Events

Seminar of International Laboratory of High Magnetic Fields and Low Temperatures PAS

Exafs spectroscopy of high temperature superconductors

Alexey Menushenkov

National Research Nuclear University “MEPhI”(Moscow Engineering Physics Institute), Russia

Temperature dependent X-ray absorption spectra of Ba_1-xK_xBiO₃, hole-doped La_2-xSr_xCuO₄ (LSCO) and electron-doped Nd_2-xCe_xCuO_4-δ (NCCO) high temperature superconductors with perovskite-like structure were investigated above L₃-Bi and K-Cu absorption edges in temperature range of 5-300K using synchrotron radiation. The nearest oxygen environment of bismuth and copper ions was studied. It was shown, that for superconductive compositions the pair radial distribution function (PRDF) of the first Bi-O (Cu-O) shell is strongly differs from the Gaussian one. So we simulated the experimental EXAFS-function with help of the vibration potential described by parametric function of interatomic distances and found that at low temperature a part of oxygen ions oscillate in a double-well potential, and their vibrations correlate with the transfer of the local hole (electron) pairs [1-3]. 

As it was shown earlier for Ba_1-xK_xBiO₃ [1], we suppose that for cuprates doping of parent compounds La₂CuO₄ and Nd₂CuO₄ with Sr or Ce leads to formation of the local hole (electron) pairs in the upper antibonding molecular orbital Cu3d_x2-y2-O2p_σ* of part of CuO_n (n = 4; 6) complexes. These pairs can tunnel between neighbouring complexes in correspondence to dynamic exchange CuL²O₆↔CuL¹O₆ (for LSCO) and CuL²O₄↔CuL¹O₄ (for NCCO), giving rise to oxygen ion vibrations in a double-well potential. Vibrations of oxygen ions in the double-well potential are correlated with the local electron (hole) pair movement between neighbouring   complexes. The phase coherence of the local pair movement is guaranteed by the peculiarities of the perovskite-like structure which includes the stiff CuO_n complexes combined by collective rotational and breathing modes .

1. A.P.Menushenkov, K.V.Klementev J. Phys.: Condens. Matter 12, 2000, pp. 3767-3786.
2. Menushenkov A.P., Kuznetsov A.V., Chernikov R.V., Ivanov A.A., Sidorov V.V., Klementiev K.V., Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques 7, 407 (2013).
3. Menushenkov A.P., Kuznetsov A.V., Chernikov R.V., Ivanov A.A., Sidorov V.V.,    Klementiev K.V., Journal of Superconductivity and Novel Magnetism 27, 925 (2014).