Site-dependent NMR Spin-lattice Relaxation in the Superconducting State of an Iron Pnictide Superconductor

TL;DR

This study reveals site-dependent spin-lattice relaxation behaviors in an iron pnictide superconductor, linked to multiple superconducting gaps and local suppression at dopant sites, challenging conventional uniform gap assumptions.

Contribution

It demonstrates that different nuclear sites in a superconductor exhibit distinct relaxation behaviors due to multiple gaps and local effects, providing new insights into the pairing mechanism.

Findings

Different nuclei show distinct power-law relaxation behaviors.

The large superconducting gap is suppressed at the cobalt dopant site.

Site-dependent relaxation is influenced by hyperfine coupling to multiple gaps.

Abstract

In a conventional superconductor, the spin-lattice relaxation rate on all nuclei should have the same temperature dependence below Tc. We performed $^{23}$Na, $^{75}$As, and $^{59}$Co NMR studies on single crystals of NaFe0.95Co0.05As, and found that spin-lattice relaxation rates show very different temperature dependent power-law behavior on three sites. We propose that such site-dependent behavior is due to the facts that the superconductor has two gaps of very different sizes. The power-law exponent of each nucleus is affected by the strength of the hyperfine coupling to the small gap. We also found that the large superconducting gap on the cobalt site is smaller than on other two sites. It suggests a local suppression of the superconducting gap on the dopant site.