Excitation of a bosonic mode by electron tunneling into a cuprate superconductor NdBa₂Cu₃O_7-δ

We performed scanning tunneling spectroscopic experiments on hole-doped NdBa₂Cu₃O_7-δ. The dI/dV curves obtained at 4.2 K are asymmetric with clear peak, dip, and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential band-structure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the CuO₂ plane.