YBCO single crystals I– V characteristics nonlinearity and Nelson–Kosterlitz jump

Temperature dependences of I– V characteristics and voltage harmonics V n ( T) ( n = 2, 3, …) have been investigated in magnetic fields up to 200 Oe for single crystals YBa 2Cu 3O 7− x . It was shown that V n ( T) had asymmetric peak form with maximum at T ∗ = 92.0 K. Amplitudes of V n ( T) were decreased in magnetic fields. The results were explained on the base of the Berezinskii–Kosterlitz–Thouless (BKT) transition model. It was shown that T ∗ coincided with the BKT transition temperature T BKT. The asymmetric peaks of V n ( T) are a consequence of the Nelson–Kosterlitz jump.