Energy Dissipation Channels Research Articles

Tuning Electronic Friction in Structural Superlubric Schottky Junctions.

Friction at sliding interfaces, even in the atomistically smooth limit, can proceed through many energy dissipation channels, such as phononic and electronic excitation. These processes are often entangled and difficult to distinguish, eliminate, and control, especially in the presence of wear. Structural superlubricity (SSL) is a wear-free state with ultralow friction that closes most of the dissipation channels, except for electronic friction, which raises a critical concern of how to effectively eliminate and control such a channel. In this work, we construct a Schottky junction between a microscale graphite flake and a doped silicon substrate in the SSL state to address the issue and achieve wide-range (by 6×), continuous, and reversible electronic friction tuning by changing the bias voltage. No wear or oxidation at the sliding interfaces was observed, and the ultralow friction coefficient indicated that electronic friction dominated the friction tuning. The mechanism of electronic friction is elucidated by perturbative finite element analysis, which shows that migration of the space-charge region leads to drift and diffusion of charge carriers at Schottky junctions, resulting in energy dissipation.

Acceptor-Induced Bulk Dielectric Loss in Superconducting Circuits on Silicon

The performance of superconducting quantum circuits is primarily limited by dielectric loss due to interactions with two-level systems (TLSs). State-of-the-art circuits with engineered material interfaces are approaching a limit where dielectric loss from bulk substrates plays an important role. However, a microscopic understanding of dielectric loss in crystalline substrates is still lacking. In this work, we show that boron acceptors in silicon constitute a TLS bath that leads to an energy dissipation channel for superconducting circuits. We discuss how the electronic structure of boron acceptors leads to an effective TLS response in silicon. We sweep the boron concentration in silicon and demonstrate the bulk dielectric loss limit from boron acceptors. We show that boron-induced dielectric loss can be reduced in a magnetic field due to the spin-orbit structure of boron. This work provides the first detailed microscopic description of a TLS bath for superconducting circuits and demonstrates the need for ultrahigh-purity substrates for next-generation superconducting quantum processors. Published by the American Physical Society 2024

Control of chiral damping in magnetic trilayers using He+ ion irradiation

In the forefront of spintronic advancements, structures with strong perpendicular magnetic anisotropy (PMA) such as Pt/Co/Pt are essential for the miniaturization and performance enhancement of high-density magnetic storage technologies. The robust PMA characteristic of these systems facilitates the development of scalable spintronic devices, crucial for next-generation magnetic memory applications. This study investigates the interplay between PMA and the Dzyaloshinskii-Moriya interaction (DMI)—an antisymmetric exchange interaction prevalent in non-centrosymmetric magnetic systems—and its dissipative counterpart, chiral damping. While chiral damping arises from the same broken inversion symmetry as DMI, it typically introduces an additional energy dissipation channel, reducing device efficiency. Our research examines the effects of controlled helium ion (He+) irradiation on a Pt/Co/Pt system. We find that ion beam irradiation enhances interfacial intermixing, which correlates with a decrease in PMA. However, domain wall velocity measurements indicate a concurrent reduction in both DMI and chiral damping, along with enhanced velocities as irradiation fluence increases. These observations suggest that ion beam irradiation can be judiciously applied to achieve a balance between lower DMI, chiral damping, and reasonable PMA, thereby optimizing the system for improved device performance.

The fluctuation-dissipation relation holds for a macroscopic tracer in an active bath.

The fluctuation-dissipation relation (FDR) links thermal fluctuations and dissipation at thermal equilibrium through temperature. Extending it beyond equilibrium conditions in pursuit of broadening thermodynamics is often feasible, albeit with system-dependent specific conditions. We demonstrate experimentally that a generalized FDR holds for a harmonically trapped tracer colliding with self-propelled walkers. The generalized FDR remains valid across a large spectrum of active fluctuation frequencies, extending from underdamped to critically damped dynamics, which we attribute to a single primary channel for energy input and dissipation in our system.

Monitoring near-field acoustic emission from confined water under shear interaction with blunt tips

Mesoscopic water adhered to a surface at ambient conditions is typically reported to behave as sticky glue. Still, Shear-force Acoustic Near-field Microscopy (SANM), complemented with the Whispering Gallery Acoustic Sensing (WGAS) technique, has demonstrated the ability of a water meniscus, formed at the nanometer-sized gap between a sharp probe and a flat surface, to remain flexible enough for emitting near-field acoustic waves while being subjected to shear interactions exerted by a laterally oscillating probe. To gain insights on the meniscus formation process, as well as to better understand its viscoelastic response, purposely blunt probes (composed of multiple sharp asperities) are used. The experimental results suggest the stochastic formation and break of water bridges at multiple asperities on the probe, each providing an energy dissipation channel from the lateral motion of the probe to the fluid. Despite heavily damping the lateral motion of the probe, the ability of the large fluid meniscus to emit acoustic wave is recovered by properly increasing the amplitude of the lateral oscillations of the probe. Overall, monitoring the behavior of large meniscus adhered to a blunt probe allows to infer the behavior of smaller volume meniscus adhered to sharp probes.

Statistical Characteristics of Electron Vortexes in the Terrestrial Magnetosheath

Utilizing the unprecedented high-resolution Magnetospheric Multiscale mission data from 2015 September to 2017 December, we perform a statistical study of electron vortexes in the turbulent terrestrial magnetosheath. On the whole, 506 electron vortex events are successfully selected. Electron vortexes can occur at four known types of magnetic structures, including 78, 42, 26, and 39 electron vortexes observed during the crossings of the current sheets, magnetic holes, magnetic peaks, and flux ropes, respectively. Except for the four types of structures, the rest of the electron vortexes are in the “Others” category, defined as unknown structures. The electron vortexes mainly occur in the subsolar region, with only a few in the flank region. The total occurrence rate of all electron vortexes is 4.86 hr–1, with, on average, 3.65 events hr−1 in the X-Y plane and 3.26 events hr−1 in the X-Z plane. The durations of most of the electron vortexes concentrate within 0.5–1.5 s and are 1.09 s on average. The electron vortexes are ion-scale structures owing to the average scale of 2.05 ion gyroradius. In addition, the means, medians, and maxima of the energy dissipation J · E′ in the electron vortexes are almost positive, implying that the electron vortex may be a potential coherent structure or channel for turbulent energy dissipation. All these results reveal the statistical characteristics of electron vortexes in the magnetosheath and improve our understanding of energy dissipation in astrophysical and space plasmas.

Intermolecular hydrogen-bonding as a robust tool toward significantly improving the photothermal conversion efficiency of a NIR-II squaraine dye

The photothermal therapy (PTT) has come across as a promising noninvasive therapeutic strategy for tumor treatment. However, low photothermal conversion efficiency (PCE) and hydrophobicity may impede the therapeutic efficacy of organic photothermal agents and an efficient PTT-agent must overcome these two major challenges. In this work, we developed a new strategy to promote higher PCE wherein the intermolecular hydrogen-bonding interaction between the single dye molecule and water facilitated the transformation of the absorbed energy into the heat. A hydrophilic squaraine dye (SCy1) with the second near-infrared region (NIR-II) absorption and extremely low emission were designed to exhibit much higher PCE than that of the analogues of pentamethine-dyes (PCy1, PCy2). The presence of the ‘–O−’ at middle of squaric cycle enabled the intermolecular H-bonding formation between the SCy1 and water to promote the energy dissipation channel. Moreover, the introduction of long-chain phenylsulfonate groups helped in to improve the water solubility apart from serving as an additional means of further enhancing PCE through fluorescence quenching. Therefore, SCy1 with a squaraine backbone and long-chain sulfonate moieties revealed outstanding photothermal stability and anti-aggregation activity apart from showing exceptionally high PCE (74%) in water. SCy1 demonstrated excellent therapeutic efficacy when applied in the PTT treatment of tumor-bearing mice under a laser irradiation of 915 nm.

Enhancing droplet rebound on superhydrophobic cones

Understanding the underlying hydrodynamics and developing strategies to control bouncing droplets on superhydrophobic surfaces are of fundamental and practical significance. While recent efforts have mainly focused on regulating the contact time of bouncing droplets, less attention was given to manipulating droplet rebound from the perspective of energy optimization, which determines the long-term successive dynamics. Here, we investigate the impact of water droplets on superhydrophobic cones at low Weber numbers, where ideally complete rebounds arise. In sharp contrast to flat superhydrophobic surfaces, an impinging droplet on a cone-shaped superhydrophobic surface undergoes almost inversion-symmetric spreading and retracting processes with prolonged contact time, and more strikingly, it rebounds with a higher restitution coefficient. Such enhanced droplet rebound is beyond the prediction of existing theoretical models, in which the viscous boundary layer was recognized as the dominant channel of energy dissipation and, thus, an increase in the contact time would result in a lower restitution coefficient; nevertheless, numerical simulations have confirmed the increase in the restitution coefficient. The quantitative energy and flow field analyses of our numerical results reveal that the suppression of the boundary layer in early impact and the weakening of the viscous flow near the moving edge in the subsequent impact phases, which were not accounted for yet in existing theoretical models, are the causes for the enhancement of droplet rebound on superhydrophobic cones.

Phononic origin of strain-controlled friction force

Employing fast Fourier transform (FFT) of instantaneous friction and interfacial phonon spectrum, phononic origin of friction between graphene layers, in which one layer suffering different biaxial strain, is disclosed. FFT spectrum indicates washboard frequencies make major contribution to friction. Peak value in FFT spectrum decreases with increase of strain, leading to drop of friction. Density of states for interface atoms proves energy exchange between tip and substrate is conditional on coupling of interfacial vibration frequencies. There is unified phonon spectrum on relative sliding surfaces at zero strain, which can establish effective energy dissipation channels. Once a strain is applied, unified phonon modes are no longer formed, which destroys conditions for establishment of energy dissipation channels and thus hinders energy dissipation.

Nonlinear Optical Radiation of a Lithium Niobate Microcavity

The nonlinear optical radiation of an integrated lithium niobate microcavity is demonstrated, which has been neglected in previous studies of nonlinear photonic devices. We find that the nonlinear coupling between confined optical modes on the chip and continuum modes in free space can be greatly enhanced on the platform of integrated microcavity, with feasible relaxation of the phase-matching condition. With an infrared pump laser, we observe the vertical radiation of second-harmonic wave at the visible band, which indicates a robust phase-matching-free chip-to-free-space frequency converter and also unveils an extra energy dissipation channel for integrated devices. Such an unexpected coherent nonlinear interaction between the free-space beam and the confined mode is also validated by the different frequency generation. Furthermore, based on the phase-matching-free nature of the nonlinear radiation, we build an integrated atomic gas sensor to characterize Rb isotopes with a single telecom laser. The unveiled mechanism of nonlinear optical radiation is universal for all dielectric photonic integrated devices, and provides a simple and robust chip-to-free-space as well as visible-to-telecom interface.

Decoding the phonon transport of structural lubrication at silicon/silicon interface

Although the friction characteristics under different contact conditions have been extensively studied, the mechanism of phonon transport at the structural lubrication interface is not extremely clear. In this paper, we firstly promulgate that there is a 90°-symmetry of friction force depending on rotation angle at Si/Si interface, which is independent of normal load and temperature. It is further found that the interfacial temperature difference under incommensurate contacts is much larger than that in commensurate cases, which can be attributed to the larger interfacial thermal resistance (ITR). The lower ITR brings greater energy dissipation in commensurate sliding, and the reason for that is more effective energy dissipation channels between the friction surfaces, making it easier for the excited phonons at the washboard frequency and its harmonics to transfer through the interface. Nevertheless, the vibrational frequencies of the interfacial atoms between the tip and substrate during the friction process do not match in incommensurate cases, and there is no effective energy transfer channel, thus presenting the higher ITR and lower friction. Eventually, the number of excited phonons on contact surfaces reveals the amount of frictional energy dissipation in different contact states.

Effect of Dopants on Laser-Induced Damage Threshold of ZnGeP2

The effect of doping Mg, Se, and Ca by diffusion into ZnGeP2 on the optical damage threshold at a wavelength of 2.1 μm has been studied. It has been shown that diffusion-doping with Mg and Se leads to an increase in the laser-induced damage threshold (LIDT) of a single crystal (monocrystal), ZnGeP2; upon annealing at a temperature of 750 °C, the damage threshold of samples doped with Mg and Se increases by 31% and 21% from 2.2 ± 0.1 J/cm2 to 2.9 ± 0.1 and 2.7 ± 0.1 J/cm2, respectively. When ZnGeP2 is doped with Ca, the opposite trend is observed. It has been suggested that the changes in the LIDT depending on the introduced impurity by diffusion can be explained by the creation of additional energy dissipation channels due to the processes of radiative and fast non-radiative relaxation through impurity energy levels, which further requires experimental confirmation.

Nanoscale imaging of super-high-frequency microelectromechanical resonators with femtometer sensitivity

Implementing microelectromechanical system (MEMS) resonators calls for detailed microscopic understanding of the devices, such as energy dissipation channels, spurious modes, and imperfections from microfabrication. Here, we report the nanoscale imaging of a freestanding super-high-frequency (3 – 30 GHz) lateral overtone bulk acoustic resonator with unprecedented spatial resolution and displacement sensitivity. Using transmission-mode microwave impedance microscopy, we have visualized mode profiles of individual overtones and analyzed higher-order transverse spurious modes and anchor loss. The integrated TMIM signals are in good agreement with the stored mechanical energy in the resonator. Quantitative analysis with finite-element modeling shows that the noise floor is equivalent to an in-plane displacement of 10 fm/√Hz at room temperatures, which can be further improved under cryogenic environments. Our work contributes to the design and characterization of MEMS resonators with better performance for telecommunication, sensing, and quantum information science applications.

Nanoscale Friction across the First-Order Charge Density Wave Phase Transition of 1T-TaS2.

Nanotribology using atomic force microscopy (AFM) can be considered as a unique approach to analyze phase transition materials by localized mechanical interaction. In this work, we investigate friction on the lamellar transition metal dichalcogenide 1T-TaS2, which can undergo first-order charge density wave (CDW) phase transitions. Based on temperature-dependent atomic force microscopy under ultrahigh vacuum conditions (UHV), we can characterize the general friction levels across the first-order phase transitions and for the different phases. While structural and electronic properties for different phases appear to be of minor influence on friction, a distinct peak in friction is observed during the phase transition when cooling the sample from the nearly commensurate CDW (NC-CDW) phase to the commensurate CDW (C-CDW) phase. By performing systematic measurements as a function of load, scan velocity, and scan time, a recently proposed friction mechanism can be corroborated, where the AFM tip gradually induces local transformations of the material close to the spinodal point in a thermally activated and shear-assisted process until the surface is fully "harvested". Our results demonstrate that repeated nanomechanical stress can trigger local first-order phase transitions constituting a so far little explored mechanical energy dissipation channel.

Velocity-dependent phononic friction in commensurate and incommensurate states

We investigate that multiple friction peaks occur with an increase in velocity in commensurate and incommensurate states between graphene layers. Fast Fourier transform analysis of instantaneous friction force confirms that frictional spectrum mainly distributes at washboard frequency of tip and its harmonics. When washboard frequency approaches natural frequency of tip and its fractional frequencies, resonance will occur in both contact cases, resulting in friction peaks. Further observations indicate that phonon spectra of tip and substrate match in commensurate state, thereby establishing effective energy dissipation channels; however, phonon spectra do not match in incommensurate state. Finally, we deduce a theoretical model for calculating number of excited phonons and quantitatively explain friction discrepancy in different contact states and velocities.

Phonon mechanism of angle-dependent superlubricity between black phosphorus layers.

Based on a combination of molecular dynamics simulations and quantum theories, this study discloses the phonon mechanism of angle-dependent superlubricity between black phosphorus layers. Friction exhibits 180° periodicity, i.e., the highest friction at 0° and 180° and lowest at 90°. Thermal excitation reduces friction at 0° due to thermal lubrication. However, at 90°, high temperature increases friction caused by thermal collision owing to lower interfacial constraints. Phonon spectra reveal that with 0°, energy dissipation channels can be formed at the interface, thus enhancing dissipation efficiency, while the energy dissipation channels are destroyed, thus hindering frictional dissipation at 90°. Besides, for both commensurate and incommensurate cases, more phonons are excited on atoms adjacent to the contact interface than those excited from nonadjacent interface atoms.

Compressive solitary waves in black phosphorene

Compressive solitons arise in crystals as a result of shock loading, and they can transfer energy over long distances, exhibiting weak damping. Propagation of compressive solitons in two-dimensional (2D) materials is studied much less than in 3D crystals. Here, molecular dynamics is used to analyze the dynamics of compressive solitons in single-layer phosphorene. The mechanisms of energy dissipation by the lattice are analyzed. The results obtained are compared with those obtained earlier for graphene and boron nitride. The damping of compressive solitons in phosphorene is stronger than in graphene and boron nitride, since it has a puckered structure and, therefore, more channels for energy dissipation. Overall, our results contribute to understanding the nonlinear dynamics of localized excitations in 2D materials. • Inducing features of compressive soliton wave are studied in black phosphorene. • Dynamics of compressive soliton wave propagation is discovered. • General channels of dissipation of the wave energy are detected.

STM Visualization of N2 Dissociative Chemisorption on Ru(0001) at High Impinging Kinetic Energies.

This paper examines the reactive surface dynamics of energy- and angle-selected N2 dissociation on a clean Ru(0001) surface. Presented herein are the first STM images of highly energetic N2 dissociation on terrace sites utilizing a novel UHV instrument that combines a supersonic molecular beam with an in situ STM that is in-line with the molecular beam. Atomically resolved visualization of individual N2 dissociation events elucidates the fundamental reactive dynamics of the N2/Ru(0001) system by providing a detailed understanding of the on-surface dissociation dynamics: the distance and angle between nitrogen atoms from the same dissociated N2 molecule, site specificity and coordination of binding on terrace sites, and the local evolution of surrounding nanoscopic areas. These properties are precisely measured over a range of impinging N2 kinetic energies and angles, revealing previously unattainable information about the energy dissipation channels that govern the reactivity of the system. The experimental results presented in this paper provide insight into the fundamental N2 dissociation mechanism that, in conjunction with ongoing theoretical modeling, will help determine the role of dynamical processes such as energy transfer to surface phonons and nonadiabatic excitation of electron-hole pairs (ehps). These results will not only help uncover the underlying chemistry and physics that give rise to the unique behavior of this activated dissociative chemisorption system but also represent an exciting approach to studying reaction dynamics by pairing the angstrom-level spatiotemporal resolution of an in situ STM with nonequilibrium fluxes of reactive gases generated in a supersonic molecular beam to access highly activated chemical dynamics and observe the results of individual reaction events.

Energy Transport during 3D Small-scale Reconnection Driven by Anisotropic Plasma Turbulence

Energy dissipation in collisionless plasmas is a long-standing fundamental physics problem. Although it is well known that magnetic reconnection and turbulence are coupled and transport energy from system-size scales to subproton scales, the details of the energy distribution and energy dissipation channels remain poorly understood. Especially, the energy transfer and transport associated with 3D small-scale reconnection that occurs as a consequence of a turbulent cascade is unknown. We use an explicit fully kinetic particle-in-cell code to simulate 3D small-scale magnetic reconnection events forming in anisotropic and decaying Alfvénic turbulence. We identify a highly dynamic and asymmetric reconnection event that involves two reconnecting flux ropes. We use a two-fluid approach based on the Boltzmann equation to study the spatial energy transfer associated with the reconnection event and compare the power density terms in the two-fluid energy equations with standard energy-based damping, heating, and dissipation proxies. Our findings suggest that the electron bulk flow transports thermal energy density more efficiently than kinetic energy density. Moreover, in our turbulent reconnection event, the energy density transfer is dominated by plasma compression. This is consistent with turbulent current sheets and turbulent reconnection events, but not with laminar reconnection.

Electronic friction and tuning on atomically thin MoS2

Friction is an energy dissipation process. However, the electronic contribution to energy dissipation channels remains elusive during the sliding friction process. The friction and dissipation on atomically thin MoS2 with semiconductive characteristics are studied and tuned by the gate-modulated carrier concentration. The electronic contribution to energy dissipation of friction on atomically thin MoS2 was confirmed and regulated through tuning the strength of the electron-phonon coupling. The electron-phonon coupling can be strengthened and depressed to increase and decrease friction by the gate-modulation of the carrier concentration. The fitting of the friction on atomically thin MoS2 and carrier concentration is approximately linear which is in accordance with Langevin equation induced friction. Then the active, dynamical, and repeated tuning of friction on atomically thin MoS2 with semiconductive properties is achieved by the active modulation of carrier concentration with gate voltage. These observations help us to understand the electronic friction in essence, provide a utility approach to tune the friction intelligently on atomically thin two-dimensional materials with semiconductive properties and achieve superlubric properties for the application in various micro-and nanoelectromechanical systems.