Evolution of superconductivity in electron-doped cuprates: Magneto-Raman spectroscopy

Abstract

The electron-doped cuprates Pr2−xCexCuO4−δ and Nd2−xCexCuO4−δ have been studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. The SC pairing strength is found to be consistent with a weak-coupling regime except in the underdoped region where we observe an in-gap collective mode at 4.5kBTc while the maximum amplitude of the SC gap is ≈8kBTc. In the normal state, doped carriers divide into coherent quasiparticles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (±π∕2a, ±π∕2a) regions of the Brillouin zone (BZ). We find that only coherent QPs contribute to the superfluid density in the B2g channel. The persistence of SC coherence peaks in the B2g channel for all dopings implies that superconductivity is mainly governed by interactions between the holelike coherent QPs in the vicinity of (±π∕2a, ±π∕2a) regions of the BZ. We establish that superconductivity in the electron-doped cuprates occurs primarily due to pairing and condensation of holelike carriers. We have also studied the excitations across the SC gap by Raman spectroscopy as a function of temperature (T) and magnetic field (H) for several different cerium dopings (x). Effective upper critical field lines Hc2*(T,x) at which the superfluid stiffness vanishes and Hc22Δ(T,x) at which the SC gap amplitude is suppressed by fields have been determined; Hc22Δ(T,x) is larger than Hc2*(T,x) for all doping concentrations. The difference between the two quantities suggests the presence of phase fluctuations that increase for x≲0.15. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.

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