| 1. |
- Bäckström, Joakim, et al.
(författare)
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Raman scattering in \RmYBa$_2$Cu$_4$O$_8$ and PrBa$_2$Cu$_4$O$_8$ : Indications of pseudogap effects in non-superconducting \RmPrBa$_2$Cu$_4$O$_8$
- 2000
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 61, s. 7049-7054
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Tidskriftsartikel (refereegranskat)abstract
- By analysis of the Raman spectra of superconducting YBa2Cu4O8 (Y-124), we observe a suppression of low-energy electronic density of states that we link to changes in the linewidth of the Ba-phonon mode. These effects occur at a temperature below 150 K, where also anomalies in resistivity and other physical properties take place. We argue that these changes in the Raman spectra of Y-124 are connected to the opening of a pseudogap. Analogous anomalies in resistivity and the Ba-phonon width in the nonsuperconducting homologue PrBa2Cu4O8 (Pr-124) below 150 K suggest that similar pseudogap effects can occur without a following transition to a superconducting state at a lower temperature.
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| 2. |
- Bäckström, Joakim, et al.
(författare)
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Raman scattering in YBa2Cu4O8 and PrBa2Cu4O8 - indications of pseudogap effects in non-superconducting PrBa2Cu4O8
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 341, s. 2251-2252
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Tidskriftsartikel (refereegranskat)abstract
- By analysis of the temperature evolution of Raman spectra, we connect an observed supression in low-energy electronic density of states to an anomalous sharpening of the Ba-phonon mode in superconducting YBa2Cu4O8 (Y-124) at around T=150 K. We link the supression of electronic excitations to an opening of a pseudogap. Similar effects observed for the Ba-phonon mode at roughly the same temperature in non-superconducting PrBa2Cu4O8 (Pr-124) suggests that a pseduogap opens up around 150 K also in this material.
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| 3. |
- Abrashev, M. V., et al.
(författare)
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Raman spectroscopy of the charge- and orbital-ordered state in La0.5Ca0.5MnO3
- 2001
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Ingår i: Physical Review B. ; 64:14
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Tidskriftsartikel (refereegranskat)abstract
- Polarized Raman spectra from microcrystals of La0.5Ca0.5MnO3 were studied as a function of temperature, excitation photon energy, and scattering configuration. At temperatures below the transition from ferromagnetic metallic phase to antiferromagnetic insulating charge- and orbital-ordered (COO) phase, several lines appear in Raman spectra excited with photon energies (h) over bar omega (L)<2.5 eV. We argue that their appearance signals ordering and freezing of the Jahn-Teller distortions at this transition temperature into a superstructure with a doubled elementary cell. We suggest a simplified structural model based on an analysis of the normal phonon modes in the COO phase, neglecting the octahedral tilts and containing ordered Mn4+O6 (undistorted) and Mn3+O6 (Jahn-Teller distorted) octahedra. The symmetries of the experimentally observed Raman lines are determined by comparing their relative intensities to those predicted by the polarization selection rules for a finely twinned quasicubic crystal. The most intensive Raman lines are assigned to definite normal modes in close comparison with corresponding modes in COO layered manganites and undoped RMnO3 (R=La, Y).
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| 4. |
- Osada, M., et al.
(författare)
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Photoinduced effects and oxygen dynamics in YBa2Cu3Ox
- 2001
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 364, s. 545-548
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen-deficient YBa2Cu3Ox (Y123) exhibits persistent photoconductivity when illuminated by visible light. We have investigated resonant Raman spectra as a function of excitation wavelength and temperature, which shed new light on photoinduced effects and chain oxygen dynamics in Y123. Experimental evidence suggests that the mechanism behind the unusual photoinduced effects primary involves photoinduced oxygen ordering. Our Raman data also constitute the non-diffraction observations of the superstructure phase transition in Y123. (C) 2001 Elsevier Science B.V. All rights reserved.
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| 5. |
- Osada, M., et al.
(författare)
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Resonant Raman scattering and photoinduced metastability in oxygen-deficient YBa2Cu3Ox
- 2000
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Ingår i: Physica C. ; 338, s. 157-160
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Tidskriftsartikel (refereegranskat)abstract
- We studied the photoinduced metastable states in oxygen-deficient YBa2Cu3Ox (Y123) (x = 6.35-6.87). The states are identified by the intensity dependences of CuO-chain resonance on photo irradiation and temperature. Below room temperature (RT) the CuO-chain resonance can be completely suppressed by irradiation with visible light polarized along the CuO chains, indicating photo-assisted oxygen ordering. Above RT, the resonance intensity returns and saturates at T > 100 degrees C. This behavior exactly reflects the temperature dependence of chain oxygen ordering in the same crystals observed by hard X-ray diffraction. The stability of the photo-induced state is in agreement with reported persistent photo-induced superconductivity effects. (C) 2000 Elsevier Science B.V. All rights reserved.
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| 6. |
- Rauer, R., et al.
(författare)
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Thickness dependent phase separation in La0.7Ca0.3MnO3 films
- 2002
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Ingår i: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 81, s. 3777-3779
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Tidskriftsartikel (refereegranskat)abstract
- The complex dielectric function of La0.7Ca0.3MnO3 films with various thicknesses and on different substrates was determined by the analysis of measured ellipsometric parameters. The temperature-dependent redistribution of low-energy spectral weight shows that the metallicity is developed gradually below an onset temperature that is found to depend strongly on global and local strain properties, not on film thickness alone. Importantly, the 2 nm film shows no metallic conduction but an optical charge-carrier response below 240 K. This suggests that ultrathin films are characterized by phase separation and a percolation-type transition. (C) 2002 American Institute of Physics.
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| 7. |
- Abrashev, M. V., et al.
(författare)
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Raman spectroscopy of CaMnO(3) : Mode assignment and relationship between Raman line intensities and structural distortions
- 2002
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 65:18, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Polarized Raman spectra of oriented finely twinned CaMnO(3) thin films and nonpolarized Raman spectra of CaMnO(3) ceramics were studied at room temperature using several excitation laser wavelengths. The selection rules for the polarized Raman spectra, obtained from samples consisting of finely twinned orthorhombic domains, were calculated and the symmetry of all observed Raman lines was determined. The relationship between the intensity of the Raman lines and the distortions in the ABO(3) perovskites with GdFeO(3)-type structure is discussed. These distortions can be described as superposition of four simple basic distortions: two MnO(6) octahedral tilts, Jahn-Teller deformation of Mn(3+) O(6) octahedra, and shift of the A ions from their sites in the ideal perovskite. Twenty of the 24 Raman-allowed modes in the real GdFeO(3)-type structure have counterparts in only one of the four simpler structures, obtained by a single basic distortion. The assignment of the Raman lines of CaMnO(3) to definite atomic vibrations, most of them activated by a single basic distortion, was made in close comparison with the results of lattice dynamical calculations and the Raman spectra of isostructural LaMnO(3) and CaGeO(3).
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| 8. |
- Bjornsson, P., et al.
(författare)
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Lattice and charge excitations in La1-xSrxMnO3
- 2000
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 61, s. 1193-1197
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Tidskriftsartikel (refereegranskat)abstract
- We employ inelastic Light scattering to study the dependence of the lattice and charge excitations in La1-xSrxMnO3 compounds on doping and temperature. The phonons reveal strong local lattice distortions in the paramagnetic state, which gradually vanish below the ferromagnetic transition. We identify charge excitations in the metallic state. They exhibit a dependence on the doping level and symmetry selection rules typical for a plasmonlike excitation. Their energy scale of 100 meV requires a low-carrier-density component of the plasma outlining the importance of electronic interactions in La1-xSrxMnO3.
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| 9. |
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| 10. |
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