Optical Polarization of Nuclear Spins in Silicon Carbide

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Optical Polarization of Nuclear Spins in Silicon Carbide

Abram L. Falk, Paul V. Klimov, Viktor Ivády, Krisztián Szász, David J. Christle, William F. Koehl, Ádám Gali, and David D. Awschalom

Phys. Rev. Lett. 114, 247603 – Published 17 June 2015

See Viewpoint: Polarizing Nuclear Spins in Silicon Carbide

Abstract

We demonstrate optically pumped dynamic nuclear polarization of ${}^{29}{Si}$ nuclear spins that are strongly coupled to paramagnetic color centers in $4 H$ - and $6 H$ -SiC. The $99 % \pm 1 %$ degree of polarization that we observe at room temperature corresponds to an effective nuclear temperature of $5 μ K$ . By combining ab initio theory with the experimental identification of the color centers’ optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.

Received 22 December 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.247603

Viewpoint

Polarizing Nuclear Spins in Silicon Carbide

Published 17 June 2015

An optical technique polarizes the spin of nuclei in silicon carbide, offering a potential new route to nuclear spin-based quantum memory.

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Authors & Affiliations

Abram L. Falk^1,2, Paul V. Klimov^1,3, Viktor Ivády^4,5, Krisztián Szász^4,6, David J. Christle^1,3, William F. Koehl¹, Ádám Gali^4,7, and David D. Awschalom^1,*

¹Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA
²IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
³Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA
⁴Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, Hungary
⁵Department of Physics, Chemistry, and Biology, Linköping University, SE-581 83 Linköping, Sweden
⁶Institute of Physics, Loránd Eötvös University, H-1117 Budapest, Hungary
⁷Department of Atomic Physics, Budapest University of Technology and Economics, H-1111 Budapest, Hungary

^*awsch@uchicago.edu

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Issue

Vol. 114, Iss. 24 — 19 June 2015

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Images

Figure 1
(a) An illustration of the $k_{1} k_{1}$ divacancy (green circles) in $6 H$ -SiC. The calculated spin density is represented by orange-lobe isosurfaces and is primarily localized at the dangling bonds of the Si vacancy’s nearest C atoms. We measure the DNP of ${}^{29}{Si}$ nuclei at the ${Si}_{IIa}$ and ${Si}_{IIb}$ sites. (b) Evolution of the ES and GS spin-sublevel energies with $B$ , showing hyperfine-mediated hybridization of the electronic and nuclear spin sublevels at the ESLAC and GSLAC. The states drawn in gray do not hybridize. (c) For ESLAC-derived DNP, a hyperfine interaction in the ES causes $| 0, ↓ ⟩$ to partially evolve into $| - 1, ↑ ⟩$ every optical cycle. Together with the electron-spin polarization provided by the intersystem crossing (green arrows), this interaction causes optical cycling (black arrows) to polarize arbitrary $| m_{S}, m_{I} ⟩$ states into $| 0, ↑ ⟩$ (highlighted). The $m_{s} = + 1$ electronic spin states do not participate in this process. (d) For GSLAC-derived DNP, the mechanism is the same as in (c), but the relevant hyperfine interaction is in the GS.
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Figure 2
(a) Upper: Low-microwave-power ODMR spectrum of the $m_{s} = 0$ to $m_{s} = 1$ spin transition of the $h h$ divacancy in $6 H$ -SiC at $T = 100 K$ . As $B$ varies from 50 to 650 G, $f_{0}$ varies from 1.5 to 3.2 GHz. Lower: Line cuts at the dashed white lines ( $B = 50$ and $B = 310 G$ ). The ${}^{29}{Si}$ nuclei at the ${Si}_{IIa}$ and ${Si}_{IIb}$ sites are unpolarized at $B = 50 G$ and nearly completely polarized at the ESLAC ( $B = 310 G$ ). The continuous lines are fits to sums of Lorentzians [44]. (b) Upper: Low-power ODMR spectrum of the $m_{s} = 0$ to $m_{s} = 1$ spin transition of the PL6 defects in $4 H$ -SiC at $T = 298 K$ . Lower: Line cuts at $B = 50$ and at $B = 330 G$ , which is the PL6 ESLAC. (c) The nuclear polarization ( $P$ ) for ${}^{29}{Si}$ nuclei at the ${Si}_{IIb}$ sites of $h h$ and $k_{1} k_{1}$ divacancies in $6 H$ -SiC at $T = 100 K$ , exhibiting peaks in $P$ at the ESLAC and GSLAC. $h h$ divacancies have stronger ESLAC-derived DNP than $k_{1} k_{1}$ divacancies. The error bars are single- $σ$ credible intervals set by the fits. The continuous lines are $P$ values simulated from our theoretical model, plotted with a resolution of 3 G. (d) ${Si}_{IIb}$ nuclear spin polarization for PL6-coupled nuclei in $4 H$ -SiC at $T = 298 K$ (experiment and theory). The origin of the small peak in $P$ at 420 G is unknown.
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Figure 3
(a) Upper: Experimental high-power ODMR spectrum of PL6 at $T = 298 K$ . Lower: Line cut of the ODMR spectrum at $B = 50 G$ (the dashed line). The curved lines correspond to basal-plane oriented defects. The different divacancy forms have nondegenerate microwave transition frequencies and are therefore individually addressable. (b) High-power ODMR spectrum of the neutral divacancies in $4 H$ -SiC and (c) of the neutral divacancies in $6 H$ -SiC, all at $T = 20 K$ . (d) Zoom-in of the red dashed rectangle drawn in (c). Because of spin mixing in the GS, each divacancy’s ES ODMR signal has a minimum at its corresponding GSLAC.
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Figure 4
(a) Temperature dependence of the high-power ES ODMR for the three $c$ -axis-oriented neutral divacancies in $6 H$ -SiC at $B = 650 G$ . The ES ODMR curves are sequentially offset from $Δ PL = 0$ , for clarity, and the bold lines are guides to the eye that follow the ES-spin resonances. (b) $T_{2, ES} *$ for divacancies in $6 H$ -SiC, calculated by fitting the curves in (a) to a sum of three Lorentzians and taking $T_{2, ES} *$ to be $1 / π$ times the inverse of the full-width at half-maximum linewidths. The single- $σ$ error bars derive from the fits. (c) Temperature dependence of $P$ at the ${Si}_{IIb}$ site of divacancies in $6 H$ -SiC, and for PL6 in $4 H$ -SiC, when $B$ is tuned to the ESLAC (270–330 G). [44].
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