100 Questions from Fermion (Issue 11)

ARPES is an experiment where light enters and electrons exit, and what is detected is the energy and momentum of the emitted photoelectrons. ARPES itself can achieve measurements at different temperatures, and the general measurement range is from liquid helium temperature to 400k. Measuring ARPES at low temperatures is mainly to obtain a relatively high electron energy resolution: due to thermal fluctuations, electrons at finite temperatures will have an electron energy broadening that is proportional to the temperature. Therefore, the lower the temperature, the smaller the thermal broadening, and the higher the resolution of the corresponding ARPES spectral line. As shown in the figure below.

30.Why does the ARPES cavity need magnetic shielding?
As an experiment of light in and electrons out, the kinetic energy and angle of the emitted electrons are detected. ARPES uses ultraviolet light, and the photon energy is relatively low, so the kinetic energy of the emitted photoelectrons is relatively small, and such photoelectrons are easily disturbed by electric and magnetic fields. The ARPES measurement cavity uses a μ metal shielding layer to shield the influence of the external geomagnetic field.
Therefore, magnetic materials cannot be used near the sample stage. Although stainless steel is a low-magnetic material, it will increase its magnetism at low temperatures, so it is generally best to avoid using stainless steel parts near the sample stage.

31.Why can't the ARPES sample holder head parts (such as the sample stage) be completely insulated?
ARPES is an experiment in which light enters and electrons exit, and it detects the kinetic energy and exit angle of electrons.
When the sample or sample stage is completely insulated, the emitted electrons cannot be replenished, which will cause the sample or sample stage to generate an electric potential, deviating from the equipotential with the analyzer. This will affect the kinetic energy and momentum of the detected electrons. Generally, we call this charging.
To determine whether the sample stage is charging, the more commonly used method is to change the intensity of ultraviolet light. Charging will change with the light intensity. The stronger the light intensity, the more photoelectrons are generated, the higher the accumulated potential, and the more the spectrum moves on the energy axis. If the Fermi edge of the spectrum line does not change with the light intensity, it is likely not charging.