INELASTIC NEUTRON STUDIES OF THE LOW-ENERGY PHONON EXCITATIONS IN THERENI2B2C SUPERCONDUCTORS (RE = LU,Y,HO,ER)

We studied the low-energy phonon excitations for wave vectors close to the Fermi surface nesting vector xi(m) congruent to 0.55a. We find th at above T-c the frequencies of the Delta(4)[zeta 00] lowest-lying opt ical and acoustic phonon modes decrease with decreasing temperature, f or xi close to xi(m), and there is a shift of intensity from the upper to the lower mode, an effect characteristic of coupled modes. From ap proximately 120 K down to temperatures in the vicinity of T-c, only a single unresolved peak is observed. Below T-c, the phonon spectra of t he Y and Lu compounds change dramatically: they consist of a sharp pea k at approximately 4.5 meV with a weak shoulder at the higher-energy s ide. No such sharp peak was observed below T-c in the Ho and Er compou nds. (C) 1998 Elsevier Science B.V. All rights reserved.