硅中金属-绝缘体过渡区以下的高场Overhauser DNP
High-field Overhauser DNP in silicon below the metal-insulator transition
摘要：
单晶硅是探索动态核极化（DNP）的优秀系统，因为它表现出作为掺杂浓度和温度的函数的从金属到绝缘的连续性质。在低掺杂浓度下，已经观察到DNP通过固体效应发生，而在非常高的掺杂浓度下，奥弗豪泽机制负责。在这里，我们报告了n型掺杂硅晶体中29 Si的超极化，掺杂浓度范围为1-3 x 10^17 /cc。在这种制度下，捐助者之间的交流互动变得极其重要。在我们的实验中的增强的迹象和它的频率依赖性表明，29 Si的自旋直接极化的施主电子通过交换耦合施主集群内的Overhauser机制。供体之间的交换相互作用仅需要大于硅超精细相互作用（通常比供体超精细耦合小得多）以实现该奥弗豪瑟机制。核极化增强观察到的范围内的捐助者集群中的交换能量是可比的捐助者超精细相互作用。的DNP动力学的特征在于由一个单一的指数时间常数，取决于微波功率，表明Overhauser机制是限速步骤。由于只有大约2%的硅核位于施主电子的一个玻尔半径内，核自旋扩散在将极化转移到所有自旋中是重要的。然而，由于相对较弱的硅超精细耦合强度，自旋扩散时间比奥弗豪瑟时间短得多。在2.35 T磁场和1.1 K下，我们观察到微波辐射两小时后$244 +/- 84 resulting in a silicon polarization of $ 10.4 +/-3.4%的DNP增强.
英文摘要：
Single crystal silicon is an excellent system in which to explore dynamic nuclear polarization (DNP), as it exhibits a continuum of properties from metallic to insulating as a function of doping concentration and temperature. At low doping concentrations DNP has been observed to occur via the solid effect, while at very high doping concentrations an Overhauser mechanism is responsible. Here we report the hyperpolarization of 29Si in n-doped silicon crystals, with doping concentrations in the range of 1-3 x 10^17 /cc. In this regime exchange interactions between donors become extremely important. The sign of the enhancement in our experiments and its frequency dependence suggest that the 29Si spins are directly polarized by donor electrons via an Overhauser mechanism within exchange-coupled donor clusters. The exchange interaction between donors only needs to be larger than the silicon hyperfine interaction (typically much smaller than the donor hyperfine coupling) to enable this Overhauser mechanism. Nuclear polarization enhancement is observed for a range of donor clusters in which the exchange energy is comparable to the donor hyperfine interaction. The DNP dynamics are characterized by a single exponential time constant that depends on the microwave power, indicating that the Overhauser mechanism is the rate-limiting step. Since only about 2 % of the silicon nuclei are located within one Bohr radius of the donor electron, nuclear spin diffusion is important in transferring the polarization to all the spins. However, the spin-diffusion time is much shorter than the Overhauser time due to the relatively weak silicon hyperfine coupling strength. In a 2.35 T magnetic field at 1.1 K, we observed a DNP enhancement of $244 +/- 84 resulting in a silicon polarization of $10.4 +/- 3.4 % following two hours of microwave irradiation.
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