$\beta$-detected nuclear quadrupole resonance with a low-energy beam of ${}^8\mathrm{Li}^{+}$

A nuclear quadrupole resonance (NQR) spectrum of ${}^8\mathrm{Li}$ has been observed in a single crystal of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ using a beam of low-energy highly polarized radioactive ${}^8\mathrm{Li}^{+}$. The resonances were detected by monitoring the $\beta$-decay anisotropy as a function of a small audio frequency magnetic field. These results demonstrate that low energy nuclear spin polarized ${}^8\mathrm{Li}$ can be used as a sensitive probe of the local magnetic and electronic environment in nanostructures and ultrathin films in zero static applied magnetic field.

Figure 1

A schematic of the experimental layout. The $30\mathrm{keV}$ ${}^8\mathrm{Li}^{+}$ ion beam is neutralized in the Na cell and then reionized in the He cell. In between, the beam is optically pumped with a laser tuned to the D1 optical transition of the ${}^8\mathrm{Li}$ atom. The resulting polarized ion beam is guided to the $\beta$-NQR spectrometer.

Figure 2

Profile of the ${}^8\mathrm{Li}$ beam stopping in a plastic scintillator taken from a CCD image of the beam. The solid line is a fit of the profile to a Lorentzian shape.

Figure 3

A schematic of the spectrometer for $\beta$-detected nuclear quadrupole resonance. The spin polarization is perpendicular to the beam direction and the oscillating $B_1$ magnetic field. The principal axis of the electric field gradient at the Li stopping site must have a component along $\widehat{z}$ in order for a signal to be detected at zero applied field.

Figure 4

The energy levels for ${\mathcal{H}}_q∕h$ (left-hand side) and $({\mathcal{H}}_q+{\mathcal{H}}_{\eta })∕h$ (right-hand side). The possible transitions induced by an oscillating field $B_1$ are also indicated.

Figure 5

The $\beta$-NQR spectrum obtained for a $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ single crystal at room temperature and zero external field. Inset: The cubic unit cell of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ showing the proposed interstitial Li site at the face center.

Figure 6

The polarization as a function of the angle between the initial spin polarization and the ⟨100⟩ lattice direction.

Figure 7

The effect of small longitudinal field on the time dependence of ${}^8\mathrm{Li}$ polarization as a function of time in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ at RT and no $B_1$.