Shozo Takeno and intrinsic localized modes

Abstract

Abstract:Research into defect lattice dynamics played an important roll in the development of our work on the local mode properties of anharmonic lattices. Using a historical perspective I show that the different theoretical and experimental studies of the impurity-induced absorption associated with the low frequency resonances in alkali halide crystals provided the kernel for our breakout into a new area.Key Words: defect induced absorption, anharmonicity, intrinsic localized modes, two elastic configurations 1. Some research at the laboratory of atomic and solid state physics in 1965When Shozo Takeno arrived at Cornell in 1965 the properties of localized vibrational modes in the acoustic spectrum of ionic crystals were being uncovered and represented one of the main laboratory thrusts. The experimental discovery by Schaefer in 1958 that a hydrogen ion substituted into a KCl crystal generated a sharp IR absorption line associated with a vibrational mode far above the plane wave phonon spectrum [1] illuminated the pathway for experimentalists to enter a field where there had been a great deal of theoretical work over the years but no experiments. This mode was strongly localized in space because of its large frequency separation from the plane wave spectrum, a result expected from the dynamical theory of lattices [2,3]. It quickly became apparent that there were very few different kinds of impurities that produced local modes so attention turned to the question of how impurities might alter the acoustic spectrum. Thermal conductivity was the experimental probe of choice at Cornell since theory suggested that impurities would scatter acoustic waves. By measuring the temperature dependence of the thermal conductivity over a large temperature interval it was discovered that, in addition to the expected Rayleigh-type scattering from the impurities, there was another contribution indicating the presence of a phonon active impurity mode that produced resonance scattering in some defect-lattice combinations [4]. Since it had been assumed that such resonances observed in the thermal conductivity temperature dependence were not IR active a complete surprise occurred when, with a far infrared (FIR) spectroscopic technique, it was discovered that KI:Ag + had a sharp absorption line at 17.3 cm −1 , far down in the acoustic spectrum [5]. Unlike the H − ion impurity mode found by Schaefer this impurity-induced vibration was in direct contact with the crystal phonon modes and such resonant 2

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