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

The electron-doped cuprates Pr_2−xCe_xCuO_4−δ and Nd_2−xCe_xCuO_4−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 4.5k_BT_c while the maximum amplitude of the SC gap is ≈8k_BT_c . 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 B_2g channel. The persistence of SC coherence peaks in the B_2g 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 H_c2^*(T,x) at which the superfluid stiffness vanishes and H_c2^2Δ(T,x) at which the SC gap amplitude is suppressed by fields have been determined; H_c2^2Δ(T,x) is larger than H_c2^*(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.