High-T$$_c$$ Cuprates: a Story of Two Electronic Subsystems

Abstract

A review of the phenomenology and microscopy of cuprate superconductors is presented, with particular attention to universal conductance features, which reveal the existence of two electronic subsystems. The overall electronic system consists of $$1+p$$ 1 + p charges, where p is the doping. At low dopings, exactly one hole is localized per planar copper–oxygen unit, while upon increasing doping and temperature, the hole is gradually delocalized and becomes itinerant. Remarkably, the itinerant holes exhibit identical Fermi liquid character across the cuprate phase diagram. This universality enables a simple count of carrier density and yields comprehensive understanding of the key features in the normal and superconducting state. A possible superconducting mechanism is presented, compatible with the key experimental facts. The base of this mechanism is the interaction of fast Fermi liquid carriers with localized holes. A change in the microscopic nature of chemical bonding in the copper oxide planes, from ionic to covalent, is invoked to explain the phase diagram of these fascinating compounds.

Optical conductivity of cuprates in a new light

“…The implication, in agreement with the observed evolution of the arcs at the Fermi surface, is that the ungapped (nodal) parts retain their FL character with an essentially universal scattering rate and Fermi velocity (effective mass), while the gapped (antinodal) states do not contribute to conductivity (5,17). Consequently, the changes in the resistivity can be simply related to the change in the carrier density, obtained from the resistivity directly by use of a standard Drude formula, and (13)(14)(15)(16). Considering the uncertainties in the absolute values of ρ and R H , and the difficulties associated with ascertaining the exact doping level for each particular sample, the agreement between n ρ and n H across different compounds is remarkable.…”

“…1c. Third, all the insights gained in the PG part of the phase diagram apply to the overdoped part as well (in agreement with the universality of µ H ), based on direct observation of evolution in the transport n eff , from n eff = p to n eff = 1 + p. Fourth, we note that the loss of the FL spectral weight associated with superconductivity in the underdoped regime tracks n eff (13,26,27), while in the overdoped regime it tracks the loss of the localized charge n loc (13,15,28). Finally, we show that the same method of separating the optical response into FL and non-FL contributions may also be applied to superconducting pnictides and to Sr 2 RuO 4 , although the physical origin of the non-FL part is different than in the cuprates: it is caused by dissipative electronic pockets with a low energy scale δ.…”

“…However, much can be concluded just by following the doping evolution of the MIR feature. As repeatedly pointed out earlier (13,15), it is obvious just by looking at the data (Ref. (42) and Supplement) that the spectral weight of exactly one charge per CuO 2 unit is transferred to the FL response from the higher energies.…”

“…a relation which notably holds across the superconducting dome (13), with a material-dependent constant O S discussed elsewhere (15). It should be noted that ρ S is proportional to the mobile concentration n eff ∝ ω 2 p here, with the constant of proportionality containing n loc as the probability to encounter the localized hole, which provides the glue.…”

“…This behavior implies that n loc is associated with an excitonic-like (short-range, short-time) glue (15).…”