Experimentally exploring quantum physics requires most sensitive instrumentation.  A scanning tunneling microscope (STM) operated at cryogenic temperature and in ultrahigh vacuum appears to be particularly well suited.  The Experimental Physics 1 / Surface Physics group at TU Ilmenau in close cooperation with the Theoretical Physics Department of TU Denmark has recently succeeded in experimentally realizing and thoroughly understanding a molecular magnifier for quantum vibrations of a two-dimensional material.  The results of this joint and fruitful study have recently been accepted for publication in Physical Review Letters, the prime journal for physical research.

Figure 1 presents the key experimental findings.  Passing the tunneling current injected from the STM tip across graphene (green) to the metal substrate (blue) (I in Fig. 1(a)) leads to a featureless spectrum of the differential conductance (dI/dV, I: current, V: sample voltage, Fig. 1(b)).  The situation changes markedly when the tunneling current is flowing across a molecule residing atop graphene (II in Fig. 1(a)).  The dI/dV spectrum now exhibits steplike changes at voltages symmetrically positioned around zero bias (Fig. 1(c)), which are typical signatures of vibration excitations.  To identify the origin of the vibrations and to understand the underlying mechanism, additional experiments were carried out.

 

Figure 2 demonstrates that for the effect to occur an orbital resonance must chemically be engineered. A phthalocyanine (2H-Pc) molecule adsorbed on graphene-covered Ir(111) gives rise to a cloverlike pattern with uniform contrast in STM images (Fig. 2(a)).  After abstraction of pyrrolic H from the molecular center by injecting high-energy tunneling electrons from the STM tip, i.e., after performing the on-surface single-molecule reaction 2H-Pc → Pc + 2H (Fig. 2(b)), the product molecule Pc appears with bright and rich submolecular contrast (Fig. 2(a)).  While intact 2H-Pc exhibits a rather flat evolution of dI/dV (Fig. 2(c)), pyrrolic-H-abstracted Pc shows a strong resonance around zero bias (Fig. 2(d)).  Intriguingly, 2H-Pc does not give rise to inelastic signals (Fig. 1(b)), while Pc does (Fig. 1(c)).  Therefore, the inelastic excitation of vibrational modes is likely related to the molecular orbital resonance covering the spectral range of vibrations.

To further shed light onto the role of the molecular resonance and the origin of vibrational excitations density functional and non-equilibrium Green function calculations were performed in Denmark.  In agreement with the experiments, only Pc exhibits a transmission resonance close to the Fermi energy (E = 0 in Fig. 3(a)), which is associated with the degenerate highest occupied molecular orbital of Pc.  Moreover, the inelastic signals are only enhanced for Pc (Fig. 3(b)), in very good agreement with the experimental observation.  In addition, the bias polarity dependence of the inelastic signal strength is well reproduced and and shown to be due to the asymmetric line shape of the calculated transmission resonance.