Kosterlitz-Thouless transition of fluxless solitons in superconductingYBa2Cu3O7−δsingle crystals

We present detailed evidence for a resistive Kosterlitz-Thouless transition in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals. The Kosterlitz-Thouless temperature $T_{\mathrm{K}\mathrm{T}}$ is the onset temperature for zero-resistance superconductivity in the $\mathrm{ab}$ planes; three-dimensional superconductivity throughout the sample follows at a lower temperature. The objects which engage in the transition do not carry flux—they are not the flux vortices of conventional theory. They are more likely normal-state excitations that pair up at $T_{\mathrm{K}\mathrm{T}}$ to form quasi-two-dimensional superconducting condensates.