Lattice Anharmonicity and Optical Absorption in Polar Crystals. III. Quantum Mechanical Treatment in the Linear Approximation

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Abstract
A quantum mechanical calculation is given for the effect of lattice anharmonicity on the optical constants of an ionic crystal in the reststrahl region of the spectrum. A modification of the Kubo formalism for treating transport coefficients is employed. Only cubic anharmonic terms in the Hamiltonian are considered. In agreement with the correspondence principle, the present results in the limit of high temperatures reduce to those of the classical treatment which was given in a previous paper. A comparison is made between the present calculation and that of Born and Huang. The available experimental data are discussed.