Dr. Bayrammurad Saparov, Oak Ridge National Laboratory will present a seminar entitled, “Effects of annealing, doping and substitution on structures and properties of ThCr₂Si₂-type pnictides.”

Abstract:

CaFe₂As₂ is an undoped ‘parent’ of iron-based superconductor, with richest magnetic and structural phase diagram among the ThCr₂Si₂-type 122 phases. In CaFe₂As₂, the pressurized phases are accessible under ambient conditions using the procedure that involves quenching from FeAs flux at high-temperature, followed by annealing at various temperatures. Samples obtained using this method can show a transition from high temperature paramagnetic tetragonal phase to a non-magnetic collapsed-tetragonal phase or an antiferromagnetic-orthorhombic. Using a combination of experimental methods, we demonstrate that there are several structural parameters that have significant influence on the transition temperatures in these materials. The mapping of the bulk and local crystal structures, and electronic structures of three different CaFe₂As₂ phases, which demonstrate markedly different magnetic and structural behaviors, will be provided in this talk.

I will also present results of our extensive studies on Pr-doped CaFe₂As₂, which exhibit the highest superconducting transition temperature of T_c = 45 K among the ThCr₂Si₂-type pnictides. Using multi-scale techniques, we determine the extent of local inhomogeneity and superconductivity in this material. We show that the inhomogeneity is manifested as a spatial variation of praseodymium concentration, local density of states, and superconducting order parameter. Furthermore, the highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of non-uniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

In the last section of the talk, I will focus on new BaMn₂Bi₂ and K-doped Ba_1-xK_xMn₂Bi₂, which are crystallized from bismuth flux. BaMn₂Bi₂ is the first bismuthide that adopts ThCr₂Si₂-type structure. It is antiferromagnetic with anisotropic magnetic susceptibility and semiconducting with a band gap of E_g = 6 meV. The high temperature magnetization and neutron diffraction results show that magnetic ordering temperature is T_N ~ 400 K. Hole-doping in BaMn₂Bi₂ via potassium in Ba_1-xK_xMn₂Bi₂ results in metallic behavior for x = 0.10(1), 0.32(1) and 0.36(1). With K-doping, more magnetically anisotropic behavior is observed.

Host: Dr. Shane Street