Dynamic nuclear polarization ͑DNP͒ of 29 Si nuclei in isotopically controlled silicon single crystals with the 29 Si isotope abundance f 29Si varied from 1.2% to 99.2% is reported. It was found that both the DNP enhancement and 29 Si nuclear spin-lattice relaxation time under saturation of the electron paramagnetic resonance transitions of phosphorus donors increase with the decrease in the 29 Si abundance. A remarkably large steadystate DNP enhancement, E ss = 2680 which is comparable to the theoretical upper limit of 3310, has been achieved through the "resolved" solid effect that has been identified clearly in the f 29Si = 1.2% sample. The DNP enhancement depends not only on the 29 Si abundance but also on the electron spin-lattice relaxation time that can be controlled by temperature and/or illumination. The linewidth of 29 Si NMR spectra after DNP shows a linear dependence on f 29Si for f 29Si Յ 10% and changes to a square-root dependence for f 29Si Ն 50%. Comparison of experimentally determined nuclear polarization time with nuclear spin diffusion coefficients indicates that the rate of DNP is limited by the polarization transfer rather than by spin diffusion.

Pulsed electron paramagnetic resonance spectroscopy of the photoexcited, metastable triplet state of the oxygen-vacancy center in silicon reveals that the lifetime of the ms=±1 sub-levels differ significantly from that of the ms=0 state. We exploit this significant difference in decay rates to the ground singlet state to achieve nearly ∼100% electron spin polarization within the triplet. We further demonstrate the transfer of a coherent state of the triplet electron spin to, and from, a hyperfine-coupled, nearest-neighbor 29 Si nuclear spin. We measure the coherence time of the 29 Si nuclear spin employed in this operation and find it to be unaffected by the presence of the triplet electron spin and equal to the bulk value measured by nuclear magnetic resonance.PACS numbers: 71.55.Cn, 76.70.Dx, 03.67.Lx Nuclear spins in solids are promising candidates for quantum bits (qubits) as their weak coupling to the environment often leads to very long spin coherence times [1][2][3][4]. However, performing fast manipulation and controlling interaction between nuclear spin qubits is often more challenging than in other, more engineered, quantum systems [5][6][7]. The use of an optically driven mediator spin has been suggested as a way to control coupling between donor electron spins in silicon: the donor spins exhibit weak direct coupling, but mutually couple through the optically excited state of the mediator [8]. Such ideas could similarly be applied to couple nuclear spins, and, if the mediator spin is a photo-excited triplet with a spinzero single ground state, it would have the added advantage that it avoids long-term impact on the nuclear spin coherence [9][10][11].Photoexcited triplets are optically-generated electron spins (S = 1) which often exhibit large (positive or negative) spin polarization, thanks to preferential population of each of the triplet sub-levels following intersystem crossing and/or the differing decay rates of these sublevels to the ground singlet state [12,13]. Nuclear spins, in contrast, have weak thermal spin polarization at experimentally accessible conditions, due to its small magnetic moment. Highly polarized electron spin triplets can be used to polarize surrounding nuclear spins, through continuous wave microwave illumination (under processes termed dynamic nuclear polarization) [14,15], or using microwave pulses [16]. Triplet states can also be used to mediate entanglement between mutually-coupled nuclear spins [9], on timescales much faster than their intrinsic dipolar coupling [17].Oxygen-vacancy (O-V ) complexes can be formed in silicon by electron beam or γ-ray irradiation of oxygen-rich silicon crystals [18,19], and can be excited to the triplet state (termed an SL1 center,) using illumination of above band gap light [20]. Magnetic resonance studies including electron paramagnetic resonance (EPR), electrically or optically detected magnetic resonance, spin dependent recombination and electrically-detected cross relaxation [20][21][22][23][24][25] have revealed that the SL1 has ...

PACS 76.70.Fz Dynamic nuclear polarization of the 29Si nuclei due to the "solid-effect" was observed clearly after saturation of the phosphorus electron paramagnetic resonance lines in three kinds of silicon crystals containing different amount of the 29 Si isotope (1 %, 4.7 %, and 99.3 %). Maximal enhancement (E) of the roomtemperature . It has been shown that in silicon containing phosphorus atoms at the concentrations lower than 5×10 16 cm -3 the dominant mechanism of DNP is so called "solid-effect" [1-3]. This mechanism is realized under saturation of the forbidden "flip-flop" and "flip-flip" transitions in the dipole-dipole coupled electron nuclear system [2,3]. The enhancement of the nuclear polarization E = P N /P N0 = 30, where P N is the DNP degree and P N0 is the equilibrium nuclear polarization, was obtained at 4.2 K under saturation of the forbidden transitions by microwave field at the frequency of 9 GHz [1]. The observed enhancement E is significantly lower than the maximal theoretical value E m = (γ e /γ N ) =3310, where γ e and γ N are electron and 29 Si nuclear gyromagnetic ratios, respectively. The low value of E can be explained by two factors. First, the long electron spin relaxation time T e ≈ 1 -10 s [4] of electrons localized at phosphorus atoms at 4.2 K reduces the value of E by factor 1/(1+ f) [3] where f = NT e /nT

We report on a pulsed electron paramagnetic resonance (EPR) study of the photoexcited triplet state (S = 1) of oxygen-vacancy centers in silicon. Rabi oscillations between the triplet sublevels are observed using coherent manipulation with a resonant microwave pulse. The Hahn echo and stimulated echo decay profiles are superimposed with strong modulations known as electron-spin-echo envelope modulation (ESEEM). The ESEEM spectra reveal a weak but anisotropic hyperfine coupling between the triplet electron spin and a 29 Si nuclear spin (I = 1/2) residing at a nearby lattice site, that cannot be resolved in conventional field-swept EPR spectra.

Electron paramagnetic resonance (EPR) of the triplet centers and dynamic29 Si nuclear polarization was studied in irradiated naturally abounded (4.7%) and 29Si isotope enriched (99.3%) silicon crystals. Saturation of the EPR lines of the photoexcited triplet centers with nonequilibrium spin polarization between m S = +1, 0, and -1 sates leads to the nuclear polarization ≈7000 times higher than equilibrium value. It was shown that the observed dynamic nuclear polarization is a result of the "solid-effect". 1 Introduction Growing interest to the investigation of spin dependent phenomena in silicon is related to the advantages in the production of the isotope enriched silicon crystals and silicon based structures. The possible future applications of these materials in spin electronics and quantum computers require high electron and nuclear spin polarization.The photoexcited spin S = 1 centers characterized by strong nonequilibrium populations of states with different spin projections m S = +1, 0, and -1 can be used for dynamic nuclear polarization. The defects with the excited triplet states are produced by electron or γ-irradiation of silicon crystals containing oxygen and carbon impurity atoms [1][2][3][4].In the present paper we report the experimental results of electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) investigations in irradiated silicon crystals containing different amount of the 29 Si lattice nuclei.

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Optical and dynamic nuclear polarization of 29 Si nuclei under saturation of the electron paramagnetic resonance (EPR) transitions of the photoexcited spin S = 1 states (Si-SL1 centers) of the oxygen + vacancy complexes (A-centers) have been investigated using silicon single crystals with various 29 Si nuclear abundances. The optical nuclear polarization alone led to the 29 Si nuclear polarization enhancement by a factor of ∼500 in the magnetic fields below 50 mT, and such enhancement was found to decrease significantly in the stronger fields up to 300 mT. On the other hand, saturation of the EPR lines of Si-SL1 centers having the electron spin polarization of 70-80% was transferred successfully to surrounding 29 Si nuclear spins to achieve the overall 29 Si nuclear spin polarization of ∼6.4%, and a method to increase up to 13-15% is proposed. The optimum condition to achieve such a high degree of nuclear polarization was found by systematic investigations of the dynamic nuclear polarization mechanisms.

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