Anomalous In-Plane Electronic Scattering in Charge Ordered ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2·0.6{\mathbf{H}}_2\mathbf{O}$

We report electronic transport measurements on high quality floating zone grown ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ and ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2·0.6{\mathrm{H}}_2\mathrm{O}$ single crystals. We find an in-plane electronic scattering minimum near 11 K and a clear charge ordering at approximately 50 K. The electronic and magnetic properties in hydrated and nonhydrated ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2$ samples are similar at higher temperature, but evolve in markedly different ways below $\sim 50\mathrm{K}$, where a strong ferromagnetic tendency is observed in the hydrated sample. Model calculations show the relationship of this tendency to the structure of the Fermi surface. The results, particularly the clear differences between the hydrated and nonhydrated material show a substantially enhanced ferromagnetic tendency upon hydration. Implications for superconductivity are discussed.

Figure 1

(a) XRD patterns of ($00l$) planes for ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2$ and ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$. Inset shows the Laue patterns after hydration. (b) and (c) give $R_c$ vs $T$ under different $H$. Inset of (b) and (c) are blowups for $H=0$ and $H//ab$.

Figure 2

${\rho }_{ab}$ vs $T$ for ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2$ and ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$. (a) and (b) $\rho (T)$ curves without field, inset of (b) is a structural model of sodium and water arrangement in ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$. a(1), a(2), and b(1), b(2): $\rho$ vs $T$ curves under various applied fields below 70 K with $H//ab$ and $H//c$.

Figure 3

Upper panel: $\rho (T)-{\rho }_{\mathrm{offset}}$ vs $T^2$ at low temperature for ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$ at $H=0$, 1, 2, 4, 6, and 9 T. ${\rho }_{\mathrm{offset}}$ is a constant arbitrary offset chosen for clarity. Lower panel: Zero field spin-orbit coupling.

Figure 4

(a) $M\sim T$ curves for ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2$ and ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$. (b) Bare ($U=J=0$, thin red dashed curves) and RPA (thick solid blue curves, $U=0.7\mathrm{eV}$ and $J=0.35\mathrm{eV}$) physical spin susceptibility $\mathrm{Re}\chi (q,\omega =0)$ vs momentum for ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2$ (left) and ${\mathrm{Na}}_{0.41}{\mathrm{CoO}}_2•0.6{\mathrm{H}}_2\mathrm{O}$ (right). Insets show corresponding FS.