59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)

TL;DR

This study uses NMR to explore electron doping effects in BaFe(2)As(2) with Co, revealing carrier donation without localized moments and indicating a different physics mechanism from cuprates, including a pseudogap phase and singlet pairing.

Contribution

It provides new insights into the electronic properties and pairing mechanism of Co-doped BaFe(2)As(2), challenging the Mott insulator model of high Tc superconductors.

Findings

Co atoms form an alloy with Fe and donate carriers without localized moments.

Identification of a pseudogap phase with Delta ~560K.

Observation of spin-singlet pairing below Tc.

Abstract

We report an NMR investigation of the superconductivity in BaFe(2)As(2) induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Delta 560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.