Temperature-dependent photoemission spectroscopy study of RExC60 (RE=Yb and Sm) films

Temperature-dependent electronic structures of RExC60 (where RE denotes Yb and Sm) films have been studied by high-resolution photoemission spectroscopy (PES). Our results indicate that Yb5C60 is the only metallic phase among all the phases in Yb doped C-60 films. For the Sm fullerides, PES results show that there is no Sm valence transition during the negative thermal expansion in Sm2.75C60 below 32 K. The Sm valence state in Sm2.75C60 films is nearly +2 at room temperature and remains stable in the films down to 16 K. In addition, the negative thermal expansion effect does not result any distinct change in valence-band electronic structures of Sm2.75C60. (c) 2007 American Institute of Physics.