We study the electronic structure of iron-based superconductors F eSe1−xT ex within the density functional theory. We pay particular attention to the pressure eects on the Fermi surface (FS) topology, which seem to be correlated with a critical superconducting temperature TC of iron chalcogenides and pnictides. A reduction of the FS nesting between hole and electron cylinders with increasing pressure is observed, which can lead to higher values of TC . The tellurium substitution into selenium sites yields FS changes similar to the pressure eect.

The influence of various strains on crystal and electronic structures of superconducting FeSe has been studied ab initio. We consider changes in the Fermi surface nesting with a vector Q = (0.5, 0.5) × (2π/a) as crucial for rising superconductivity (SC) mediated by spin-fluctuations (SF). Our results indicate that the c-axis strained FeSe exhibits the most imperfect nesting, which enhances SF and, hence, also SC. In turn, the ab-plane compressive strain slightly weakens this nesting while the tensile strain destroys it completely. These findings are consistent with reported earlier experimental dependencies of superconducting transition temperatures on strain in FeSe thin films.

The influence of hydrostatic pressure and ab-plane strain on the magnetic structure of FeTe is investigated from first principles. The results of calculations reveal a phase transition from antiferromagnetic double-stripe ordering at ambient pressure to ferromagnetic ordering at 2 GPa, or under compressive strain reducing the lattice parameter a by about 3%. In turn, a tensile strain of less than 2% induces the phase transition to antiferromagnetic single-stripe ordering. It corresponds to the superconducting FeTe thin films, thereby confirming that the superconducting state is positively linked to single-stripe antiferromagnetic fluctuations. Both types of transition indicate that the position of Te atoms in the crystal is crucial for the magnetic and superconducting properties of iron chalcogenides.

Depopulation of the Mn2+ state in ZnO:Mn upon illumination, monitored by quenching of the Mn 2+ EPR signal intensity, was observed at temperatures below 80 K. Mn 2+ photoquenching is shown to result from the Mn 2+ → Mn 3+ ionization transition, promoting one electron to the conduction band. Temperature dependence of this process indicates the existence of an energy barrier for electron recapture of the order of 1 meV. GGA+U calculations show that after ionization of Mn 2+ a moderate breathing lattice relaxation in the 3+ charge state occurs, which increases energies of d(Mn) levels. At its equilibrium atomic configuration, Mn 3+ is metastable since the direct capture of photo-electron is not possible. The metastability is mainly driven by the strong intra-shell Coulomb repulsion between d(Mn) electrons. Both the estimated barrier for electron capture and the photoionization energy are in good agreement with the experimental values.

No abstract