Soft phonons reveal the nematic correlation length in Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$

TL;DR

This study uses soft phonon modes to measure the nematic correlation length in a high-temperature superconductor, revealing a temperature-dependent change at the superconducting transition that links nematicity to superconductivity.

Contribution

It introduces a method to probe finite-momentum nematic fluctuations via phonon softening, providing new insights into nematic correlation length behavior in Fe-based superconductors.

Findings

Nematic correlation length sharply changes at T_c.

Soft phonon modes can reveal nematic fluctuations at finite momentum.

Strong connection between nematicity and superconductivity.

Abstract

Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. We determine the nematic correlation length and find that it sharply changes its $T$-dependence at $T_c$, revealing a strong connection between nematicity and superconductivity.