Theory of induced quadrupolar order in tetragonal YbRu2Ge2

The tetragonal compound YbRu2Ge2 exhibits a nonmagnetic transition at T-0=10.2 K and a magnetic transition at T-1=6.5 K in zero magnetic field. We present a model for this material based on a quasiquartet of Yb3+ 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 ferroquadrupole order of either O-2(2)- or O-xy-type are prime candidates for the nonmagnetic transition. Considering the matrix element of these quadrupole moments, we show that the lower CEF states of the level scheme consist of a Gamma(6) and a Gamma(7) doublet. This leads to induced type of O-2(2) and O-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.