Optical excitations in Bi sub 2 Sr sub 2 CuO sub 6 and Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8 : Evidence for localized (excitonic) and delocalized charge-transfer gaps

Journal Article · Tue Jan 01 04:00:00 EST 1991 · Physical Review, B: Condensed Matter; (USA)

DOI:https://doi.org/10.1103/PhysRevB.43.1241· OSTI ID:5945189

Wang, Y; Ritter, A L ^[1]

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1. Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (US)

The optical properties of Bi{sub 2}Sr{sub 2}CuO{sub 6} (2:2:0:1) and Bi{sub 2}Sn{sub 2}CaCu{sub 2}O{sub 8} (2:2:1:2) have been investigated by transmission electron-energy-loss spectroscopy. At low energy, {ital E}{sub loss}{le}10.0 eV, common features are observed in the spectra of these two materials at {ital E}{sub loss}=0.0,{similar to}2.7,3.6, and 4.6 eV. The effective number of charges associated with these excitations has been estimated using the optical-sum rule, and from the ratio of effective charges in the two materials, the origin of this feature has been inferred based on the relative number of Cu-O{sub 2} and Bi-O layers per formula unit. The effective-charge ratio for the free carriers at zero energy loss, {ital N}{sub 2:2:1:2}/{ital N}{sub 2:2:0:1}=5.0, could not be used to determine whether the carriers were in the Cu-O{sub 2} or Bi-O planes because the oxygen doping in the two materials was not known. But at {ital E}{sub loss}=2.7 and 3.6 eV, the effective-charge ratio is 2.6 indicating that these transitions are associated with the Cu-O{sub 2} planes since the ratio is close to 2:1. The effective-charge ratio is 0.9 for {ital E}{sub loss}=4.6 eV suggesting that this excitation is localized in the Bi-O planes. If the two excitations in the Cu-O{sub 2} layers of 2:2:0:1 and 2:2:1:2 are identified with the delocalized and localized charge-transfer reactions (Mark S. Hybertsen, Michael Schluter, and Niels E. Christensen, Phys. Rev. B 39, 9028 (1989)), then the three-band Hubbard parameters {var epsilon}={var epsilon}{sub {ital p}}{minus}{var epsilon}{sub {ital d}} and {ital U}{sub {ital p}{ital d}} ({var epsilon} is the energy difference between the Cu 3{ital d} and O 2{ital p} levels and {ital U}{sub {ital p}{ital d}} is the Coulomb repulsion between two holes occupying adjacent Cu and O sites) are 1.8 and 0.9 eV, respectively, with an uncertainty of {plus minus}0.5 eV.

• OSTI ID: 5945189
• Journal Information: Physical Review, B: Condensed Matter; (USA), Journal Name: Physical Review, B: Condensed Matter; (USA) Vol. 43:1; ISSN 0163-1829; ISSN PRBMD
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360204* -- Ceramics
Cermets
& Refractories-- Physical Properties
ALKALINE EARTH METAL COMPOUNDS
BISMUTH COMPOUNDS
BISMUTH OXIDES
CALCIUM COMPOUNDS
CALCIUM OXIDES
CHALCOGENIDES
COPPER COMPOUNDS
COPPER OXIDES
ELECTRIC CONDUCTIVITY
ELECTRICAL PROPERTIES
ELECTRON SPECTROSCOPY
ENERGY-LOSS SPECTROSCOPY
EXCITONS
MINERALS
OPTICAL PROPERTIES
OSCILLATOR STRENGTHS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PEROVSKITE
PEROVSKITES
PHYSICAL PROPERTIES
QUASI PARTICLES
SPECTROSCOPY
STRONTIUM COMPOUNDS
STRONTIUM OXIDES
SUPERCONDUCTIVITY
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS