Quasiparticle GWGW band structures and Fermi surfaces of bulk and monolayer NbS2_2

In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level, and that our calculations are in good agreement with recent ARPES measurements. In the case of a free-standing monolayer NbS$_2$, we observe a strong increase of the screened Coulomb interaction and the quasiparticle corrections as compared to bulk. In this case we also perform calculations to include the effect of screening by a substrate. We report in detail the results of our convergence tests and computational parameters, to serve as a solid basis for future studies.Comment: 15 pages, 18 figure

Absence of superconductivity in iron polyhydrides at high pressures

Recently, C. M. Pépin et al. [Science 357, 382 (2017)] reported the formation of several new iron polyhydrides FeHx at pressures in the megabar range and spotted FeH5, which forms above 130 GPa, as a potential high-Tc superconductor because of an alleged layer of dense metallic hydrogen. Shortly after, two studies by A. Majumdar et al. [Phys. Rev. B 96, 201107 (2017)] and A. G. Kvashnin et al. [J. Phys. Chem. C 122, 4731 (2018)] based on ab initio Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We conversely find, on the same theoretical-numerical basis, that neither FeH5 nor its precursor, FeH3, shows any conventional superconductivity and explain why this is the case. We also show that superconductivity may be attained by transition-metal polyhydrides in the FeH3 structure type by adding more electrons to partially fill one of the Fe-H hybrid bands (as, e.g., in NiH3). Critical temperatures, however, will remain low because the d-metal bonding, and not the metallic hydrogen, dominates the behavior of electrons and phonons involved in the superconducting pairing in these compounds

Effect of the iron valence in the two types of layers in LiFeO2_2Fe2_2Se2_2

We perform electronic structure calculations for the recently synthesized iron-based superconductor LiFeO$_2$Fe$_2$Se$_2$. In contrast to other iron-based superconductors, this material comprises two different iron atoms in 3$d^5$ and 3$d^6$ configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical mean-field theory. The contributions from the half-filled 3$d^5$ atoms in the LiFeO$_2$ layer are suppressed and the 3$d^6$ states from the FeSe layer restore the standard iron-based superconductor fermiology.Comment: 9 pages, 11 figure

Origin of superconductivity and latent charge density wave in NbS2_2

We elucidate the origin of the phonon-mediated superconductivity in 2$H$-NbS$_2$ using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electron-phonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide definitive evidence for two-gap superconductivity in 2$H$-NbS$_2$, and show that the low- and high-energy peaks observed in tunneling spectra correspond to the $\Gamma$- and $K$-centered Fermi surface pockets, respectively. The present findings call for further efforts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides.Comment: 6 pages, 5 figures and Supplemental Materia

Temperature and quantum anharmonic lattice effects in lutetium trihydride: stability and superconductivity

In this work, we resolve conflicting experimental and theoretical findings related to the dynamical stability and superconducting properties of $Fm\overline{3}m$-LuH$_3$, which, according to Dasenbrock-Gammon \textit{et al.}~[Nature 615, 244 (2023)], is the parent phase harboring room-temperature superconductivity at near-ambient pressures. Including temperature and quantum anharmonic lattice effects in our calculations, we demonstrate that the theoretically predicted structural instability of the $Fm\overline{3}m$ phase near ambient pressures is suppressed for temperatures above $200\,\text{K}$. We provide a $p$--$T$ phase diagram for stability up to pressures of $6\,\text{GPa}$, where the required temperature for stability is reduced to $T>80\,\text{K}$. We also determine the superconducting critical temperature $T_\text{c}$ of $Fm\overline{3}m$-LuH$_3$ within the Migdal-Eliashberg formalism, using temperature- and quantum-anharmonically-corrected phonon dispersions, finding that the expected $T_\text{c}$ for electron-phonon mediated superconductivity is in the range of $50$--$60\,\text{K}$, i.e., well below the temperatures required to stabilize the lattice. When taking into account moderate doping based on rigidly shifting the Fermi level, $T_\text{c}$ decreases for both hole and electron doping. Our results thus provide evidence that any observed room-temperature superconductivity in pure or doped $Fm\overline{3}m$-LuH$_3$, if confirmed, cannot be explained by a conventional electron-phonon mediated pairing mechanism.Comment: 8 pages, 5 figure