Periodic Reporting for period 4 - ULTRADISS (Ultra-sensitive mechanical dissipation in classical, quantum and non-equilibrium nanocontacts)
Reporting period: 2023-11-01 to 2024-10-31
Nanomechanical dissipation, experienced by the tip of Force Microscopy (AFM) instruments, provides an innovative probe of the physics of classical and quantum materials, solids and surfaces. Experimental and conceptual advances by exploiting and adapting advanced AFM techniques were made, especially the ultra-sensitive pendulum-AFM (p-AFM), to detect collective phenomena, including structural, electronic and magnetic phase transitions. So far, dissipation spectroscopy was applied mostly at the equilibrium physics of 3D classical solids. Our goal is to go a step beyond by exploring nanomechanical dissipation to sense much weaker effects caused by non-equilibrium perturbations, during nanomanipulations or involving quantum effects in ground-breaking case studies. The examples of those are the measurements of the energy dissipation in strongly correlated electron system of twisted bilayer graphene or study of imperceptible wind force exerted on a noncontact tip by a thermal or electrical current in the surface below, or the minute mechanical cost of creating and dismantling a single spin Kondo state, or a topological surface state. The goal of these investigations is to get a more fundamental understanding of energy dissipation and friction processes, as well as to link them to generally understood solid state surface phenomena. Friction and energy loss are of great concern for the society, because it is the main source of energy losses of all kind of machines, including significant concern for future electronics. The main objective is to explore energy loss mechanisms on a local scale with emphasis on non-classical effects. Identifying new methods of spectroscopy is of wider scientific and technological interest. An important methodological goal of our project is to demonstrate and consolidate further both experimentally and theoretically the potential of nanotribological and nanomechanical dissipation in performing, often without physically touching a material's surface, a delicate form of spectroscopy. Covering disparate phenomena – electronic and magnetic, structural and phononic, on or off-equilibrium, static or dynamic – that occur inside or at the surface of a solid material. With that perspective, theoretical calculations, simulations and predictions form an important part of the project, regarding ongoing and planned experiments in the group, as well as wider range explorations of tribological and related physical problems in a broader context.
The topological insulator Bi2Te3 is investigated by the pendulum AFM. It is found that due to topological character of the surface, the classical Joule dissipation is reduced on this surface and that novel energy dissipation channels are observed, which have quantum-mechanical origins. Dissipation peaks are observed at discrete energies, which is related to image potential states. A number of molecular networks and molecular wires were synthesized by on-surface chemistry or by electro-spray deposition in vacuum. A surprising effect is the observation of giant thermal expansion of polyphenylene molecules equipped with peripheral dodecyl chains. The thermal motion of the dodecyl chains seems to be responsible for this effect. Kagome lattices and porous graphene nanoribbons were grown under vacuum conditions, where both systems reveal semiconducting character as compared to pristine graphene. Interestingly, DFT calculations of the Kagome lattice indicate the presence of electronic flat bands suggesting exotic electron correlations. The study of magic angle twisted bilayer graphene samples with back electrode were investigated by the pendulum AFM. The preparation of tetraazapyrene-derivatives on superconductive Pb(111) was a major achievement. Yu-Shiba-Rusinov (YSR) states are observed in the superconducting gap. The application of positive voltages leads to the discharging of the molecules. The self-assembly of these molecules gives 2d-spin arrays, which show bands of YSR-states and low energy modes at the border of the islands. Simulation and theory: A variety of surface and potentially nanotribological systems were analyzed and simulated, when possible in comparison with existing or fresh data, with mechanical nanodissipation as the goal. Among them are the sliding motion of flakes, where the role of moiré patterns is elucidated, an example of amplitude-resolved AFM sliding mechanical spectroscopy, a study of carbon nanoribbon sliding, which is parallel and coordinated with those in the experimental group. We also conducted preparatory theory work on several problems and systems, such as nanowire mechanics, ice , electronic softening, Chevrel superconductors, the Si(7x7) surface as well as quantum dissipation systems.
The observation of dissipation on topological insulators is a first example of a dissipative process closely related to quantum-mechanical image potential states. This novel energy dissipation mechanism goes beyond state of the art and is observable due to the reduction of classical Joule dissipation. The observation of a giant thermal expansion in a molecular networks with alkyl chains has been reported by temperature dependent AFM imaging. The thermal motions of the alkyl chains seems responsible for the expansion due to entropic forces.
We investigated molecular networks assembled on surfaces that revealed a number of quantum phenomena such as Coulomb blockade, Kondo effect on metals and Yu-Shiba-Rusinov (YSR)-states on superconducting surfaces We are currently exploring possible energy dissipation channels probed by AFM spectroscopy during charge/spin manipulations in these systems.
An unexpected results was obtained for twisted bilayer graphene, where the p-AFM responded to single electrons entering the mini-Brillouin zone, which leads to presence of strongly correlated insulating phases. Moreover, we observed strong dependence of dissipation on magnetic fields.
Theoretically, we are moving along the three main backbone lines of the project, namely novel nanomechanical effects in graphene, nanoribbon peeling, and quantum effects such as Kondo dissipation in AFM spectroscopy as well as quantum-dot-like mechanical spectroscopy of suspended graphene in magnetic field. In the first line, we are conducting a thorough re-examination of mutual sliding in graphene bilayers, where fundamentally different regimes are discovered for decreasing twist angle and where different static and kinetic frictional behaviors are analyzed as a function of area, temperature, velocity and load. In the second line, we examine as discussed with the experimental partners the peeling of tethered nanoribbons, where a surprising power law regime emerges, both analytically and in simulation. In the third line, we are calculating the electronic spectrum of graphene flakes in a magnetic field, for direct comparison with the experimental partner's p-AFM spectra. We are also devising a new quantum simulation scheme that will permit to evaluate the dissipation caused by quenching of a Kondo impurity as a function of the switching time, for future application to a real system such as a surface adsorbed radical swept over by a magnetic tip.
We investigated molecular networks assembled on surfaces that revealed a number of quantum phenomena such as Coulomb blockade, Kondo effect on metals and Yu-Shiba-Rusinov (YSR)-states on superconducting surfaces We are currently exploring possible energy dissipation channels probed by AFM spectroscopy during charge/spin manipulations in these systems.
An unexpected results was obtained for twisted bilayer graphene, where the p-AFM responded to single electrons entering the mini-Brillouin zone, which leads to presence of strongly correlated insulating phases. Moreover, we observed strong dependence of dissipation on magnetic fields.
Theoretically, we are moving along the three main backbone lines of the project, namely novel nanomechanical effects in graphene, nanoribbon peeling, and quantum effects such as Kondo dissipation in AFM spectroscopy as well as quantum-dot-like mechanical spectroscopy of suspended graphene in magnetic field. In the first line, we are conducting a thorough re-examination of mutual sliding in graphene bilayers, where fundamentally different regimes are discovered for decreasing twist angle and where different static and kinetic frictional behaviors are analyzed as a function of area, temperature, velocity and load. In the second line, we examine as discussed with the experimental partners the peeling of tethered nanoribbons, where a surprising power law regime emerges, both analytically and in simulation. In the third line, we are calculating the electronic spectrum of graphene flakes in a magnetic field, for direct comparison with the experimental partner's p-AFM spectra. We are also devising a new quantum simulation scheme that will permit to evaluate the dissipation caused by quenching of a Kondo impurity as a function of the switching time, for future application to a real system such as a surface adsorbed radical swept over by a magnetic tip.