First Order Preemptive Ising-nematic Transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$

TL;DR

This study reveals a unique preemptive Ising-nematic transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$, where nematic order undergoes a first-order transition before magnetic ordering, highlighting a novel quasi-two-dimensional behavior.

Contribution

We identify and characterize a first-order preemptive nematic transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$, the first such observation in iron-based materials with a nematic tricritical point.

Findings

Nematic transition is strongly first-order upon cooling.

Magnetic transition occurs second-order at ~34.6 K.

Distinct temperature gap (~1 K) between nematic and magnetic transitions.

Abstract

Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference between these two phase transitions is $\sim$ 1 K. The observed phenomenon can be attributed to a distinctive first-order preemptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies establish K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$ as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling.