稀土化合物Faraday效应的理论研究

A THEORETICAL STUDY OF THE FARADAY EFFECT IN RARE EARTH COMPOUNDS

以ＣｅＦ３为例，研究了晶场强度及自旋─轨道耦合对Ｖｃｒｄｅｔ常数（Ｖ）的影响，发现晶场强度在很大的范围内变化时，若温度Ｔ＞２００Ｋ，Ｖ基本上保持不变，若Ｔ＜１００Ｋ，随晶场增强，Ｖ的绝对值缓慢增大。当晶场增强到一定值时，Ｖ出现共振现象，其值显著增大并突然变号。还发现，自旋─轨道耦合强度的变化对Ｖ影响很小。当晶场或自旋─轨道耦合不存在时，Ｖ值并不趋于零，而是一有限值，这表明晶场和自旋─轨道耦合不是决定Ｆａｒａｄｎｙ旋转存在与否的关键因素。

Centering around the case of CeF3 crystals, the paper presents a study of the effect of the crystal field and the spin-orbit coupling on the Verdet constant (V). It is found that, at and above 200K, as the strength parameter of the crystal field Dq varies in a large region, the absolute value of the V nearly keeps unchanged and it reduces slowly as the Dq increases at and below 100K. However, when the Dq increases beyond a certain value, there occurs a resonance phenomenon. Owing to the resonance dispersion effect, V varies rapidly and it's sign changes suddenly. This feature can be explained with the factor ω2/(ω2g -- ω2). At and above 200K, when the value of the strength of the spin-orbit coupling of the 4f configuration of a Ce3+ ion ζ4f varies in a large rigion the absolute value of the V nearly keeps unchanged.However, when T is below 100K, it increases as ζ4f approaches zero. The relation between the V and ζ5d, which has similar meaning as ζ4f, is the same as the relation between the V and the ζ4f. Above discussion about the dependance of the V on the spin--orbit coupling strength is under the condition that the crystal field parameters are obtained with the point charge model,which are near to the actual vaines. However when the value of Dq is very forge, resonance phenomenon may occur as ζ5d increases. This feature can also be explained with the factor ω2/(ω2g--ω2).We have discovered that the value of V never approaches zero when Dq, ζ4f, or ζ5d approaches zero. This result is important. Our calculation also shows that both the crystal field and the spin-orit coupling are not the key factor in the occurrence of the Faraday rotaion.