Microscopic properties of vortex states inYB6probed by muon spin rotation

Abstract

The local magnetic field distribution $B(\mathbf{r})$ in the mixed state of a boride superconductor, $\mathrm{Y}{\mathrm{B}}_{6}$, is studied by muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$. A comparative analysis using the modified London model and Ginzburg-Landau (GL) model indicates that the latter exhibits a better agreement with $\ensuremath{\mu}\mathrm{SR}$ data at higher fields and thus demonstrating the importance of reproducing the field profile near the vortex cores when the intervortex distance becomes closer to the GL coherence length. The temperature and field dependence of the magnetic penetration depth $(\ensuremath{\lambda})$ does not exhibit any indications of the nonlocal effect or of low-lying quasiparticle excitation. This suggests that the strong coupling of electrons to the rattling motion of Y ions in the boron cage, as suggested by bulk measurements, induces a conventional superconductivity with isotropic $s$-wave pairing. Considering the present results, a review is provided for probing the anisotropy of the superconducting order parameters by the slope of $\ensuremath{\lambda}$ against the field $(\ensuremath{\eta}\ensuremath{\propto}d\ensuremath{\lambda}∕dH)$.