Electron-lattice-spin correlations and many-body phenomena in 2D semiconductors and related heterostructures

Project Information

FLATLAND

Grant agreement ID: 682843

DOI

10.3030/682843

Project closed

EC signature date 23 September 2016

Start date 1 October 2016

End date 30 September 2021

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 640 632,89

EU contribution

€ 2 640 632,89

Coordinated by

MAX-PLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN EV
Germany

Periodic Reporting for period 4 - FLATLAND (Electron-lattice-spin correlations and many-body phenomena in 2D semiconductors and related heterostructures)

Reporting period: 2021-04-01 to 2021-09-30

Summary of the context and overall objectives of the project

The emergence of atomically flat two-dimensional crystals opened novel perspectives for condensed matter physics: the fabrication of functional devices from building blocks as thin as a single atom as well as the fabrication of new materials by stacking fundamentally different materials into arbitrary order. These two scenarios represent the device physics and material science perspectives on van der Waals-bonded heterostructures. From both perspectives, a microscopic understanding of the fundamental interactions between electrons, lattice and spins in 2D materials and heterostructures is desired. FLATLAND addresses these phenomena by developing and applying ultrafast techniques, which allow to observe the temporal evolution of non-equilibrium states on the level of single quantum states. This is achieved by a combination of different ultrafast techniques, namely time- and angle-resolved photoemission spectroscopy (trARPES), femtosecond electron diffraction (FED) and femtosecond point-projection electron microscopy (fsPPM). The experimental data achieved with these techniques can be compared to electronic and atomic structure calculations and provides benchmarks for validating computational approaches as well as for designing functional van der Waals heterostructures.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Choosing WSe2 as a model system, we obtained a microscopic understanding of the nature of excited states in this prototypical transition metal dichalcogenide (TMDC) semiconductor. This includes the spin-, valley- and layer-polarisation of bright excitons, their binding energy as well as their interaction strength with the lattice, and the size of the exciton wavefunctions. The accompanying investigation of the lattice excitations generated by the interaction of the excitons with the atomic structure yields a comprehensive picture of the non-equilibrium states in TMDC materials.

We established FED as a methodology for investigating the flow of energy through electrons and vibrations in nanoscale heterostructures. In addition, we pioneer utilizing the inelastic scattering signal for retrieving momentum-resolved information of transient phonon distributions.

In parallel, we developed four-dimensional photoemission spectroscopy, which provides a view of the electronic structure resolved in energy, time and both momentum directions. Based on this, we established excited stated mapping as an extension of the band mapping concept with ARPES to excited / non-equilibrium states. The attached image shows a snapshot of the energy and momentum-distribution of excitons in WSe2 tens of femtosecond after the creation of the exciton with a short laser pulse.
A central aspect of this progress is the development of data analytics tools, which we share with the community.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We developed four-dimensional trARPES as new technique providing an unprecedented view of the electronic structure of crystals in transient non-equilibrium states. Based on this technique, we study the momentum- and energy-distribution of excitons in TMDC materials with approximately 10 femtosecond temporal resolution, which provides basic information of the exciton’s properties like exciton binding energy, the exciton self-energy and the real-space distribution of the exciton wave function. In contrast to optical techniques, our approach is sensitive to optically bright as well as dark excitonic states. Future work will extend this approach to interfacial excitons and hybrid excitons, where the exciton wave functions extend over qualitatively different materials like inorganic semiconductors and molecules. We anticipate recording a movie of the birth and evolution of excited states in molecular crystals and of excitons in van der Waals heterostructures.

The response of the atomic structure to the presence of excitons is investigated by FED. We established femtosecond inelastic electronic scattering as a technique to obtain momentum-resolved information of lattice excitations. This approach is currently applied to van der Waals heterostructures in order to retrieve the pathways of interfacial charge and energy transfer and to identify the dominating interfacial interactions.

Snapshot of the excited state distribution in momentum space of a 2D semiconductor.