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

本文通过对核热容、电子热容和磁化率的测量，获得了阻挫磁系统的基态和相变信息，对自旋s=3/2的VX_2（X：Br，Cl），估算了反映低温下磁相的自旋约减为40%。与从时间尺度为10 μ sec的核衍射实验中获得的70%的减少相比，该值具有显著的差异<-13>。结果表明，系统和动力学观测是理解阻挫体系基态的必要条件，对于共振价键态的费米液体，热容与低温之间存在线性关系.然而，VX_2的热容并不显示T^1项。对于s=1/2的受抑系统，这一点的关键检验是可能的。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>= 240 K和T_<N2>= 15 K处可能存在连续相变，这是三角形反铁磁体的一个特征。在CuO_2平面中，电子空穴的共振价键态起着重要作用。我们也开始研究有机自由基晶体中的共振价键态，它与具有3d电子的无机化合物相比有一些显著的特点，第一个例子是不成对电子体系三苯基Verdazyi的磁性。

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.