Electronic and magnetic properties of the kagome systems YBaCo4O7 and
  YBaCo3MO7 (M=Al, Fe)

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Abstract

We present a combined experimental and theoretical x-ray absorption spectroscopy (XAS) study of the new class of cobaltates YBaCo4O7 and YBaCo3MO7 (M= Al, Fe). The focus is on the local electronic and magnetic properties of the transition metal ions in these geometrically frustrated kagome compounds. For the mixed valence cobaltate YBaCo4O7, both the Co2+ and Co3+ are found to be in the high spin state. The stability of these high spin states in tetrahedral coordination is compared with those in the more studied case of octahedral coordination. For the new compound YBaCo3FeO7, we find exclusively Co2+ and Fe3+ as charge states.

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Superconductivity in two-dimensional CoO2 layers.

Kazunori Takada, Hiroya Sakurai, Eiji Takayama-Muromachi … (2003)

Since the discovery of high-transition-temperature (high-T(c)) superconductivity in layered copper oxides, many researchers have searched for similar behaviour in other layered metal oxides involving 3d-transition metals, such as cobalt and nickel. Such attempts have so far failed, with the result that the copper oxide layer is thought to be essential for superconductivity. Here we report that Na(x)CoO2*yH2O (x approximately 0.35, y approximately 1.3) is a superconductor with a T(c) of about 5 K. This compound consists of two-dimensional CoO2 layers separated by a thick insulating layer of Na+ ions and H2O molecules. There is a marked resemblance in superconducting properties between the present material and high-T(c) copper oxides, suggesting that the two systems have similar underlying physics.