BKT transition observed in magnetic and electric properties of YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals

TL;DR

This study observes the Berezinskii-Kosterlitz-Thouless transition in YBa₂Cu₃O₇−δ single crystals, revealing a two-dimensional superconducting phase influenced by the crystal's layered structure.

Contribution

It provides experimental evidence of the BKT transition in high-temperature superconductors and links it to the layered crystal structure of YBa₂Cu₃O₇−δ.

Findings

Superconducting transition temperature varies with magnetic field orientation.

Two-dimensional superconductivity observed between 0.6-0.8 K.

BKT transition occurs approximately 0.15 K below T_c.

Abstract

Superconductivity of YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals was investigated in small magnetic fields. In magnetic measurements the superconducting transition for $\textbf{H} \| c$ appears 0.4 K higher than for $\textbf{H} \bot c$. 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 \textit{ab}-plane and along the \textit{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 $T_c$, the mean-field transition temperature.