Methods and Instrumentation

• ARPES band mapping and Fermi surface mapping
  
  • photon energy range: 18 – 240 eV
  • energy resolution: 10 meV, angular resolution 0.1°
  • use of selection rules through variable photon polarisation (linear horizontal/vertical, circular left/right)
  • smallest sample size 500 x 500 μm²
• Spectral function spectroscopy
  
  • photon energy range 18 – 80 eV.
  • energy resolution 2 – 5 meV, angular resolution 0.1°
  • hole spectral function at any accessible momentum
  • self energy as a function of binding energy
• Nano-ARPES
  
  • spatially resolved band mapping and Fermi surface mapping 
  • photon energy range: 50 – 100 eV.
  • energy resolution: 50 meV, angular resolution 0.5°
  • feature size: 0.5 μm for single domain ARPES
  • scanning resolution: 0.2 μm
• Sample preparation
  
  • cleavage
  • sputter/anneal
  • direct heating
  • evaporation sources

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 μm² given 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).

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 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.

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:

• cleavage of layered materials by knocking off a post glued to the top surface;
• Ar-ion etching and annealing for polished metal surfaces;
• Evaporation sources for the preparation of ultrathin films;
• direct heating flash for passivated semiconductor surfaces;
• port for attaching user-supplied preparation equipment.