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

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In a prior study of the boron isotope effect on the transition temperature (Tc) of RNi₂B₂C (R = Y, Lu) system, a positive isotope effect was observed, e.g., Tc(¹⁰B)> Tc(¹¹B). BCS theory predicts that the superconducting gap at zero temperature (Δ₀) is proportional to Tc. 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 Ω of the gap-like feature in B₂g symmetry, e.g., Ω(¹⁰B)< Ω(¹¹B). 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.