Electron Correlation Effects in the Nickel-Sulfide-Selinide System

摘要：本项目是对NiS2-x SeX系统物理特性研究的延续。随着x的增加，NiS2的强绝缘特性不断向NiS2的强金属特性转变。因此，我们可以预料到一个组成区域，在这个区域中，系统会表现出有趣的电异常。这确实是0.3 x 0.5的情况。此外，在整个x范围内，阳离子子晶格保持完整。此外，除了晶格参数随x的增加而持续增加外，晶体结构几乎没有变化。因此，该体系非常适合研究具有基本相同结构和固定阳离子价的体系中电子相关效应的变化。使用从Te熔剂中生长并经过退火以控制化学计量学的单晶，建议进行详细的x射线和中子散射研究，磁化测量，交流电导率研究，比热研究，光学反射率测量以及施加足以改变该系统中遇到的相变的压力的影响的研究。实验结果，以及根据先前的补助金获得的早期测量结果，将根据处理电子相关现象的现有理论加以合理化。人们对金属-绝缘体跃迁和相关现象的表征一直很感兴趣。NiS2Sex系统与同类中几乎所有其他系统的区别在于，通过改变阴离子亚晶格的组成，同时保持阳离子亚晶格的完整，可以实现电学性质的显著变化和近乎不连续。这是因为NiS2是很好的绝缘体而NiS2是金属；这些材料可以按各种比例混合。因此，预计在合金的某些中间成分范围内可能会遇到有趣的电异常。这确实是0.3 x 0.5范围的情况。正是在这一基础上，为开始这一领域的研究请求了经费。上述变化发生在相对复杂的相图中，当材料随着温度的变化经历各种相变时，晶体结构没有明显变化。任何晶体学的变化，可能成为检测非常仔细的研究，致力于这个主题，不太可能在各种电子相变中发挥重要作用，这是提出的研究的对象。因此，人们可以研究改变电子相关性的影响，而无需大规模伴随结构改变或阳离子亚晶格阵列中任何成分变化的复杂性。寻找和研究显示从弱相关电子态到强相关电子态跃迁的单价系统是很重要的，这个问题通常被认为是至关重要的。建议的系统满足这些要求。

Abstract 9612130 Honig This project is the continuation of research completed under terms of the predecessor grant on the physical characteristics of the NiS2-x SeX system. As x is increased there is a continuing shift away from the strongly insulating characteristics of NiS2 toward the strongly metallic properties of NiSe2. One thus anticipates a compositional region in which the system exhibits interesting electrical anomalies. This is indeed the case for 0.3 x 0.5. Further, over the entire x range, the cation sublattice is left intact. Moreover, there is almost no change in the crystal structure except for the continuing increase of lattice parameter with x. Thus, the system lends itself very well to a study of changes in electron correlation effects in a system with essentially the same structure and with fixed valency of the cation. Using single crystals that have been grown from a Te flux and annealed for control of stoichiometry, it is proposed to conduct detailed X-ray and neutron scattering studies, magnetization measurements, AC conductivity investigations, specific heat studies, optical refectivity measurements, and studies on the effects of applying pressures sufficient to alter the phase transformations that are encountered in this system. The experimental results, as well earlier measurements obtained under terms of the predecessor grant, are to be rationalized in terms of existing theories dealing with electron correlation phenomena. %%% There is continuing interest in the characterization of metal-insulator transitions and related phenomena. What sets the NiS2Sex system apart from almost all others in the category is the remarkable alterations and neardiscontinuities in electrical properties that can be achieved by changes of composition in the anion sublattice while keeping the cation sublattice intact. This comes about because NiS2 is a very good insulator whereas NiSe2 is a metal; these materials can be mixed in all proportions. T hus, it is anticipated that interesting electrical anomalies might be encountered over some intermediate composition range of the alloy. This is indeed the case for the range 0.3 x 0.5. It was on this basis that funding was requested for initiation of research in this area. The above variations occur in the absence of significant alterations in crystal structure when the material passes through various phase transitions with changing temperature in the relatively complex phase diagram. Any crystallographic changes that might become detectable in very careful studies devoted to this topic are not likely to play a significant role in the various electronic phase changes that are the object of the proposed study. One can thus investigate the effect of changing electron correlations without the complications of large scale concomitant structural alterations or of any compositional changes in the cation sublattice array. It is important to search for and study singlevalent system that display transitions from a weakly to a strongly correlated electron regime - a problem that is generally conceded to be of fundamental importance. These requirements are met by the proposed system.