A local metallic state in globally insulating La1.24Sr1.76Mn2O7 well above the metal–insulator transition

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.