Nuclear spin orientation via electron spin locking (NOVEL)

In this communication we present what we believe to be the first observation of resonant transfer of electron spin polarization to a nuclear spin system. Our method is called nuclear spin orientation via electron spin locking (NOVEL) and is, in some aspects, similar to the technique of dynamic nuclear polarization (I). The nuclear spin system is present in a solid doped with paramagnetic centers and microwave irradiation is used to transfer the electron spin polarization of these paramagnetic centers to the nuclear spins. However, instead of cw microwave irradiation, we apply pulsed microwave fields in the NOVEL experiment. Our experimental procedure is closely related to the method of Hartmann and Hahn (2) for transferring spin polarization from one nuclear spin species to another. As we will explain in more detail below, our method consists of bringing the electron spins in a so-called spin-locked state in the rotating frame where they have the same resonance frequency as the nuclear spins in the laboratory frame. The presence of dipolar interaction between the two spin species then allows for the resonant transfer of the electron spin polarization to the nuclear spins. The sample in our experiment is a single crystal of silicon doped with 1017 cms3 boron acceptors. The paramagnetic centers are the holes bound to these acceptors. We apply a uniaxial stress of 4 kbar along the (111) axis of the crystal. Then the ground state of the holes forms a Kramers doublet described by an effective electron spin S = 1 with an axially symmetric g tensor. The principal values of this tensor are gll = 1.21 and g, = 2.43 for the directions parallel and perpendicular to the direction of the uniaxial stress (3). The nuclear spin system is formed by the 29Si I = 1 nuclei having a gyromagnetic ratio y1 = 2~8.458 MHz/T and present at a natural abundance of 4.7%. Our NOVEL experiments are performed at 1.2 K with an electron spin-echo (ESE) spectrometer operating at 9.5 GHz equipped with a specially designed double, splitring resonator which allows NMR measurements to be performed and the uniaxial stress to be applied to the sample. The details of the construction of this resonator will be described elsewhere. The magnetic field & is oriented perpendicular to the (111) axis of the crystal and has an amplitude of 0.28 T. In these circumstances the electron spins reach a thermal equilibrium polarization Pp = (&)/S = 0.19 with a time constant Ti, = 20 ms, as a result of electron spin-lattice relaxation processes. With a repetition time long compared with Tr, we apply composite microwave pulses