ENHANCED PLASMON-LOSS EMISSION ALONG AL[011] CHAINS UPON HEATING

The Mg K-alpha-excited Al 2s core line and the subsequent inelastic plasmon-loss peaks have been measured as a function of temperature in an angle-resolved photoemission experiment. Along the [011] direction and along a lower-symmetry direction of an Al(001) crystal, the plasmon frequency decreases with increasing temperature, which is explained with the thermal expansion of Al. In contrast to the elastic Al 2s line, the plasmon peak intensities along [011] increase upon heating. This anomaly is interpreted as evidence for a temperature-induced destruction of the defocusing along high-density atomic chains.