Condensed-Matter Physics & Materials Science Seminar

Electronic structure and mechanisms of high temperature superconductivity
S.G. Ovchinnikov
L.V. Kirensky Institute of Physics, Siberian Branch of RAS, Krasnoyarsk, 660036, Russia

Strong electron correlations are responsible for the formation of unusual electronic structure and its doping dependence in cuprates. A review of recent theoretical results obtained by GTB method is given. Undoped La2CuO4 and Nd2CuO4 appear to be the antiferromagnetic charge transfer insulators with the first removal electron state in La2CuO4 at k=(p/2, p/2) and the first electron addition state in Nd2CuO4 at k=(p, 0) in agreement to ARPES. In-gap states and its evolution with doping and temperature are obtained. In the underdoped regime short magnetic order correlation functions decrease with doping that results in a quantum phase transition near optimal doping.
The low energy effective Hamiltonian in the form of model is constructed with parameters calculated ab initio within the LDA+GTB scheme. The electron-phonon interaction for strong correlated electrons is considered with matrix elements wave vector dependence for breathing, buckling and apical breathing mode. A mean field theory of d-type superconductivity is developed in the Hubbard operator representation. Without phonons pure magnetic mechanism provides too high TC (250 K for La2-xSrxCuO4). Phonon contribution to pairing may either increase or decrease TC because breathing and buckling modes give opposite contribution. Corrections beyond mean-field approximation are necessary both for magnetic and phonon mechanisms.