材料期刊网

利用基于密度泛涵的平面波赝势方法,研究了Cs2AgF4中Jahn-Teller（JT）畸变导致的轨道有序,以及铁磁态存在的机制.通过比较各种磁性态的总能和原胞实空间的电荷分布,我们发现：因为在a-b平面内Ag-F键长存在长短不同的交替变换,所以Ag的4d电子会交替占据dz2-x2和dz2-y2轨道.这种相互正交的轨道有序态使体系稳定到铁磁态.同时还发现当增大Jahn-Teller畸变大于0.03时会使层内反铁磁成为基态,这提示人们可以通过增大压力或掺杂等方法提高Jahn-Teller畸变,从而实现层内反铁磁的Cs2AgF4,而这种层内反铁磁态有可能出现超导.

The Jahn-Teller distortion induced orbital order and optical-conductivity in Cs2AgF4 are investigated by the first-principle calculations.Cs2AgF4 contains 4d9 Ag^2＋,similar to 3d9 Cu2＋ of La2CuO4.Although Cs2AgF4 is anisoelectronic analogue of La2CuO4,their magnetism are radically different： La2CuO4 is antiferromagnetism（AFM）,while Cs2AgF4 is proposed to be ferromagnetism（FM） based on recent neutron scattering data.The great difference can be understood from the JT distortions in the AgF6 octahedron,which favors the formation of staggered orbital order of dz2-x2 and dz2-y2.Such orbital order favors FM state,according to Goodenough-Kanamori rules.We find when the JT distortion is larger than 0.03,the ground state of Cs2AgF4 is AFM.This antiferromagnetism might induce superconductivity,just as La2CuO4.In order to confirm such orbital order of Ag,we present the calculated optical-conductivity,which could be examined by the experiment.