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 E-F, which separates the occupied from the unoccupied electron energy levels. For a metal, E-F lies in the middle of a band of electronic states, whereas E-F 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 excitations(1). 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 T-C, when the sample is globally metallic, and above T-C, when it is globally insulating. Further, small amounts of metallic spectral weight survive up to temperatures more than twice T-C. Such behaviour may also have close ties to a variety of exotic phenomena in correlated electron systems, including the pseudogap temperature in underdoped cuprates(2).