Vector susceptibility and QCD phase transition in AdS/QCD models

We calculate the vector isospin susceptibility in AdS/QCD models to study QCD phase transition. In the hard wall model, we show explicitly that the infalling boundary condition at the horizon can be treated as a Dirichlet boundary with a fine-tuned boundary value in the zero frequency and momentum limit. With the infalling boundary condition, we uniquely determine the overall normalization of the vector isospin susceptibility in the hard wall model. In the framework of the soft wall model, we obtain the vector isospin susceptibility with and without Hawking-Page transition and compare our results with lattice QCD. We briefly discuss the chiral symmetry restoration in the AdS/QCD models.