# Breakdown of single spin-fluid model in heavily hole-doped superconductor CsFe2As2

## TL;DR

This study reveals the breakdown of the single spin-fluid model in heavily hole-doped CsFe2As2 using site-selective NMR, highlighting coexistence of localized and itinerant spins and the emergence of a coherent state at low temperatures.

## Contribution

First observation of the breakdown of the single spin-fluid model in CsFe2As2, demonstrating coexistence of localized and itinerant spins and proposing a Kondo liquid scenario.

## Key findings

- Distinct temperature-dependent behavior of Knight shift and relaxation rates at 133Cs and 75As.
- Breakdown of single spin-fluid model due to coexistence of localized and itinerant spins.
- Recovery of single spin-fluid behavior at low temperatures with a coherent state.

## Abstract

Although Fe-based superconductors are multiorbital correlated electronic systems, previous nuclei magnetic resonance (NMR) measurement suggests that a single spin-fluid model is sufficient to describe its spin behavior. Here, we firstly observed the breakdown of single spin-fluid model in a heavily hole-doped Fe-based superconductor CsFe2As2 by site-selective NMR measurement. At high temperature regime, both of Knight shift and nuclei spin-lattice relaxation at 133Cs and 75As nuclei exhibit distinct temperature-dependent behavior, suggesting the breakdown of single spin-fluid model in CsFe2As2. This is ascribed to the coexistence of both localized and itinerant spin degree of freedom at 3d orbits, which is consistent with orbital-selective Mott phase. However, single spin-fluid behavior is gradually recovered by developing a coherent state among 3d orbits with decreasing temperature. A Kondo liquid scenario is proposed for the low-temperature coherent state. The present work sets strong constraint on the theoretical model for Fe-based superconductors.