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Jahn-Teller inactivity and magnetic frustration in GeCo_2O_4 probed by ultrasound velocity measurements

机译：超声速度测量探测GeCo_2O_4的Jahn-Teller失活和磁性受挫

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Ultrasound velocity measurements of cubic spinel GeCo_2O_4 in single crystal were performed for the investigation of shear and compression moduli. The shear moduli in the paramagnetic state reveal the absence of Jahn-Teller activity despite the presence of orbital degeneracy in the Co~(2+) ions. Such a Jahn-Teller inactivity indicates that the intersite orbital-orbital interaction is much stronger than the Jahn-Teller coupling. The compression moduli in the paramagnetic state near the Neel temperature T_N reveal that the most relevant exchange path for the antiferromagnetic transition lies in the [111] direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector q ‖ [ 111 ], suggesting the presence of bond frustration due to competition among a direct ferromagnetic interaction and several distant-neighbor antiferromagnetic interactions. In the Jahn-Teller-inactive condition, the bond frustration can be induced by geometrical orbital frustration of t_(2g)-t_(2g) interaction between the Co~(2+) ions, which can be realized in the pyrochlore lattice of the high-spin Co~(2+) with t_(2g)-orbital degeneracy. In GeCo_2O_4, the tetragonal elongation below T_N releases the orbital frustration by quenching the orbital degeneracy.
机译：进行了单晶立方尖晶石GeCo_2O_4的超声速度测量，以研究剪切模量和压缩模量。尽管在Co〜（2+）离子中存在轨道简并性，顺磁性状态下的剪切模量显示不存在Jahn-Teller活性。这种Jahn-Teller失活表明站点间轨道-轨道相互作用比Jahn-Teller耦合强得多。在Neel温度T_N附近的顺磁性状态下的压缩模量表明，与反铁磁性转变最相关的交换路径位于[111]方向。这种交换路径的各向异性与波矢量为​​q的反铁磁结构相一致[111]，这表明由于直接铁磁相互作用和若干远邻反铁磁相互作用之间的竞争而导致键合受挫。在Jahn-Teller惰性条件下，键的挫折可以通过Co〜（2+）离子之间的t_（2g）-t_（2g）相互作用的几何轨道挫折引起，这可以在分子的烧绿石晶格中实现。 t_（2g）-轨道简并的高自旋Co〜（2+）。在GeCo_2O_4中，T_N以下的四边形伸长通过淬灭轨道简并性来释放轨道受挫。

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