Directed polymer in a random medium of dimension 1+3 : multifractal properties at the localization/delocalization transition

We consider the model of the directed polymer in a random medium of dimension 1+3, and investigate its multifractal properties at the localization/delocalization transition. In close analogy with models of the quantum Anderson localization transition, where the multifractality of critical wavefunctions is well established, we analyse the statistics of the position weights $w_L(\vec r)$ of the end-point of the polymer of length $L$ via the moments $Y_q(L) = \sum_{\vec r} [w_L(\vec r)]^q$. We measure the generalized exponents $\tau(q)$ and $\tilde \tau(q)$ governing the decay of the typical values $Y^{typ}_q(L) = e^{\bar{\ln Y_q(L)}} \sim L^{- \tau(q)}$ and disorder-averaged values $\bar{Y_q(L)} \sim L^{- \tilde \tau(q)}$ respectively. To understand the difference between these exponents, $\tau(q) \neq \tilde \tau(q)$ above some threshold $q>q_c \sim 2$, we compute the probability distributions of $y=Y_q(L)/Y^{typ}_q(L)$ over the samples : we find that these distributions becomes scale invariant with a power-law tail $1/y^{1+x_q}$. These results thus correspond to the Ever-Mirlin scenario [Phys. Rev. Lett. 84, 3690 (2000)] for the statistics of Inverse Participation Ratios at the Anderson localization transitions. Finally, the finite-size scaling analysis in the critical region yields the correlation length exponent $\nu \sim 2$.