Contact : Antonio TEJEDA

Next generation spintronics efficiently targets ultra-low power memories for green electronics and on a longer term full-spin information processing. The spin-orbit coupling (SOC) plays a fundamental role in spintronics as it allows controlling the spin in the conduction channels through an electrostatic manipulation [1-3].

SOC is greatly enhanced at reduced dimensions since the inversion symmetry is broken at surfaces or interfaces, and the resultant electric field couples to the spin of itinerant electrons, a phenomenon known as Rashba effect. Spin-orbit coupling is being intensively studied in two dimensional systems as in transition metal dichalcogenides, hybrid perovskites or in molecular layers on ferromagnetic substrates [4-6].

In this internship, we will tune the SOC in 2D materials by structural modification, for instance by introducing defects in the structure (vacancies or impurities) or by introducing strain in the lattice. The effect of the induced structural modification will be studied by electron diffraction and the impact on the electronic bands will be determined by angle-resolved photoemission (occupied states) and by spin- and angle-resolved inverse photoemission (unoccupied states) [7].

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Contact: Vincent JACQUES

The present subject aims at studying strain-induced exotic electronic phases in correlated electron systems presenting such states as charge density waves (CDW), spin density waves (SDW) and/or superconductivity (SC). Thanks to a unique cryogenic biaxial tensile strain device, compatible with optical, tansport and x-ray diffraction (XRD) measurements, we can study lamellar systems that exhibit various electronic orders under strain. We recently showed that application of mechanical strain can induce exotic phase transitions in lamellar systems, like CDW orientational transition only governed by in-plane lattice parameter symmetry, and with a linear evolution of transition temperatures with strain, reaching 40K at maximum deformation. This evolution of Tc is apparently not at all proportional with the gap, and new studies have to be performed in those systems under strain to understand the very nature of these CDW. In this project, we plan to develop and use new types of measurements under strain: the direct band gap measurement by photoemission spectroscopy under strain, the evolution of electron-phonon coupling by time-resolved optical and XRD techniques, and the study of local strain-induced CDW structure by x-ray microdiffraction. Finally, we plan to extend these techniques to other systems presenting competing exotic electronic phases: CDW/CDW and CDW/SC in transition metal dichalcogenides and CDW/SDW in chromium thin metallic films.

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Contact : Olivier PLANTEVIN

We will investigate exciton physics and their relationship with the presence of structural defects and deformation in 2D hybrid perovskites single crystals. In a previous internship, we demonstrated an original contribution of self-trapped excitons (STE) at low temperatures. The controlled introduction of defects will be performed by ion irradiation at IJCLab (MOSAIC platform). We also showed, in 3D perovskites, that irradiation defects were inducing a crystal deformation along with a modulation of optoelectronic properties. In parallel, we will use a direct method to induce crystal deformations (up to ~1%) with a biaxial traction device developped in the LUTECE team of LPS ( In the two cases (direct and irradiation-induced deformation), we will study both strucural properties (by XRD) and optoelectronic properties (by photoluminescence spectroscopy) at different temperatures. We will study the evolution of structure under biaxial strain in-situ and the photoluminescence in the same conditions. The understanding of the coupling between structure (deformation) and properties (dynamics) will bring unprecedented view on these 2D semiconducting materials and help optimization for photovoltaic devices. We will compare the permanent deformation induced by irradiation to the reversible one we can access in the elastic deformation regime with the traction device. The results will help improve photovoltaic devices based on 2D perovskites that are a good alternative to 3D perovskites, that undergo degradation and stability issues in time. In contrast the 2D perovskites are indeed much more robust regarding chemical degradation of their structure.

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Contact: Olivier PLANTEVIN

The effect of confinement in porous matrices on the optoelectronic properties of perovskites will be studied. The samples will be synthesized by the Ph.D. student at the LPICM-Ecole Polytechnique in collaboration with Frederic Oswald and Hindia Nahdi. We will focus on single halogen (Iode or Brome) or mixed halogen (Iode and Brome) perovskites with varying relative concentrations. We will study separately thin layers made by perovskite impregnation (liquid phase synthesis) in different mesoporous media, with priority TiO2, ZrO2 and Carbon graphite to compare the role of the media, whether be it semiconductor, insulator or metal on the opto-electronic properties of the thin layer. The architecture of complete cells will also be discussed. The two sides of the cell (graphite and TiO2, the ZrO2 layer playing an insulating role between these two sides) will be studied in a complementary way. A characterization by low temperature photoluminescence spectroscopy will reveal the induced defect levels in the material. The «blue-shift» already observed by our team for confined perovskites, will allow to better characterize the confinement in the different layers of the cell (mesoporous graphite, TiO2 and ZrO2 mesoporous). In particular, the photoinduced mobility of halogen ions (iodine and/or bromine) will be investigated by studying the evolution of emission spectra and the influence of temperature on ion diffusion mechanisms.

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