Study on strongly correlated electron systems by means of angle-resolved magnetization measurements at low temperatures

1.研制了一种能在0.5K以下低温下连续旋转磁场方向的新型磁强计。利用这台磁强计，我们研究了反铁四极有序系统PrPb_3的相图，得到了相变温度的场角变化。根据计算结果，我们可以确定四极相互作用各向异性的序参量。2.我们研制了一种能在0.5 K以下的低温下连续旋转磁场方向的新型量热计。利用该热量计，我们研究了各向异性超导体CeCoIn_5在超导混合态下的低温比热。我们观察到一个明确的四倍角振荡的电子比热，表明d波超导。从比热最小值的角位置，得出了d_xy型带隙对称性的结论。3.研究了自旋冰化合物Dy_2 Ti_2 O_7在（111）磁场中的磁化过程。我们观察到一个明确的磁化高原在戈薇冰态，其次是一个一阶过渡到饱和状态。研究了方钴矿化合物PrOs_4Sb_（12）的低温场-温度相图，发现在所有的主场方向上都存在场诱导有序。结果表明该体系的基态为γ_1单重态。

1.We developed a new magnetometer that can rotate the field direction continuously at low temperatures below 0.5 K. Using this magnetometer, we examined the phase diagram of an antiferro-quadrupole ordering system PrPb3 and obtained the field-angle variation of the transition temperature. From the results, we could identify the order parameter, which requires the quadrupole interaction to be anisotropic.2.We developed a new calorimeter that can rotate magnetic field direction continuously at low temperatures below 0.5 K. Using this calorimeter, we investigated the low temperature specific heat of the anisotropic superconductor CeCoIn5 in the superconducting mixed state. We observed a clear fourfold angular oscillation of the electronic specific heat, indicating d wave superconductivity. From the angular position of the minima in the specific heat, the d_xy type gap symmetry is concluded.3.Magnetization process of a spin-ice compound Dy2Ti2O7 has been examined in a (111] magnetic field. We observed a clear magnetization plateau in the kagome ice state, followed by a first-order transition to a saturated sate. The transition is found to be of a liquid-gs type, which is new for the localized spin systems.4.Field-temperature phase diagrams of the skutterudite compound PrOs4Sb12 has been examined at low temperatures and a field-induced ordering is observed for all the principal field directions. The results indicate the ground state of this system to be a Gamma_1 singlet.