PHONON DENSITY OF STATES

The results of the neutron measurements and calculations for the phonon density of states are summarized in Fig.1 (click here to see it above).

The top curve (black) is the generalized phonon density of states for MgB₂ determined from inelastic neutron scattering measurements. The bottom curve (blue) is the calculated DOS.

The experimental data indicate two bands of phonons, one below 40 meV corresponding primarily to the acoustic phonon modes, and one above 50 meV that mostly involves the boron motions. The high-energy portion of the spectrum shows a sharp cutoff at about 100 meV.

We also have measured the temperature dependence of the spectrum and find no substantial changes in the basic features from 7 K up to 200 K, while a very modest softening of some of the modes was observed in going to 325 K.

In order to understand the origin of the features observed in the neutron data, we have carried out detailed first-principles calculations of the electronic band structure and lattice dynamics.

The calculated density of states obtained from these calculations is shown in Fig.1 and we see that the agreement between experiment and theory for the energies of the modes is excellent. The agreement for the intensities is also good considering that the observed spectrum is a weighted density of states rather than the actual DOS, and the wave vector averaging may include some coherency effects.