The double life of electrons in magnetic iron pnictides as revealed by NMR

H. H. Wills Physics Laboratory, University of Bristol - Tyndall Av, BS8-1TL, Bristol, UK, EU

^a Andrew.Smerald@bristol.ac.uk

Received: 14 September 2010
Accepted: 29 October 2010

Abstract

We present a phenomenological, two-fluid approach to understanding the magnetic excitations in Fe pnictides, in which a paramagnetic fluid with gapless, incoherent particle-hole excitations coexists with an antiferromagnetic fluid with gapped, coherent spin wave excitations. We show that this two-fluid phenomenology provides an excellent quantitative description of NMR data for magnetic “122” pnictides, and argue that it finds a natural justification in LSDA and spin density wave calculations. We further use this phenomenology to estimate the maximum renormalisation of the ordered moment that can follow from low-energy spin fluctuations in Fe pnictides. We find that this is too small to account for the discrepancy between ab initio calculations and neutron scattering measurements.