Electronic topological transition in 2D electron system on a square   lattice and the line T^*(\delta) in the underdoped regime of high - T_c   cuprates
  It is shown that a 2D system of free fermions on a square lattice with hoping between more than nearest neighbours undergoes a fundamental electronic topological transition (ETT) at some electron concentration n_c. The point \delta=\delta_c, T=0 (\delta=1-n) is an exotic quantum critical point with several aspects of criticality. The first trivial one is related to singularities in thermodynamic properties. An untrivial and never considered aspect is related to the effect of Kohn singularity (KS) in 2D system: this point is the end of a critical line T=0, \delta>\delta_c of static KS in free fermion polarizability. This ETT is a motor for anomalous behaviour in the system of interacting electrons. The anomalies take place on one side of ETT, \delta<\delta_c, and have a striking similarity with the anomalies in the high T_c cuprates in the underdoped regime. The most important consequence of ETT is the appearance of the line of characteristic temperatures, T^*(\delta) \propto \delta_c-\delta, which grows from the point \delta=\delta_c on the side \delta<\delta_c. Below this line the metal state is anomalous. Some anomalies are considered in the present paper. The anomalies disappear in the case t'=t''=... \to 0. 
F. Onufrieva, P. Pfeuty (Laboratoire Leon Brillouin, CE-Saclay,   France)