Spin resonance measurements of the clock molecule 15N@C60

Green, Matthew and Laird, Edward (2025) Spin resonance measurements of the clock molecule 15N@C60. PhD thesis, Lancaster University.

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Abstract

Atomic clocks form a vital part of modern infrastructure and are the most accurate frequency standards available. One of the most common applications of atomic clocks in global navigation satellite systems (GNSS). The current integration of atomic clocks into this system is through highly accurate satellite-based frequency standards. Another possible application in GNSS is on the receiver end of the system, which also requires a frequency standard. An atomic clock receiver offers a number of benefits in securing GNSS networks. This presents a challenge of combining high accuracy and stability, but also reducing the size and power demands to make the clock easily portable. The endohedral fullerene molecule 15N@C60 is a potentially suitable material for such a clock. Here, the transitions of 15N@C60 are measured and the magnetic field-resistant clock transition is observed. A value for the hyperfine constant A/h = −22.32±0.03MHz was extracted, in agreement with previous measurements of A/h both at the clock transition of ∼38.6MHz and at ∼10GHz. Estimates for a potential clock’s stability described by the Allan deviation at the clock transition gave a value of σy(τ) = 2.1 × 10−4τ−1/2, a poor standard of stability compared to current commercial standards. To determine the potential improvements to this estimate, pulsed spin resonance measurements of 15N@C60 were performed to obtain the spin-spin relaxation time T2, which is connected to the width of the transition by 1/πT2. Measurements of the spin echo at 102MHz gave a T2 = 8.6 ± 0.7µs. A transition width as narrow as 1/π × 8.6µs offers a marginal improvement to the clock stability. Progression towards lower frequencies closer to the clock transition is required to obtain the absolute limit on how low the Allan deviation might be reduced to.