Directed polymers on hierarchical lattices with site disorder

We study a polymer model on hierarchical lattices very close to the one introduced and studied in Derrida and Griffith (1989) [19] and Cook and Derrida (1989) [16]. For this model, we prove the existence of free energy and derive the necessary and sufficient condition for which very strong disorder holds for all [beta], and give some accurate results on the behavior of the free energy at high temperature. We obtain these results by using a combination of fractional moment method and change of measure over the environment to obtain an upper bound, and a second moment method to get a lower bound. We also get lower bounds on the fluctuation exponent of logZn, and study the infinite polymer measure in the weak disorder phase.