Pressure‐Driven Enthalpic and Lifshitz Transition in 122‐Pnictides

Abstract

Iron-arsenic pnictides are a class of compounds in which strong Coulomb interactions manifest in novel properties. We discuss basic ideas behind a Lifshitz transition and review salient properties of 122-pnictides (AFe 2As2; A = Ca, Sr, Ba, Ra). Using T = 0 first principles total energy calculations, as a function of hydrostatic pressure, we show that several pressure-driven anomalies, and the tetragonal (T) to collapsed tetragonal (cT) phase transition in the 122-pnictides family, can be understood as consequences of Lifshitz transitions arising from nontrivial changes in Fermi surface topology. We also show that several features found in our calculations, namely, enthalpic, magnetic and T-cT transitions, are universal to the 122-pnictides.

Electronic structures and spin states of BaFe2As2 and SrFe2As2 probed by x-ray emission spectroscopy at Fe an

“…The theory predicted the transition to the zero-magnetic moment at the cT phase. 18,57 We measured the pressure dependence of the Fe Kβ emission spectra for SrFe 2 As 2 , from which we know the pressure-induced change in the IAD values, corresponding to the magnetic moment. The results indicate a gradual decrease of the magnetic moment with pressure up to 11.8 GPa at 17 K and up to 15.6 GPa at 300 K. A similar trend has been observed in K x Fe 2−y Se 2 .…”

Elastic Instability of the Orthorhombic Antiferromagnetic Phase of 122-Pnictides Under Pressure

“…Our previous work [13], based on first principles density functional theory calculations, showed that the pressure-driven T = 0K transition from OR-AFM to tetragonal (cT or T) can be understood as a 1 st -order enthalpic transition that occurs at a pressure we denote P H . Subsequent work [14,15], also based on first principles total energy calculations, proposed that the T -cT transition and anomalies in lattice parameters and elastic properties, observed experimentally at finite temperatures, can be interpreted as arising from proximity to T = 0K Lifshitz transitions, at pressures denoted as P L , wherein pressure-driven changes in lattice constants alter the Fermi surface topology in these materials. Finally, our work revealed the existence of a metastable OR-AFM phase that remains mechanically stable until it transitions discontinuously to the tetragonal phase at a pressure we denoted P M .…”