Angle-Resolved Photoemission Spectroscopy of Ca1-xLaxB6

We performed angle-resolved photoemission spectroscopy (ARPES) on Ca 1- x La x B 6 ( x =0.01) to study the electronic structure and the origin of the anomalous high-temperature ferromagnetism. We found a large energy gap of about 1 eV at the Fermi level at the X point in the Brillouin zone in contrast to the LDA band calculation, which has predicted the semimetallic nature of CaB 6 with a small overlap of an electron and a hole pocket at the X point. The band structure determined by ARPES shows a good agreement with the GW band calculation which predicts the semiconductive character. The present ARPES results request reexamination of the excitonic-insulator model based on a small overlap of two bands at the X point to explain the ferromagnetism in Ca 1- x La x B 6 .