Study of Low-Temperature Excitation and Phase Transition in the Frustrated Systems by Nuclear and Electronic Heat Capacities.

通过测量核热容量和电子热容量以及磁化率，获得了受挫磁系统的基态和相变信息。在低温下，VX_2（X:Br,Cl）的自旋s=3/2时，自旋还原率估计为40%。这一数值与时间尺度为10^<-13>秒数量级的核衍射实验所得的70%的还原值相比，有很大的不同。揭示了系统和动力学观测是理解受挫系统基态的必要条件。期望在共振价键态的费米液体的热容量与低温的线性关系。然而，VX_2的热容不显示T^1项。对于s=1/2的受挫系统，可以进行关键的检查。在LiNiO_2的热容测量中，不仅观察到热容的线性关系，而且还观察到相变的任何痕迹：磁化率测量表明，在T_<N1>=240K和T_<N2>=15K处可能发生连续相变，这是三角形反铁磁体的一个特征。在最近的超导性研究中，强调共振价键态在具有电子空穴的CuO_2平面中起重要作用。我们也开始研究有机自由基晶体中的共振价键态，与具有三维电子的无机化合物相比，它有一些显著的特点。本文首先给出了未配对电子体系三苯甲醛的磁性行为。

Information of the ground state and phase transition in frustrated magnetic systems has been obtained by the measurements of nuclear and electronic heat capacity, and magnetic susceptibility.The spin reduction which reflects the magnetic phase at low temperatures is estimated to be 40% for spin s=3/2 of VX_2(X:Br,Cl). This value makes a significant difference when compared with the reduction of 70% obtained from the nuclear diffraction experiment with the time scale of the order 10^<-13> sec.. It is revealed that systematic and dynamical observations are necessary for understanding the ground state of frustrated systems.The linear dependence of the heat capacity on low-temperatures is expected for the Fermi liquid in the resonating valence bond state. The heat capacity of VX_2, however, does not show the T^1 term. The crucial check of this is possible with the frustrated system with s=1/2. Not only the linear dependence of the heat capacity, but also any trace of the phase change is observed in the heat capacity measurements for LiNiO_2: the magnetic susceptibility measurements indicate the possibility of the successive phase transitions at T_<N1>=240K and T_<N2>=15K, which is a feature of the triangular antiferromagnet.In the recent study of the superconductivity, the resonating valence bond state is stressed to play an important role in the CuO_2 plane with the electronic holes. We have begun investigating the resonating valence bond state also in the crystal of organic free radical which has some remarkable points when compared with inorganic compounds with the 3d electron. The magnetic behavior of the unpaired electron system ,triphenylverdazyi, is given as the first example.