Optical constants of copper and nickel as a function of temperature

The optical constants were determined for copper and nickel from reflection and transmission measurements on vacuum-evaporated thin films, in the spectral range 0.5-6.5 eV and at temperatures of 78, 293, and 423 K. The imaginary part of the dielectric constant was nearly independent of temperature for nickel, but for copper it increased with temperature in the intraband region below 2 eV and decreased above 4 eV in the interband region. Interpretation of the increase below 2 eV according to the Drude free-electron expression suggests a temperature and frequency dependence of the relaxation time, which is not completely explained. The thermal behavior in the interband region can be largely understood, however, if the zero-temperature theory of Williams, Janak, and Moruzzi is modified by including a Debye-Waller factor in transitions between nearly-free-electron-like bands.