Optical absorption by mixed polarons. II. Application to SrTiO₃

Abstract

For pt.I see ibid., vol.17, no.4, p.637 (1984). The theory of optical absorption by mixed polarons developed in the previous paper is used to interpret results of Bursian et al. (1976) on infrared absorption for photon energies between 0.18 and 0.72 eV at temperatures of 200 and 330K in a crystal of SrTiO3 with a conductivity of 110 Omega -1 m-1. Five adjustable parameters are used in the data fitting, and four of these are permitted to vary independently with temperature. Three of the parameters are: (i) and (ii) S1 and S2, measuring the preferred net numbers of phonons to be emitted for each photon absorbed for transitions for types N to N and W to or from N, where W and N denote the parts of the mixed-polaron wavefunctions associated with wide (large-polaron) and narrow (nearly-small-polaron) bands, and (iii) mb/me, the ratio of the bare electron mass to that of the free electron. Two possibilities occur for a degeneracy factor zeta associated with the W to W contribution to the absorption, according to whether a multivalley or single-valley conduction-band structure is appropriate. Good fits to the results can be found with either model, but values of parameters for zeta =1, corresponding to the multivalleyed model, are closer to those expected. For this case, S1 approximately=10.1, S2 approximately=3.8 and mb/me approximately=1.2, at both temperatures.