Electron Mass Enhancement near a Nematic Quantum Critical Point in NaFe$_{1-x}$Co$_{x}$As

TL;DR

This study uses NMR to investigate how electron mass is enhanced near a nematic quantum critical point in NaFe$_{1-x}$Co$_{x}$As, revealing a divergence in effective mass at the nematic QCP beneath the superconducting dome.

Contribution

It provides direct evidence of electron mass enhancement at a nematic quantum critical point in an iron-based superconductor, linking nematic fluctuations to quantum critical behavior.

Findings

Electron mass $m^*$ is enhanced near the nematic QCP.

Two peaks in London penetration depth squared at specific doping levels.

Nematic QCP is located beneath the superconducting dome.

Abstract

A magnetic order can be completely suppressed at zero temperature($T$), by doping carriers or applying pressure, at a quantum critical point(QCP), around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance(NMR) studies on NaFe$_{1-x}$Co$_{x}$As where magnetic and nematic transitions are well separated. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state, which is related to London penetration depth $\lambda_{\rm L}$ that measures the electron mass $m^*$. We discovered two peaks in the doping dependence of $\lambda_{\rm L}^2$($T\sim$0); one at $x_{\rm M}$=0.027 where the spin-lattice relaxation rate shows quantum critical behavior, and another at $x_{\rm c}$=0.032 around which the nematic transition temperature extrapolates to zero and the electrical resistivity shows a $T$-linear variation. Our results indicate that a nematic QCP lies beneath the superconducting dome at $x_{\rm c}$ where $m^*$ is enhanced. The impact of the nematic fluctuations on superconductivity is discussed.