Surprises from NMR in sodium cobaltates

  Superconductivity

A large part of the interest in superconducting Na_xCoO₂ is that the CoO₂ layers are reminiscent of the CuO₂ layers in high-T_c cuprates, except that the 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 Na_xCoO₂ displays a rich medley of ordered states: superconductivity when intercalated with water for 1/4<x<1/3, 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<x<0.62, 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