Nature of the Antiferromagnetic and Nematic Transitions in Sr$_{1-x}$Ba$_x$Fe$_{1.97}$Ni$_{0.03}$As$_2$

摘要

We have systematically studied the antiferromagnetic and nematic transitionsin Sr$_{1-x}$Ba$_x$Fe$_{1.97}$Ni$_{0.03}$As$_2$ by magnetic susceptibility anduniaxial-pressure resistivity measurements, respectively. The derivatives ofthe temperature dependence of both magnetic and nematic susceptibilities showclearly sharp peak when the transitions are first-order. Accordingly, we showthat while both of the magnetic and nematic transitions change from first-orderto second-order with increasing Barium doping level, there is a narrow dopingrange where the former becomes second-order but the latter remains first-order,which has never been realized before in other systems. Moreover, theantiferromagnetic and nematic transition temperatures become different and thejump of nematic susceptibility becomes small in this intermediate doping range.Our results provide key information on the interplay between magnetic andnematic transitions. Concerning the current debate on the microscopic modelsfor nematicity in iron-based superconductors, these observations agree with themagnetic scenario for an itinerant fermionic model in excellent details.