Ab initio modelling of the photoemission properties of Cs2Te as photocathode material for particle accelerators

The overarching goal of this doctoral project, with planned duration of 36 months, is to understand, predict, and control the photoemission properties of the photocathode material Cs2Te. To accomplish this task, state-of-the-art methods of solid-state theory and computational materials science will be employed. The knowledge of the electronic structure of the material is the key to disclose the fundamental physical mechanisms ruling the creation of photo-excited charge-carriers upon light absorption, their scattering mechanisms, and their final emission from the cathode. Due to the little existing information about the nanoscale electronic properties of Cs2Te, the proposed study will begin with a detailed characterization of the bulk material in the framework of density-functional theory (DFT) and many-body perturbation theory (MBPT) following the path proposed in the recent publication C. Cocchi et al., J. Phys.: Condens. Matter 31, 014002 (2018). A quantitative comparison with experimental results is enabled by the adopted parameter-free theoretical scheme. The information about the electronic structure of bulk Cs2Te can be used as an input to describe photoemission by adopting and extending the Spicer’s three step model. The response of the material to photo-absorption (step 1) is given by the dielectric tensor computed from MBPT. Scattering mechanisms of the photo-excited electrons (step 2) that are relevant in semiconductors involve the electron-phonon interaction, which is accessible within the framework of DFT. Emission rates (step 3) can be inferred consequently. The extension of Spicer’s model includes the revision of the relevant physical quantities (absorption, scattering, and emission rates) including quantum-mechanical many-particle effects, which are included in the aforementioned first-principles scheme. The application of the extended photoemission model on bulk Cs2Te is the first step to validate it. Refinements will be needed to enhance the comparison with experiments and operational characteristics, taking into account defects and surface effects. To identify relevant structures within such a huge configurational space, we will perform a computational high-throughput screening. Surfaces will be automatically created and optimized using DFT. After assessing and ranking their stability, the electronic structure of the most stable surfaces will be investigated in view of modeling their emission properties. Due to the significantly larger amount of atoms that is needed to describe a surface compared to a bulk, MBPT calculations will be performed only in selected critical cases where DFT qualitatively fails. The prior assessment of the photoemission properties of bulk Cs2Te will serve as a benchmark for the surfaces. Defected structures will be investigated adopting the same approach.

Principal Investigators
Cocchi, Caterina Prof. Dr. (Details) (Theoretical Physics / Theory of Excitations in low-dimensional Systems)

Financer
Helmholtz-Gemeinschaft

Duration of Project
Start date: 06/2019
End date: 06/2022

Last updated on 2020-09-11 at 08:53