$d$-wave superconducting gap observed in protect-annealed electron-doped cuprate superconductors $\mathrm{Pr}{}_{1.3-x}\mathrm{La}{}_{0.7}\mathrm{Ce}{}_x\mathrm{CuO}{}_4$

For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the “protect-annealing” method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors ${\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, suggesting that strong electron correlation persists and hence antiferromagnetic spin fluctuations remain a candidate that mediates Copper pairing in the protect-annealed electron-doped cuprates.

Figure 1

ARPES spectra of protect-annealed PLCCO ($x=0.10,0.15$) recorded within 1 h after cleavage. (a)–(c) ARPES spectra of sample 2 with $x=0.10$ along cuts A–C indicated in the inset of (d). The spectra were recorded at $T=12$ K with $h\nu =16.5$ eV incident photons. (d) EDCs at $k_{\mathrm{F}}$ points extracted from (a)–(c). The spectra have been normalized to the intensity at the high binding energies of 45–60 meV. (e)–(h) The same as (a)–(d), but for sample 5 with $x=0.15$.

Figure 2

Leading-edge shift ${\mathrm{\Delta }}_{\mathrm{LE}}$ observed for PLCCO samples. EDCs of (a) samples $1$ and $2$ with $x=0.10$, (b) sample $3$ with $x=0.10$ measured three times on different cleavage surfaces, (c) sample $4$ with $x=0.10$, and (d) samples $5$ and $6$ with $x=0.15$ measured at temperatures above (red curves) and below (blue curves) $T_c$. Insets of (a), (b), and (d) indicate the momentum cuts and $k_{\mathrm{F}}$ positions where the EDCs were measured. For each EDC in (c), the Fermi surface angle $\varphi$ defined in the inset is indicated. Estimated ${\mathrm{\Delta }}_{\mathrm{LE}}$ values are shown beside each set of the EDCs.

Figure 3

$d$-wave SC gap of PLCCO. (a) Leading-edge shift ${\mathrm{\Delta }}_{\mathrm{LE}}$ plotted against the $d_{x^2-y^2}$-wave order parameter $|\cos k_x-\cos k_y|/2$. A line obtained by fitting the data is also plotted. (b) ${\mathrm{\Delta }}_{\mathrm{LE}}$ at the antinode $(0.3\pi ,$ $\pi )$, ${\mathrm{\Delta }}_{\mathrm{LE}}^0$, plotted against $T_c$ values. A line representing $2{\mathrm{\Delta }}_{\mathrm{LE}}^0=1.18k_{\mathrm{B}}{T}_c$ is superimposed.

Figure 4

SC gap estimated from TDOS. (a) and (b) Off-nodal TDOS of the $x=0.10$ sample derived from laser-ARPES data at $T=2.7$ and 30 K, respectively. The $T=2.7$ K spectra are fitted to the Dynes formula (shown in black). (c) SC gap $\mathrm{\Delta }$ and pair-breaking scattering rate $\mathrm{\Gamma }$ obtained through fitting and plotted against the $d_{x^2-y^2}$-wave order parameter $|\cos k_x-\cos k_y|/2$.