Columnar Defects as a Probe of the Vortex Liquid in YBa_2Cu_3O_7.

The Bose-glass transition in YBCO single crystals with columnar defects produced by irradiation with 5.8 GeV Pb ions was studied both experimentally and theoretically. Experimentally, the effect was tracked by the onset of the third harmonic generation at low frequencies (f=1-7 Hz). The Bose-glass transition line B_BG(T) progressively shifts upwards with increasing dose of irradiation φ, up to 10^11 ions/cm^2. For larger φ, the B_BG(T) line does not shift anymore, and saturates at B_BG^max(T) which is still considerably below H_c2(T). Theoretically, we show that the observed evolution of the Bose-glass transtion line provides a strong evidence for an intermediate, or disentangled vortex liquid phase, sandwiched between the melting line B_m(T) and B_BG^max(T), in virgin samples.