Coulomb Gap and Correlated Vortex Pinning in Superconductors

The positions of columnar pins and magnetic flux lines determined from a decoration experiment on B${\mathrm{i}}_2$S${\mathrm{r}}_2$CaC${\mathrm{u}}_2$ ${\mathrm{O}}_8$ were used to calculate the distribution of pinning energies in the Bose glass phase. A wide Coulomb gap is found, with effective gap exponent $s_{\mathrm{eff}}\approx 1.2$, as a result of interactions between the vortices. As a consequence, the variable-range hopping transport of flux lines is considerably reduced with respect to the noninteracting case, the effective Mott exponent being enhanced from $p_0=1\mathrm{}/3$ to $p_{\mathrm{eff}}\approx 0.5$ for this sample.