The physical properties of materials are dictated by their electrons, especially the interaction of electrons with other entities like other electrons, phonons, magnons, impurities and etc. In order to understand the unique properties of advanced materials, a good understanding of the electronic states (energy, momentum and spin) is a prerequisite. Angle-resolved photoemission spectroscopy (ARPES) is the most direct method to detect the electronic structures in materials and it has become a most powerful tool to directly probe many-body effects in correlated systems, for example, in cuprate superconductors.

ARPES is basically based on the photoelectric effect originally observed by Hertz and later explained by Einstein. When light is incident on a sample, an electron can absorb a photon and escape from the material with a maximum kinetic energy hν-Ф (hν is the photon energy andФ is the material work function).

During the photoemission process, the electrons are governed by the following momentum and energy conservation laws, thus we can obtain the information about the initial state of the electrons in the materials by analyzing the energy and angle of the photoelectrons.


Here, Ekin is the kinetic energy , EB is the binding energy, ħK|| is the component parallel to the surface of the electron crystal momentum.

Figure 1 shows a typical ARPES setup, that the photoelectrons from the single crystal surface are captured by an analyzer which can detect the energy and angle of the electrons.


Fig.1

To the physics point of view, the absorption of a photon by an electron, the photoemission, and the detection of the photoelectrons should be considered as a whole process, which is called "one step model". However, it is too complicated to calculate them as a whole. In practice, people use "three step model" to deal with the problem. This model, although phenomenological, has proven to be successful. Within this approach, the whole process has been divided into three steps:
1. Optical excitation of the electron in the bulk.
2. Travel of the excited electron to the surface.
3. Escape of the photo electron into vacuum.

It is obvious that the first step contains the key information of the photoemission. Under "Sudden approximation" which regards the photoemission process as "sudden" and the system has not been relaxed, the N- particle final state can be simplified as a combination of the photoelectron and N-1 particle. Then the photoemission intensity can be written as:


Which is proportional to the electron self-energy A(k,ω ). This is why we can use ARPES to study the electron self-energy through which many body effects can be analyzed.

To do ARPES people first need a chamber with ultra-high vacuum (10-12~10-11mbar) where the measurement can be carried out. Otherwise, the photoelectrons will be scattered by the atoms in air which makes the accurate detection of their momentum impossible. The ultra-high vacuum is also critical for the sample surface to stay fresh. Then, people need a good analyzer. VG scienta has made a lot of progress on the development and improvement of the electron analyzer, and the ones in our lab are the state-of-the-art analyzers which are newly developed by scienta, named DA30, R8000, R4000 etc. Another critical part of ARPES system is the light source. Based on different light sources, ARPES systems can be mainly divided into three categories: Synchrotron ARPES, Gas discharge lamp ARPES, and Laser ARPES.

Laser ARPES is a most recent developed technique which has the highest resolution among all the ARPES systems. Our group is working at the cutting edge of the laser ARPES. We also have many different kinds of gas lamps.

References:
  • S. Huefner, Photoelectron Spectroscopy-Principles and Applications, Third edition, Springer (2003).
  • A. Damascelli, Z. Hussain and Z.-X. Shen, Review of Modern Physics 75 (2003) 473.
  • J. C. Campuzano, M. R. Norman and M. Randeria , In "Physics of Superconductors", Vol. II, edited by K. H. Bennemann and J. B. Ketterson (Springer, Berlin, 2004), Page167-273.
  • X. J. Zhou, T. Cuk, T. Devereaux, N. Nagaosa and Z.-X. Shen, in “Handbook of High Temperature Superconductivity—Theory and Experimet", edited by J. Robert Schrieffer, (Springer, Berlin, 2007), Page 87-144.