$\beta$-NMR of isolated ${}^8\mathrm{Li}^{+}$ implanted into a thin copper film

Depth-controlled $\beta$-NMR was used to study highly spin-polarized ${}^8\mathrm{Li}^{+}$ in a Cu film of thickness $100\mathrm{nm}$ deposited onto a MgO substrate. The positive Knight shifts and spin relaxation data show that ${}^8\mathrm{Li}^{+}$ occupies two sites at low temperatures, assigned to be the substitutional $\left(S\right)$ and octahedral $\left(O\right)$ interstitial sites. Between 50 and $150\mathrm{K}$, there is a site change from $O$ to $S$. The temperature dependence of the Knight shifts and spin-lattice relaxation rates at high temperatures, i.e., when all the Li are in the $S$ site, is consistent with the Korringa law for a simple metal.

Figure 1

(Color online) Representative resonance signals of ${}^8\mathrm{Li}^{+}$ in the $\mathrm{Cu}∕\mathrm{Mg}\mathrm{O}$ sample in an applied field of $4.1\mathrm{T}$. Each spectrum was obtained at an implantation energy of $10.6\mathrm{keV}$, except the bottom-most one, which was obtained at the full implantation energy of $30.5\mathrm{keV}$. The three vertical dashed lines indicate the peak frequencies of the MgO, $S$ and $O$ signals. The zero shift in frequency is taken to be that in MgO at room temperature. The long-dashed lines indicate the $S$ and $O$ contributions of the signal at each temperature.

Figure 2

(Color online) Temperature dependences of the normalized amplitudes for an applied field of $4.1\mathrm{T}$.

Figure 3

(Color online) The temperature dependence of $1∕T_1$ at $4.1\mathrm{T}$ (circles) and $6.55\mathrm{T}$ (stars). The straight line is a best fit to the data above $100\mathrm{K}$ through the origin while the dashed line is the calculated effective relaxation rate (see text).