Recently, using X-ray free-electron laser radiation, the first direct experimental evidence of the existence of a realspace paired state of charge carriers was obtained in the parent compound BaBiO 3 of the bismuthate family of high-temperature superconductors (HTSCs), whose perovskite-like structure is similar to that of cuprate HTSCs.This confirmed the fundamental premises of our previously formulated model of a spatially separated Fermi±Bose mixture, implying a new, original mechanism for high-temperature superconductivity in bismuthates.In this review, we discuss the progress achieved with this model based on results obtained with an X-ray free-electron laser and go through details of the complete phase diagram of the superconducting and normal states in Ba 1Àx K x BiO 3 bismuth oxides at various potassium doping concentrations.We also discuss new, unique quantum states of matter in the form of a bosonic insulator (semiconductor) with initially paired charge carriers and two energy gaps, and a bosonic metal shunted by a fermionic component.We also provide experimental evidence that local pairing of electrons and holes is responsible for the set of the main anomalous properties of the bismuthate family.Given the numerous similarities in the behavioral patterns of various families of perovskite superconductors, we believe that our work will provide new impetus to understanding the nature of high-temperature superconductivity in bismuth oxides and other families, including cuprate HTSCs.
For doped two-dimensional Mott insulators in their normal state, the challenge is to understand the evolution from a conventional metal at high doping to a strongly correlated metal near the Mott insulator at zero doping. To this end, we solve the cellular dynamical mean-field equations for the two-dimensional Hubbard model using a plaquette as the reference quantum impurity model and continuous-time quantum Monte Carlo method as impurity solver. The normal-state phase diagram as a function of interaction strength
U, temperature
T, and filling
n shows that, upon increasing
n toward the Mott insulator, there is a surface of first-order transition between two metals at nonzero doping. That surface ends at a finite temperature critical line originating at the half-filled Mott critical point. Associated with this transition, there is a maximum in scattering rate as well as thermodynamic signatures. These findings suggest a new scenario for the normal-state phase diagram of the high temperature superconductors. The criticality surmised in these systems can originate not from a T
= 0 quantum critical point, nor from the proximity of a long-range ordered phase, but from a low temperature transition between two types of metals at finite doping. The influence of Mott physics therefore extends well beyond half-filling.
The mechanism controlling the exotic behavior of the transport properties in the strange metallic phase of high temperature superconductors is one of the main unresolved problems in condensed matter physics. Recent experimental observations have proven that space-time symmetry breaking due to charge density wave orderings is an ubiquitous feature of the phase diagram of high temperature superconductors and plays a dominant role in determining the transport properties of these materials. In this paper we show that in the strange metal phase of the cuprates, a whole set of transport coefficients are described by a universal hydrodynamic framework once one accounts for the effects of quantum critical charge density waves. We corroborate our theoretical prediction by measuring the DC transport properties of Bi-2201 close to optimal doping, proving the validity of our approach. Our argument does not assume any specific microscopic mechanism, relying only on the symmetries of the system, and can be used as a consistency check to understand the universality class governing the behavior of cuprate high temperature superconductors.