Comment on "Strong vortex liquid correlation from multiterminal measurements on untwinned YBa(2)Cu(3)O(7-delta) single crystals".

YBa 2 Cu 3 O 7−δ Single Crystals " From recent transport experiments A.Rydh and¨O.Rapp [1] (RR) claim that the vortex liquid in untwinned YBa 2 Cu 3 O 7−δ crystals is correlated above the melting transition, in striking contrast to previous work [2]. In this Comment we present new measurements using the same experimental technique on twinned and untwinned YBa 2 Cu 3 O 7−δ crystals with similar overall characteristics as those reported by RR. The comparison of the vor-tex correlation response in both cases indicates that the central conclusion of the work of RR is not correct. Our results reconfirm the work by López et al. [2] and points on the origin of the misinterpretation in the work of RR. The results of the voltage response, V(T), for a twinned crystal in a magnetic field of H = 6 T, are shown Fig.1(a). At high temperatures the dissipation at the top, V top , is larger than at the bottom, V bot , as a result of the inho-mogeneous current distribution acting upon an uncorre-lated vortex system. As the temperature is lowered V top equals V bot at T th. This result could be either a trivial consequence of the limited experimental voltage sensitivity (indicating an ohmic decrease of the resistance in the c direction) or associated with the stablishment of vortex velocity correlation wich implies vortex phase correlation along the field direction across the sample thickness. In the first case the I-V curves are ohmic, in the second a non-linear behavior indicates the existence of a finite cutting current [3], I cut , for TT th , ∆V(I)= V top-V bot extrapolates to zero at I=0, for T<T th a well defined I cut is detected. For an untwinned crystal [4] equivalent results are shown in Fig. 1(b). From the V(T) curves alone one is tempted to conclude, as RR did, that the vortex liquid is correlated between T m and the " apparent " T th. However, the behavior of the I-V curves shows that in this case the result V top =V bot is just a consequence of the limited voltage sensitivity. The inset shows a linear ∆V(I) that extrapolates to zero for I=0 at T < " T th " , consistent with …