Resistivity of High Temperature Superconductors in Magnetic Field

Temperature dependence of resistivity observed in high temperature superconductors such as YBCO under a magnetic field is shown to be in marked disagreement with the standard theory of fluctuation enhanced conductivity. By estimating relevant energies, we suggest that vortex pairs or rings can be important fluctuations in these superconductors and lead to melting of the flux lattice near H c2 . A mechanism due to phase slippage at grain boundaries is also discussed.