Effect of Field Tilting on the Vortices in Irradiated Bi-2212

We report on transport measurements in a Bi-2212 single crystal with columnar defects parallel to the c-axis. The tilt of the magnetic field away from the direction of the tracks is studied for filling factors f = Bz/B φ < 1. Near the Bose Glass transition temperature TBG, the angular scaling laws are verified and we find the field independent critical exponents ν ′ = 1.1 and z ′ = 5.30. Finally, above H ⊥C we evidence the signature of a smetic-A like vortex phase. These experimental results provide support for the Bose Glass theory. Columnar defects have been introduced in HTCS in order to avoid dissipation due to vortex motion. In the case of parallel tracks Nelson and Vinokur[1] have predicted a transition between a so-called Bose Glass (BG) and vortex liquid, at a critical temperature T = T BG. An important result is that field tilting does not destabilize the BG phase. Above a threshold transverse field H ⊥C (T) perpendicular to the columnar defects (CD), the flux lines accomodate simultaneously to CD and to the transverse field direction, defining a smectic-A like phase as proposed by Hwa and Nelson[2]. Increasing further H ⊥ leads to a vortex-liquid state [3]. Recently, Grigera et al. [4] have evidenced a threshold transverse field H ⊥C (T) with resistivity measurements performed on a twinned Y-123 single crystal. In this paper, we present results obtained on a Bi-2212 single crystal, irradiated with CD parallel to the c-axis with 5.8 GeV P b ions in GANIL (Caen-France). The CD density corresponds to a matching field B φ = 0.75 T. Isothermal I-V curves have been obtained varying the tilt angle between the magnetic field and the c-axis of the crystal. Measurements were performed keeping the B z component constant, where the z-axis is along the c-axis of the sample. In the insert of Fig. 1, the log-log plot of isothermal ohmic resistance versus H ⊥ /H z is displayed above and below T BG. For T > T BG , ohmic behaviour is detected, within the experimental sensitivity, even for vanishing transverse field. In contrast, for T < T BG the ohmic resistance goes to zero at some critical tilt showing the existence of a critical transverse magnetic field H ⊥C (T). In the BG theory, scaling functions f + and f − are predicted to describe the resistivity in the presence of …