# Synopsis: Surprises from NMR in sodium cobaltates

#### Spin correlations and cobalt charge states: Phase diagram of sodium cobaltates

G. Lang, J. Bobroff, H. Alloul, G. Collin, and N. Blanchard

Published October 14, 2008

A large part of the interest in superconducting ${\text{Na}}_{x}{\text{CoO}}_{2}$ is that the ${\text{CoO}}_{2}$ layers are reminiscent of the ${\text{CuO}}_{2}$ layers in high-${T}_{c}$ cuprates, except that the $\text{Co}$ ions sit on a triangular lattice, which enhances the role of magnetic frustration between the Co spins. Also like the cuprates, the layered cobaltate ${\text{Na}}_{x}{\text{CoO}}_{2}$ displays a rich medley of ordered states: superconductivity when intercalated with water for $1/4, insulating charge-order at $x=1/2$, and spin-density wave order at $x=3/4$. The insulating state at $x=1/2$ separates two distinct metallic states: a paramagnetic metal below $x=1/2$ and a “Curie-Weiss” metal with antiferromagnetically coupled spins above $x=1/2$.

While no consensus exists on a theoretical picture, this experimental phase diagram is generally believed to be true. Now, Guillaume Lang and colleagues from Laboratoire de Physique des Solides at Université Paris-sud and Laboratoire Léon Brillouin in Saclay report a rather different phase diagram based on nuclear magnetic resonance experiments. Writing in Physical Review B, the authors find that at low temperatures there exists a critical doping range, ${x}^{*}=$ 0.63–0.65, below and above which antiferromagnetic and ferromagnetic correlations are, respectively, dominant. This contradicts the nonmagnetic behavior reported earlier for $x<1/2$.

For $0.5, Lang et al. also identify a doping-dependent temperature scale ${T}^{*}$, which separates a high-temperature region with ferromagnetic correlations and a low-temperature region with antiferromagnetic correlations. The ${T}^{*}$ line slopes away from the insulating limit ($x=1/2$) in the same way the pseudogap crossover line does in the cuprates. The physical origin of this doping- and temperature-dependent crossover in magnetic correlations is a striking new puzzle for theory to address. – Sarma Kancharla