Nernst effect in the electron-doped cuprate superconductor PCCO: Superconducting fluctuations, upper critical field Hc2, and the origin of the Tc dome

摘要

The Nernst effect was measured in the electron-doped cuprate superconductorPr2-xCexCuO4 (PCCO) at four concentrations, from underdoped (x=0.13) tooverdoped (x=0.17), for a wide range of temperatures above the criticaltemperature Tc. A magnetic field H up to 15 T was used to reliably access thenormal-state quasiparticle contribution to the Nernst signal, Nqp, which issubtracted from the total signal, N, to obtain the superconductingcontribution, Nsc. As a function of H, Nsc peaks at a field H* whosetemperature dependence obeys Hc2* ln(T/Tc), as it does in a conventionalsuperconductor like Nb1-xSix. The doping dependence of the characteristic fieldscale Hc2* - shown to be closely related to the upper critical field Hc2 -tracks the dome-like dependence of Tc, showing that superconductivity isweakened below the quantum critical point where the Fermi surface isreconstructed, presumably by the onset of antiferromagnetic order. Our data atall dopings are quantitatively consistent with the theory of Gaussiansuperconducting fluctuations, eliminating the need to invoke unusualvortex-like excitations above Tc, and ruling out phase fluctuations as themechanism for the fall of Tc with underdoping. We compare the properties ofPCCO with those of hole-doped cuprates and conclude that the domes of Tc andHc2 vs doping in the latter materials are also controlled predominantly byphase competition rather than phase fluctuations.