Comment on "History Effects and Phase Diagram near the Lower Critical   Point in YBa_2Cu_3O_{7-\delta} Single Crystals"
  This is a Comment on an experimental paper by Zhukov et al. on the issue of a lower critical point, seen only in YBCO clean samples, of the first order transition line implying a vortex liquid-freezing. An explanation with a firm theoretical basis of the strange doping dependence of the lower critical point is presented. 
Ryusuke Ikeda