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OptoDNPcontrol Report Summary

Project ID: 306719
Funded under: FP7-IDEAS-ERC
Country: France

Final Report Summary - OPTODNPCONTROL (Optically controlled carrier and Nuclear spintronics: towards nano-scale memory and imaging applications)

The project has allowed mayor advances towards all ERC project goals and well beyond as documented in 41 publications (6 Physical Review Letters, 15 Physical Review B, 1 Reviews of Modern Physics, 2 Nature Communications, 2 Nano Letters, ...).

The ERC project OptoDNPcontrol has developed experiments to optically control the charge carrier spins and valley index and to manipulate nuclear spins in semiconductor nano-structures. The polarization is transferred from the photons to the electrons and via the hyperfine interaction to the nuclear spins. From a fundamental point of view, we have advanced our understanding of how to maximize the optically generated dynamic nuclear polarization. From a more applied perspective, we have shown that transferring nuclear spin polarization to a target region in the sample (here surface ligands) allows us to increase the NMR signal from this region to a detectable level, not possible without this transfer.

(*) Nuclear spin initialization
In addition to the established control by laser polarization and applied magnetic fields, we have demonstrated voltage control of the hyperfine interaction between a single electron spin and the nuclear spin bath in a single quantum dot, which is fast and practical for applications. Also efficient nuclear spin polarization of a strain free GaAs quantum dot has been demonstrated at zero magnetic field, showing that nuclear quadrupole fields, present in strained dots, are not necessary to stabilize dynamic nuclear polarization.
Using magnetized CoFeB electrodes, also electrical spin injection at zero magnetic field has been demonstrated to lead to dynamic nuclear polarization in a SpinLED device, without the need of optical pumping with a polarized laser.

(*) Nuclear spin transfer
The target was to investigate how the NMR signal coming from surface ligands, which is far too weak for direct imaging, can be amplified by using transfer of Nuclear Polarization. Here our main result is the proof of concept of a new NMR technique. The investigation of ligands on the surface of Cd3P2 and also InP nano-particles is a clear first demonstration. This novel NMR pulse sequence based on polarization transfer (i) can be implemented in commercial NMR systems, no specialized equipment is needed and (ii) is applicable to a wide range of nanocrystal/nanostructures.

(*) Valence states with optimized hyperfine coupling
Controlling the coupling of a single valence hole spin to the nuclear spin bath is a promising route towards long spin coherence times for spintronics applications. Here we developed a new approach to measuring the hole hyperfine interaction in a single 111 grown quantum dots. In addition, we measured the optical bandgap and exciton resonances in new 2D materials for spintronics and valleytronics with optimized valence band spin states. We report the longest spin memory times (several micro seconds) to date in atomically thin WSe2 layers.

(*) Knight field control for NMR
The main idea is to replace the AC external magnetic field by an internal magnetic field provided by the electron spin. The corresponding field is called the Knight field and we show clear tuning of both amplitude and direction of the Knight field, as demanded for the implementation of all optical NMR experiments. The remaining challenge is to separate the optical process that probes the dynamic nuclear spin polarization from the process that generates this polarization, as we recently demonstrated in bi-directional nuclear spin polarization experiments [M. Manca et al, arXiv:1802.00629]

The funding in terms of equipment and consumables has allowed us to work under best possible conditions, with optical components well adapted to each of the different experimental challenges. The recruitment of 3 post-docs and 1 PhD student has been a great success. The 4 motivated young scientists have really themselves driven this project towards the pre-defined goals, benefiting from the excellent working conditions provided by the ERC funding.

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