Electron-phonon coupling and exchange-correlation effects in superconducting H3S under high pressure

We investigate the H3S phase of sulfur hydride under high pressure similar or equal to 200 GPa by means of ab initio calculations within the framework of the density-functional theory with the PBE0 hybrid exchange-correlation (E-xc) approximation. The choice of E-xc has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high-pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation E-xc. Our principle finding is that the calculated PBE0 T-c is enhanced by 25% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of T-c by anharmonic effects [I. Errea et al., Phys. Rev. Lett. 114, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials

New inversion boundary structure in Sb-doped ZnO predicted by DFT calculations and confirmed by experimental HRTEM

Today, ab-initio calculations are becoming a powerful tool to perform virtual experiments that have the capacity to predict and to reproduce experimentally observed non-periodic features, such as interfaces, that are responsible for quantum properties of materials. In our paper we investigate 2D quantum-well structures, known as inversion boundaries OM. Combining atomistic modeling, DFT calculations and HRTEM analysis we provide a new fundamental insight into the structure and stability of Sb-rich basal-plane IBs in ZnO. DFT screening for potential IB model was based on the known stacking deviations in originating wurtzite structure. The results show that the model with A beta-B alpha-A beta C-gamma B-beta C sequence (IB3) is the most stable translation for Sb-doping, as opposed to previously accepted A beta-B alpha-A beta C-gamma A-alpha C (IB2) model. The key to the stability of IB structures has been found to lie in their cationic stacking. We show that the energies of constituting stacking segments can be used to predict the stability of new IB structures without the need of further ab-initio calculations. DFT optimized models of IBs accurately predict the experimentally observed IB structures with lateral relaxations down to a precision of similar to 1 pm. The newly determined cation sublattice expansions for experimentally confirmed IB2 and IB3 models, Delta(IB(zn-zn)) are +81 pm and +77 pm, whereas the corresponding O-sublattice contractions Delta(IB(0-0)) are -53 pm and -57 pm, respectively. The refined structures will help to solve open questions related to their role in electron transport, phonon scattering, p-type conductivity, affinity of dopants to generate IBs and the underlying formation mechanisms, whereas the excellent match between the calculations and experiment demonstrated in our study opens new perspectives for prediction of such properties from first principles

Orthorhombic fulleride (CH3NH2)K3C60 close to Mott-Hubbard instability: Ab initio study

We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkali-doped fullerene (CH3NH2)K3C60 within the density functional theory. As in the simpler ammonia intercalated compound (NH3)K3C60, the orthorhombic crystal-field anisotropy \Delta lifts the t1u triple degeneracy at the \Gamma point and drives the system deep into the Mott-insulating phase. However, the computed \Delta and conduction electron bandwidth W cannot alone account for the abnormally low experimental N\'eel temperature, T_N = 11 K of the methylamine compound, compared to the much higher value T_N = 40 K of the ammonia one. Significant interactions between CH3NH2 and C60^{3-} are responsible for the stabilization of particular pseudo-Jahn-Teller fullerene-cage distortions and the ensuing low-spin S = 1/2 state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of pseudo-Jahn-Teller distortions of neighboring C60^{3-} molecules. For the ferro-orientational order of CH3NH2-K^+ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed N\'eel temperature. The disorder in exchange interactions caused by orientational disorder of CH3NH2-K^+ groups could further contribute to this suppression.Comment: 8 pages, 5 figures, 1 tabl