Point-contact spectroscopy of the phononic mechanism
Here, we present direct point-contact spectroscopy studies of the superconducting interaction in YB6. As a result, the normalized superconducting gap reveals a strong coupling with 2Δ/kBTc ≈ 4 and, moreover, the spectra contain nonlinearities typical of the electron–phonon interaction at energies around 8 meV. The measurements in a
magnetic field evidence that the phonon features found in the second derivative of the current–voltage characteristics are due to the energy dependence of the superconducting energy gap as their energy position shrinks equally as the gap is closed. This provides direct
proof that the superconducting coupling in the system is due to the low energy Einstein-like phonon mode associated with the yttrium ion vibrations, in perfect agreement with determinations from bulk measurements.
Figure 1: The second derivative, d2V/dI2(V), spectrum measured at T = 4.2 K on a Pt–YB6 point-contact in the superconducting (at 0 and 0.1 T) and normal (at 0.4 T) states. The down-most curve plots the EPI function of a Pt–Pt homo-contact.
Figure 2: The magnetic-field dependence of the superconducting energy gap of YB6 (left ordinate) resulting from the measurements. The solid line is a square-root field dependence. The full circles indicate a position development of the phonon mode of YB6 in a magnetic field from the spectrum in figure 1 (right ordinate).