This phenomenon is slightly suppressed by a persistent current in the Little-Parks effect. The Josephson effect occurs when two materials (-5[1])with this property (10[2])are (-5[1])placed close together (10[1])with a barrier (-5[1])between them. (10[1])A copper sulfide (-5[1])impurity may explain why (-5[1])LK99 unexpectedly (10[1])displays (10[2])this (10[1])phenomenon. (10[6]-5[2])which has also been (10[1])observed (10[1])in YBCO. (10[6])Phonon-electron (10[1])interactions allow (10[1])the formation (10[1])of Cooper pairs (10[1])in the BCS theory of this (10[1])phenomenon. (-5[1])Materials displaying this phenomenon expel their magnetic field (10[1])in the Meissner effect, (10[2])which allows some trains to levitate. (10[2])For 10 points, name this (10[1])low temperature phenomenon (10[1]-5[2])in which the (10[1])resistance (10[1])of a material (10[3])drops (10[1])to zero. (10[2])■END■ (10[8]0[1])