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arXiv · 0708.2872

Theory of induced quadrupolar order in tetragonal YbRu_{2}Ge_{2}

Tetsuya Takimoto·Peter Thalmeier
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Abstract

The tetragonal compound YbRu$_{2}$Ge$_{2}$ exhibits a non-magnetic transition at $T_0$=10.2K and a magnetic transition at $T_1$=6.5K in zero magnetic field. We present a model for this material based on a quasi-quartet of Yb$^{3+}$ crystalline electric field (CEF) states and discuss its mean field solution. Taking into account the broadening of the specific heat jump at $T_0$ for magnetic field perpendicular to [001] and the decrease of $T_0$ with magnetic field parallel to [001], it is shown that ferro-quadrupole order of either O$_{2}^{2}$ or O$_{\rm xy}$ - type are prime candidates for the non-magnetic transition. Considering the matrix element of these quadrupole moments, we show that the lower CEF states of the level scheme consist of a $Γ_{6}$ and a $Γ_{7}$ doublet. This leads to induced type of O$_{2}^{2}$ and O$_{\rm xy}$ quadrupolar order parameters. The quadrupolar order introduces exchange anisotropy for planar magnetic moments. This causes a spin flop transition at low fields perpendicular [001] which explains the observed metamagnetism. We also obtain a good explanation for the temperature dependence of magnetic susceptibility and specific heat for fields both parallel and perpendicular to the [001] direction.

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Tetsuya Takimoto, Peter Thalmeier. 2008-01-14. Theory of induced quadrupolar order in tetragonal YbRu_{2}Ge_{2}. https://doi.org/10.1103/physrevb.77.045105

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