Thermal conductivity of layered organic superconductorβ−(BDA−TTP)2SbF6in a parallel magnetic field: Anomalous effect of coreless vortices

Abstract

Thermal conductivity $\ensuremath{\kappa}$ of the organic superconductor $\ensuremath{\beta}\text{\ensuremath{-}}{(\mathrm{BDA}\text{\ensuremath{-}}\mathrm{TTP})}_{2}\mathrm{Sb}{\mathrm{F}}_{6}$ was studied down to 0.3 K in magnetic fields $H$ of varying orientation with respect to the superconducting plane. Anomalous plateau shape of the field dependence, $\ensuremath{\kappa}$ vs $H$, is found for orientation of magnetic fields precisely parallel to the plane, in contrast to usual behavior observed in the perpendicular fields. We show that the lack of magnetic-field effect on the heat conduction results from coreless structure of vortices, causing both negligible scattering of phonons and constant in field electronic conduction up to the fields close to the upper critical field ${H}_{c2}$. Usual behavior is recovered on approaching ${H}_{c2}$ and on slight field inclination from parallel direction, when normal cores are restored. This behavior points to the lack of bulk quasiparticle excitations induced by magnetic field, consistent with the conventional superconducting state.