1. S. A. Al’tshuler and B. M. Kozyrev, Electron Paramagnetic Resonance of Compounds of Elements of Intermediate Groups (Nauka, Moscow, 1972; Wiley, New York, 1974).
2. A. Carrington and E. McLachlan, Introduction to Magnetic Resonance: With Applications to Chemistry and Chemical Physics (Chapman and Hall, London, 1979; Mir, Moscow, 1970).
3. E. van Wynsberghe and A. Turak, Candidate Materials as Gain Media in Organic, Triplet-Based, Room-Temperature Masers Targeting the ISM Bands, Ed. by S. Pyshkin (IntechOpen, London, 2017).
4. J. Breeze, K. Tan, B. Richards, J. Sathian, M. Oxborrow, and N. M. Alford, Nat. Commun. 6, 6215 (2015).
5. D. V. Azamat, A. G. Badalyan, A. Dejneka, L. Jastrabik, and J. Lančok, Appl. Phys. A 122, 1026 (2016).