Nernst effect in the electron-doped cuprate superconductor Superconducting fluctuations, upper critical field and the origin of the dome

The Nernst effect was measured in the electron-doped cuprate superconductor Pr2-xCexCuO4 (PCCO) at four concentrations, from underdoped (x = 0.13) to overdoped (x = 0.17), for a wide range of temperatures above the critical temperature T-c. A magnetic field H up to 15 T was used to reliably access the normal-state quasiparticle contribution to the Nernst signal N-qp, which is subtracted from the total signal N, to obtain the superconducting contribution N-sc. As a function of H, N-sc peaks at a field H-star whose temperature dependence obeys H(c2)(star)ln(T/T-c), as it does in a conventional superconductor such as NbxSi1-x. The doping dependence of the characteristic field scale H-c2(star), shown to be closely related to the upper critical field H-c2, tracks the domelike dependence of T-c, showing that superconductivity is weakened below the quantum critical point where the Fermi surface is reconstructed, presumably by the onset of antiferromagnetic order. Our data at all dopings are quantitatively consistent with the theory of Gaussian superconducting fluctuations, eliminating the need to invoke unusual vortexlike excitations above T-c, and ruling out phase fluctuations as the mechanism for the fall of T-c with underdoping. We compare the properties of PCCO with those of hole-doped cuprates and conclude that the domes of T-c and H-c2 versus doping in the latter materials are also controlled predominantly by phase competition rather than phase fluctuations.