First-order preemptive Ising-nematic transition in K5Fe4Ag6Te10
Employing inelastic x-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K5Fe4Ag6Te10. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a magnetic transition at TN≈34.7K that is consistent with a second-order transition. The temperature difference between these two phase transitions is ∼1.0(8) 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 K5Fe4Ag6Te10 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.