Brillouin scattering study of lattice-stiffness changes due to ion irradiation: Dramatic softening inNb3Ir

We demonstrate a novel approach for studying progressive disorder which couples the versatility of charged-particle irradiation with subsequent measurements of elastic properties using Brillouin scattering. Irradiated films of ${\mathrm{Nb}}_3$Ir, which remain crystalline, show a dramatic reduction in an average shear elastic constant of up to 40%. The large softening of this long-wavelength shear mode is difficult to explain with the conventional resonant-mode defect model used in simple metals and alloys. However, in qualitative agreement with existing theoretical models, the softening shows a strong correlation with the measured electronic density of states at $E_F$, which changes significantly with irradiation dose.