| 1. |
- Blomgren, Fredrik, 1973, et al.
(författare)
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Exploring the potential energy surface of retinal, a comparison of the performance of different methods
- 2005
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 26:7, s. 738-742
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Tidskriftsartikel (refereegranskat)abstract
- The ground state structure of retinal has been investigated. We found that DFT and CASSCF produce different results for the bond length alternation in a model system of retinal. Quantum mechanics/molecular mechanics calculations including the closest surrounding amino acids have been performed, using DFT and CASSCF to calculate the structure of retinal in the protein cavity. The planarity of the retinal molecule is affected by the surrounding protein. DFT and CASSCF produce different twist angles. The difference between CASSCF and DFT appears to be related to the positively charged nitrogen of the Schiff base, which leads to different π-bond orders produced by the two methods.
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| 2. |
- Blomgren, Fredrik, 1973, et al.
(författare)
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Primary photoprocess in vision: Minimal motion to reach the photo- and Bathorhodopsin Intermediates
- 2005
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 109:18, s. 9104-9110
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Tidskriftsartikel (refereegranskat)abstract
- According to time-resolved spectroscopic measurements, the initial step of the photoreaction of rhodopsin occurs with a time constant of approximately 200 fs. The whole or a part of the retinal molecule cannot move any significant distance in such a short time. In this paper, we propose instead a minimal motion that accomplishes the important task of guiding the molecule to a configuration where it can decay to the ground-state surface, with a minimal loss of strain energy. This motion is proposed to involve a -90° twisting of the C11=C12 double bond and a simultaneous twisting around two other double bonds in retinal to minimize the geometrical changes along the reaction path. The ONIOM method (complete active space self-consistent field for retinal and AMBER for the peptides) is used in a chromophore-cavity model to elucidate and confirm important features of the mechanism. The potential energy surface (PES) obtained according to the proposed mechanism show all of the characteristics of a fast photoreaction, meaning a downhill reaction path from the Franck-Condon point to an avoided crossing between S1 and S0. In this motion, only a few carbon and hydrogen atoms move more than 0.3 Å, and the retinal structure is conserved in the protein cavity. We propose that the photorhodopsin intermediate is a retinal molecule formed on the excited-state PES. Bathorhodopsin, however, is a ground-state intermediate, still located inside the protein cavity.
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| 3. |
- Ivashin, N., et al.
(författare)
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Trapped water molecule in the charge separation of a bacterial reaction center
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 112:38, s. 12124-12133
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Tidskriftsartikel (refereegranskat)abstract
- Low-frequency oscillations in the absorption spectrum at 1020 nm, connected to the primary charge separation process in Rhodobacter sphaeroides, have been shown by Yakovlev et al. to be caused by rotational motion of an interstitial water molecule called "water-A". The same water molecule was shown by Potter et al. to increase the rate of charge separation by a factor of 8. We have carried out geometry optimization of water-A and its nearest atoms in the protein pocket, using density functional theory (DFT). There are strong hydrogen bonds to the axial imidazol group of the B part of the special pair (P = P A P B ) and to the keto carbonyl group of ring V of the accessory chlorophyll (B A ). Rotation of water-A is thus impossible in the electronic ground state. We have tried to support our speculations on other possible mechanisms by calculations. The P + B A - charge transfer state is stabilized by proton transfer from water-A and simultaneous proton transfer from the axial group of P B to water-A. After double proton transfer the hydrogen bond to the keto group disappears whereby a possibility opens up for almost free water rotation. The results therefore would explain the 32 cm -1 oscillation of Yakovlev et al. The proposed mechanism assumes, however, that the general assumption that the activation energy disappears in the primary charge separation of bacterial photosynthesis, holds also for this special case. © 2008 American Chemical Society.
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| 4. |
- Kilså, Kristine, et al.
(författare)
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Enhanced Intersystem Crossing in Donor/Acceptor Systems Based on Zinc/Iron or Free-Base/Iron Porphyrins
- 2001
-
Ingår i: Chemistry, A European Journal. - 1521-3765 .- 0947-6539. ; 7:10, s. 2122-33
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Tidskriftsartikel (refereegranskat)abstract
- The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(iii) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 Å donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Förster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f2779_s.pdf or from the author. 1H NMR spectra of FeP, FeP-OB, FeP-BB, FeP-NB, FeP-AB, ZnP-OB-FeP, ZnP-BB-FeP, ZnP-NB-FeP, and ZnP-AB-FeP.
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| 5. |
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| 6. |
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| 7. |
- Larsson, Sven, 1941
(författare)
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Applications of CASSCF
- 2011
-
Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 111:13, s. 3424-3430
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Tidskriftsartikel (refereegranskat)abstract
- Multiconfigurational wave functions are necessary in a number of applications such as breaking of bonds and curve crossing problems. One of the most important multiconfigurational methods is CASSCF. Three applications are discussed here: cis-trans isomerization, as a reasonably simple curve crossing problem. Curve crossing connected to electron-transfer contains the additional problem that the electronic interaction vertical bar Delta vertical bar may tend to zero. The electron pair transfer problem includes multiconfigurational wave functions and requires a very accurate calculation of the interaction between two configurations, the charged and spin-coupled configurations.
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| 8. |
- Larsson, Sven, 1941
(författare)
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Charge Disproportionation and Superconductivity in Tl- and In-doped Lead Telluride
- 2012
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 25:2, s. 319-323
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Tidskriftsartikel (refereegranskat)abstract
- The "negative-U model" is analyzed and applied to Tl- and In-doped lead telluride, Pb1-x Tl (x) Te, and Pb1-x In (x) Te, respectively. The former, but not the latter, is superconducting (T (c) 0.003). At very low concentrations, carriers are created in the valence band and the conductivity is metallic, of the ordinary kind. Matsushita et al. and Erickson et al. explain this as due to disproportionation 2Tl(2+)-> Tl+ and Tl3+. The latter authors are of the opinion that indium also disproportionates in the same way as thallium. If this would be the case, it would be hard to explain the absence of superconductivity at indium doping. In this paper it is shown that In2+ does not disproportionate under the given circumstances. The Hubbard model, extended and generalized, is therefore consistent with all experimental results.
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| 9. |
- Larsson, Sven, 1941
(författare)
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Charge transfer transitions in cuprates
- 2010
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 491:1-3, s. 49-53
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Tidskriftsartikel (refereegranskat)abstract
- Absorption spectra of cuprates are discussed. Persistent photo-induced conductivity occurs in the visible spectrum (similar to 2 eV) and is commonly assigned to ligand-metal (LM) charge transfer (CT) transitions. However, LM CT is site local and cannot possibly generate persistent charges. The assignment in this Letter is 'metal to adjacent metal' (MM) CT transitions, while the absorption at hv > 3 eV is still assigned to mainly LM CT. Only MM CT, defining the Mott-Hubbard gap, is exclusively polarized in the CuO2 plane, as found experimentally. Since MM CT is strongly affected by the local electric field, doping transfers spectral weight to the IR region. (C) 2010 Published by Elsevier B.V.
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| 10. |
- Larsson, Sven, 1941
(författare)
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Conductivity in Cuprates Arises from Two Different Sources: One-Electron Exchange and Disproportionation
- 2017
-
Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 30:2, s. 275-285
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Tidskriftsartikel (refereegranskat)abstract
- Simulation of the resistivity in the normal state of doped La2-x Sr (x) CuO4 has been performed using a hopping model based on Marcus theory. The results are in substantial agreement with experimental results. At oxidative doping, Cu(III) sites are formed and electron mobility possible due to hopping: Cu(III)Cu(II) -> Cu(II)Cu(III) (one-electron exchange). In the underdoped, non-metallic region, the resistivity (rho) decreases from almost insulation at T = 0 to a minimum at about T = 100 K. rho then increases more than linearly with T (similar to T (3/2)) in the region 100 Cu(I) + Cu(III) is responsible for the strong absorption in the visible spectrum of La2CuO4. The down-shift of spectral density with doping (x) in La2-x Sr (x) CuO4 depends on the appearance of Cu(III) sites which makes optical as well as thermal one-electron exchange transitions possible with lower energy. Disproportionation occurs spontaneously for x > 0.06, opening up for electron pair formation. Configuration interaction between two-electron states of low chemical potential, but strong vibrational coupling, gives rise to the superconductor and pseudogaps. Data from photo-induced conductivity and absorption spectra are used in the simulation, which gives results in good agreement with experiments. Possible explanations for Raman and MIR absorption suggest themselves.
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| 11. |
- Larsson, Sven, 1941
(författare)
-
Conductivity Properties of Perovskite Nickelates and Cuprates Depend on the Oxidation States of the Metal Ions
- 2022
-
Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 35:11, s. 3101-3107
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Tidskriftsartikel (refereegranskat)abstract
- Superconducting nickelates in thin film perovskite structures, designed to be isostructural with the Cu(II) cuprates, have been discovered recently by Zhou et al. (Appl. Mater. Interfaces 10, 1463-1467, 1) and Li et al. (Nature 572, 624-627, 2) The strategies of both groups, to make nickelates as similar to the cuprates as possible by using planar NiO2 superlattice structures, proved successful. Ni(I) and Ni(III) are superconducting in superlattices, although TC is lower than in most Cu(II) cuprates. Here we find that a “3-oxidation states rule” applies to nickelate superlattices based on Ni(I) and Ni(III). It is predicted that the oxides of Ni(II), Cu(I), and Cu(III) cannot superconduct, because of violation of the 3-rule. This rule is derived here, along with other rules where the number of interacting oxidation states is decisive for the conduction properties of the metal ion. The Mott model is replaced by a free energy model.
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| 12. |
- Larsson, Sven, 1941
(författare)
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Correlation effects and superconductivity in cuprates: A critical account
- 2011
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Ingår i: Superconducting Magnets and Superconductivity: Research, Technology and Applications. - 9781607410171 ; , s. 3-53
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Electronic structure of transition metal oxides and salts is reviewed, particularly what concerns electron transfer, electron correlation, electron-nuclear coupling, and inter-metal interaction in cuprates. A short resume of electronic correlation in molecular systems is first given. Electron pair transfer is treated in a many-electron real space approach using standard mixed-valence theories. The latter models have been successful in the past in describing the response of the electrons to the motion of the nuclei (electron - phonon interaction). The possibility for multiple oxidation states of the metal ion is typical for transition metal compounds. Mott-Hubbard-U is strictly defined, its dependence on breathing mode coordinates analysed, and the connection between U and the energy gap for superconductivity clarified. Delocalization is treated in terms of an extended Hush model and electron pair supercurrents derived at the Van Hove degeneracy. d-wave gap anisotropy is found to be consistent with the general atomic level model presented here. Softening of phonon half-breathing modes in inelastic neutron scattering (INS) can be connected to mixed-valence. The fundamental vibronic interaction between spin density wave (SDW) and charge density wave (CDW) states leads to a new phase with energy gap and electron pair carriers. Finally comments are made on why MV-2 systems delocalize as Fermi- Dirac systems (ordinary metals) while MV-3 systems delocalize and condense as Bose-Einstein systems (superconductors).
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| 13. |
- Larsson, Sven, 1941
(författare)
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Correlations between Spectra and Resistivity in Transition Metal Oxides
- 2019
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 123:44, s. 9449-9455
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Tidskriftsartikel (refereegranskat)abstract
- A comparison between photoconductivity spectra and resistivity in two transition metal oxides, La2-xSrxCuO4 and La1-xSrxVO3, is presented. The resistivities ρ(T) for x < 0.05 in the cuprate and x < 0.28 in the vanadate are typical for single electron transfer. For T > 100 K, ρ(T) - ρ(0) ∼T3/2. For higher dopings (x) the cuprate is a superconductor (x < 0.25) and the vanadate an ordinary metal. This tallies with the number of oxidation states and their spins when the electrons transfer locally. The insulator-metal transition and the vanishing of Cooper pairs are discussed in the conclusion.
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| 14. |
- Larsson, Sven, 1941
(författare)
-
Effect of Pressure on Superconducting Properties
- 2015
-
Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 28:6, s. 1693-1698
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Tidskriftsartikel (refereegranskat)abstract
- It is shown that the existence of superconductivity in a material, and its critical temperature, depends strongly on pressure. Several parameters are pressure-dependent: (1) structure, particularly bond distances, (2) Hubbard U, (3) coupling between sites, and (4) orbital occupation number. Eliashberg theory often leads to incorrect predictions, for example in A(3)C(60) with A = K, Rb, and Cs. While T (C) is correctly predicted to be higher for Rb3C60 than for K3C60 and decreasing with pressure in both cases, Cs3C60 is not superconducting at ambient pressure. The same is the case for pure metals such as Cs and Ca (superconducting at high pressure). A theory for electron pairs, similar to the Marcus model for single electrons, appears to agree with the experiment in most cases.
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| 15. |
- Larsson, Sven, 1941
(författare)
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Electronic and vibrational structure of one-dimensional conductors and superconductors
- 2006
-
Ingår i: Faraday Discussions. - 1359-6640 .- 1364-5498. ; 131, s. 69-77
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Tidskriftsartikel (refereegranskat)abstract
- An attempt is made to treat molecular wires by quantum chemical methods. What is the electronic difference between systems (molecules, stacks of molecules, or polymers) that are conducting and almost identical systems that are insulating? At 50% band filling a one-dimensional crystal undergoes a Peierls transition and becomes an insulator (without doping). In the case of 75% band filling, on the other hand, two phases are possible: charge density wave (CDW) and spin-density wave (SDW). The transition between these two insulating phases should be connected to a high conductivity. If CDW and SDW are energetically possible at zero T, vibrational coupling leads to stabilization of a superconducting (SC) ground state and the formation of an energy gap. This hypothesis is exemplified on (SN)(x) and (TMTSF)(2)X (X = ClO4, PF6).
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| 16. |
- Larsson, Sven, 1941
(författare)
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Fysikalisk kemi
- 2008
-
Bok (övrigt vetenskapligt/konstnärligt)
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| 17. |
- 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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| 18. |
- 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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| 19. |
- 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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| 20. |
- 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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| 21. |
- 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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| 22. |
- 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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| 23. |
- 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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| 24. |
- 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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| 25. |
- Larsson, Sven, 1941
(författare)
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Strong electron correlation and phonon coupling in high T-c superconductors
- 2007
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Ingår i: Physica C: Superconductivity and its Applications. - : Elsevier BV. - 0921-4534. ; 460, s. 1063-1065
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Tidskriftsartikel (refereegranskat)abstract
- Systems with one active electron (or hole) per site may be spin-alternating (SDW) or charge alternating (CDW). Typical SDW systems (CuO) cannot be described with a single Slater determinant. Electron correlation is also important in typical CDW systems such as BaBiO3 (on-site correlation). If the CDW and the SDW phases have similar free energies, they are either connected by a phase transition, or, if the zero-point vibrations overlap sufficiently, mix quantum mechanically and form a new phase. It will be shown, using very accurate quantum chemical calculations that a four-site Cu-O cluster has local energy minima for both phases. The properties of the mixed phase are those of a superconductor: The electrons are coupled in pairs, move in a delocalized non-activated way, and give a current in a magnetic field equivalent to the Meissner effect. (c) 2007 Elsevier B.V. All rights reserved.
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| 26. |
- Larsson, Sven, 1941
(författare)
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Superconductivity in Cuprates: Details of Electron Phonon Coupling
- 2010
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 110:5, s. 1117-1126
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Tidskriftsartikel (refereegranskat)abstract
- The Bardeen-Cooper-Schrieffer (BCS) model explains superconductivity (SC) as due to correlation between electronic momentum and nuclear momentum (phonons) in a free electron gas. The BCS model lacks chemical specificity, however, since the coupling mechanism is left unspecified. After the discovery of high T-C superconductivity in 1986 it was concluded that electron-phonon interactions are insufficient to explain electron pairing. A large part of theoretical research has since been aiming at finding another mechanism that would allow us to consider the superconducting system as a gas of charged free bosons. However, there appears to be no reason to assume free electrons in oxides. In this article the free-electron criterion is therefore replaced by the criterion that a pair of electrons can move freely between sites without resistance, i.e., without activation energy. Electron pair transfer is treated in a many-electron real space approach using standard mixed-valence theories. Mott-Hubbard-U is strictly defined, its dependence on breathing mode coordinates analyzed, and the connection between U and the energy gap for superconductivity clarified. d-wave gap anisotropy is found to be consistent with the general atomic level model presented here. Softening of phonon half-breathing modes in inelastic neutron scattering (INS) is connected to mixed-valency. The fundamental vibronic interaction between spin density wave (SDW) and charge density wave (CDW) states leads to a new phase with energy gap and electron pair carriers that can only be the superconducting phase. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 1177-1126, 2010.
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| 27. |
- Larsson, Sven, 1941
(författare)
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T-C Dependence on Hubbard U in the Fullerenes
- 2015
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1947 .- 1557-1939. ; 28:2, s. 315-317
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Tidskriftsartikel (refereegranskat)abstract
- Pair formation occurs when the free energy for charge disproportionation is negative, i.e., U 0 is the difference between ionization energy and electron affinity, and R is the distance between the lattice sites. -1/R (atomic units) is the particle-hole attraction after charge separation in the case of two C-60 ions, the much needed negative term. In the case of alkali C-60 salts of type A(3)C(60), the attractive term 1 /R is the smallest for the largest ion A(+), i.e., Cs+. Hence, U is increasing with the lattice constant R, i.e., with the size of the alkali ion. Lack of superconductivity in Cs3C60 can be interpreted as U > 0 for this salt, while U 4 kbar.
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