Optical and Dielectric Constants

At higher photon energies the n vs. hv curve derived from reflectivity data shows considerable structure, see Fig. 122 [5]. For a figure of the absorption index k vs. hv between 1.78 eV (=Eg) and 6eV, see [4]. 

At 660 nm, n increases linearly with decreasing temperature from n = 2.55 at 300 K to n = 2.58 at 20 K and below this temperature nonlinearly to n = 2.59 at 4.6 K (=T ) and (extrapolated) to --2.592 at 0 K. In a 14.5 kOe magnetic field n is elavated over the zero field value below 30 K n = 2.59 at 10 K, rising to the extrapolated value of 2.63 at 0 K. The refractive index at 4.2 K as a function of magnetic field shows an intermediate range of partial saturation due to change in the spin structure [1] (AF-I- ferrimagnetic transition). 

The thermal changes of the complex dielectric constants, Ae, Ae , have been obtained from Kramers-Kronig transformation of the thermally modulated reflectivity at 8 K, cf. p. 256, see figure in the paper, Mitani, Koda [11]. 

Kramers-Kronig analysis of reflectivity data between 10 and 400 cm for as-grown crystals at 4.2 K reveal no contribution to e due to free carriers. As an external field is applied (H = 46 kOe) an absorption ramp of free carriers appears below 150 cm cf. p. 251, Faymon-ville etal. [12]. 

---

EQNS (1), (2) and (3) are commonly used as the basis of the molecular interpretation of the static permittivities measured as a function of temperature (e.g. [6,12-15,20,21,28-30,36,37]) and/or pressure [30,38]. Recently, quite successful predictions of anisotropic optical and dielectric constants from molecular modelling calculations were achieved with the aid of the Maier-Meier theory [39,40]. It seems worthwhile, therefore, to analyse these equations in order to point out the weak and strong points of the theory. EQN (3) is the most convenient for this purpose (both components of the permittivity are discussed by Jadzyn et al in a recent paper [41]). The parameters N, F and h in the Maier-Meier equations vary little with temperature. Therefore, the contribution from the polarisability anisotropy Aa to A8 varies with temperature in the same way as the order parameter S, whereas that connected with the orientation polarisation varies like S/T. Especially interesting seems to be the case of constant temperature discussed in [38] where As was measured as a function of pressure, p. The discussion of the measured permittivities, Sj and the anisotropy As as a function of the order parameter S obtained from the independent experiment seems to be the best way of verifying the assumptions on which the theory is based. 

---