Schematic of phononic, conduction electron, and electrostatic tribo-electric friction mechanisms for surface molecules excited by an AFM tip sliding at speed V. (a) Molecules at the surface of a substrate that is sufficiently far from the tip are unaffected by it. (b)–(d) Molecular vibrations excited by the passing tip give rise to phononic contributions to friction. (c)For conducting materials, conduction electronic effects such as electron-hole pair creation or electron wind drag effects may be detected if the phononic background is sufficiently reduced. (c)–(e) Contact electrification and charge transfer results in large electrostatic forces for pinned charge scenarios. (e) Molecules far from the tip after it has passed gradually return to an uncharged and unexcited vibrational states. When the substrate enters the superconducting state, the ability to pin or transfer charge in the sliding contact may be significantly altered even if the contact temperature is locally elevated.
(Left) Distance dependence of the AFM normal force, which was measured during the magnetic tip approach. (Right) Approach-retract hysteresis of normal AFM force (approach: green curve; retract: red curve). The levitation force is not observed after the physical contact was established between the probe and the YBCO surface.
Maximum tip-sample levitation force (blue symbols) and friction coefficient μ (green symbols) versus temperature. The μ(T) curve abruptly changes slope near Tc = 92.5 K.
Lateral friction force versus normal force, measured at different temperatures, for a magnetic AFM probe in sliding contact with 0.5 μm thick YBCO film on SrTiO3. Inset: 100 × 100 nm2 AFM image of the YBCO surface exhibiting atomic terraces. Measurements were performed at normal forces sufficiently low so as to not cause damage or wear to the surface.
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