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

摘要: Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in KFeAgTe. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at 34.6 K. The temperature difference between these two phase transitions is 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 KFeAgTe 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.