[Paper Review] NMR evidence for inhomogeneous nematic fluctuations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

A. P. Dioguardi, T. Kissikov+8 more|arXiv (Cornell University)|Oct 5, 2015

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This study provides direct NMR evidence for inhomogeneous nematic fluctuations in P-doped BaFe2(As1-xPx)2, showing that quadrupolar fluctuations of the As nuclear spin contribute to spin-lattice relaxation beyond magnetic fluctuations. The stretched-exponential relaxation and enhanced 75As T1−1 at Ts indicate glassy nematic dynamics, suggesting universal nematic fluctuations in iron-based superconductors.

ABSTRACT

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe$_2$As$_2$ single crystals. Both the $^{75}$As and $^{31}$P sites exhibit stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a universal phenomenon in the iron-based superconductors.

Motivation & Objective

• To investigate the origin of anomalous spin-lattice relaxation in P-doped BaFe2(As1-xPx)2 using NMR on 75As and 31P sites.
• To determine whether nematic fluctuations—beyond magnetic spin fluctuations—contribute to relaxation at the 75As site via coupling to its quadrupolar moment.
• To assess the universality of glassy nematic dynamics by comparing results with Co- and Cu-doped systems.
• To examine the role of local disorder and inhomogeneous strain in driving inhomogeneous relaxation dynamics.

Proposed method

• Performed 75As and 31P NMR measurements on single crystals of BaFe2(As1-xPx)2 at x = 0.26 and x = 0.33.
• Analyzed spin-lattice relaxation rates (T1−1) and hyperfine fields to distinguish magnetic vs. quadrupolar contributions.
• Used stretched-exponential relaxation functions to model inhomogeneous dynamics in relaxation rates.
• Compared relaxation ratios T1−1(75As)/T1−1(31P) to isolate contributions from nematic fluctuations to As relaxation.
• Applied theoretical modeling based on spin-fluctuation theory and susceptibility scaling to interpret relaxation behavior.
• Evaluated the role of local electric field gradient (EFG) fluctuations via quadrupolar coupling to nematic order.

Experimental results

Research questions

• RQ1 Do nematic fluctuations contribute to 75As nuclear spin-lattice relaxation beyond magnetic hyperfine coupling?
• RQ2 Why is the 75As relaxation rate enhanced relative to 31P despite similar magnetic hyperfine fields?
• RQ3 Is the observed inhomogeneous relaxation in P-doped BaFe2(As1-xPx)2 consistent with glassy nematic dynamics?
• RQ4 How does isovalent P-doping affect the universality of nematic fluctuations compared to electron or hole doping?
• RQ5 What is the origin of the stretched-exponential relaxation behavior in both 75As and 31P sites?

Key findings

• The 75As spin-lattice relaxation rate exhibits a maximum in (T1T)−1 at the structural transition temperature Ts, indicating strong coupling to nematic fluctuations.
• The ratio T1−1(75As)/T1−1(31P) exceeds predictions from magnetic fluctuations alone, demonstrating a significant contribution from quadrupolar fluctuations due to nematic order.
• Both 75As and 31P sites show stretched-exponential relaxation, indicating inhomogeneous dynamics consistent with glassy nematic fluctuations.
• The relaxation behavior is unchanged by isovalent P-doping, suggesting that electronic frustration—not chemical disorder—drives the glassy dynamics.
• Nematic fluctuations persist above Ts without a clear phase transition, consistent with critical fluctuations in the tetragonal phase.
• The results support a universal role for nematic fluctuations in iron-based superconductors, with implications for the superconducting pairing mechanism.