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講演者：Prof. Ernst Z. Kurmaev（Institute of Metal Physics, Russian Academy of Sciences）

日時： ４月２０日（金）　午後４時半から
場所： 青山学院大学　理工学部（世田谷キャンパス）　一号館　５階　１５３８号室

講演題目： X-ray emission spectra and electronic structure of new superconductors
要旨：Results of X-ray fluorescence measurements of MA₂Ca_n-1Cu_nO_2n+3 compounds with [M-12(n-1)n] type structures; (C_0.8N_0.2Sr₂CuO_5.3, [Cu_0.25(C,N)_0.75]Sr₂CaCu₂O_y, Cu_0.5V_0.5Sr₂CaCu₂O_7.5, Cu_0.5V_0.5Sr₂Ca₂Cu₃O_9.4, Cu_0.5V_0.5Sr₂(Ca_3.6Sr_0.4)Cu₅O_13.5 and Cu_0.5V_0.5Sr₂(Ca_4.7Sr_0.3)Cu₆O_15.3) are presented. The experiments show that C-atoms form a distorted CO₃ oxyacid group with a partial substitution of copper and nitrogen atoms in the M-sites. We conclude that the CO₃^2- group in (C,N) Sr₂Ca_n-1Cu_nO_y and (Cu,C,N) Sr₂Ca_n-1Cu_nO_y is substituted by the NO₂^- which induces the creation of hole carriers in these compounds. We have found that V-atoms in the given compounds form (VO₄)^3- tetrahedrons and have a pentavalent state. A spectral estimation of the oxygen concentration shows that the oxygen content under high-pressure and high temperature synthesis conditions is not changed considerably from the beginning to the final product. Measurements of X-ray emission and absorption spectra of constituents of new superconductor MgB₂ (Tc=39 k) are found in a good agreement with band structure calculations, in particular calculations of intensities of X-ray spectra taling into account dipole matrix elements. The comparison of X-ray emission spectra of graphite, AlB₂ and MgB₂ in the binding energy scale supports the idea of charge transfer from sigma to pai bands which creates holes at the top of the bonding sigma bands and provides high-Tc superconductivity of MgB₂.