Point-Contact Spectroscopy of Heavy Fermion Systems at Low Temperature.

点接触光谱学目前已成为研究块状材料微观激发或电子态密度的有力工具。科学研究补助金(B) 1进行了以下三点。考虑到点接触光谱数据的再现性和可靠性，完成了低温点接触光谱仪。采用浮区法制备了多种高质量的六硼化稀土单晶。与其他机构就这些样品进行了许多联合工作。我们的点接触实验的主要结果如下：(1)实验得出障壁型隧道与纯金属点接触隧道存在明显差异。(2)用金属点接触光谱法测定了价态波动化合物SmB_6的间隙。(3)点接触光谱分析表明，密集的CeB_6化合物存在比近藤温度大一个数量级的特征能。(4)低温下CeB_6在磁场作用下的电流-电压特性出现了电压跳变。我们现在正在研究它与密集的近藤材料中的近藤间隙的相关性。

Point-contact spectroscopy is now becoming a powerful tool for the investigation of the microscopic excitation or the electronic density of states of the bulk-materials.Following three points were proceeded by the Grant-in-Aid for Scientific Research (B).1. A point-contact spectrometer for low temperature was completed taking into account the reproducibility and the reliability of the data of point-contact spectroscopy.2. Various kinds of high quality single crystals of rare earth hexaborides were grown by the floating zone method. Many joint works with other institutions were carried out on these samples.3. Main results of our point-contact experiments are as follows. (1) It was concluded experimentally that there is a obvious difference between the barrier-type tunneling and the pure metal point-contact. (2) We decided the gap of valence fluctuating compound SmB_6 by the metal point-contact spectroscopy. (3) It was revealed by the point-contact spectroscopy that there should be some characteristic energy which is one order of magnitude larger compared with the Kondo temperature in the dense-Kondo CeB_6 compound. (4) We observed a voltage jump in the current-voltage characteristic of CeB_6 at low temperature under the magnetic field. We are now investigating the correlation of it with the Kondo gap in dense Kondo material.