Anomalous phonon behavior: blueshift of the surface boson peak in silica glass with increasing temperature.

We present helium atom scattering measurements of the boson peak at the surface of vitreous silica between 127.0 and 368.5 K. The most probable energy shows a strong temperature dependence and increases linearly with temperature in the measured range. The observed blueshift of the surface boson peak (shift rate 0.008+/-0.002 meV/K) is a factor of 4 to 10 times stronger than shift rates measured in the bulk by inelastic neutron and Raman scattering. We suggest that the anomalous shift direction of the boson peak to higher energies with increasing temperature has the same origin as the unusual temperature dependence of the bulk modulus of silica glass.