Electron Spin Relaxation Mechanisms of Atomic Hydrogen Trapped in Silsesquioxane Cages: The Role of Isotope Substitution

Abstract

Encapsulated atomic hydrogen in silsesquioxane cages is a promising candidate for applications in emerging technologies like spin-based quantum computing, magnetic field sensing, and atomic clock devices. Previous studies on different polyhedral octasilsesquioxanes (POSS) of the type Si8O12R8 have shown that key parameters for quantum computing like electron spin relaxation times T 1 and T M depend strongly on the type of peripheral organic substituents. Herein we examine for the first time the effect of deuterium isotopic substitution on the spin relaxation properties of H@h 72Q8M8, the derivative with R = OSi­(CH3)3, by applying pulsed electron paramagnetic resonance (EPR) methods on its deuterated analogues H@d 72Q8M8 and D@d 72Q8M8. For the latter species we measure a phase memory time of 60 μs at 190 K, the largest obtained so far for this family of molecular spins. We show that substitution of peripheral hydrogen atoms with deuterium reveals high-temperature relaxation mechanisms that were previously hidden by proton nuclear spin diffusion. Unusually short T M values observed for all deuterated species even at liquid helium temperatures are discussed in terms of tunneling reorientation of methyl groups.

Long Electron Spin Coherence Times of Atomic Hydrogen Trapped in Silsesquioxane Cages

“…The linear fit of data with eq gives α M = 11.1 ± 0.7, i.e., C loc ≈ 11 C H , implying a nonuniform spatial distribution of paramagnetic centers ( C loc ≥ C H ), which is a well-known result of track effects in irradiated solids . We have previously observed similar differences between C loc and C av for low-dose γ-irradiated POSS cages . Interestingly, the method of electric discharge used in the present work favors the trapping of H atoms mainly on the surfaces of the molecular crystals, a fact that can adequately justify the aforementioned nonuniform spatial distribution of encapsulated H atoms.…”

“…33 We have previously observed similar differences between C loc and C av for low-dose γ-irradiated POSS cages. 15 Interestingly, the method of electric discharge used in the present work favors the trapping of H atoms mainly on the surfaces of the molecular crystals, 19 a fact that can adequately justify the aforementioned nonuniform spatial distribution of encapsulated H atoms.…”

“…At temperatures below 200 K all studies published so far reported a shortening of T 2 to about 1 μs which was not reversible even at liquid helium temperatures. Using R groups with different rotational degrees of freedom we showed that this peculiarity, initially ascribed to changes in cage symmetry, has its origin in the methyl rotation of organic groups. , Moreover, our recent study with deuterated methyl groups provided strong evidence that the short T 2 values observed at very low temperatures could be assigned to quantum rotational tunneling …”

“…13,14 Moreover, our recent study with deuterated methyl groups provided strong evidence that the short T 2 values observed at very low temperatures could be assigned to quantum rotational tunneling. 15 Herein we study for the first time the electron spin relaxation properties of H@Si 8 O 12 H 8 , also known as H@T 8 H 8 , which is the smallest derivative of octa-silsesquioxanes. Interestingly, this species is the least studied among all H@POSS systems, presumably because the proximal proton nuclear spins of R and the larger delocalization of the spin wavefunction were assumed to contribute much more to decoherence compared to larger species.…”