Development of an ESR/NMR Double-Magnetic-Resonance System for Use at Ultra-low Temperatures and in High Magnetic Fields and Its Use for Measurements of a Si Wafer Lightly Doped with 31P

Dynamic nuclear polarization–nuclear magnetic resonance (DNP–NMR) and electron–nuclear double resonance (ENDOR) provide useful information about the magnetic properties of dilute spin systems. One such system is a Si wafer lightly doped with 31P (Si:P) which is a candidate for quantum-computing devices. The “Si:P” model was proposed by Kane in 1998. To date, however, the details of the nuclear magnetism of 31P, which is important information for the use of these nuclei as quantum bits for computing, are still unknown. The reason is because the spins are diluted, and there has been no report about 31P in Si from direct NMR detection. It is thus necessary to overcome the dilution to show the usefulness of Si:P. The DNP–NMR method provides a way to improve the NMR sensitivity to 31P by controlling the relative magnetization with the Overhauser effect. We have developed magnetic-resonance equipment for ultra-low temperatures and high magnetic fields with the goal of using DNP to detect 31P directly in NMR measurements. We have carried out 31P-DNP–NMR at 139.03 MHz, 220 mK using this system. We have successfully detected by one-shot measurement the spin-echo NMR signal of approximately 1.9 × 1014 fully polarized 31P nuclear spins which is estimated from the size and the concentration of the sample. In this report, we describe a new electron spin resonance (ESR)/NMR double-magnetic-resonance system constructed in a 3He–4He dilution refrigerator, and demonstrate its use to obtain ENDOR and DNP–NMR measurements of 31P in a Si wafer.