Thermal Effects in the Inverse Photoemission of Ni(110) Surface

We have studied the influence of thermal effects on the unoccupied electronic structure of Ni(110) surface using k-resolved inverse photoemission spectroscopy. The temperature dependence of the peak intensity has been measured for both the bulk and surface states. Our results can be successfully fit with the Debye-Waller model in the temperature region where the surface atoms undergo harmonic vibrations. We find the minimum value of the characteristic temperature (T_c) of ~ 600 K for the crystal-derived surface state of Ni(110). This surface state is located inside the projected bulk band gap in the ΓY direction of the surface band structure and reveals a free-electron-like energy dispersion with m*/m_e = 0.5± 0.1, where the vertex of its parabola is situated at 2.3 ± 0.1 eV above the Fermi level. The above reported minimum T_c value is found to be at the center of the projected bulk band gap. The characteristic temperature increases from this minimum as the surface state approaches the bulk band boundary. We attribute this to the surface state gaining bulk-like character near the boundary.