Prof. Dr. Kai Roßnagel (Kiel University & DESY): Quantum TMDC-tronics
In this talk, I will show how electron dynamics in one prominent class of quantum materials, layered transition-metal dichalcogenides (TMDCs), can be visualized and deconstructed in energy-momentum space using advanced photoelectron spectroscopy techniques. Quantum materials are expressions of our desire to find and explain new physical phenomena as well as our need for faster, smaller, more powerful and energy-efficient devices in our digital world. In quantum materials, it is the complexity of the quantum mechanical interactions of the constituent electrons that causes qualitatively new and unexpected behavior to emerge, providing a research field of inexhaustible depth and at the same time promising great technological potential.
Layered transition-metal dichalcogenides (TX2) are prime examples of quasi-2D quantum materials. With a restricted set of atomic ingredients (T = Ti, Zr, Hf, V, Nb, Ta, Mo, W; X = S, Se, Te) and arrangements (octahedral versus trigonal prismatic coordination of one T atom by six neighboring X atoms), an immense diversity of intriguing electronic phases in single TX2 layers results, from correlated and topological insulators over non-centrosymmetric semiconductors and charge-density-wave metals to Ising, topological, and charge-density-wave superconductors. What's more, with their exceptional further tunability through van der Waals layer stacking and twisting in combination with electrical gating in device-like structures, TMDC heterostructures provide a most powerful synthetic quantum simulation and discovery platform for 2D electronic phenomena, similar to ultracold atoms in optical lattices.
Yet, to explain how these new kinds of quantum matter and devices work, we need to go beyond transport measurements, which often provide first evidence of new electronic phenomena, and use spectroscopic tools that can take direct snapshots of electron behavior in material interiors. Arguably the most powerful such tool is angle-resolved photoelectron spectroscopy (ARPES), which has become a mainstay of mapping the momentum-dependent electronic structure of materials. Excitingly, using nanofocused as well as ultrashort-pulsed photon beams, ARPES has recently been turned into an in operando technique able to probe nonequilibrium electronic function in materials and devices directly at relevant nanometer length and femtosecond time scales, respectively.
In the first part of my talk, I will introduce TMDCs as paradigmatic quantum materials focusing on novel forms of nano and quantum electronics ("TMDC-tronics") such as spin-valleytronics, Mottronics, and twistronics. In the second half of the talk, I will present experimental studies of TMDC-tronic materials and devices using soft x-ray micro-ARPES and free-electron-laser-based femtostroboscopic ARPES.