Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).

Bismuth ferrite has been under intense research for many years as it can exhibit first- and second-order transitions where all the phases have distinct properties encapsulating various exciting phenomena. This work reports a computational study of bismuth ferrite and its varied phases using density functional theory with the implementation of Hubbard correction for increased accuracy. The proposed method is validated through Linear Response Theory using Quantum ESPRESSO. The phase transition and the mechanical properties are explored by calculating elastic tensors for different polymorphs. A negative Poissons ratio for the tetragonal phase supporting its growth in compressive environments is predicted. The electronic properties of different phases of bismuth ferrite are explored, which helps in understanding properties such as charge transfer excitation, metal-insulator transition, ferroelectric nature based on lone pair charges and orbital hybridization. The phonon modes of different phases are also investigated.

Recent experiments [Barbero et al. Phys. Rev. B 95, 094505 (2017)] have established that bulk superconductivity (T_c ∼ 8.3-8.7 K) can be induced in AlB₂-type ZrB₂ and HfB₂, highly covalent refractory ceramics, by vanadium (V) doping. These AlB₂-structured phases provide an alternative to earlier diamon-like or diamond-based superconducting and superhard materials. However, the underlying mechanism for doping-induced superconductivity in these materials is yet to be addressed. In this paper, we have used first-principles calculations to probe electronic structure, lattice dynamics, and electron-phonon coupling (EPC) in V-doped ZrB₂ and consequently examine the origin of the superconductivity. We find that, while doping-induced stress weakens the EPC, the concurrently induced charges strengthen it. The calculated critical transition temperature (T_c) in electron (and V)-doped ZrB₂ is at least one order of magnitude lower than experiments, despite considering the weakest possible Coulomb repulsion between electrons in the Cooper pair, hinting a complex origin of superconductivity in it.

The superconducting transition temperature (T-C) of rock-salt type niobium nitride (delta - NbN) typically varies between 9 to 17 K and the theoretically predicted value of 18 K has not been achieved hitherto. The low T-C in delta - NbN has been assigned to some structural disorder which is always present irrespective of the microstructure (polycrystalline or epitaxial), methods or conditions adopted during the growth of NbN thin films. In this work, we investigate the atomic origin of such suppression of the T-C in delta - NbN thin films by employing combined methods of experiments and ab initio simulations. Sputtered delta - NbN thin films with different disorder were studied using the synchrotron-based N and Nb K-edge x-ray absorption spectroscopy techniques. A strong correlation between the superconductivity and the electronic structure reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and results into a smeared electronic structure around Fermi-level. The role of electronic smearing on the T-C is thoroughly discussed.