Weak phase separation and the pseudogap in the electron-doped cuprates

– We study the quantum transition from an antiferromagnet to a superconductor in a model for electronand hole-doped cuprates by means of a variational cluster perturbation theory approach. In both cases, our results suggest a tendency towards phase separation between a mixed antiferromagnetic-superconducting phase at low doping and a pure superconducting phase at larger doping. However, in the electron-doped case the energy scale for phase separation is an order of magnitude smaller than for hole doping. We argue that this can explain the different pseudogap and superconducting transition scales in holeand electron-doped materials. Introduction. – High-temperature superconducting materials (HTSC) are characterized by strong electronic correlations which are responsible for a number of anomalous properties and competing phases. The occurrence of these anomalous phases is probably related to the close proximity of the Mott-insulating and antiferromagnetic (AF) phases at half-filling. This is particularly true for hole(p-)doped materials which, besides the AF and the superconducting (SC) states, display a number of unconventional phases such as stripes [1,2], checkerboard structures [3], non-Fermi liquid phases, etc. It has been argued that these inhomogeneous phases originate from an instability of the AF phase towards phase separation [2], which is intrinsic in models with short-range interactions such as Hubbard or t−J models. On the other hand, long-range Coulomb repulsion enforces charge homogeneity at long distances so that stripes, other short-range charge inhomogeneities, or strong charge fluctuations can originate as a compromise between the two competing effects [2, 4, 5]. A number of theories support the notion that fluctuations of these inhomogeneities, or of related order parameters, are responsible for the pseudogap observed in p-doped materials [2, 6, 7]. In this case, one would expect the corresponding (low) pseudogap temperature scale T ∗ to be indirectly related to the phase-separation energy scale. The situation is not clear in electron(n-)doped cuprates (especially in the Ndand PrCeCuO compounds). These materials display a more continuous transition from the AF to the SC state [8]. Stripes have not been observed so far, although there are indications of electronic phase inhomogeneities [9, 10]. In some mean-field calculations based upon the single-band