Condensed-Matter Physics & Materials Science Seminar

Based on the ab-initio LDA+GTB band structure calculations of La2-xSrxCuO4 in the regime of strong electron correlations the effective low energy Hamiltonian is obtained. Within this model the normal and superconducting states are studies by a perturbation theory in the Hubbard X-operators representation. The self-energy is obtained in the non-crossing diagramm approximation. At low temperature it is static, and is determined by the short antiferromagnetic order.
There are two quantum Lifshitz-type phase transitions between underdoped region with small hole pocket and overdoped region with large Fermi surface. The first one at is related to the log singularity in the density of states and determines the optimal doping. The second one at results in the Heviside step singularity in and is resulted to the crossover from the pseudogap at to the Fermi liquid at .
Both magnetic and phonon mechanisms of pairing are considered in the mean field BCS-type theory with symmetry gap. The only empirical parameter of the theory is the effective electron-phonon interaction G. This parameter is found from fitting the calculated isotope effect to experimental data. Then we compare the magnetic and phonon contributions in Tc and it appears that there is no dominant mechanism, both are approximately equal. For the two CuO2 layer cuprate like YBCO we have calculated the effect of the interlayer hopping and do not found the increase of Tc.
By new norm-conserving cluster perturbation theory for the Hubbard model we have studied whether ARPES measure the true Fermi surface. The answer is not for the modern energy resolution. The order of magnitude improvement of the resolution results in the momentum distribution curves in agreement with Fermi surface and its concentration dependence.