Chemical potential dependent gap-opening at the Dirac surface states of   Bi2Se3 induced by aggregated substitutional Cr atoms
  With angle-resolved photoemission spectroscopy, gap-opening is resolved at up to room temperature in the Dirac surface states of molecular beam epitaxy grown Cr-doped Bi2Se3 topological insulator films, which however show no long-range ferromagnetic order down to 1.5 K. The gap size is found decreasing with increasing electron doping level. Scanning tunneling microscopy and first principles calculations demonstrate that substitutional Cr atoms aggregate into superparamagnetic multimers in Bi2Se3 matrix, which contribute to the observed chemical potential dependent gap-opening in the Dirac surface states without long-range ferromagnetic order. 
Cui-Zu Chang, Peizhe Tang, Yi-Lin Wang, Xiao Feng, Kang Li, Zuocheng   Zhang, Yayu Wang, Li-Li Wang, Xi Chen, Chaoxing Liu, ...
cond-mat.mtrl-sci