Steric repulsion and van der Waals attraction between flux lines in   disordered high Tc superconductors
  We show that in anisotropic or layered superconductors impurities induce a van der Waals attraction between flux lines. This attraction together with the disorder induced repulsion may change the low B - low T phase diagram significantly from that of the pure thermal case considered recently by Blatter and Geshkenbein [Phys. Rev. Lett. 77, 4958 (1996)]. 
Sutapa Mukherji and Thomas Nattermann