Seminars Archive

Many-body effects and electronic reconstructions in misfit cobaltates investigated by ARPES

Abstract
The interpretation of the phase diagram of Na cobaltates (NaXCoO2), from the Mott-insulator limit at X=0 (one spin ½ hole per Co) to the band insulator limit at X=1, is still far from clear. Surprisingly, these materials seem to behave like a non correlated metal near X=0, when many magnetic holes are present, whereas magnetic orderings and magnetic correlations appear when more than 60% of the magnetic Co holes are filled, and increase as the number of holes is further reduced. In this region of the phase diagram, Curie-Weiss susceptibility and high thermoelectric-power (TEP) are observed, despite the fact that they remain good metals [1]. We present an angle-resolved photoemission (ARPES) study on a parent family of compounds, the “misfit‿ cobaltates. In these materials, the same CoO2 layers are present, but they are separated by a rigid rock-salt structure (RS), instead of a Na layer. The “misfits‿ are located in the “magnetic part‿ of the phase diagram (X=0.7-1) and they show high TEP and a Curie-Weiss susceptibility, as for the Na cobaltates with the same doping. However, some properties are different: some “misfits‿ show a negative magneto-resistance, no magnetic ordering is reached at low temperature, and the transport properties suggest an evolution toward an insulating phase before the band insulator limit [2,3]. By ARPES, we have established the similarity of the low energy electronic structure between the two families [4]. We have observed a narrow quasi-particle peak (QP) crossing the Fermi level, and tracing a Fermi surface qualitatively similar to that of Na cobaltates. Moreover, by using different polarizations of the light, we were able to select the band that crosses the Fermi level, and to extract precisely its lineshape. It has a peak-dip-hump structure, which indicates the presence of strong many-body effects. We have further observed the progressive disappearance of the QP when the number of holes decreases, which is not observed in Na cobaltates. This suggests that the metal-insulator transition present in the “misfits‿ is due to increasing many-body effects. Moreover, we have detected some weak replicas of the Fermi surface with the periodicity of the RS subsystem, which proves its non negligible influence on the CoO2 electronic structure. This questions the role of the 3D couplings in cobaltates, for “misfits‿ as well as for Na cobaltates. [1]. M.L. Foo et al. Phys. Rev. Lett., 92, 247001 (2004) [2]. A.C. Masset et al. Phys. Rev. B 62 166 (2000) [3]. A. Maignan et al. J. Phys: Condens. Matter 15 2711 (2003) [4]. V. Brouet, A.Nicolaou et al. Phys. Rev. B 76 100403 (2007)