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Register a new userPolaronic Contributions to Friction in a Manganite Thin Film
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Despite the huge importance of friction in regulating movement in all natural and technological processes, the mechanisms underlying dissipation at a sliding contact are still a matter of debate. Attempts to explain the dependence of measured frictional losses at nanoscale contacts on the electronic degrees of freedom of the surrounding materials have so far been controversial. Here, it is proposed that friction can be explained by considering damping of stick-slip pulses in a sliding contact. Based on friction force microscopy studies of La$_{(1-x)}$Sr$_x$MnO$_3$ films at the ferromagnetic-metallic to paramagnetic-polaronic conductor phase transition, it is confirmed that the sliding contact generates thermally-activated slip pulses in the nanoscale contact, and argued that these are damped by direct coupling into phonon bath. Electron-phonon coupling leads to the formation of Jahn-Teller polarons and a clear increase in friction in the high temperature phase. There is no evidence for direct electronic drag on the atomic force microscope tip nor any indication of contributions from electrostatic forces. This intuitive scenario, that friction is governed by the damping of surface vibrational excitations, provides a basis for reconciling controversies in literature studies as well as suggesting possible tactics for controlling friction.
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We report active control of the friction force at the contact between a nanoscale asperity and a La$_{0.55}$Ca$_{0.45}$MnO$_3$ (LCMO) thin film using electric fields. We use friction force microscopy under ultrahigh vacuum conditions to measure the friction force as we change the film resistive state by electric field-induced resistive switching. Friction forces are high in the insulating state and clearly change to lower values when the probed local region is switched to the conducting state. Upon switching back to an insulating state, the friction forces increase again. Thus, we demonstrate active control of friction without having to change the contact temperature or pressure. By comparing with measurements of friction at the metal-to-insulator transition and with the effect of applied voltage on adhesion, we rule out electronic excitations, electrostatic forces and changes in contact area as the reasons for the effect of resistive switching on friction. Instead, we argue that friction is limited by phonon relaxation times which are strongly coupled to the electronic degrees of freedom through distortions of the MnO6 octahedra. The concept of controlling friction forces by electric fields should be applicable to any materials where the field produces strong changes in phonon lifetimes.
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The surface charge associated with the spontaneous polarization in ferroelectrics is well known to cause a depolarizing field that can be particularly detrimental in the thin-film geometry desirable for microelectronic devices. Incomplete screening of the surface charge, for example by metallic electrodes or surface adsorbates, can lead to the formation of domains, suppression or reorientation of the polarization, or even stabilization of a higher energy non-polar phase . A huge amount of research and development effort has been invested in understanding the depolarizing behavior and minimizing its unfavorable effects. Here we demonstrate the opposite behavior: A strong polarizing field that drives thin films of materials that are centrosymmetric and paraelectric in their bulk form into a non-centrosymmetric, polar state. We illustrate the behavior using density functional computations for perovskite-structure potassium tantalate, KTaO$_3$, which is of considerable interest for its high dielectric constant, proximity to a quantum critical point and superconductivity. We then provide a simple recipe to identify whether a particular material and film orientation will exhibit the effect, and develop an electrostatic model to estimate the critical thickness of the induced polarization in terms of well-known material parameters. Our results provide practical guidelines for exploiting the electrostatic properties of thin-film ionic insulators to engineer novel functionalities for nanoscale devices.
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