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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 $B_Φ$. Repulsive vortex interactions destroy the Mott insulator phase predicted to occur at $B=B_Φ$. For ratios of the penetration depth to average defect distance $λ/ d \leq 1$ remnants of the Mott insulator singularities remain visible in the magnetization, the bulk modulus, and the magnetization relaxation, as B is varied near $B_Φ$. For $λ\geq d$, the ensuing weakly pinned Bose glass is characterized by a soft Coulomb gap in the distribution of pinning energies.