Point-contact spectroscopy on aluminium atomic-size contacts: longitudinal and transverse vibronic excitations

We report a point-contact spectroscopy investigation of aluminium atomic-size contacts produced with the help of the mechanically controllable break-junction technique at low temperature. The derivatives of the differential conductance traces (d2I/dV2 versus V) display point-symmetric structures at certain voltage values, which move upon stretching the contact. These excitations may give rise to either enhancements or reductions of the conductance, indicating that they couple to highly or weakly transmissive electronic channels, respectively. By analyzing the sign of the conductance change and the strain dependence of the amplitudes and the energies we suggest that transverse as well as longitudinal vibronic modes are detected in the transport through single-atom contacts. Our findings are in agreement with symmetry arguments for the coupling of electronic and vibronic excitations.