Anomalous electron-phonon coupling probed on the surface of ZrB$_{12}$ superconductor
Magnetization measurements under hydrostatic pressure up to 10.5 kbar in zirconium dodecaboride ZrB$_{12}$ superconductor ($T_c\simeq6.0$ K at $p=0$) were carried out. A negative pressure effect on $T_c$ with ${\rm d} T_c/{\rm d} p =-0.0225(3)$ K/kbar was observed. The electron-phonon coupling constant $λ_{el-ph}$ decreases with increasing pressure with ${\rm d}\lnλ_{el-ph}/{\rm d}p\simeq-0.20$%/kbar. The magnetic field penetration depth $λ$ was studied in the Meissner state and, therefore, probes only the surface of the sample. The absolute values of $λ$ and the superconducting energy gap at ambient pressure and zero temperature were found to be $λ(0)=$140(30) nm and $Δ_0=$1.251(9) meV, respectively. $Δ_0$ scales linearly with $T_c$ as $2Δ_0/k_BT_c=4.79(1)$. The studies of the pressure effect on $λ$ reveal that $λ^{-2}$ increases with pressure with ${\rm d}\lnλ^{-2}(0)/{\rm d}p=0.60(23)$ %/kbar. This effect can not be explained within the framework of conventional adiabatic electron-phonon pairing, suggesting that close to the surface, an unconventional {\it non-adiabatic} character of the electron-phonon coupling takes place.