Magnetization Relaxation via Quantum and Classical Vortex Motion in a Bose Glass Superconductor

I show that in Bose Glass superconductor with high $j_c$ and at low $T$ the magnetization relaxation (S), dominated by quantum tunneling, is $\propto{\sqrt j_c}$, which crosses over to the conventional classical rate $\propto T/j_c$ at higher $T$ and lower $j_c$, with the crossover $T^*\sim j_c^{3/2}$. I argue that due to interactions between flux lines there exist three relaxation regimes, depending on whether $B<B_\phi$, $B\approx B_\phi$, $B>B_\phi$, corresponding to Strongly-pinned Bose Glass (SBG) with large $j_{c2}$, Mott Insulator (MI) with vanishing S, and Weakly-pinned Bose Glass (WBG) characterized by small $j_{c1}$. I discuss the effects of interactions on $j_c$ and focus attention on the recent experiment which is consistently described by the theory.