ARTICLES The loss of vortex line tension sets an upper limit to the irreversibility line in YBa 2 Cu 3 O 7 J. FIGUERAS 1 , T. PUIG 1 , X. OBRADORS 1 *, W. K. KWOK 2 , L. PAULIUS 2,3 , G. W. CRABTREE 2 AND G. DEUTSCHER 4 1 Institut de Ci ` encia de Materials de Barcelona, C.S.I.C., Campus U.A. Barcelona, 08193 Bellaterra, Catalonia, Spain 2 Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, USA 3 Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008, USA 4 Department of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel *e-mail: xavier.obradors@icmab.es Published online: 21 May 2006; doi:10.1038/nphys311 In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field H m ( T ) below the upper critical field H c2 ( T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be ‘pinned’ individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line H l ( T ) between H m ( T ) and H c2 ( T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field H l ( T ) sets an upper limit to the irreversibility line H irr ( T ) marking the onset of dissipation. For that reason it determines a new magnetic-field–temperature region in which a superconductor can remain useful. I n the mixed state of conventional type-II low-T c superconductors, vortices form the Abrikosov lattice 1 up to the upper critical field H c2 ( T ) where superconductivity is destroyed. In the mixed state, flowing electrical current will generate a Lorentz force acting on vortices that, unless pinned at structural defects, would generate vortex motion and hence the loss of the zero-resistance state. Strong shear forces in the vortex lattice allow pinning of a few vortices to fix the entire lattice. The discovery of a vortex lattice melting at a field H m ( T ) considerably lower than H c2 ( T ) showed that the magnetic phase diagram of high-temperature superconductors is much more complex because of the combined effects of enhanced thermal fluctuations and a large anisotropy 2–4 . Vortex pinning is most effective when linear or planar defects are introduced 5,6 . Hence, the irreversibility line H irr ( T ), where a continuous transition from the vortex liquid phase to a non-dissipative state occurs, is shifted upwards in temperature when these defects are introduced in the superconductor 7–9 . Studies of the upwards shift of the irreversibility field H irr ( T ) and of the vortex correlation along the magnetic field in the vortex liquid state of ion-irradiated or twinned YBa 2 Cu 3 O 7−δ (YBCO) single crystals have suggested that a maximum magnetic field exists where this correlation can be maintained 10–14 , even if the correlation can actually be lost at H m ( T ) in clean crystals where any defect can stabilize the vortices 15–17 . Here we report further investigations that show that these apparently unrelated phenomena are actually a manifestation of the same universal behaviour of a single vortex line, which, due to thermal fluctuations, loses its line tension and hence all of the linear defects become ineffective for pinning 9 . We have investigated the vortex liquid state of twinned YBCO crystals, with different types of linear (dislocations) and planar (twin boundaries) defects, and also untwinned single crystals with linear defects created by ion irradiation. Several types of electrical transport measurements, such as anisotropic magnetoresistance 402 nature physics VOL 2 JUNE 2006 www.nature.com/naturephysics Nature Publishing Group ©2006