Charge Density Fluctuations with Enhanced Superconductivity at the Proposed Nematic Quantum Critical Point

TL;DR

This study reveals persistent charge density fluctuations near a nematic quantum critical point in a superconductor, suggesting charge fluctuations can enhance superconductivity independently of magnetism.

Contribution

It demonstrates charge density fluctuations at a nematic QCP in a non-magnetic superconductor, highlighting their potential role in boosting superconductivity.

Findings

Charge density fluctuations persist down to 15 K.

CDF softens completely at 25 K with critical behavior similar to antiferromagnetic Fermi liquids.

Nematic fluctuations are not driven by lattice softening, as shown by phonon measurements.

Abstract

A quantum critical point (QCP) represents a continuous phase transition at absolute zero. At the QCP of an unconventional superconductor, enhanced superconducting transition temperature and magnetic fluctuations strength are often observed together, indicating magnetism-mediated superconductivity. This raises the question of whether quantum fluctuations in other degrees of freedom, such as charge, could similarly boost superconductivity. However, because charge is frequently intertwined with magnetism, isolating and understanding its specific role in Cooper pair formation poses a significant challenge. Here, we report persistent charge density fluctuations (CDF) down to 15 K in the non-magnetic superconductor Sr$_{0.77}$Ba$_{0.23}$Ni$_{2}$As$_{2}$, which lies near a proposed nematic QCP associated with a six-fold enhancement of superconductivity. Our results show that the quasi-elastic CDF does not condense into resolution-limited Bragg peaks and displays non-saturated strength. The CDF completely softens at 25 K, with its critical behavior described by the same mathematical framework as the antiferromagnetic Fermi liquid model, yielding a fitted Curie-Weiss temperature of $\theta \approx 0$ K. Additionally, we find that the nematic fluctuations are not lattice-driven, as evidenced by the absence of softening in nematic-coupled in-plane transverse acoustic phonons. Our discovery positions Sr$_{x}$Ba$_{1-x}$Ni$_{2}$As$_{2}$ as a promising candidate for charge-fluctuation-driven nematicity and superconductivity.