ARPES Method

Angle Resolved Photoemission Spectroscopy (ARPES) is a most powerful and direct method to study the ectronic structure of a material. Recent improvement of energy and angular resolution has made ARPES a leading technique as far as the experimental determination of the single particle excitations in solids is concerned.

The very idea of the technique is based on the photoelectric effect. Monochromatic light of known energy (hn) impinges on the sample surface and those electrons which have absorbed incident photons can be excited from the initial state with energy E_i to a final state with energy E_f. If the frequency of the light is high enough electron gains the energy which is sufficient to escape from the solid. Such an electron is called then a photoelectron. With a certain degree of approximation one can write:

E_i = hn - f - E_kin

where E_kin is the kinetic energy of an outgoing photoelectron, hn - the energy of incident photon, f - the work function of the analyzer. Thus, analyzing the kinetic energies of the emitted electrons one can gather information about the energy distribution of the electronic states in the studied material. If, in addition, one measures such distribution for the various emission angles we obtain the information about the electron's momentum. It is generally believed that the parallel component of the electron's momentum is conserved during the photoemission process:

k_||=(2mE_kin)^1/2sinQ

Therefore ARPES is especially attractive in application to quasi-2D materials since in this case direct and complete experimental determination of the band structure becomes possible.