Preempted phonon-mediated superconductivity in the infinite-layer nickelates

Nickelate superconductors are outstanding materials with intriguing analogies with the cuprates. These analogies suggest that their superconducting mechanism is similarly unconventional, although this fundamental question is currently under debate. Here, we scrutinize the role played by electronic correlations in enhancing the electron-phonon coupling in the infinite-layer nickelates and the extent to which this may promote superconductivity. Specifically, we use $ab$ $initio$ many-body perturbation theory to perform state-of-the-art $GW$ and Eliashberg-theory calculations. We find that the electron-phonon coupling is in effect enhanced compared to density-functional-theory calculations. This enhancement may lead to low-$T_c$ superconductivity in the parent compounds already. However, it remains marginal in the sense that it cannot explain the record $T_c$s obtained with doping. Conventional superconductivity then appears to be preempted by another mechanism in the infinite-layer nickelates.Comment: 6 pages, 4 figures, 1 table + Supplementary Informatio

Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening

We present an calculation of the electronic and optical excitations of an isolated polythiophene chain as well as of bulk polythiophene. We use the GW approximation for the electronic self-energy and include excitonic effects by solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain screening in the case of bulk polythiophene drastically reduces both the quasi-particle band gap and the exciton binding energies, but the optical gap is hardly affected. This finding is relevant for conjugated polymers in general.Comment: 4 pages, 1 figur

Temperature effects on dislocation core energies in silicon and germanium

Temperature effects on the energetics of the 90-degree partial dislocation in silicon and germanium are investigated, using non-equilibrium methods to estimate free energies, coupled with Monte Carlo simulations. Atomic interactions are described by Tersoff and EDIP interatomic potentials. Our results indicate that the vibrational entropy has the effect of increasing the difference in free energy between the two possible reconstructions of the 90-degree partial, namely, the single-period and the double-period geometries. This effect further increases the energetic stability of the double-period reconstruction at high temperatures. The results also indicate that anharmonic effects may play an important role in determining the structural properties of these defects in the high-temperature regime.Comment: 8 pages in two-column physical-review format with six figure

Ab-initio prediction of the electronic and optical excitations in polythiophene: isolated chains versus bulk polymer

We calculate the electronic and optical excitations of polythiophene using the GW approximation for the electronic self-energy, and include excitonic effects by solving the electron-hole Bethe-Salpeter equation. Two different situations are studied: excitations on isolated chains and excitations on chains in crystalline polythiophene. The dielectric tensor for the crystalline situation is obtained by modeling the polymer chains as polarizable line objects, with a long-wavelength polarizability tensor obtained from the ab-initio polarizability function of the isolated chain. With this model dielectric tensor we construct a screened interaction for the crystalline case, including both intra- and interchain screening. In the crystalline situation both the quasi-particle band gap and the exciton binding energies are drastically reduced in comparison with the isolated chain. However, the optical gap is hardly affected. We expect this result to be relevant for conjugated polymers in general.Comment: 15 pages including 4 figures; to appear in Phys. Rev. B, 6/15/200

The relationship between the perception of distributed leadership in secondary schools and teachers' and teacher leaders' job satisfaction and organizational commitment

This study investigates the relation between distributed leadership, the cohesion of the leadership team, participative decision-making, context variables, and the organizational commitment and job satisfaction of teachers and teacher leaders. A questionnaire was administered to teachers and teacher leaders (n=1770) from 46 large secondary schools. Multiple regression analyses and path analyses revealed that the study variables explained significant variance in organizational commitment. The degree of explained variance for job satisfaction was considerably lower compared to organizational commitment. Most striking was that the cohesion of the leadership team and the amount of leadership support was strongly related to organizational commitment, and indirectly to job satisfaction. Decentralization of leadership functions was weakly related to organizational commitment and job satisfaction

New metallic allotropes of planar and tubular carbon

We propose a new family of layered sp2-like carbon crystals, incorporating five-, six-, and seven-membered rings in 2D Bravais lattices. These periodic sheets can be rolled so as to generate nanotubes of different diameter and chirality. We demonstrate that these sheets and tubes are metastable and more favorable than C60, and it is also shown that their mechanical properties are similar to those of graphene. Density of states calculations of all structures revealed an intrinsic metallic behavior, independent of orientation, tube diameter, and chirality