${}^{17}\mathrm{O-NMR}$ Knight shift study of the interplay between superconductivity and pseudogap in $({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}\left)\right({\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}){\mathrm{Cu}}_3{\mathrm{O}}_y$

We report systematic ${}^{17}\mathrm{O-NMR}$ measurements on the high-$T_c$ cuprate $({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}\left)\right({\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}){\mathrm{Cu}}_3{\mathrm{O}}_y$, for four different families (different $x)$. Using Knight shift data, we show that the pseudogap opening temperature $T^{*}$ is much higher than $T_c$ near optimal doping, unlike structurally similar YBCO. In addition, at constant doping the pseudogap temperature does not vary with $x$, in contrast to $T_c$. This puts constraints on the nature of the pseudogap and position of the quantum critical point inside the superconducting dome.

Figure 1

CLBLCO unit cell, which is tetragonal and very similar to YBCO cuprate. There is no ordering of chain oxygen O(1).

Figure 2

(a) Central NMR transition of ${}^{17}\mathrm{O}$ with two distinct lines. Powder spectrum recorded with repetition times $\left(t_{\mathrm{rep}}\right)$ of 3 (black), 10, 150, and 300 ms (red). The apical oxygen site O(4) has much slower spin-lattice relaxation than in-plane oxygen O(2,3). Sample: $x=0.4$, $y=7.1$. Inset: Comparison of two different families $(x$ = 0.1 and 0.4) at (approximately) the same oxygen doping $y$. The difference between linewidths are 6% for apical and negligible (less than 1%) for in-plane oxygen. (b) Spectra at different temperatures. The thick gray line follows in-plane oxygen peaks. The shift of the apical oxygen is temperature independent, as is denoted by the dashed line. Intensity of apical oxygen varies because its relaxation time shortens with temperature. Sample: $x=0.1$, $y=7.05$.

Figure 3

In-plane oxygen line position in reference to apical oxygen position for sample $x=0.1$, $y=7.035$. For fitting, only points above the superconducting transition $T_c$ are taken into account. As the gap opening is not abrupt, the pseudogap temperature $T^{*}$ is not easily distinguished in the plots, but is acquired as a parameter from a fit function (dotted line; see text). Inset: Knight shifts for $x=0.1$ samples with various $y$. Each sample is offset on the vertical axis for better display. The topmost sample is near-optimal doping, and its shift still strongly depends on temperature.

Figure 4

(a) CLBLCO phase diagram. Comparison between O-NMR measurements presented in this paper and Ca-NMR and SQUID susceptibility measurements from Refs. [27, 39]. Presented are only $x=0.1$ and $x=0.4$ samples. Abscissa is oxygen doping parameter $y$. Lines are guides to the eye. (b) Phase diagram for all measured samples. Chemical doping $y$ is replaced with the in-plane hole concentration relative to in-plane optimal doping, $\Delta p$ (see text). The dashed pseudogap line is a guide to the eye.