Origin of the variable-range vortex hopping in Bi2Sr2Ca1-xYxCu2O8 with columnar defects

We report on the vortex transport in Bi2Sr2Ca1-xYxCu2O8 single crystals irr adiated in a parallel direction with the c axis with 5.8-GeV Ph ions at flu ences of 3.75 x 10(10) cm(-2) and 7.5 x 10(10) cm(-2). A detailed investiga tion of the vortex transport using current-voltage measurements is carried out with the magnetic field applied along the c axis. First, we investigate the critical behavior of the linear and nonlinear conductivity near the Bo se-glass melting line. We obtain field and sample-independent critical expo nents z' and v' consistent with a compressible Bose glass (i.e., v(0) =2v(p erpendicular to) equivalent to 2v'). Using the values z' = 5.28 +/- 0.05 an d v' = 1.04 +/- 0.06, the data collapse into two single-scaling functions. Second, we report on conductivity measurements over a wide filling factor ( B/B-phi) range from 0.04 up to 0.9 within the Bose-glass phase. Our data pr ovide support for a variable-range hopping mechanism for low current densit ies, in accordance with the ideas of Nelson and Vinokur [Phys. Rev. B 48, 1 3 060 (1993)]. We determine a glass exponent value of 1/3 for all the filli ng factors investigated at large ratios of the penetration depth to the ave rage defect distance (lambda(ab)/d similar to 10) This finding implies that no Coulomb gap occurs in the pinning energy spectrum. Furthermore, our res ults show that for low filling: factors (<1/2) the on-site disorder plays a major role in the bandwidth of flux-binding energies. Finally, the appeara nce of a well-defined crossover near half filling is consistent with the fi eld B* separating the strongly pinned Bose-glass regime from the weal;ly pi nned Bose-glass regime.