Common origin of superconductivity in hole- and electron-doped cuprates based on indirect-exchange coupling. Doping dependence of chemical potentials

Abstract

The discovery in 1989 of superconductivity in “electron-doped” cuprates of composition Ln 2− x Ce x CuO 4− z : Ln=Nd, Pr, Sm, by Tokura et al. (Nature 337 (1989) 345) appeared at first to imply the existence of a novel branch of superconducting cuprates. Later, however, strong similarities with hole-doped cuprates were found at low temperatures such as positive Hall coefficients and striking analogies regarding (in-plane) resistivities. In the present paper we show, first of all, that also shifts in chemical potential upon doping, deduced from photoelectron spectra, are similar. They do not obey rules prescribed by a “semiconductor model” for either category of cuprates. Instead, the shifts appear to vary continuously in accordance with the value of a parameter in the (theta) function for conduction states, specifying the relative position of conduction and (main) valence bands. In a second step, we assert that these electron-doped cuprates are, in fact, hole-doped because of overcompensation of doped excess positive charge by oxygen anions. Taking account of established facts that the CuO 2 layers in this ( T′) structure are strained and locally heavily distorted in the superconductors we follow, in the framework of an indirect-exchange formalism for superconductivity earlier extensively applied for hole-doped cuprates, the development of critical temperature T C as a function of doping x. We reason that on this basis the superconducting doping range must be considerably less wide than in La 2− x Sr x CuO 4 since the unstrained, undistorted ( T′) structure does not afford superconductivity. In addition, analogies with T C( x) in the (La,Sr) cuprate for the Ce-underdoped, and with oxygen-over-doped Tl 2Ba 2CuO 6+ y in the Ce-overdoped range, must be expected. Detailed calculations of pressure gradients d T C/d P for the over(hole-)doped (Tl, Ba) compound strongly support this correspondence. In a recent paper by Fournier et al. [Phys. Rev. Lett. 81 (1998) 4720] both these analogies, with Pr 2− x Ce x CuO 4− z as the electron-doped cuprate, had been deduced from (in-plane) resistivities at low temperatures in high magnetic fields. The results of the present paper are also taken to imply that electron doping into insulating cuprates is not possible.