Systematic investigation of wear-induced cold welding in ultrahigh vacuum piezoelectric motors with non-metallic coatings.

Piezoelectric motors are widely used in various applications where both precision positioning and miniaturization are required. Inertial or quasi-static motors are commonly employed because of their high accuracy, which demands consistent sliding friction between moving sliders and their static counterparts for reliable operation. In general, slider wear is unavoidable after long-term use. This wear can often lead to more serious cold welding in vacuum, which is also referred to as friction welding induced by direct contact between similar metal surfaces. Non-metallic coatings can prevent such unwanted cold welding in ultrahigh vacuum (UHV) applications. However, the practical reliability of available coatings under UHV conditions still remains to be elucidated. Here, we systematically investigate the practical reliability of commonly used, UHV-compatible lubricant coatings for piezoelectric motors in vacuum. We demonstrate that polytetrafluoroethylene (PTFE) shows the most reliable long-term operation in vacuum, while other coatings eventually lead to wear-induced cold welding and motor failure. Our findings provide a simple and effective method to improve the long-term performance of UHV piezoelectric motors by coating the slider surface with PTFE.

Imaging single electron spin in a molecule trapped within a nanocavity of tunable dimension.

Control of magnetism at the nanoscale is shown by a reversible transfer of an electron to and from a single molecule within the tunable gap of a scanning tunneling microscope. The addition of an electron to magnesium porphine changes the molecule from the diamagnetic state to the paramagnetic state. The existence of the single unpaired electron in the molecule is confirmed by spectroscopy and spatial imaging of the many body Kondo state and inelastic spin excitation between the Zeeman levels at 600 mK and up to 9 Tesla magnetic field. Here, we show that the spin is delocalized in an extended molecular orbital, in contrast to the spatially confined d and f states in atoms and magnetic centers in molecules. Furthermore, by tuning the dimension of the tunneling gap and visualizing the spectroscopic images, the inelastic spin-flip scatterings are shown to underlie the formation of the Kondo state.

Spin splitting unconstrained by electron pairing: the spin-vibronic states.

Spin splitting of individual vibronic states was observed in a single molecule where all the electrons are paired, as well as a molecule with one extra electron injected. This observation was made possible by the use of a scanning tunneling microscope capable of reaching ∼800 mK in a magnetic field up to 9 T and the sharpness of the vibronic states, ∼1 meV. These conditions also led to the resolution of spectral diffusion caused by minute fluctuations at the probing location of the molecule.