DISPLACIVE VIBRONIC PHASE TRANSITIONS IN NARROW-GAP SEMICONDUCTORS

Displacive phase transitions are considered in small‐gap semiconductors due to the electron‐phonon interaction of two bands. The Helmholtz free energy of the system is obtained on the basis of the electron spectrum renormalized by the interaction. The temperature dependent low‐symmetry lattice distortion, renormalized frequencies of the active “soft”; optical branch in both phases, the Curie temperature, etc., are obtained. By means of the Green's function method combinative vibronic frequencies of the system are found. The dielectric susceptibility which is essentially determined by active vibrations is calculated and the general picture of similar (ferroelectric) phase transitions is discussed. Copyright © 1968 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim