Weakly Pinned Bose Glass vs. Mott Insulator Phase in Superconductors

We study the properties of the Bose glass phase of localized flux lines in irradiated superconductors near the matching field $B_{\Phi}$. Repulsive vortex interactions destroy the Mott insulator phase predicted to occur at $B=B_\Phi$. For ratios of the penetration depth to average defect distance $\lambda / d \leq 1$ remnants of the Mott insulator singularities remain visible in the magnetization, the bulk modulus, and the magnetization relaxation, as B is varied near $B_{\Phi}$. For $\lambda \geq d$, the ensuing weakly pinned Bose glass is characterized by a soft Coulomb gap in the distribution of pinning energies.