BKT transition observed in magnetic and electric properties of YBa2Cu3O7−δ single crystals

Abstract

Superconductivity of YBa2Cu3O 7−δ single crystals was investigated in small magnetic fields. In magnetic measurements the superconducting transition for H c appears 0.4 K higher than for H⊥c. In this temperature range superconductivity is two-dimensional and the total thickness of superconducting layers is about 0.83 of the sample thickness, which is a consequence of the occurrence of the quasi-insulating plane in the unit cell of the crystal structure. Resistivity in the ab-plane and along the c-axis was measured simultaneously. In these measurements two-dimensional superconductivity was observed in a temperature range of 0.6-0.8 K with the clear signs of the Berezinskii-Kosterlitz-Touless (BKT) transition which occurs approximately 0.15 K below Tc, the mean-field transition temperature.

Approaching two-dimensional superconductivity in ultrathin DyBa$_2$Cu$_3$O$_{7-δ}$

“…Even in the presence of strong anisotropy, the CuO 2 -bilayer material RBa 2 Cu 3 O 7-δ (RBCO), where R = rare earth, displays critical fluctuations belonging to the 3D-XY universality class, where the fluctuation correlations extend over many CuO 2 planes along the c-axis [1][2][3]. Close to optimal doping the 2D-like properties of the SC condensate appear only within a very narrow temperature range just below T c [4],where uncorrelated fluctuations in adjacent CuO 2 layers are described by the Berezinskii-Kosterlitz-Thouless (BKT) vortex unbinding transition [5,6].…”