Bulk superconductivity at 84 K in the strongly overdoped regime of cuprates

By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized ${\mathrm{Cu}}_{0.75}{\mathrm{Mo}}_{0.25}{\mathrm{Sr}}_2{\mathrm{YCu}}_2{\mathrm{O}}_{7.54}$, in which overdoping is achieved up to $p\sim 0.46$ hole/Cu, well beyond the $T_c\text{−}p$ superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with $T_c=84$ K and superfluid density similar to those of optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. These results point at an unusual high-$T_c$ phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high $T_c$ of cuprates originates from a non-Fermi-liquid ground state.

Figure 1

Zero-field-cooling and field-cooling (ZFC, FC) magnetization (solid symbols) and ac susceptibility $\chi$ (open symbols) of high-pressure oxidized (Cu,Mo)1212 powders. The two curves are compared using the same scale as the small dc field of 100 Oe allows one to approximate $M/H$ with $\chi$. The inset shows a closeup of the transition.

Figure 2

Isobaric specific heat $C_p\left(T\right)$ measured on the same (Cu,Mo)1212 powder sample of Fig. 1. The main panel is a closeup of the transition showing the jump at $T_c$. The inset shows the behavior in the whole 2–300 K temperature range measured.

Figure 3

Correlation between $T_c$ and the $c$-axis parameter for a series of ${\mathrm{Cu}}_{1-x}{\mathrm{Mo}}_x{\mathrm{Sr}}_2{\mathrm{YCu}}_2{\mathrm{O}}_{7+\delta }$ samples prepared under different conditions. The red solid square corresponds to the present result obtained on a $x=0.25$ high-pressure (HP) oxidized sample. Black symbols indicate previous results by Ono [5] on $x=0.20$ annealed HP samples (squares), HP samples (circles), and starting samples prepared at ambient pressure (triangles). The crystal structure is shown in the inset; blue, green, yellow, and red spheres indicate Cu/Mo, Sr, Y, and oxygen atoms, respectively; white-red spheres indicate partially occupied oxygen sites in the basal plane. The structure is described in detail in Ref. [9] and in the Supplemental Material.

Figure 4

Comparison of the low-temperature specific heat of the high-pressure oxidized (HPO) (Cu,Mo)1212 sample of the previous figure (black dots) with previously reported data on overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x=0.259$ $(T_c=6.5$ K, blue squares) and $x=0.290$ (nonsuperconducting, red diamonds) and on optimally doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x=0.178$ (green open circles) and ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ with $\delta =0.05$ (black open squares). La214 and Y123 data are taken from Refs. [24, 25], respectively.

Figure 5

Temperature dependence of the $\mu \mathrm{SR}$ rate $\sigma$ at two different magnetic fields for an oxidized (Cu,Mo)1212 powder sample similar to that of Figs. 1 and 2.