RESUMO
We show that a quantum phase transition, generating flat bands and altering Fermi surface topology, is a primary reason for the exotic behavior of the overdoped high-temperature superconductors represented by La2-xSrxCuO4, whose superconductivity features differ from what is predicted by the classical Bardeen-Cooper-Schrieffer theory. This observation can open avenues for chemical preparation of high-Tc materials. We demonstrate that (1) at temperature T = 0, the superfluid density ns turns out to be considerably smaller than the total electron density; (2) the critical temperature Tc is controlled by ns rather than by doping, and is a linear function of the ns; (3) at T > Tc the resistivity ρ(T) varies linearly with temperature, ρ(T) â αT, where α diminishes with Tc â 0, whereas in the normal (non superconducting) region induced by overdoping, Tc = 0, and ρ(T) â T2. Our results are in good agreement with recent experimental observations.
RESUMO
We show that the merging of the spin- and valley-split Landau levels at the chemical potential is an intrinsic property of a strongly interacting two-dimensional electron system in silicon. Evidence for the level merging is given by available experimental data.
RESUMO
On the example of two-dimensional (2D) 3He we demonstrate that the main universal features of its experimental temperature T-density x phase diagram [see Neumann, Nyéki, and Saunders, Science 317, 1356 (2007)10.1126/science.1143607] look like those in the heavy-fermion metals. Our comprehensive theoretical analysis of the experimental situation in 2D 3He allows us to propose a simple expression for the effective mass M*(T,x), describing all the diverse experimental facts in 2D 3He in a unified manner and demonstrating that the universal behavior of M*(T,x) coincides with that observed in heavy-fermion metals.
