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arXiv 1402.4288cond-mat.mes-hall
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硅中磷核在强磁场和低温下的有效动态极化

Efficient Dynamic Polarization of Phosphorus Nuclei in Silicon in Strong Magnetic Fields and Low Temperatures

J. Järvinen, D. Zvezdov, J. Ahokas, S. Sheludyakov, O. Vainio, L. Lehtonen, S. Vasiliev, Y. Fujii, S. Mitsudo, T. Mizusaki, M. Gwak, SangGap Lee, Soonchil Lee, L. Vlasenko

摘要:

我们证明了硅中磷施主的动态核极化(DNP)在磁场为4.6 T和温度低于1 K时是非常有效的。DNP的发生是由于通过禁止触发器或触发器跃迁的交叉弛豫后的Overhauser效应。在用0.4 $μ$ W的微波功率泵浦20分钟后达到核极化值$P>0.98$。我们评估了在2小时的泵浦后,超精细态布居的比率增加了三个数量级,并且可以创建包含另一个自旋态的$<10^{-11}$的极纯核自旋系统。通过抽运P的低场ESR谱线,然后进行翻转交叉弛豫,得到了反转的DNP。这种转变具有小得多的弛豫速率,并且需要相当长的泵送时间。我们发现,核极化动力学大大偏离一个简单的指数函数。极化的演化由两个时间常数$T^{'}_{ac}\approx$ 15 s和$T^{"}_{ac}\approx$ 1100 s表征。研究了$^{31}$ P的核弛豫速率的温度依赖性,直到0.75 K,低于该温度,弛豫时间变得太长而无法测量。在弛豫过程中,核极化遵循双指数时间依赖关系。我们认为,非指数行为的DNP动力学和随后的弛豫介导的$^{29}$ Si周围的$^{31}$ P施主的核,这影响了禁止交叉弛豫过程的跃迁几率。

英文摘要:

We demonstrate that the dynamic nuclear polarization (DNP) of phosphorus donors in silicon can be very effective in a magnetic field of 4.6 T and at temperatures below 1 K. The DNP occurs due to the Overhauser effect following a cross relaxation via the forbidden flip-flop or flip-flip transitions. Nuclear polarization values $P>0.98$ were reached after 20 min of pumping with 0.4 $μ$W of microwave power. We evaluated that the ratio of hyperfine state populations increased by three orders of magnitude after 2 hours of pumping, and an extremely pure nuclear spin system containing $<10^{-11}$ of the other spin state can be created. An inverted DNP has been obtained by pumping the low field ESR line of P followed by the flip-flip cross relaxation. This transition has much smaller relaxation rate and required substantially longer pumping times. We found that the nuclear polarization dynamics deviates substantially from a simple exponential function. The evolution of the polarization is characterized by two time constants $T^{'}_{ac}\approx$15 s in the beginning, and $T^{"}_{ac}\approx$1100 s for long pumping time. Temperature dependence of the nuclear relaxation rate of $^{31}$P was studied down to 0.75 K, below which the relaxation time became too long to be measured. The nuclear polarization followed a bi-exponential time dependence during relaxation. We suggest that the non-exponential behavior of DNP dynamics and the subsequent relaxation is mediated by the nuclei of $^{29}$Si surrounding $^{31}$P donors, which affect the transition probabilities of the forbidden cross-relaxation processes.

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