Commensurate Spin Dynamics in the Superconducting State of an Electron-Doped Cuprate Superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped ($n$-type) superconducting (SC) cuprate ${\mathrm{N}\mathrm{d}}_{2-x}{\mathrm{C}\mathrm{e}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ ($x=0.15$) with $T_c=18$ and 25 K. Unlike the hole-doped ($p$-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the $n$-type cuprate shows commensurate magnetic fluctuations at the tetragonal ($1/21/20$) reciprocal points both in the SC and in the normal state. A spin gap opens up when the $n$-type cuprate becomes SC, as in the optimally doped $p$-type ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$. The gap energy, however, increases gradually up to about 4 meV as $T$ decreases from $T_c$ to 2 K, which contrasts with the spin pseudogap behavior with a $T$-independent gap energy in the SC state of $p$-type cuprates.

Figure 1

(a) $T$ dependence of the elastic magnetic peak intensities of NCCO ($x=0.15$) at $(3/21/20)$ measured from an as-grown insulating (crosses) and the two reduced SC single crystals with $T_c=18\mathrm{K}$ (open circles) and 25 K (open triangles). (b) $T$ dependence of dynamical susceptibility ${\chi }^{\prime \prime }(\omega =0.8\mathrm{m}\mathrm{e}\mathrm{V})$ at $(1/21/20)$ obtained from the reduced sample of NCCO ($x=0.15$, $T_c=18\mathrm{K}$). Dotted curves are to guide the eye.

Figure 2

Constant-$\omega$ scans around $(1/21/20)$ obtained from two reduced samples of NCCO [$x=0.15$; (a) $T_c=25\mathrm{K}$ and (b) $T_c=18\mathrm{K}$] in the normal ($T=26\mathrm{K}$, open circles) and SC ($T=1.5\mathrm{K}$, closed circles) phases. (a) (upper) 26 K, normal, $\omega =2\mathrm{m}\mathrm{e}\mathrm{V}$, with scans along the $(h00)$ direction and (lower) 26 K, normal, $\omega =2\mathrm{m}\mathrm{e}\mathrm{V}$, with scans along the $(hh0)$ direction. (b) (upper) 1.5 K, SC, $\omega =4.5\mathrm{m}\mathrm{e}\mathrm{V}$, with scans along the $(hh0)$ direction and (lower) 26 K, normal and 1.5 K, SC, $\omega =3\mathrm{m}\mathrm{e}\mathrm{V}$, with scans along the $(hh0)$ direction. Horizontal bars indicate the full width at half maximum of the instrumental resolution. Solid curves are explained in the text.

Figure 3

Energy spectra of ${\chi }^{\prime \prime }(\omega )$ obtained from the normal and SC phases (a) of NCCO ($x=0.15$, $T_c=18\mathrm{K}$) and (b) of NCCO ($x=0.15$, $T_c=25\mathrm{K}$). Dotted curves are to guide the eye.

Figure 4

$T$ dependence of the spin gap, $2\Delta$ for the $n$-type SC cuprate NCCO ($x=0.15$) with $T_c=18\mathrm{K}$ (filled circles) and 25 K (filled triangles). Dotted lines are guides to the eye. At $T_c$, the spin gap is absent for both NCCO samples. The inset shows $2{\Delta }_{\max \mathrm{}}$ versus $k_B{T}_c$ obtained from our samples and LSCO ($x=0.15$) (open square, data taken from Ref. 12).