Strongly Correlated Quantum Spin Liquids versus Heavy Fermion Metals: A Review.

This review considers the topological fermion condensation quantum phase transition (FCQPT) that explains the complex behavior of strongly correlated Fermi systems, such as frustrated insulators with quantum spin liquid and heavy fermion metals. The review contrasts theoretical consideration with recent experimental data collected on both heavy fermion metals (HF) and frustrated insulators. Such a method allows to understand experimental data.

The influence of topological phase transition on the superfluid density of overdoped copper oxides.

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 LaSrCuO, 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-T materials. We demonstrate that (1) at temperature T = 0, the superfluid density n turns out to be considerably smaller than the total electron density; (2) the critical temperature T is controlled by n rather than by doping, and is a linear function of the n; (3) at T > T the resistivity ρ(T) varies linearly with temperature, ρ(T) ∝ αT, where α diminishes with T → 0, whereas in the normal (non superconducting) region induced by overdoping, T = 0, and ρ(T) ∝ T.

Merging of Landau levels in a strongly interacting two-dimensional electron system in silicon.

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.

Universal behavior of two-dimensional 3He at low temperatures.

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.