Exciton and fermion-pair condensation at non-zero temperature

A thermodynamic study of a many-particle two-level system with theSU(2) ⊗SU(2)-symmetry interaction in the particle-particle and particle-hole channels is carried out. Using the symmetry of the system, its free energy and phase diagram are calculated. It is demonstrated that the superconducting (particle-particle) and excitonic (particle-hole) condensates are both formed in the considered system which undergoes two phase transitions. The calculated phase diagram represents qualitatively the phase diagram typical for materials with the high-temperature superconducting properties, including the existence of two phase transitions, the first and second order phase transitions, triple and tricritical points.

The Physics of Life and Quantum Complex Matter: A Case of Cross-Fertilization

Progress in the science of complexity, from the Big Bang to the coming of humankind, from chemistry and biology to geosciences and medicine, and from materials engineering to energy sciences, is leading to a shift of paradigm in the physical sciences. The focus is on the understanding of the non-equilibrium process in fine tuned systems. Quantum complex materials such as high temperature superconductors and living matter are both non-equilibrium and fine tuned systems. These topics have been subbjects of scientific discussion in the Rome Symposium on the “Quantum Physics of Living Matter”.

Metal-insulator quantum critical point beneath the high Tc superconducting dome

An enduring question in correlated systems concerns whether superconductivity is favored at a quantum critical point (QCP) characterized by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high T(c) cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultrastrong magnetic fields to measure quantum oscillations in underdoped YBa(2)Cu(3)O(6+x), revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in T(c) in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centered at a critical Fermi surface instability.

Filamentary microstructure and linear temperature dependence of normal state transport in optimized high temperature superconductors

A filamentary model of "metallic" conduction in layered high temperature superconductive cuprates explains the concurrence of normal state resistivities (Hall mobilities) linear in T (T-2) with optimized superconductivity. The model predicts the lowest temperature T0 for which linearity holds and it also predicts the maximum superconductive transition temperature Tc. The theory abandons the effective medium approximation that includes Fermi liquid as well as all other nonpercolative models in favor of countable smart basis states.