Vortex Lock-In Deep in the Bose Glass

We use a Bi gaussmeter of micron dimensions to explore the magnetic field dependence of the magnetization relaxation rate and the critical current down to millikelvin temperatures in untwinned single crystals of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ with columnar defects. The response separates into three regimes as a function of the ratio of vortex density to columnar defect density $\frac{B}{B_{\phi }}$: enhancements in both critical current and quantum creep in the dilute limit, vanishing magnetization relaxation at the matching density (the proposed "Mott insulator" phase line), and the emergence of temperature-dependent vortex motion for $B\gg B_{\phi }$.