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Mott‐Insulation, High Frequencies, and the High Transition Temperatures of the New Ceramic Superconductors
Author(s) -
Hyland G. J.
Publication year - 1987
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.2221440235
Subject(s) - condensed matter physics , superconductivity , ionic bonding , dopant , cuprate , doping , stoichiometry , ceramic , hubbard model , high temperature superconductivity , materials science , physics , chemistry , ion , quantum mechanics , composite material
A recent proposal of Fröhlich concerning the origin of the high transition temperatures, T c , which characterize the newly discovered ceramic superconductors, is examined for the case of materials based on La 2 CuO 4 . Whilst one of the conditions specified by Fröhlich — namely the existence of a narrow energy band based on incomplete inner shells of certain metal atoms is indeed realized in terms of a quasi two‐dimensional lower Hubbard band of Cu d‐orbitals — it is noted that certain ionic (Cu–O) vibrational breathing modes exist whose frequency is already high enough (≈ 10 × 10 13 Hz) to account, via BCS theory, for the high T c 's, without having to invoke the higher frequency excitonic‐like modes envisaged by Fröhlich. Evidence is adduced that the carriers responsible for the superconductivity are essentially Cu 3d holes introduced into the lower Hubbard band by divalent dopants, and/or deviations from La stoichiometry; their density, n , is thus rather low, which could well account for the low critical currents, J c , found experimentally. It is noted that n is most probably less than the degree of doping, x , on account of the possible contributions of oxygen vacancies to the maintenance of charge neutrality; this, in turn, imposes a lower limit on x if n is to be > 0.