Nearly-critical spin and charge fluctuations in KFe2As2 observed by high-pressure NMR

TL;DR

This study uses high-pressure NMR to reveal nearly-critical spin and charge fluctuations in KFe2As2, highlighting their potential role in its superconductivity and proximity to a magnetic quantum critical point.

Contribution

It provides the first detailed high-pressure NMR analysis showing the evolution of spin and charge fluctuations in KFe2As2, linking them to superconductivity and quantum criticality.

Findings

Spin fluctuations decrease then increase with pressure, correlating with Tc.

Evidence of a magnetic quantum critical point at negative pressure.

Detection of charge order emergence above 2.4 GPa.

Abstract

We report a high-pressure 75As NMR study on the heavily hole-doped iron pnictide superconductor KFe2As2 (Tc~3.8 K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then increase again, changing in lockstep with the pressure-induced evolution of Tc. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of P~-0.6 GPa. Above 2.4 GPa, the 75As satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly-critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity.