Nematic Fluctuations and Electronic Correlations in Heavily Hole-Doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ Probed by Elastoresistance

TL;DR

The paper investigates how nematic fluctuations and electronic correlations evolve with heavy hole doping in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ using longitudinal and transverse elastoresistance measurements that are decomposed into the symmetry channels $A_{1g}$ and $B_{2g}$. It combines these experiments with a five-orbital Hubbard model and a renormalization scheme to compute low-energy effective interactions, applying a Kubo-Greenwood framework to obtain symmetry-resolved elastoresistive responses. A crossover from $B_{2g}$-dominated nematic fluctuations at low $x$ to a pronounced $A_{1g}$–channel enhancement at high $x$ is found, with the high-doping peak near $x \approx 0.8$ attributed to an orbital-selective Kondo-like resonance involving the $d_{xy}$ orbital; Lifshitz transitions modulate but do not solely drive this behavior. These findings connect enhanced $A_{1g}$ correlations to changes in the Sommerfeld coefficient and the emergence of a time-reversal-symmetry–breaking superconducting phase, suggesting strong orbital-selective correlations play a crucial role in the superconducting state of the strongly hole-doped regime.

Abstract

This work investigates nematic fluctuations and electronic correlations in the hole-doped iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ by means of longitudinal and transverse elastoresistance measurements over a wide doping range ($0.63 < x < 0.98$). For this purpose, the orbital character of the electronic response was revealed by decomposition of the elastoresistance into the $A_{1g}$ and $B_{2g}$ symmetry channels. It was shown that at lower doping levels nematic fluctuations in the $B_{2g}$ channel dominate, while for $x > 0.68$ the $A_{1g}$ channel becomes dominant and reaches a pronounced maximum at $x \approx 0.8$ which indicates strong orbital-selective electronic correlations. Despite the dominance of the $A_{1g}$ signal at high doping, a weak contribution in the $B_{2g}$ channel persists, which can be interpreted as a remnant of nematic fluctuations. Model calculations based on a five-orbital tight-binding Hamiltonian with interactions attribute the observed enhancement in the $A_{1g}$ channel to an orbital-selective Kondo-like resonance, predominantly involving the $d_{xy}$ orbital. We discuss our results in relation to the evolution of the Sommerfeld coefficient reported in the literature and a reported change of the superconducting order parameter. All this indicates that for $x > 0.68$ qualitatively new physics emerges. Our findings suggest that electronic correlations in the strongly hole-doped regime play an important role in superconductivity, while the detectable weak nematic fluctuations may also be of relevance.

Figures (8)

• Figure 1: (a) Schematic of the fully contacted piezoelectric actuator. (b) Schematic representation of the $A_{1g}$ and $B_{2g}$ symmetry channels of the $D_{4h}$ point group.
• Figure 2: Longitudinal elastoresistance measurements of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ samples with 0.63 $\leq \times \leq$ 0.98 (blue) and the corresponding Curie-Weiss fits (red). The vertical lines indicate the fitting range for each sample.
• Figure 3: Summary of the Curie-Weiss fit parameters extracted from the longitudinal elastoresistance measurements shown in Fig. \ref{['fig:Elastomessungen']} for the different doping levels. The dataset has been complemented with values from the literature. The orange rectangle is based on values from Ref. Daten1, the green rectangle on Ref. Daten2, and the red triangles correspond to the measurements reported in Ref. Xiaochen1. The connecting lines between the data points are guides to the eye.
• Figure 4: The previously shown longitudinal elastoresistance measurements (blue) were complemented by the transverse resistance measurements shown in orange.
• Figure 5: Results of the elastoresistance decomposition into the $A_{1g}$ and $B_{2g}$ symmetry channels, determined using Equation \ref{['eq:A1g']} and \ref{['eq:B2g']}, for samples with doping levels of 0.74, 0.80, and 0.95. All samples exhibit a divergent increase in both the $A_{1g}$ and $B_{2g}$ channels, which can also be described by a Curie-Weiss-like behavior. However, the signal in the $A_{1g}$ channel is significantly more pronounced than in the $B_{2g}$ channel for all samples.