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

Abstract

The electron-doped cuprates ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ and ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ 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 $4.5{k}_{B}{T}_{c}$ while the maximum amplitude of the SC gap is $\ensuremath{\approx}8{k}_{B}{T}_{c}$. 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 ($\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2a$, $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2a$) 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 ($\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2a$, $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2a$) 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 ${H}_{c2}^{2\ensuremath{\Delta}}(T,x)$ at which the SC gap amplitude is suppressed by fields have been determined; ${H}_{c2}^{2\ensuremath{\Delta}}(T,x)$ 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 $x\ensuremath{\lesssim}0.15$. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.