Friction on the atomic scale across phase transitions

• Forschungsstelle: COST
• Projektnummer: C14.0040
• Projekttitel Englisch: Friction on the atomic scale across phase transitions

Schlüsselwörter (Englisch)

Atomic friction; nanotribology; atomic stick-slip

Forschungsprogramme (Englisch)

COST-Action MP1303 - Understanding and Controlling Nano and Mesoscale Friction

Kurzbeschreibung (Englisch)

The aim is to explore the influence of phononic vs. electronic friction in small nanometer-sized contacts. Friction and energy dissipation are explored by state-of-the-art force microscopy under ultrahigh vacuum conditions at variable temperatures. The influence of the order parameter in phase transitions, such as superconductive and charge density waves, is investigated in contact and non-contact mode. Dissipation via phononic degrees of freedom is explored by the coverage of metallic and graphene samples with adsorbates. Damping on semiconductors with different doping concentrations and quantum dot systems is studied to probe the influence of electronic effects. Molecular systems in the form of single molecules or small aggregates are prepared to explore anisotropy effects and to determine the range of relevant interactions.

Partner und Internationale Organisationen (Englisch)

AT; CZ; DK; EE; FI; FR; DE; EL; IE; IL; IT; LV; NL; NO; PL; PT; RS; ES; SE; TR; UK; RU; UA

Abstract (Englisch)

Friction and related energy dissipation are studied by well-defined experimental setups, where parameters, such as surface structure, temperature and velocity can be controlled with high accuracy. The first experiment is related to a phase transition of SrTiO3, an oxide of great scientific and technological interest. A sharp probing tip is approached to this surface in the pendulum geometry. The dissipation of this small contact is observed as a function of temperature. In a narrow regime around the phase transition temperature, an increase of energy dissipation is observed. The dissipative forces are related to a phonon peak at low frequencies, the so called 'central peak'. A model was developed in collaboration with the partners from ICTP/SISSA Trieste, which yields a temperature dependence of dissipation with a critical exponent. The theoretical description is found to be in good agreement with the model. In a second study, single porphyrin molecule were attached to a probing tip and scanned across a copper surface, where atomic friction was detected with unprecedented resolution. A characteristic saw-tooth pattern with local instabilities is observed, which seems to be related to conformal changes of the molecule. In collaboration with the partners from Tel Aviv University a theoretical model was developed, which is based on a generalized Tomlinson model including parameters obtained from density functional theory (DFT) that describe the internal degrees of freedom of the side-group with respect to the molecule core. We demonstrate that the frictional changes is induced by variations of the bond lengths and dihedral angle of the di-cyanophenyl side group. Most probably one carbonitrile end group (CN) is in contact with the metallic surface, which leads in combination with the molecular flexibility to the occurrence of atomic friction. Combined experiments and simulations demonstrate the crucial role of the molecule mechanics during lateral and vertical manipulations.

Datenbankreferenzen (Englisch)

Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. [phone] Swiss Project-Number: C14.0040