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 spot size 50 x 50 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 (for standard zone plates)
  • energy resolution: ~30 meV, angular resolution 0.2°
  • feature size: 0.7 μm for single domain ARPES
  • option for 5 (or 4) electrical connections to sample plate
  • Chip package available for ease of preparation in home facility
• 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 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.

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:

• 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.