Experimental evidence for vortex lines in the vortex-liquid phase of YBa2Cu3O6.95 in a geometry of tilted vortices

Using a transport method with a geometry of tilted magnetic field, we have studied the direction of the electric field generated by the motion of vort ices. This experiment gives evidence of uncut vortex lines in slightly over doped untwinned YBa2Cu3O6.95 above the melting first order transition, show ing that the first order transition does not correspond to a decoupling of the vortex lines. The gradual loss of the line correlation appears close to T-c2 on a width which is typically that of the critical fluctuations. Thos e results are similar to what we have found previously in a heavily twinned sample and lead to the conclusion that the twin planes are not necessary t o induce uncut vortex lines in the liquid phase, even in a thick sample. Mo reover, we observe a nonzero transverse voltage below the critical current, which is not explained unless referring to surface nondissipative currents . [S0163-1829(99)10201-7].