Investigation of anisotropic spin phenomena in doped single nanocrystals probed by luminescence spectroscopy in a vector magnetic field

• Applicant: Professor Dr. Gerd Bacher
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435349552

The overall goal of this project is to explore collective and individual spin effects in transition metal doped colloidal nanocrystals of defined crystal and/or shape anisotropy. This requires a main innovation in methodology, namely luminescence experiments on single nanocrystals in a vector magnetic field. As a model system, we will predominantly concentrate on Mn2+-doped II-VI shaped-engineered nanocrystals of either wurtzite or zincblende structure. The main scientific questions can be split into three subitems. In nanocrystals doped with a collection of transition metal ions it is intended to unravel the impact of crystal and/or shape anisotropy on the formation of an exciton magnetic polaron complex – an effect which has been hypothesized since more than a decade, but never been proven. Hereby, the application of a magnetic field with varying orientation with respect to the dominant anisotropy axis will allow to purposely stabilize / destabilize magnetic polaron formation. We in addition intend to address the role of statistical magnetic fluctuations on polaron formation and emission linewidth in individual nanocrystals, separating between transversal and longitudinal spin fluctuations. Nanocrystals doped with single impurities will be investigated to elaborate the combined action of s-p and sp-d exchange interactions in single-atom doped nanocrystals. Here, we hypothesize that the fine structure of the lowest exciton state well-established for undoped nanocrystals has most likely to be revised in case of transition metal doping due to pronounced sp-d exchange interactions. Adjusting the exciton bandgap carefully with respect to the internal 4T1 – 6A1 transition of the Mn2+ will allow for dual emission because of the quite rapid energy transfer between these states. It is planned to study single dual emitting nanocrystals, where an enhanced sensitivity of the emission to temperature and magnetic field is expected due to the suppressed inhomogeneous broadening. As the single exciton – single dopant interaction leads to pronounced energy splitting between different Mn2+ spin states within the exciton - Mn2+ complex, the study of dual emission in single-atom doped nanocrystals may shed light on a possible spin-selective energy transfer.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Daniel R. Gamelin