Classifying Charge Carrier Interaction in Highly Compressed Elements and Silane

Since the pivotal experimental discovery of near-room-temperature superconductivity (NRTS) in highly compressed sulphur hydride by Drozdov et al. (Nature 2015, 525, 73–76), more than a dozen binary and ternary hydrogen-rich phases exhibiting superconducting transitions above 100 K have been discovered to date. There is a widely accepted theoretical point of view that the primary mechanism governing the emergence of superconductivity in hydrogen-rich phases is the electron–phonon pairing. However, the recent analysis of experimental temperature-dependent resistance, R(T), in H3S, LaHx, PrH9 and BaH12 (Talantsev, Supercond. Sci. Technol. 2021, 34, accepted) showed that these compounds exhibit the dominance of non-electron–phonon charge carrier interactions and, thus, it is unlikely that the electron–phonon pairing is the primary mechanism for the emergence of superconductivity in these materials. Here, we use the same approach to reveal the charge carrier interaction in highly compressed lithium, black phosphorous, sulfur, and silane. We found that all these superconductors exhibit the dominance of non-electron–phonon charge carrier interaction. This explains the failure to demonstrate the high-Tc values that are predicted for these materials by first-principles calculations which utilize the electron–phonon pairing as the mechanism for the emergence of their superconductivity. Our result implies that alternative pairing mechanisms (primarily the electron–electron retraction) should be tested within the first-principles calculations approach as possible mechanisms for the emergence of superconductivity in highly compressed lithium, black phosphorous, sulfur, and silane.

Quantifying the Charge Carrier Interaction in Metallic Twisted Bilayer Graphene Superlattices

The mechanism of charge carrier interaction in twisted bilayer graphene (TBG) remains an unresolved problem, where some researchers proposed the dominance of the electron–phonon interaction, while the others showed evidence for electron–electron or electron–magnon interactions. Here we propose to resolve this problem by generalizing the Bloch–Grüneisen equation and using it for the analysis of the temperature dependent resistivity in TBG. It is a well-established theoretical result that the Bloch–Grüneisen equation power-law exponent, p, exhibits exact integer values for certain mechanisms. For instance, p = 5 implies the electron–phonon interaction, p = 3 is associated with the electron–magnon interaction and p = 2 applies to the electron–electron interaction. Here we interpret the linear temperature-dependent resistance, widely observed in TBG, as p→1, which implies the quasielastic charge interaction with acoustic phonons. Thus, we fitted TBG resistance curves to the Bloch–Grüneisen equation, where we propose that p is a free-fitting parameter. We found that TBGs have a smoothly varied p-value (ranging from 1.4 to 4.4) depending on the Moiré superlattice constant, λ, or the charge carrier concentration, n. This implies that different mechanisms of the charge carrier interaction in TBG superlattices smoothly transition from one mechanism to another depending on, at least, λ and n. The proposed generalized Bloch–Grüneisen equation is applicable to a wide range of disciplines, including superconductivity and geology.

Photoluminescence and Boosting Electron–Phonon Coupling in CdS Nanowires with Variable Sn(IV) Dopant Concentration

High-quality Sn(IV)-doped CdS nanowires were synthesized by a thermal evaporation route. Both XRD and Raman scattering spectrum confirmed the doping effect. The room temperature photoluminescence (PL) demonstrated that both near bandgap emission and discrete trapped-state emission appeared simultaneously and significantly, which were attributed to the strong exciton trapping by impurities and electron–phonon coupling during the light transportation. The PL intensity ratio of near bandgap emission to trapped-state emission could be tune via doped Sn(IV) concentration in the CdS nanowires. It is interesting that the trapped-state emission shows well separated peaks with the assistance of 1LO, 2LO, 4LO phonons, demonstrating the boosting electron–phonon coupling in these doped CdS nanowires. The influence of Sn(IV) dopant is further revealed by PL lifetime decay profile. The optical micro-cavity also plays an important role on this emission process. Our results will be helpful to the understanding of doping modulated carrier interaction, trapping and recombination in one-dimensional (1D) nanostructures.

Photoluminescence and Boosting Electron-phonon Coupling in CdS Nanowires With Variable Sn(IV) Dopant Concentration

High-quality Sn(IV) doped CdS nanowires were synthesized by a thermal evaporation route. Both XRD and Raman scattering spectrum confirmed the doping effect. The room temperature photoluminescence (PL) demonstrated that both near bandgap emission and discrete trapped-state emission appeared simultaneously and significantly, which were attributed to the strong exciton trapping by impurities and electron-phonon coupling during the light transportation. The PL intensity ratio of near bandgap emission to trapped-state emission could be tune via doped Sn(IV) concentration in the CdS nanowires. It is interesting that the trapped-state emission shows well separated peaks with the assistance of 1LO, 2LO, 4LO phonons, demonstrating the boosting electron-phonon coupling in these doped CdS nanowires. It is interesting that the trapped-state emission shows well separated peaks with the assistance of 1LO, 2LO, 4LO phonons, demonstrating the boosting electron-phonon coupling in these doped CdS nanowires. The influence of Sn(IV) dopant is further revealed by PL lifetime decay profile. The optical micro-cavity also plays an important role on this emission process. Our results will be helpful to the understanding of doping modulated carrier interaction, trapping and recombination in one-dimensional (1D) nanostructures.

How to Carry Loads Economically: Analysis Based on a Predictive Biped Model

Carrying heavy loads costs additional energy during walking and leads to fatigue of the user. Conventionally, the load is fixed on the body. Some recent studies showed energy cost reduction when the relative motion of the load with respect to the body was allowed. However, the influences of the load's relative motion on the user are still not fully understood. We employed an optimization-based biped model, which can generate human-like walking motion to study the load–carrier interaction. The relative motion can be achieved by a passive mechanism (such as springs) or a powered mechanism (such as actuators), and the relative motion can occur in the vertical or fore-aft directions. The connection between the load and body is added to the biped model in four scenarios (two types × two directions). The optimization results indicate that the stiffness values affect energy cost differently and the same stiffness value in different directions may have opposite effects. Powered relative motion in either direction can potentially reduce energy cost but the vertical relative motion can achieve a higher reduction than fore-aft relative motion. Surprisingly, powered relative motion only performs marginally better than the passive conditions at similar peak interaction force levels. This work provides insights into developing more economical load-carrying methods and the model presented may be applied to the design and control of wearable load-carrying devices.