Condensed-Matter Physics & Materials Science Seminar

I will report on two different projects that used inelastic neutron scattering measurements to determine 1. The origin of the charge gap in LaMnPO [1] 2. The origin of the magnetic frustration in CaMn2Sb2 [2] 1. I present my inelastic neutron scattering measurements over a wide range of temperatures on a powder sample of the antiferromagnetic insulator LaMnPO, that exhibits long range antiferromagnetic order below TN = 375 K. I used these measurements to determine a) the strength of the inter-site magnetic exchange interactions SJ1 = 34 meV, SJ2 = 10 meV b) the temperature Tmax = 700 K (>> TN) where 2D magnetic correlations are no longer important. I then present high temperature optical spectroscopy measurements on single crystals of LaMnPO that show a charge gap persists above TN and Tmax. These experimental results show that long range magnetic order and exchange interactions play only a limited role in the formation of the charge gap in LaMnPO. Instead, density functional theory plus dynamical mean field theory calculations show that Hund's coupling is critical for the formation of the charge gap in LaMnPO, as well as related square net Mn pnictide compounds such as BaMn2As2 [D.E. McNally et al. PRB 92 115142 (2015)]. I will show that this work supports the view that multi-orbital electronic correlations are important in the isostructural iron pnictide based superconductors. 2. I present my inelastic neutron scattering measurements on single crystals of the antiferromagnetic insulator CaMn2Sb2, that forms a corrugated honeycomb lattice of Mn spins that order below TN = 85 K, rather low for a Mn pnictide compound. I observed sharp dispersive 3D spin wave excitations up to energy transfers of 25 meV. I used a Heisenberg model to analyze the excitations and determine the first, second and c-axis exchange interactions J1, J2, Jc. I use the determined ratio J2/J1 = 0.17 to situate CaMn2Sb2 on the theoretical phase diagram of the ho