Figure 1

Temperature dependence of the dc magnetic susceptibility for the parent compound ${\mathrm{N}\mathrm{a}}_{0.6}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ (filled circles), intermediate phase ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·0.9{\mathrm{H}}_2\mathrm{O}$ (unfilled circles), superconducting ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{H}}_2\mathrm{O}$ (filled diamonds), and ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{D}}_2\mathrm{O}$ (unfilled diamonds) above the superconducting transition $T_c$, and overdeuterated ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.8{\mathrm{D}}_2\mathrm{O}$ (solid line). All measurements were carried out by applying $H=20\mathrm{O}\mathrm{e}$ using a commercial SQUID magnetometer by quantum design. The inset shows the ${\mathrm{H}}_2\mathrm{O}/{\mathrm{D}}_2\mathrm{O}$ content $y$ dependence of the room-temperature magnetic susceptibility slope $d\chi /dT(290\mathrm{K})$ (filled triangles) and crossover temperature $T_x$ (unfilled triangles) (see the definition in the text). Note that both $d\chi /dT(290\mathrm{K})$ and $T_x$ exhibit extrema at $y=1.4$, indicating that the susceptibility data correlate with both the Na content (Co valence) and the degree of hydration/deuteration.Reuse & Permissions

Figure 2

Temperature dependence of the magnetic susceptibility $\chi$ of ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{H}}_2\mathrm{O}$ (a) and ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{D}}_2\mathrm{O}$ (b) between 1.9 and 10 K at $H=20\mathrm{O}\mathrm{e}$. The measurements were performed under both zero-field-cooling (ZFC) (filled circles) and field-cooling (FC) (unfilled circles) conditions. Shown in the right panels are the temperature dependences of the electrical resistivity of polycrystalline ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{H}}_2\mathrm{O}$ (c) and ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{D}}_2\mathrm{O}$ (d).Reuse & Permissions

Figure 3

Temperature dependence of specific heat $C_p$ of ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{H}}_2\mathrm{O}$ (filled circles) and ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·1.4{\mathrm{D}}_2\mathrm{O}$ (unfilled circles) plotted as $C_p/T$ versus $T^2$ between 2 and 10 K. The solid lines are the fit to experimental data between 5.5 and 10 K using $C_p/T=\gamma +\beta T^2$ ($\beta$ and $\gamma$ are constants). The insets show the temperature dependence of (a) $C_p$ between 2 and 60 K and (b) electronic specific heat $C_p^{\mathrm{e}\mathrm{l}}$ plotted as $C_p^{\mathrm{e}\mathrm{l}}/T$ versus $T^2$ between 2 and 10 K.Reuse & Permissions

Figure 4

Temperature dependence of the in-plane and out-of-plane resistivity ${\rho }_{ab}$ (a) and ${\rho }_c$ (b) of ${\mathrm{N}\mathrm{a}}_{0.3}{\mathrm{C}\mathrm{o}\mathrm{O}}_2·y{\mathrm{H}}_2\mathrm{O}$ single crystals. The inset (c) is an enlargement of the low-temperature data, showing a superconducting transition in both ${\rho }_{ab}$ and ${\rho }_c$ at $T_c=5\mathrm{K}$.Reuse & Permissions