Excitonic Wigner crystal and high Tc ferromagnetism in RB6

The mechanisms for the high Tc ferromagnetism in La-doped divalent hexaborides DB6 are studied in detail comparing with similar family materials, in particular with YbB6, EuB6 and Ce monopnictides. It is shown that in DB6 the light-electron-heavy-hole paired excitonic states form the Wigner crystal, or Wigner glass in actual materials, in which the conventional intersite electron exchange interactions similar to that in Ni dominate the pair singlet formation due to the intra pair mixing causing a ferromagnetic spin glass-like ordering of electron spins. In the La-doped system LaxD1-xB6, the population of molecular La impurity states with giant moments increases as x approaches the optimal value x0∼0.005 for high Tc providing vacant states for the roton-like fluctuations, which cause the high Tc at the boundary of the delocalization of electron carriers. Therefore, the critical La concentration for delocalization coincides with the optimal La population x0. In this mechanism, the lattice polarons play an essentially important and delicate role to stabilize the excitonic Wigner crystal but not to prevent the roton-like fluctuation of electron carriers. It is shown that in YbB6 and EuB6 the lattice polaron effect is too strong for the roton-like fluctuation