Magnetic field dependence of triplet-state ONP: theoretical analysis in terms of level anti-crossings

ABSTRACT

A theoretical approach to Optical Nuclear Polarisation (ONP) is described, which is based on the analysis of Level Anti-Crossings (LACs) in triplet states. Here we consider ONP formed in molecular crystals doped with suitable guest molecules and ONP generated in diamond crystals containing negatively charged nitrogen-vacancy (NV^–) centres. In both cases, electron spin polarisation of triplet states generated by light excitation is transferred to nuclei giving rise to ONP. Polarisation transfer is most efficient at LACs; for this reason, we consider in detail crossings of electron–nuclear energy levels and the role of different perturbation terms (coming from isotropic and anisotropic hyperfine coupling, zero-field splitting and sample orientation), which turn these crossings into LACs and give rise to ONP. Analytical results are supported by numerical calculations of the ONP field dependences. Thus, the outlined LAC analysis is a useful approach for interpreting the ONP magnetic field dependence.

GRAPHICAL ABSTRACT

KEYWORDS:

• Spin hyperpolarisation
• optical nuclear polarisation
• triplet states
• molecular crystals
• NV-centres

Acknowledgements

We are thankful to Prof. Hans-Martin Vieth and Dr Sergey Anishchik for stimulating and useful discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Investigation of ONP in molecular crystals has been supported by the Russian Science Foundation [project No. 15-13-20035]. We acknowledge the Russian Foundation for Basic Research [grant number 17-03-00932] and Federal Agency for Scientific Organizations of Russian Federation [project 0333-2014-0001].