High-field Overhauser DNP in silicon below the metal-insulator transition

摘要

Single crystal silicon is an excellent system in which to explore dynamicnuclear polarization (DNP), as it exhibits a continuum of properties frommetallic to insulating as a function of doping concentration and temperature.At low doping concentrations DNP has been observed to occur via the solideffect, while at very high doping concentrations an Overhauser mechanism isresponsible. Here we report the hyperpolarization of 29Si in n-doped siliconcrystals, with doping concentrations in the range of 1-3 x 10^17 /cc. In thisregime exchange interactions between donors become extremely important. Thesign of the enhancement in our experiments and its frequency dependence suggestthat the 29Si spins are directly polarized by donor electrons via an Overhausermechanism within exchange-coupled donor clusters. The exchange interactionbetween donors only needs to be larger than the silicon hyperfine interaction(typically much smaller than the donor hyperfine coupling) to enable thisOverhauser mechanism. Nuclear polarization enhancement is observed for a rangeof donor clusters in which the exchange energy is comparable to the donorhyperfine interaction. The DNP dynamics are characterized by a singleexponential time constant that depends on the microwave power, indicating thatthe Overhauser mechanism is the rate-limiting step. Since only about 2 % of thesilicon nuclei are located within one Bohr radius of the donor electron,nuclear spin diffusion is important in transferring the polarization to all thespins. However, the spin-diffusion time is much shorter than the Overhausertime due to the relatively weak silicon hyperfine coupling strength. In a 2.35T magnetic field at 1.1 K, we observed a DNP enhancement of $244 +/- 84resulting in a silicon polarization of $10.4 +/- 3.4 % following two hours ofmicrowave irradiation.