The long-lasting joint research activities of the Experimental Physics 1 / Surface Physics Group at TU Ilmenau with theoretical physicist Prof. Mads Brandbyge from TU Denmark have succeeded in exploring and understanding a molecular magnifier for quantum vibrations of a two-dimensional material. The findings have recently been published in Physical Review Letters, the prime journal for physical research.
In everyday life, we use a magnifying glass to visualize objects that are difficult or even impossible to see with the naked eye. A pertinent question arises as to the magnified visualization of quantum excitations, i.e., of the transfer of smallest energy portions. Physicists from Ilmenau and Lyngby have found the answer and showed how a single molecule can magnify the spectroscopic signature of graphene vibrations, that is, the oscillatory motion of carbon atoms in the honeycomb lattice of the two-dimensional material, and how the signal of the vibrational excitation can be detected by means of a scanning tunneling microscope (STM).
In contrast to optical microscopes, which deploy lenses for imaging, an STM uses the quantum mechanical tunneling current, which is injected from an atomically sharp tip into the sample and, thereby, passes through the vacuum barrier between tip and sample, which for a classical current would be impossible. Owing to the local injection of the current and its exponential dependence on the tip-surface distance the STM is capable of atomic resolution, i.e., atoms of surfaces or adsorbed molecules can be imaged. Moreover, the STM represents a tool that enables the manipulation of matter atom by atom, which provides a direct connection between the macroscopic world and the nanocosmos.
The information on quantum excitations, which play the central role in the present studies, can be extracted from current-voltage characteristics acquired atop deliberate surface sites: the applied bias voltage provides energy for the tunneling electrons that can be transferred to, e.g., atom vibrations of the graphene lattice – a process that is referred to as inelastic electron tunneling. Upon energy transfer the current changes at a specific bias voltage. By means of phase-sensitive detectors the minute energy transfer can be measured.
The importance of the present work is its contrast to an emerging physical picture that describes the excitation of graphene vibrations. According to this picture, the excitation by tunneling electrons is hampered if graphene resides on a metal surface. The generally accepted rationale is the good electric coupling of graphene to the metal that favors the elastic electron transport at the expense of the inelastic current. The latter, however, is required for exciting quantum vibrations. The results obtained in the German-Danish studies are at odds with this picture.
The magnifier for graphene vibrations as discovered by the scientists of TU Ilmenau and TU Denmark is a single molecule on graphene with a chemically engineered orbital that covers the spectral range of graphene vibrations. Passing the tunneling current across this molecule into graphene efficiently excites graphene vibrations, which without the molecular magnifier do not surmount the detection limit. The supporting simulations unveil that besides the molecular orbital an ample coupling between symmetry-equivalent vibration modes of the molecule and the graphene lattice is required. The graphene-metal coupling, however, is irrelevant.
“We were simply baffled by the presence of graphene vibrational signatures in the spectra of the adsorbed molecule”, says Jörg Kröger. “With the help of our theory partner in Denmark, the quantum physical mechanism underlying the experimental observation could be clarified. This mechanism is novel, extends our view on inelastic electron transport across quantum objects in general and on graphene phonon excitation via tunneling electrons in particular. These lattice vibrations play an important role in the superconducting phase of graphene, which we study in a project funded by the German Science Foundation. Moreover, the vibrations impact the electron transport in graphene, which is relevant for graphene applications in electronic circuits.”
Parts of the discussed experiments were performed with the scanning probe apparatus funded by the Federal Ministry of Education and Research within the ForLab project at the Centre of Micro- und Nanotechnologies. The continuous supply with liquefied helium from the helium recovery used in the ForLab project was of key importance to the success of the experiments.
To the article X. Wu et al., Phys. Rev. Lett. 130, 116201 (2023)
Prof. Jörg Kröger
Head of Experimental Physics 1 / Surface Physics