| 1094421. |
- Larsson, Sven, 1941
(författare)
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Fysikalisk kemi
- 2008
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Bok (övrigt vetenskapligt/konstnärligt)
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| 1094422. |
- Larsson, Sven, 1941
(författare)
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Hubbard-U and disproportionation in superconducting boron doped diamond
- 2012
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Ingår i: Diamond and Related Materials. - : Elsevier BV. - 0925-9635. ; 26, s. 71-77
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Tidskriftsartikel (refereegranskat)abstract
- Synthetic and natural diamonds containing small amounts of boron are conducting with apparent activation energy of 0.37 eV at high temperature and approximate to 0.01 eV at low temperature. If the boron/carbon ratio is increased above about 1 parts per thousand, the higher activation energy starts to decrease to zero. By applying high pressure and high temperature Ekimov et al. managed to raise the B/C ratio above 1%. This material is superconducting below a few Kelvin. It is shown here that at low B/C ratio the MIR absorption at 0.37 eV can be identified with vertical Hubbard-U. Increasing the boron concentration Hubbard-U decreases to zero. Apparently B+/B- becomes more stable than B/B as a sub-lattice phase. It is shown that this is due to the smaller distance between the boron sites. The presence of B+, B, and B-_(-) in highly boron doped diamond is supported by NMR, Raman, and IR data in combination with simple calculations.
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| 1094423. |
- Larsson, Sven, 1941
(författare)
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Localization of electrons and excitations
- 2006
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 326:1, s. 115-122
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Tidskriftsartikel (refereegranskat)abstract
- Electrons, electron holes, or excitations in finite or infinite 'multimer systems' may be localized or delocalized. In the theory of Hush, localization depends on the ratio Delta/lambda(Delta/2 = coupling lambda = reorganization energy). The latter theory has been extended to the infinite system [S. Larsson, A. Klimkans, Mol. Cryst. Liq. Cryst. 355 (2000) 217]. The metal/insulator transition often takes place abruptly as a function of,Delta/lambda. It is argued that localization in a system with un-filled bands cannot be determined on the basis of Mott-Hubbard U alone, but depends on the number of accessible valence states, reorganization energy lambda and coupling Delta (=2t). In fact U = 0 does not necessarily imply delocalization. The analysis here shows that there are many different situations for an insulator to metal transition. Charge transfer in doped NiO is characterized by Ni2+-Ni3+ exchange while charge transfer in pure NiO is characterized by a disproportionation 2Ni(2+)Ni(+)-> Ni3+. In spite of the great differences between these two cases, U has been applied without discrimination to both. The relevant localization parameters appear to be Delta and lambda in the first case, with only two oxidation states, and U,Delta and lambda in the second case with three oxidation states. The analysis is extended to insulator-metal transitions, giant magnetic resistance (GMR) and high T-c superconductivity (SC). lambda and Delta can be determined quite accurately in quantum mechanical calculations involving only one and two monomers, respectively. (c) 2006 Elsevier B.V. All rights reserved.
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| 1094424. |
- Larsson, Sven, 1941
(författare)
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Microscopic theory for insulator to metal transition in cuprates
- 2012
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 112:7, s. 1829-1837
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Tidskriftsartikel (refereegranskat)abstract
- Hubbard U is equal to the energy of the insulator to metal transition in Mott insulators. Hubbard U is, therefore, also the excitation energy for metal atom to other metal atom transitions and the key concept in photoinduced conductivity spectra of pure and doped cuprates. In this article, the electronic structure of Cu(II) and Cu(III) sites is first discussed. It is shown that Hubbard U depends on the location of the excitation in the CuO2 plane relative to the Cu(III) sites. Far from the Cu(III) sites, the ground-state wave function continues to be of spin-coupled type. Near to a Cu(III) site, it changes character and is mixed with charge components. The Hubbard gap thus depends on locality in the CuO2 plane. Close to a Cu(III) site, it tends to zero and induces local conductivity. Far from a Cu(III) site, it is large but converges to zero as the doping levels are raised. In fact, the Hubbard gap has many features in common with the pseudogap.
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| 1094425. |
- Larsson, Sven, 1941
(författare)
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Mixed valence and superconductivity
- 2008
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Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-503X .- 1471-2962. ; 366:1862, s. 47-54
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Tidskriftsartikel (refereegranskat)abstract
- Mixed-valence (MV) systems are referred to here as MV-2 and MV-3 depending on whether two or three consecutive valence states are involved. MV-3 systems range from systems with Hubbard U≫0, corresponding to a single stable, intermediate valence state, and U≪0, corresponding to stable alternating valences differing by two units. Experiments using inelastic neutron scattering or inelastic X-ray scattering show softening of breathing phonon modes in MV systems compared with related systems with a single valence. It is hypothesized that softening is due to coupling between potential energy surfaces, corresponding to differing localizations of the electron. As predicted, softening is larger in the delocalized case. A mechanism for superconductivity is suggested. © 2007 The Royal Society.
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| 1094426. |
- Larsson, Sven, 1941
(författare)
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Phonons in A3C60 Lattice and Structural Dynamics
- 2010
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Ingår i: Advances in Condensed Matter Physics. - : Hindawi Limited. - 1687-8108 .- 1687-8124. ; 2010, s. Art. no. 627452-
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Tidskriftsartikel (refereegranskat)abstract
- The critical temperature (TC) of superconductivity in A3C60 compounds is generally lower smaller with alkali atoms (A).Furthermore TC decreases with applied pressure. In the BCS model, these trends are explained by the lower density of states at the Fermi level for a decreased lattice constant (R). There is more than one counterexample, however, suggesting that BCS does not give the whole truth. The most important one is that the compound with the largest lattice constant, Cs3C60, is not superconducting at all at ambient pressure. In this paper we derive a novel model where a negative lattice contribution to Hubbard U, proportional to 1/R, is taken into account. It is possible to explain why A3C60 compounds with A = Li, and Na have a low TC or are not superconducting at all, and why Cs3C60 is superconducting only at applied pressure and then with the highest TC of all C60 alkali fullerides. It is concluded that the density of states mechanism derived in the BCS model is in doubt. Nevertheless superconductivity in A3C60 depends on electron-phonon coupling. The dominating phonon is the bond stretching Ag phonon, a breathing phonon for the whole fullerene molecular ion.
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| 1094427. |
- Larsson, Sven, 1941
(författare)
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Pseudogap in Boron-Doped Diamond and Cuprates
- 2013
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 26:4, s. 1089-1091
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Tidskriftsartikel (refereegranskat)abstract
- The Marcus model, well known as a general model for electron transfer, is applied to electron pair formation and transfer in systems characterized by neighboring superconducting and local phases, particularly boron-doped diamond and cuprates. Hubbard-U is identified with the adiabatic free energy difference between charged and spin-coupled configurations. Typical for the Marcus model is the coupling of electronic and nuclear coordinates due to structural changes with number of electrons during electron transfer or excitation.
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| 1094428. |
- Larsson, Sven, 1941
(författare)
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Quantum Chemistry and Superconductors
- 2017
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Ingår i: Advances in Quantum Chemistry. - : Elsevier. - 0065-3276. ; 74, s. 209-226
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Thirty years after the discovery of high temperature (HT) superconductivity (SC), no by all accepted theory exists. The Bardeen, Cooper, Schrieffer (BCS) model, hewed into the Bloch theory for metals, is unfit for local systems such as cuprates and organic superconductors. In this chapter, we will use a theory that dates back to Landau and Pekar, but we will avoid the effective mass approach by using a total free energy model, as designed for electron transfer problems by Marcus and Jortner. A diffusion equation is used to derive the resistivity in the local case. The original definition of Hubbard U by Mott as a metal-to-metal (or molecule-to-molecule) charge transfer energy will be updated by including the neglected negative terms. It will be shown that the absorption at 2 eV in the cuprates is indeed due to Cu–Cu charge transfer, identical to the Hubbard U or Mott transition. The model accounts for bond-length fluctuations due to occupancy of d-orbitals (extended over the ligands), or in the molecular case the ? orbitals, and this makes it necessary to make a distinction between adiabatic and vertical Hubbard U. Uvert = 1.5–3 eV while Uad may be a few hundred times smaller. Organic SC in aromatic hydrocarbons will be shortly reviewed and found consistent with the general model. Finally, we will discuss SC in tungsten bronzes discovered in 1964 by Matthias.
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| 1094429. |
- Larsson, Sven, 1941
(författare)
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Resistivity in Cuprates and the Pseudogap
- 2017
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 30:3, s. 777-782
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of resistivity (rho) in the normal state of La2-x Sr (x) CuO4 is calculated from electron transfer (ET) rates. The minimum at about 100 K for doping levels less than 6 % is characteristic for all cuprates. The resistivity depends on ET between Cu(III) and Cu(II) sites. For T 100 K depends on increased kinetic energy in a Landau avoided crossing. The agreement with experimental data is convincing. The behaviour for T -> 0 can be explained as nuclear tunnelling. For T > 400 K, rho bends down, presumably because the pseudogap is overcome at a higher kinetic energy.
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| 1094430. |
- Larsson, Stefan, 1976, et al.
(författare)
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Self-optimising control of an SI-engine using a torque sensor
- 2008
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Ingår i: Control Engineering Practice. ; 16:5, s. 505-514
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Tidskriftsartikel (refereegranskat)abstract
- Control of combustion phasing using a torque sensor is developed for a spark-ignited engine to demonstrate a method to increase its efficiency. Combustion phasing is quantified based on the torque signal using the torque ratio concept for combustion modelling. The combustion phasing is adjusted using the spark advance as control signal. An extremum-seeking controller finds the optimal combustion phasing in each cylinder. The combustion phasing is extracted using an algorithm, which considers the torsional resonances in the crankshaft. The torque-based control system is successfully evaluated on-line using internal EGR changes as combustion disturbances.
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