Negative isotope effect of a BCS-like gap; an inelastic light scattering study

Abstract

In a prior study of the boron isotope effect on the transition temperature ($T_c$) of $R$Ni${}_2$B${}_2$C ($R$ = Y, Lu) system, a positive isotope effect was observed, e.g., $T_c{(}^{10}B)>T_c{(}^{11}B)$. BCS theory predicts that the superconducting gap at zero temperature (${\Delta }_0$) is proportional to $T_c$. Therefore, the gap should also show a positive boron isotope effect. On the contrary, in an inelastic light scattering study, we report a negative boron isotope effect on the energy $\Omega$ of the gap-like feature in B${}_{2g}$ symmetry, e.g., $\Omega {(}^{10}B)<\Omega {(}^{11}B)$. The origin of the effect is discussed in terms of selective probing of a non-conventional electron-phonon interaction on part of the Fermi surface, probably near the X point in the Brillouin zone.

4 pages, 4 figures