Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 13: Symposium “Superconductivity and Magnetism in Lamellar Cobaltates”

TT 13.6: Talk

Tuesday, March 27, 2007, 16:30–16:55, H18

Magnetic ordering and excitations in Na_xCoO₂ — •Sibel Bayrakci¹, Isabelle Mirebeau², Philippe Bourges², Yvan Sidis², Mechthild Enderle³, Joel Mesot⁴, Dapeng Chen¹, Chengtian Lin¹, and Bernhard Keimer¹ — ¹Max-Planck-Institut für Festkörperforschung, Stuttgart — ²Laboratoire Léon Brillouin, C.E.A./C.N.R.S., Gif-sur-Yvette, France — ³Institut Laue-Langevin, Grenoble, France — ⁴Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institute, Villigen, Switzerland

Superconductivity in the layered cobalt oxide Na_xCoO₂:yH₂O (x∼0.30, y∼1.4) is the subject of intensive current investigation. This material is particularly interesting given its structural similarity to the high-T_c copper oxide superconductors, with the added attribute of triangular symmetry. The unhydrated parent compound Na_xCoO₂ is itself intriguing due to its exceptionally high thermopower over the range 0.5 ≤ x ≤ 0.9, which occurs in unusual combination with low resistivity and low thermal conductivity. Na_xCoO₂ has been shown to exhibit bulk antiferromagnetic order below ∼20 K for 0.75 ≤ x ≤ 0.9. In neutron scattering experiments on the composition with x = 0.82, we have observed Bragg reflections which correspond to A-type antiferromagnetic order and characterized the corresponding three-dimensional spin wave dispersion. The in- and out-of-plane exchange constants resulting from a fit to a simple Heisenberg model are similar in magnitude, which is unexpected given the layered crystal structure of Na_xCoO₂. Possible explanations will be discussed. Recent inelastic measurements on the x = 0.5 composition will also be mentioned.