The development of new materials is one of the key challenges in information technology. Electronic many-body effects, which give rise to fascinating phenomena such as superconductivity and spontaneous magnetic ordering, have inspired modern research to search for even more exotic effects to exploit for applications. Thus, understanding the correlation between unconventional superconductivity and spatial electronic modulations, such as magnetic, chiral, or charge density wave (CDW) orders, is one of the key challenges in current condensed matter research.
On the other hand, the understanding of magnetic, thermal, optical and electrical properties cannot be achieved without understanding the behaviour of the electrons within these materials and how they control the observable effects. The distribution of electronic states, the band structure, of a given material is unique, like a fingerprint. By tuning the band structure of materials, it is possible to dramatically change or even create new 'exotic' physical properties of advanced materials. Modulating band structures for the needs of modern electronics and materials science in general is therefore one of the main tasks of solid-state physics.
Angle-resolved photoelectron spectroscopy (ARPES) is one of the most powerful experimental techniques for probing the electronic states that determine most physical properties of materials. The technique is based on the photoelectric effect. By analysing the kinetic energy and angular distribution of photoelectrons emitted by light with photon energy above the work function, one gains direct access to the electronic band structure of a material.
The latest and most efficient electron spectrometer for ARPES is based on the time-of-flight (ToF) recording scheme. The time-of-flight momentum microscope (ToF-MM, is a new way of performing ARPES) allows the parallel detection of the 3D photoelectron distribution, i.e. the photo-emitted electron intensity as a function of the parallel momenta kx and ky and the final kinetic energy I(Ekin, kx, ky).
The light source plays an important role in any ARPES measurement and defines the probing depth and energy resolution. Gas discharge lamps, continuous wave or pulsed lasers and synchrotron radiation sources can be used to generate photons. This allows the full range from ultraviolet (UV) to soft and hard X-rays to be covered and the sensitivity of the ARPES technique to be tuned from the surface to the bulk state depending on the scientific task.
As a result, angle-resolved photoemission spectroscopy is now one of the world's most important techniques for studies in solid-state physics and materials’ science in general.
ToF-MM is an ideal technique that allows band structure mapping to be combined with X-ray photoemission spectroscopy (XPS), X-ray photoelectron diffraction (XPD), real space imaging in photoelectron emission microscopy (PEEM) mode, circular and/or linear dichroism in the angular distribution of photoelectrons (CDAD and LDAD), and spin asymmetry measurements (using a spin-filter crystal). All these methods can be applied under external stimuli such as magnetic field, strain, electric field, temperature, and light (pump-probe experiments). The measurements can be performed in a static or dynamic (ultrafast) mode over a wide range of photon energies from 6 eV in the laboratory with a laser, to tens of eV using high-harmonic-generation (HHG) based VUV laboratory sources, hundreds of eV in the soft X-ray regime at free-electron lasers (FELs) and thousands of eV in the hard X-ray regime at synchrotrons and high-energy beamlines of FELs. Typically, researchers have to use a variety of methods to collect experimental data from all of the above approaches.
To give an overview, ToF-MM is a unique technique which enables multi-dimensional recording of photoemission data and measurements with different methods in one setup (each of which increases the dimensionality of the information obtained). ToF-MM provides a wealth of useful information about the geometric and electronic structures of advanced materials and can be used as a tool for: