Spin coherence time temperature anomaly of the V_Si and V_C vacancy centers in 4H-SiC

An increase in the spin coherence time of the spin defect systems, such as the silicon (V_Si) and carbon (V^±_C) vacancy centers in 4H−SiC, is crucial in quantum information applications. Recent work by Embley et.al [1]. has measured the T2 for the vacancy in 4H-SiC and seen a strange anomaly: the T2 increases with increasing temperature until it peaks, then decreases. We theoretically investigated the mechanisms which cause the T2 temperature anomaly. Our findings show that a spontaneous symmetry, lowering due to a motional Jahn-Teller distortion of the vacancy center, generates a polaron. We see for low temperatures (8 to 20 − 40K), the coherence temperature dependence is dominated by phonon-assisted spin relaxation as expected. As temperatures reach 20 − 40K, depending on the vacancy center, polaron hopping turns on from thermal activation, and motional narrowing dominates, increasing T2. The energy barrier gets high enough to slow this polaron hopping as temperatures reach 120 − 160K, causing a peak in coherence time. Decoherence then takes over due to the Larmor precession. Then we compared to the experiment of Embley et.al [1], and found that they are in good agreement. Our theoretical model can extend to explain a more universal phenomenon which has been seen in other defect center spin systems.

[1] J. S. Embley, J. S. Colton, K. G. Miller, M. A. Morris, M. Meehan, S. L. Crossen, B. D. Weaver, E. R. Glaser, and S. G. Carter, Phys. Rev. B 95, 045206 (2017).