Direct Correlation of Phonon Anomalies in NbC with Fermi-Surface-Induced Maxima in Generalized Susceptibilities

The generalized susceptibility of NbC, determined from its augmented plane wave energy band structure (with constant matrix elements), shows large maxima, arising in large part from Fermi surface "nesting," to occur at precisely those →q values at which soft modes have been found in the phonon acoustic branches by Smith. These maxima can be represented by a warped cube of dimension ~1.2(2πa) in momentum space, in striking agreement with the soft mode surface proposed by Weber.