Kai Rossnagel 
Universität Kiel und  DESY

A world from a quantum material sandwich and how we can explore it with soft x-rays

Without materials, there is nothing. Without quantum materials, there is nothing interesting. Quantum materials express our desire to find and explain new states of matter and physical phenomena. In these materials, it is the complexity of the quantum mechanical interactions of the constituent electrons that leads to qualitatively new and unexpected behaviors, offering a field of research of intriguing richness and great technological potential. Paradigmatic examples are quantum material sandwiches fabricated by stacking and twisting single layers of layered materials, which currently represent one of the richest, cleanest, and most tunable, discovery and engineering platforms in all of condensed matter physics.

On the experimental side, to see and understand how these and other quantum materials work, we need to go beyond electrical transport measurements, which often provide the first clues to new electronic phenomena, and use spectroscopic tools that can provide direct snapshots of electron behavior in the machine room of materials. The most powerful toolbox in this regard is soft x-ray spectroscopy, and the single most powerful tool is angle-resolved photoelectron spectroscopy (ARPES), which has become a mainstay for imaging the momentum-dependent electronic structure of materials. Excitingly, ARPES has recently been transformed into a true in operando technique using both nanofocused and ultrashort-pulsed soft x-rays to directly probe nonequilibrium electronic functions in materials and devices on relevant nanometer length and femtosecond time scales, respectively.

Here, I will give an overview of recent innovations in quantum material sandwiches from the transition-metal dichalcogenide family and the nanoscopic and femtostroboscopic imaging of their electronic structures.