Since the first phonomenon of photoelectric effect was deteced by Hertz in 1887, photoemission has evolved into a more sophisticated scientific technique. However, the principle of experiment remains simple. As shown in the figure on the left, a beam of monochromic light with a sufficiently high frequency w (energy hw ) incidents onto a well-prepared solid sample, electrons inside the sample can be emitted. The energy conservation law requires
Ekin=h w - f -E B
where Ekin is the kinetic energy of an outgoing electron, h w the energy of incident photon, f the work function of the sample, and EB the binding energy of the same electron in the original band inside the sample.
The momentum conservation law is also hold for the case of a two-dimensional system. The discontinuity of the surface breaks the conservation of momentum along the normal direction. Because the momentum of photon is neiligable compared to that of the electron in a solid, the momentum of the outgoing electron is approximately equal to that of the electron inside the solid. By detecting the direction of the outgoing electron, one obtain the in-plane momentum which is same as the one inside the solid. ARPES can probe the properties of electronic structures, such as band sturcture, Fermi surface, energy gap, self energy etc.
What ARPES measures, under the sudden approximation, is closely related to the spectral function, A(k,w), which is the imaginary part of the single-particle Green's function . The Green's function describes the propagation of electrons in an interacting system. Thus ARPES can also provides information of many-body interaction in solids.
As mentioned above, ARPES on a two-dimensional system is straight and easy to understand. Since the high-Tc superconductors(HTSCs) are nearly two dimensional, the application of ARPES on HTSCs is an ideal tool. In addition, the large anisotropy of the HTSCs requires the capability of momentum-resolved measurement. The improvement of the instrumental resolutions in energy and momentum has made ARPES a powerful technique for studying of HTSCs. It has contributed significantly to the understanding of the HTSCs problem. Some of our contributions can be found here.