Joan van der Waals colloquium
- 15 March 2019
- Drinks afterwards (17.00h)
Niels Bohrweg 2
2333 CA Leiden
- De Sitterzaal
Accessing non-equilibrium states at the atomic scale
Laerte L. Patera, Dominik Peller, Thomas Buchner, Fabian Queck, Philipp Scheuerer,
Lukas Kastner, Carmen Rölcke, Tyler L. Cocker, Rupert Huber and Jascha Repp
Department of Physics, University of Regensburg, 93040 Regensburg, Germany
Scanning probe microscopy (SPM) has revolutionized our understanding of the atomistic world. Conventional SPM, however, is an inherently slow technique – too slow to capture transition states in excitation processes in most cases. Two complementary approaches that allow accessing non-equilibrium phenomena with SPM will be discussed, opening a new arena for atomistic studies.
We introduce a novel variant of SPM by combining principles of scanning tunneling (STM) and atomic force microscopy (AFM). Instead of the usual direct current in conventional STM, we drive a tiny alternating current between the microscope’s tip and a single molecule under study. We exploit the single-electron sensitivity of AFM  in detecting the current which consists of only a single electron per AFM-cantilever oscillation cycle, tunneling back and forth between tip and molecule. This enables operation in absence of any conductance of the underlying substrate, while retaining the capability of imaging electronic states with Angstrom resolution. Thereby, we can access out-of-equilibrium charge states that are out of reach for conventional STM. Our results unveil the effects of electron-transfer and polaron formation on the single-orbital scale .
Accessing ultra-fast non-equilibrium phenomena is enabled by terahertz (THz) scanning tuneling microscopy  (THz-STM) through combining STM with lightwave electronics. In THz-STM, the electric field of a phase-stable single-cycle THz waveform acts as a transient bias voltage across an STM junction. In analogy to the all-electronic pump-probe scheme introduced recently in STM  these voltage transients may result in a net current that can be detected by time-integrating electronics.
By means of a low-noise low-temperature THz-STM we enter an unprecedented tunneling regime, in which the peak of a terahertz electric-field waveform opens an otherwise forbidden tunneling channel through a single molecular orbital. In this way, the terahertz peak removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window of ~100 fs – faster than an oscillation cycle of the terahertz wave. This quantum process allows us to capture a microscopic real-space snapshot of the molecular orbital on a sub-cycle time scale. By correlating two successive state-selective tunneling events, we directly track coherent THz vibrations of a single molecule in the time domain .
Finally, we induce an in-plane rotation of individual phthalocyanine molecules via the THz-driven current. Here, we reduce the repetition rate of the laser source to single-shot experiments, which allows us to follow each individual electron-induced motion by combining lightwave-driven STM with action spectroscopy.
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 T. L. Cocker et al., Nature 539, 263 (2016).