Optically pumped spin polarization as a probe of many-body thermalization

Abstract

Disorder and many body interactions are known to impact transport and thermalization in competing ways, with the dominance of one or the other giving rise to fundamentally different dynamical phases. Here we investigate the spin diffusion dynamics of 13C in diamond, which we dynamically polarize at room temperature via optical spin pumping of engineered color centers. We focus on low-abundance, strongly hyperfine-coupled nuclei, whose role in the polarization transport we expose through the integrated impact of variable radio-frequency excitation on the observable bulk 13C magnetic resonance signal. Unexpectedly, we find good thermal contact throughout the nuclear spin bath, virtually independent of the hyperfine coupling strength, which we attribute to effective carbon-carbon interactions mediated by the electronic spin ensemble. In particular, observations across the full range of hyperfine couplings indicate the nuclear spin diffusion constant takes values up to two orders of magnitude greater than that expected from homo-nuclear spin couplings.

Nuclear spin temperature reversal via continuous radio-frequency driving

“…Part of the problem is the large energy mismatch between nuclei featuring different hyperfine couplings, a regime that truncates nuclear flip-flop terms in the Hamiltonian and thus (presumably) quenches spin diffusion. We found in prior work [26] that, contrary to expectations, nuclear spins strongly coupled to the NV contribute to transporting polarization to the bulk, a process mediated by interactions between electronic spins. For completeness, we reproduce part of these results in Fig.…”

“…We work under conditions similar to those reported previously [26,31]. Briefly, we implement a field-cycling sequence that starts with simultaneous laser illumination and RF drive of variable frequency and amplitude for a time interval t OP , followed by sample shuttling to a high-field magnet [Fig.…”

“…Often hosting a dilute set of nuclear spins, materials such as diamond or silicon carbide have also emerged as versatile systems to investigate fundamental questions on the spin dynamics governing DNP [21][22][23][24], most notably the transport of spin polarization from strongly hyperfine-coupled to bulk nuclei. For example, diamond was recently exploited to show how coupled electron spin networks can effectively mediate interactions between remote nuclei, in the process circumventing the transport restrictions created by the spin diffusion barrier surrounding individual paramagnetic defects [25][26][27][28].…”

Direct observation of hyperpolarization breaking through the spin diffusion barrier

“…The question of the spin diffusion barrier is also raised outside of dDNP and MAS-DNP. We note the recent work of Pagliero et al (50) in the field of color centers in diamonds used for 13 C hyperpolarization. Although they are concerned with a different physical system, they draw conclusions that are similar to ours.…”

Low-field microwave-mediated optical hyperpolarization in optically pumped diamond

“…DNP polarization is enhanced $720 times over 7T, corresponding to a $1% level. Polarization builds up in $40s of optical pumping, typically limited by 13 distances in the lattice compared to direct 13 C interactions [32]. Simultaneously, the P1 centers can cause P1 À 13 C À P1 threebody flip-flop processes that can contribute to 13 C T1.…”