Anomalous Bloch sphere dynamics in a crystalline organic radical observed by pulsed electron paramagnetic resonance at 240 GHz

Pulsed electron paramagnetic resonance (EPR) enables one to create and probe states of interacting spins far from thermal equilibrium. Free electron laser-powered pulsed EPR experiments performed at 240 GHz / 8.56 T on crystalline BDPA, an organic radical, reveal a nonlinear dependence on sample magnetization not typically encountered in EPR, which manifests as a tip-angle dependent resonant frequency. Frequency shifts as large as 11 MHz (45 ppm) are observed during a single Rabi oscillation. These frequency shifts are attributed to a large sample magnetization and to strong spin-spin interactions. A semi-classical model which includes a demagnetizing field is developed, and subsequently used to map out the magnetization's Bloch sphere dynamics during anomalous Rabi oscillations. Nonlinear interactions are tunable, by modifying temperature and sample geometry. Measurements of anomalous Bloch sphere dynamics provide the first steps towards using pulsed EPR to measure long-range electron spin quantum correlations.