Angle-Resolved Photoemission Spectroscopy (ARPES) maps the dispersion of electronic bands near the Fermi level and, in particular, the Fermi surface itself by exciting the bound electrons in a metal with a given photon energy hv. The momentum parallel to the surface is fully conserved, thus making the method suitable for layered low-dimensional materials. The three-dimensional momentum distribution is also reflected in the photoelectron features thus making the spectroscopy applicable to metallic single crystals, provided that a well-defined clean surface can be prepared in ultra-high vacuum. The minimum samples size is 500 x 500 μm2given by the light spot (50 x 50 μm2) and the sphere of confusion of the sample goniometer.
The fine structure of the energy spectra of photoelectrons reflect the interaction of the photo-hole left behind with the re-arranging electronic and vibronic structure in the sample. The width of the band as a function of binding energy corresponds to the imaginary, the deviation from a bare dispersion to the real part of the spectral function. High sensitivity to excitations on a thermal energy scale require measurements at low temperatures (less than 10 K) at an energy resolution of a few meV (2 – 5 meV) and a momentum resolution at a fraction of the Brillouin zone dimensions (0.1° angular resolution).
Samples have to be prepared in situ in ultrahigh vacuum to achieve atomically controlled, clean surfaces. The following methods will be provided in the vacuum system of HR-ARPES or nanoARPES:
Spectro-microscopy combines the power of momentum and energy resolution in the electron analyser with a microscopic light spot and raster sample scanner. The reduced transmission of the nano-focusing optics necessitates the reduction of energy resolution to about 30-50 meV, but the power of electron band mapping allows either a highly selective contrast in microscopy or a band and Fermi surface mapping at high spatial resolution. It can thus serve to image grains, domains of artificial structure of micro- or nanometric dimensions or to illuminate selectively micro- and nano-domains for ARPES analysis.
At the same time the instrument is used a lot as an ARPES spectrometer with sub-micron spot.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2020 Diamond Light Source
Diamond Light Source Ltd
Harwell Science & Innovation Campus