The Raman spectrum of the same sample is plotted in Fig. ). The phonon density of states (DOS) of CuP 2 , as calculated by density functional perturbation theory in good agreement with recent experiments, is also shown for comparison. Since Raman spectra of bulk CuP 2 are not available in the literature, we briefly discuss some qualitative aspects here. Raman features originating from the phonon bands centered around 300 cm -1 and 450 cm -1 can clearly be seen in the experimental spectrum. In particular, the most intense Raman peak at 425 cm -1 probably arises from one of the lowestenergy phonon branches within the highest-energy band in the calculated DOS. All modes in this band essentially involve vibrations of P atoms with nearly static Cu atoms. The lower the phonon energy, the larger the contribution from Cu vibrations, as expected from the larger mass of Cu.