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Abstract. In the limit of strong electron-phonon coupling, we provide a unified insight into the stability criterion for bipolaron formation in low-dimensionally confined media. The model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an anisotropic parabolic potential box, whose barrier slopes can be tuned arbitrarily from zero to infinity. Thus, encompassing the bulk and all low-dimensional geometric configurations of general interest, we obtain an explicit tracking of the critical ratio ηc of dielectric constants below which bipolarons can exist. PACS. 71.38.+i Polarons and electron-phonon interactions

The Faraday rotation (FR) of cerium substituted yttrium iron garnets single crystals Ys,C+Fe>On (YIG:Ce) with x=0.055 has been investigated at 1152 and 632.8 nm in the 6 K-600 K temperature range and magnetic field up to 20 kOe applied along the [Ill] and [loo] directions. In contrast to magnetic roperties, the FR of MG.Ce is strongly different fiom FR of pure MG at both wavelengths. The FR contribution of the Cegf ions, mainly attributed to intra-ionic electric dipole transitions between the 4P and 4P 5d1 configurations, is discussed within the fiame of the one-ion model of fehagnetic gamets.

Linear birefringence of iron garnet bulk crystals were investigated at a wavelength A=1.151!m and at room temperature. In Bi-substituted iron garnet c!yst~ls, the linear birefringence was composed of the magnetic linear b~refnngence (MLB) and some met astable birefringence. The metastable buefringence was suppressed by annealing, and was attributed to the growth-mduced. birefringence. The notable growth-induced birefringence was not observed In Bi-free iron garnet crystals. These results indicate that Bi ion plays an. impo!ta~t role to induce large growth-in~uced birefringence. Effect of the BI-substltutlOn on the MLB effect was also lOvestigated. The anisotropy parameter of the MLB effect increased with amount of Bi.

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