We analyze the hydrogenation of the Si-terminated surfaces $3\text{C-SiC}\left(100\right)\text{−}3\times 2$ and $3\text{C-SiC}\left(100\right)\text{−}c\left(4\times 2\right)$ using density-functional-theory techniques. For the Si-rich $3\times 2$ case, after saturation of the Si dangling bonds, hydrogen atoms break Si-Si bonds between atoms in the first two layers, replacing them by Si-H bonds, up to a hydrogen adsorption of eight hydrogen atoms per $3\times 2$ unit cell, with the formation of ${\text{SiH}}_3$ groups on the surface. At higher hydrogen adsorptions Si-H bonds replace also Si-Si bonds between second- and third-layer Si atoms. We find that all the stable structures as a function of the hydrogen chemical potential are insulating and propose that the observed hydrogen-induced metallization of the Si-rich $\text{SiC}\left(100\right)\text{−}3\times 2$ surface is related to the dynamical equilibrium between adsorption and abstraction of hydrogen atoms close to the point where hydrogen atoms start to break Si-Si bonds. We also find that desorption of ${\text{SiH}}_4$ molecules is not energetically favorable until values of the hydrogen chemical potential above half the energy of the ${\text{H}}_2$ molecule. Regarding the Si-poor $c\left(4\times 2\right)$ surface, our results show that the hydrogenated $\text{H}/\text{SiC}\left(100\right)\text{−}2\times 1$ surface corresponds to a monohydride surface with a Si coverage of one monolayer above the last C layer.

Two-dimensional melting is one of the most fascinating and poorly understood phase transitions in nature. Theoretical investigations often point to a two-step melting scenario involving unbinding of topological defects at two distinct temperatures. Here, we report on a novel melting transition of a charge-ordered K-Sn alloy monolayer on a silicon substrate. Melting starts with short-range positional fluctuations in the K sublattice while maintaining long-range order, followed by longer-range K diffusion over small domains, and ultimately resulting in a molten sublattice. Concomitantly, the charge order of the Sn host lattice collapses in a multistep process with both displacive and order-disorder transition characteristics. Our combined experimental and theoretical analysis provides a rare insight into the atomistic processes of a multistep melting transition of a two-dimensional materials system.

The quest to understand and mimic proton translocation mechanisms in natural channels has driven the development of peptide‐based artificial channels facilitating efficient proton transport across nanometric membranes. It is demonstrated here that hierarchical peptide self‐assembly can form micrometers‐long proton nanochannels. The fourfold symmetrical peptide design leverages intermolecular aromatic interactions to align self‐assembled cyclic peptide nanotubes, creating hydrophilic nanochannels between them. Titratable amino acid sidechains are positioned adjacent to each other within the channels, enabling the formation of hydrogen‐bonded chains upon hydration, and facilitating efficient proton transport. Moreover, these chains are enriched with protons and water molecules by interacting with immobile counter ions introduced into the channels, increasing proton flow density and rate. This system maintains proton transfer rates closely resembling those in natural protein channels over micrometer distances. The functional behavior of these inherently recyclable and biocompatible systems opens the door for their exploitation in diverse applications in energy storage and conversion, biomedicine, and bioelectronics.

Atomic charge is one of the most important concepts in Chemistry. Mulliken population analysis is historically the most important method to calculate atomic charges and is still widely used. One basic hypothesis of this method is the half-and-half partition of the overlap populations, Q(μ, v), into equal charges in orbitals μ and v. This partition preserves the monopole moment of the overlap density but, other than that, is arbitrary. In this work we derive a new population analysis (which we designate Mulliken-Dipole population analysis) based on the conservation of both the monopole moment and the dipole moment along the bond direction. Test calculations show that the Mulliken-Dipole atomic charges are in accord to the chemical intuition; also they are very different from the Mulliken ones, being quite similar to the Hirshfeld atomic charges. Mulliken-Dipole atomic charges are conceptually appealing and very easy to calculate. In a further step, we also show how this Mulliken-Dipole population analysis can be used to derive atomic charges for atomistic simulations that reproduce the total dipole moment of the molecule, yielding at the same time a good description of the local charges and dipole moments for the molecular fragments.

In order to elucidate the mechanism creating superconductivity in the 2-dimensional layer of a p-doped Sn/Si(111) surface, we have analyzed the many-body effects associated with the electron-phonon (e-ph) coupling and the electron–electron interaction. First, we have calculated the DFT surface band of the system and the coupling associated with the different interactions. In our calculations we find a mean field (DFT) electron bandwidth of 0.54 eV, an attractive coupling $U^{neg}=0.32$ eV associated with the e-ph coupling and an effective electron–electron Hubbard repulsion of $U=0.83$ eV. Then, we analyze the Hubbard Hamiltonian, neglecting in this step the e-ph coupling that is much smaller than the Hubbard coupling, by considering a p-doping in this Hamiltonian of 10%; by means of a Dynamical Mean Field (DMF) approach combined with an interpolative calculation for the self-energy, we deduce the local density of states (DOS) and show that the quasi-particle DOS induced by the doping is not large enough to induce magnetism in the Sn-monolayer. This leads us to analyze the possibility of having superconductivity by considering the attractive interaction induced by the e-ph coupling within an appropriate BCS-Hamiltonian. Our calculations show that the quasiparticle metallic system has a superconductivity critical temperature of ≈7–9 K, in good agreement with experiments.