuation simulations1985In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 32, no 5, p. 3337-3340Article in journal (Refereed)
  • 26.
    Paul, Jan
    Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, Route 22 East, United States.
    Hydrogen adsorption on Al(100)1988In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 37, no 11, p. 6164-6174Article in journal (Refereed)
  • 27. Paul, Jan
    et al.
    Rosén, A.
    Electronic structure of CO adsorbed on a Cu(111) surface analyzed with molecular cluster models1982In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 26, no 8, p. 4073-4077Article in journal (Refereed)
    Abstract [en]

    Using the self-consistent Hartree-Fock-Slater model we have calculated the electronic structure for various Cux(CO)y clusters symbolizing not only CO bound to "on-top" and "bridge" sites but also some lateral CO-CO interaction on a Cu(111) surface. By comparison with experimental photoemission data we are able to reproduce the observed energies of the occupied CO 4σ, 1π, and 5σ orbitals as well as the partly occupied 2π orbital. In our model we assume CO to be adsorbed on "top" sites for coverages less than ⊖=0.33 [(sqrt[3]×sqrt[3])R30°] and on both top and bridge sites for ⊖>0.33. The experimentally observed peak of intensity at the Fermi edge which increases with coverage above 0.33 is in our model explained by the occupation of CO orbitals of the b1 and b2 symmetry types, i.e., "π" orbitals, degenerate for top positions (C3v), split by the change to bridge positions (C2v). Our results further indicate that the broadened 5σ-1π intensity peak at high coverages is a result of CO bound to top and bridge sites.

  • 28.
    Pritchard, R E
    et al.
    Imperial College of Science Technology and Medicine.
    Newman, R C
    Imperial College of Science Technology and Medicine.
    Wagner, J
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Fuchs, F
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Bonding of H-CAs pairs in AlxGa1-xAs alloys1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 15, p. 10628-10636Article in journal (Refereed)
    Abstract [en]

    The local structure of CAs acceptors in AlxGa1-xAs has been investigated by studying the nondegenerate localized vibrational modes of H-CAs pairs with A1 symmetry, rather than those of isolated CAs impurities. Infrared absorption and Raman scattering measurements have been made on AlxGa1-xAs: 12C epilayers that (a) had been exposed to a radio-frequency hydrogen (deuterium) plasma or (b) contained hydrogen incorporated during growth. Arguments are advanced that indicate that the H(D) atom should occupy a bond-centered site between CAs and Ga atoms rather than between CAs and Al atoms at low temperatures. An ab initio local-density-functional calculation indicates that the energy is then lowered by 0.24 eV. This analysis has led to the assignment of five antisymmetric stretch modes and five symmetric (X) modes to H-CAs pairs at sites where the carbon atom that was originally unpaired had zero, one, two, three, or four Al nearest neighbors

  • 29.
    Rasmussen, F Berg
    et al.
    University of Aarhus.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Nitrogen in germanium: identification of the pair defect1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, no 7, p. 4378-4384Article in journal (Refereed)
    Abstract [en]

    Several nitrogen-related centers have been introduced by ion implantation of nitrogen into germanium and studied by infrared-absorption spectroscopy. Two local vibrational modes at 825.3 and 658.6 cm-1 were especially prominent. Measurements on annealed samples implanted with either pure 14N or 15N, or implanted with both isotopes showed these modes to arise from a nitrogen pair defect analogous to one previously suggested to occur in silicon. Based on this and ab initio pseudopotential cluster theory, a model of the pair is proposed which is consistent with the observed infrared absorption. This model is similar to that of the nitrogen pair in silicon and offers an explanation of the low donor efficiency of nitrogen in germanium. Several other nitrogen-related local vibrational modes are also observed in the implanted material.

  • 30.
    Rasmussen, F Berg
    et al.
    University of Aarhus.
    Nielsen, B Bech
    University of Aarhus.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Comment on "Role played by N and N-N impurities in type-IV semiconductors"1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, no 20, p. 14756-14757Article in journal (Other academic)
    Abstract [en]

    In a recent paper [Phys. Rev. B 48, 17 806 (1993)] Cunha, Canuto, and Fazzio reported ab initio Hartree-Fock calculations on nitrogen impurities in group-IV semiconductors. In their paper it is suggested that nitrogen pairs form from substitutional atoms on adjacent lattice sites. However, the experimentally observed configuration in silicon and germanium is different from this. The aim of this Comment is to clarify the situation of the nitrogen pair in these materials.

  • 31.
    Stich, P.
    et al.
    Department of Physics, University of Exeter.
    Jones, R
    Department of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, M.I.
    Department of Computer Science, University of Exeter.
    Structures of dislocations in GaAs and their modification by impurities1994In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 50, p. 17717-17720Article in journal (Refereed)
    Abstract [en]

    Local density functional theory is used to show that both α and β dislocations in GaAs are reconstructed. This is done by relaxing large 158-atom H-terminated clusters of GaAs containing 90° partial dislocations. The reconstruction is strongly influenced by impurities: acceptor pairs destroy the reconstruction of β partials but strengthen it for α dislocations. Donors have opposite effects. The implication of these results for the pinning of dislocations in GaAs is discussed.

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  • 32.
    Stich, P. K.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Heggie, M. I.
    University of Exeter.
    Ab initio investigation of the dislocation structure and activation energy for dislocation motion in silicon carbide1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 52, no 7, p. 4951-4955Article in journal (Refereed)
    Abstract [en]

    The structures of straight 90° glide partial dislocations in SiC are calculated using an ab initio local density functional cluster method. Si partials containing core Si atoms are found to be strongly reconstructed with a Si-Si bond of comparable length to that in bulk silicon. The C partial possessing core C atoms is more weakly reconstructed with a bond length 16% longer than that in bulk diamond. The formation and migration energies of kinks on the partials are calculated and indicate that the C partial is the more mobile. The calculations also predict that n-type doping leads to an increase in the mobility of C partials whereas p-type doping increases the mobility of Si partials.

  • 33.
    Tange, A.
    et al.
    Max-Planck-Institut für Mikrostrukturphysik.
    Gao, C.L.
    Max-Planck-Institut für Mikrostrukturphysik.
    Yarovsky, B.Y.
    Martin Luther University of Halle-Wittenherg.
    Maznichenko, Igor
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Etz, Corina
    Max-Planck-Institut für Mikrostrukturphysik.
    Ernst, A.
    Max-Planck-Institut für Mikrostrukturphysik.
    Hergert, W.
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg.
    Mertig, Ingrid
    Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Max-Planck-Institut für Mikrostrukturphysik.
    Wulfhekel, Wulf
    Max-Planck-Institut für Mikrostrukturphysik, Karlsruher Institut of Technology KIT.
    Kirschner, J.
    Max-Planck-Institut für Mikrostrukturphysik.
    Electronic structure and spin polarization of the Fe(001)-p(1X1)O surface2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 81, no 19, article id 195410Article in journal (Refereed)
    Abstract [en]

    We present a combined experimental and theoretical study on electronic and magnetic properties of the Fe(001)-p(1X1)O surface. The ordered p(1X1) surface is investigated with spin-polarized scanning tunneling microscopy and spectroscopy accompanied by first-principles calculations. The atomic registry of the Fe(001)-p(1X1)O surface was confirmed in real space from the atomically resolved images. Tunneling spectroscopy reveals two oxygen induced features in the local density of states, around -0.7 eV and at the Fermi level, the origin of which is discussed based on first-principles calculations. Due to the hybridization of oxygen p(z) states with the Fe states near the Fermi level, the spin polarization in tunneling experiments is inverted upon oxygen adsorption.

  • 34.
    Wallin, Mats
    et al.
    Kungliga tekniska högskolan, KTH.
    Weber, Hans
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Resistance scaling at the Kosterlitz-Thouless transition1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, no 9, p. 6163-6166Article in journal (Refereed)
    Abstract [en]

    We study the linear resistance at the Kosterlitz-Thouless transition by Monte Carlo simulation of vortex dynamics. Finite-size scaling analysis of our data shows excellent agreement with scaling properties of the Kosterlitz-Thouless transition. We also compare our results for the linear resistance with experiments. By adjusting the vortex chemical potential to an optimum value, the resistance at temperatures above the transition temperature agrees well with experiments over many decades.

  • 35.
    Weber, Hans
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Jensen, Henrik Jeldtoft
    Nordic Institute of Theoretical Physics, Blegdamsvej.
    Crossover from three- to two-dimensional behavior of the vortex energies in layered XY models for high-Tc superconductors1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 1, p. 454-457Article in journal (Refereed)
    Abstract [en]

    We use Monte Carlo simulations of a layered XY model to study phase fluctuations in high-Tc superconductors. A vortex-antivortex interaction dominated by a term linear in the vortex separation is found in the low-temperature regime. This is in agreement with a zero-temperature variational calculation. At temperature just above the two-dimensional (2D) vortex-unbinding temperature, the linear term vanishes and an ordinary 2D vortex behavior is found. This explains the finding that high-Tc superconductors show 2D properties in the vortex fluctuations responsible for the resistivity transition close to the critical temperature.

  • 36.
    Weber, Hans
    et al.
    Nordic Institute for Theoretical Physics (NORDITA), Copenhagen, Denmark.
    Jensen, Henrik Jeldtoft
    Nordic Institute for Theoretical Physics (NORDITA), Copenhagen, Denmark.
    Monte Carlo study of the mean-field theory of the three-dimensional anisotropic XY model1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 46, no 18, p. 11826-11829Article in journal (Refereed)
    Abstract [en]

    We use Monte Carlo simulations of a two-dimensional XY model in a magnetic field to study a self-consistent mean-field theory for the three-dimensional anisotropic XY model. The relation between the critical temperature Tc and the interplane coupling J⊥ is determined. The magnetization exponent β is discussed and results for the specific heat cv are presented.

  • 37.
    Weber, Hans
    et al.
    Department of Theoretical Physics, Umeå University, S-90 187 Umeå, Sweden.
    Minnhagen, Petter
    Department of Theoretical Physics, Umeå University, S-90 187 Umeå, Sweden.
    Monte Carlo determination of the critical temperature for the two-dimensional XY model1988In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 37, no 10, p. 5986-5989Article in journal (Refereed)
  • 38.
    Weber, Hans
    et al.
    Department of Theoretical Physics, Umeå University, S-90187 Umeå, Sweden.
    Minnhagen, Petter
    Department of Theoretical Physics, Umeå University, S-90187 Umeå, Sweden.
    Monte Carlo simulations for a nonlocal sine-Gordon theory, vortex fluctuations, and connections to superconducting films1988In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 38, no 13, p. 8730-8738Article in journal (Refereed)
  • 39.
    Weber, Hans
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wallin, Mats
    Department of Theoretical Physics, Royal Institute of Technology.
    Jensen, Henrik Jeldtoft
    Department of Mathematics, Imperial College, London.
    Monte Carlo calculation of the current-voltage characteristics of a two-dimensional lattice Coulomb gas1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 53, no 13, p. 8566-8574Article in journal (Refereed)
    Abstract [en]

    We have studied the nonlinear current-voltage characteristic of a two-dimensional lattice Coulomb gas by Monte Carlo simulation. We present three different determinations of the power-law exponent a(T) of the nonlinear current-voltage characteristic, V∼Ia(T)+1. The determinations rely on both equilibrium and nonequilibrium simulations. We find good agreement between the different determinations, and our results also agree closely with experimental results for Hg-Xe thin-film superconductors and for certain single crystal thin-film high-temperature superconductors.

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  • 40.
    Winter, J.
    et al.
    Institut für Materialphysik, Universität Wien Strudlhofgasse 4, A-1090 Wien, Austria.
    Kuzmany, H.
    Institut für Materialphysik, Universität Wien Strudlhofgasse 4, A-1090 Wien, Austria.
    Soldatov, Alexander
    Department of Experimental Physics, Umeå University, S-901 87 Umeå, Sweden.
    Persson, P-A
    Department of Experimental Physics, Umeå University, S-901 87 Umeå, Sweden.
    Jacobsson, P.
    Department of Experimental Physics, Umeå University, S-901 87 Umeå, Sweden.
    Sundqvist, B.
    Department of Experimental Physics, Umeå University, S-901 87 Umeå, Sweden.
    Charge transfer in alkali-metal-doped polymeric fullerenes1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 54, no 24, p. 17486-17492Article in journal (Refereed)
  • 41.
    Öberg, Sven
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Stich, P K
    University of Exeter.
    Jones, R
    University of Exeter.
    Heggie, M I
    University of Exeter.
    First-principles calculations of the energy barrier to dislocation motion in Si and GaAs1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, no 19, p. 13138-13145Article in journal (Refereed)
    Abstract [en]

    The single kink formation and migration energies Fk and Wm of 90° glide partial dislocations in Si and GaAs are calculated using an ab initio local density-functional cluster method. Kink migration occurs via a concerted exchange of an atom at a dislocation core with one of its glide plane nearest neighbors. By constraining these atoms to sit in high-energy positions and relaxing a surrounding cluster of atoms, sufficient points in configuration space can be sampled for the energy barrier for the first step in kink pair formation to be estimated. By including an estimate of the elastic energy of the interaction of kink pairs, the single kink formation energy is calculated. It is found that Fk and Wm for Si are 0.1 and 1.8 eV, respectively. For the 90° α glide partial in GaAs, these quantities are 0.07 and 0.7 eV, respectively, and 0.3 and 1.1 eV for β partials

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