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MA: Fachverband Magnetismus

MA 3: Magnetic Materials I

MA 3.8: Talk

Monday, March 26, 2007, 12:00–12:15, H22

Bulk and surface magnetism in Na_0.75CoO2: why is Luttinger’s theorem violated? — •Michelle Johannes¹, Igor Mazin¹, and George Sawatzky² — ¹Naval Research Laboratory, Washington, D.C. USA — ²University of British Columbia, Vancouver, Canada

The properties of Na_xCoO₂ vary substantially with the sodium concentration, x. Though strong spin fluctuations are proposed to exist throughout the full range of sodium, long range magnetic order sets in only at low hole concentration (x ∼ 0.7). Also around this doping, ARPES results show substantial disagreement with Luttinger’s theorem (LT). We show that this is definitively not the result of deviations from two-dimensionality and propose that the two effects (magnetism and violation of LT) are related. We present a careful analysis of the various possible terminations of a cleaved surface of Na_0.75CoO₂, and conclude that the observed Fermi surfaces are inconsistent with ANY physically possible bulk system. We suggest surface layer magnetism as a possible explanation for the discrepancy between Fermi surface size and electron count.