Nematic Quantum Critical Fluctuations in BaFe$_{2-x}$Ni$_x$As$_2$

TL;DR

This study investigates nematic quantum critical fluctuations in BaFe$_{2-x}$Ni$_x$As$_2$ using resistance measurements under uniaxial pressure, revealing complex behaviors and potential higher symmetry near optimal doping.

Contribution

It provides new insights into nematic fluctuations and their doping dependence, challenging simple Ising models and suggesting strong coupling or higher symmetry at optimal doping.

Findings

Resistivity along (100) direction peaks at optimal doping.

Thermal fluctuations dominate in the underdoped regime.

Evidence of complex nematic behavior beyond simple models.

Abstract

We have systematically studied the nematic fluctuations in the electron-doped iron-based superconductor BaFe$_{2-x}$Ni$_x$As$_2$ by measuring the in-plane resistance change under uniaxial pressure. While the nematic quantum critical point can be identified through the measurements along the (110) direction as studied previously, quantum and thermal critical fluctuations cannot be distinguished due to similar Curie-Weiss-like behaviors. Here we find that a sizable pressure-dependent resistivity along the (100) direction is present in all doping levels, which is against the simple picture of an Ising-type nematic model. The signal along the (100) direction becomes maximum at optimal doping, suggesting that it is associated with nematic quantum critical fluctuations. Our results indicate that thermal fluctuations from striped antiferromagnetic order dominate the underdoped regime along the (110) direction. We argue that either there is a strong coupling between the quantum critical fluctuations and the fermions, or more exotically, a higher symmetry may be present around optimal doping.