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

Abstract

The electron-doped cuprates Pr2−xCexCuO 4−δ (PCCO) and Nd2−xCexCuO 4−δ (NCCO) 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 under-doped region where we observe an in-gap collective mode at 4.5kB Tc while the maximum amplitude of the SC gap is ≈ 8kBTc. In the normal state, doped carriers divide into coherent quasi-particles (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 hole-like 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 hole-like 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 2∆ c2 (T, x) at which the SC gap amplitude is suppressed by field have been determined; H 2∆ c2 (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.

Hole superconductivity in the electron-doped superconductorPr2−xCexCuO4

“…Qazilbash et al 20 have shown from Raman spectroscopy measurements that superconductivity in the electron-doped is primarily due to pairing and condensation of hole-like carriers. It was also theoretically predicted that superconductivity will be favored by having hole states rather than electron ones at the Fermi energy 4 .…”

Two-band model of Raman scattering on electron-doped high-Tcsuperconductors

“…Pairing symmetry of electron-doped high-T c cuprate superconductors such as Nd 2−x Ce x CuO 4 and Pr 2−x Ce x CuO 4 is a long standing problem [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Although no consensus has been reached yet, more and more recent experimental results have suggested that the order parameter of electron-doped cuprates is likely to have d x 2 −y 2 -wave pairing symmetry [1,2,3,5,9,10,14,16,17], in close resemblance to that of hole-doped materials.…”

Weak-coupling Bardeen-Cooper-Schrieffer superconductivity in the electron-doped cuprate superconductors

“…Unfortunately, the superconducting gap of n-type cuprates is relatively small and determination of its exact value over a large doping range is an experimental challenge [12,13,14,15,16,17]. Previous estimates suggest that the superconducting pairing strength of the n-type cuprates is close to a weakcoupling regime in the optimally doped and overdoped region [18,19]. Furthermore, tunneling data suggest that …”