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Register a new userStrongly Temperature Dependent Sliding Friction for a Superconducting Interface
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A sudden drop in mechanical friction, between an adsorbed nitrogen monolayer and a lead substrate, occurs when the lead passes through the superconducting transition temperature. We attribute this effect to a sudden drop at the superconducting transition temperature of the substrate Ohmic heating. The Ohmic heating is due to the electronic screening current that results from the sliding adsorbed film.
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The effects of a step defect and a random array of point defects (such as vacancies or substitutional impurities) on the force of friction acting on a xenon monolayer film as it slides on a silver (111) substrate are studied by molecular dynamic simulations and compared with the results of lowest order perturbation theory in the substrate corrugation potential. For the case of a step, the magnitude and velocity dependence of the friction force are strongly dependent on the direction of sliding respect to the step and the corrugation strength. When the applied force F is perpendicular to the step, the film is pinned forF less than a critical force Fc. Motion of the film along the step, however, is not pinned. Fluctuations in the sliding velocity in time provide evidence of both stick-slip motion and thermally activated creep. Simulations done with a substrate containing a 5 percent concentration of random point defects for various directions of the applied force show that the film is pinned for the force below a critical value. The critical force, however, is still much lower than the effective inertial force exerted on the film by the oscillations of the substrate in experiments done with a quartz crystal microbalance (QCM). Lowest order perturbation theory in the substrate potential is shown to give results consistent with the simulations, and it is used to give a physical picture of what could be expected for real surfaces which contain many defects.
Spin-orbit torques (SOT) in thin film heterostructures originate from strong spin-orbit interactions (SOI) that, in the bulk, generate a spin current as the result of extrinsic spin-dependent, skew or/and side-jump, scattering, or in the intrinsic case due to Berry curvature in the conduction band. While most SOT studies have focused on materials with heavy metal components, the oxide perovskite SrRuO3 has been predicted to have a pronounced Berry curvature. Through quantification of its spin current by the SOT exerted on an adjacent Co ferromagnetic layer, we determine that SrRuO3 has a strongly temperature (T) dependent spin Hall conductivity which becomes particularly high at low T, e.g. sigma_{SH} geqslant (hbar/2e)3x10^{5} Omega^{-1}m^{-1} at 60 K. Below the SrRuO3 ferromagnetic transition, non-standard SOT components develop associated with the magnetic characteristics of the oxide, but these do not dominate as with spin currents from a conventional ferromagnet. Our results establish a new approach for the study of SOI in epitaxial conducting oxide heterostructures and confirm SrRuO3 as a promising candidate material for achieving new and enhanced spintronics functionalities.
The friction of a nanosized sphere in commensurate contact with a flat substrate is investigated by performing molecular dynamics simulations. Particular focus is on the distribution of shear stress within the contact region. It is noticed that within the slip zone, the local friction coefficient defined by the ratio of shear stress to normal pressure declines monotonically as the distance to contact center increases. With the lateral force increasing, the slip zone expands inwards from the contact edge. At the same time, the local friction coefficient at the contact edge decreases continuously, while at the dividing between the slip and stick zones keeps nearly invariant. These characteristics are distinctly different from the prediction of the conventional Cattaneo-Mindlin model assuming a constant local friction coefficient within the slip zone. An analytical model is advanced in view of such new features and generalized based on numerous atomic simulations. This model not only accurately characterizes the interfacial shear stress, but also explains the size-dependence of static friction of single nanosized asperity.
Markov State Modeling has recently emerged as a key technique for analyzing rare events in thermal equilibrium molecular simulations and finding metastable states. Here we export this technique to the study of friction, where strongly non-equilibrium events are induced by an external force. The approach is benchmarked on the well-studied Frenkel-Kontorova model, where we demonstrate the unprejudiced identification of the minimal basis microscopic states necessary for describing sliding, stick-slip and dissipation. The steps necessary for the application to realistic frictional systems are highlighted.
We propose a method of achieving large temperature sensitivity in the Casimir force that involves measuring the stable separation between dielectric objects immersed in fluid. We study the Casimir force between slabs and spheres using realistic material models, and find large > 2nm/K variations in their stable separations (hundreds of nanometers) near room temperature. In addition, we analyze the effects of Brownian motion on suspended objects, and show that the average separation is also sensitive to changes in temperature . Finally, this approach also leads to rich qualitative phenomena, such as irreversible transitions, from suspension to stiction, as the temperature is varied.
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