Quantum phase transition from a Antiferromagnetic-Insulator to a   Paramagnetic-Metal laying beneath the superconducting dome
  The effect of doping on the TB model of the CuO planes in the La2CuO4 constructed in previous works is investigated. Firstly, it is noted that the model employed constitutes a generalization of the Hubbard one for the same system. Thus, the former predictions of the insulator gap, antiferromagnetic character and the existence of a paramagnetic-pseudogap state, become natural ones to be expected from this more general picture. The effect of hole doping on the antiferromagnetic-insulator state (AFI) and the paramagnetic-pseudogap one, is investigated. The results predict a quantum phase transition (QPT) from the AFI state at low doping to a paramagnetic-metallic state (PM) at higher hole densities. Therefore, a clear description of the hidden QPT laying beneath the dome in high critical temperature superconducting materials is found. At low doping, the system prefers the AFI state, and at the critical value of the doping density 0.2, the energy of a metallic state becomes lower. The evolution with small doping values of the band spectrum of the AFI state, shows that the holes tend to become localized at the middle of the sides of the reduced Brillouin zone (BZ). Then, around the critical value, the holes of the AFI state move to become situated at the corners of the same reduced BZ, showing a structural change at the phase transition point. Thus, the PM state appearing at the QPT acquires the same behavior with respect to the position of holes as the pseudogap state. In the small doping limit a clear difference between the degree of convergence of the iterative self-consistent solution is associated to an even or odd number of electrons. It suggests that the Kramers degeneration in combination with the spin-spatial entangled nature of the hole states, leads to a new kind of pair interaction between two holes. The binding energy value is estimated as a function of the screening. 
Victor M. Martinez Alvarez, Alejandro Cabo-Bizet and Alejandro Cabo   Montes de Oca
cond-mat.str-el cond-mat.mtrl-sci cond-mat.supr-con