Abstract
The distinction between metals, semiconductors and insulators depends on the behaviour of the electrons nearest the Fermi level EF, which separates the occupied from the unoccupied electron energy levels. For a metal, EF lies in the middle of a band of electronic states, whereas EF in insulators and semiconductors lies in the gap between states. The temperature-induced transition from a metallic to an insulating state in a solid is generally connected to a vanishing of the low-energy electronic excitations1. Here we show the first direct evidence of a counter-example, in which a significant electronic density of states at the Fermi energy exists in the insulating regime. In particular, angle-resolved photoemission data from the colossal magnetoresistive oxide La1.24Sr1.76Mn2O7 show clear Fermi-edge steps, both below the metal–insulator transition temperature TC, when the sample is globally metallic, and above TC, when it is globally insulating. Further, small amounts of metallic spectral weight survive up to temperatures more than twice TC. Such behaviour may also have close ties to a variety of exotic phenomena in correlated electron systems, including the pseudogap temperature in underdoped cuprates2.
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Acknowledgements
The authors thank K. Gray for the resistivity data of Fig. 1a and Y. Tokura and T. Kimura for providing preliminary samples, and are grateful to D. N. Argyriou, A. Bansil, E. Dagotto, K. Gray, A. Moreo, R. Osborn, L. Radzihovsky, D. Reznik, S. Rosenkranz, Y. Tokura and M. Veillette for helpful discussions. Primary support for this work came from US National Science Foundation grant DMR 0402814, with other support from the US Department of Energy under grant DE-FG02-03ER46066. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, US Department of Energy, under contract No. DE-AC02-05CH11231. Argonne National Laboratory, a US Department of Energy Office of Science Laboratory, is operated under contract No. DE-AC02-06CH11357. The US Government retains for itself, and others acting on its behalf, a paid-up non-exclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public and perform publicly and display publicly, by or on behalf of the government.
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Sun, Z., Douglas, J., Fedorov, A. et al. A local metallic state in globally insulating La1.24Sr1.76Mn2O7 well above the metal–insulator transition. Nature Phys 3, 248–252 (2007). https://doi.org/10.1038/nphys517
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DOI: https://doi.org/10.1038/nphys517
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