Nuclear magnetic resonance study of Li implanted in a thin film of niobium

We report results of beta-detected NMR of ${{}^8\text{L}\text{i}}^{+}$ implanted in Nb at high magnetic field. We identify two distinct sites for ${}^8\text{L}\text{i}$ in the body-centered lattice. At low temperature, the site is characterized by a well-defined quadrupolar splitting. At about 50 K this site becomes unstable. Close to room temperature, Li occupies the cubic substitutional site. Spin-lattice relaxation measurements are consistent with a site-dependent coupling to the Nb conduction electrons and suggest that the site change proceeds in two steps. We report Knight shifts for the two well-defined sites and perform a Korringa analysis.

Figure 1

(Color online). Representative $\beta \text{-NMR}$ spectra with Lorentzian fits of ${}^8\text{L}\text{i}$ in Nb (oriented film) with $H_0=4.1\text{T}$. (a) At room temperature a single resonance from the substitutional sites is observed $\left(H_1\approx 10\mu \text{T}\right)$. (b) The broadened resonance at 150 K appears to contain an additional unresolved resonance. (c) For $T<70\text{K}$, a quadrupole split resonance due to the noncubic interstitial site appears, becoming better resolved at lower temperature. For (b)–(d), $H_1\approx 40\mu \text{T}$.

Figure 2

(Color online) Resonances in the film for ${}^8\text{L}\text{i}$ polarized parallel and antiparallel to $H_0$. (a) The spectrum at 290 K shows no quadrupolar splitting. (b) At 150 K, the line is broader and one helicity has a high-frequency shoulder. (c) At low temperature the quadrupolar splitting of the interstitial site is clearly revealed as satellites on opposite sides of the Larmor frequency for opposite helicities. The solid vertical line marks the resonance position at high temperature, while the dashed line is the center of mass of the outer satellites.

Figure 3

(Color online) The bcc unit cell of Nb, indicating the high-symmetry interstitial sites and the substitutional $\left(S\right)$ site. The face center and edge center are equivalent and are at the center of a distorted octahedron $\left(O\right)$. A representative $\mathcal{T}$ site is the center of the dashed quasitetrahedron at $\left(\frac{1}{4},\frac{1}{2},0\right)$ In the right panel a trigonal interstitial site $\left(\Delta \right)$ is shown. It is equidistant from three near neighbors at the vertices of the isosceles triangle at $\left(\frac{3}{8},\frac{5}{8},0\right)$. The lattice constant $a=3.30\text{Å}$, so the near neighbor distances are $1.65\text{Å}$ $\left(O\right)$, $2.86\text{Å}$ $\left(S\right)$, $1.84\text{Å}$ $\left(\mathcal{T}\right)$, and $1.75\text{Å}$ $\left(\Delta \right)$.

Figure 4

(Color online) (a) Resonance frequencies as a function of temperature: main line (filled triangles); outer quadrupole satellites (filled squares); inner satellites (open circles). (b) Main linewidth (FWHM) and (c) normalized amplitudes, of the main line. In (b) the stars are data taken at lower power $H_1\approx 10\mu \text{T}$ (less power broadened) and indicate the narrowing trend continues to room temperature. Lines are guides to the eye.

Figure 5

(Color online). A composite spectrum of Nb and the cubic insulator MgO at 6.55 T and room temperature fit to two Lorentzians yielding the Knight shift of ${}^8\text{L}\text{i}$ at the $S$ site.

Figure 6

(Color online) Fits of the spin relaxation in the Nb film at $H_0=4.1\text{T}$ for several temperatures. The dashed vertical line is the trailing edge of the beam pulse. Solid lines are fits to a single exponential relaxation.

Figure 7

(Color online) Spin-lattice relaxation rates as a function of temperature in the Nb film at $H_0=4.1\text{T}$. The light curve is a guide to the eye. The linear fits are $0.01271\left(39\right){\left(\text{sK}\right)}^{-1}\times T$ below 20 K (dotted), $0.00554{\left(\text{sK}\right)}^{-1}\times T-0.138{\text{s}}^{-1}$ from 60 to 220 K (dot dash), and $0.0028{\left(\text{sK}\right)}^{-1}\times T$ (solid line), an upper limit for the slope above 300 K. The dark solid curve is a fit to the temperature dependence below 150 K using the activated site change model described in the text.

Figure 8

(Color online) The magnetic field dependence of (a) the relaxing amplitude and (b) the spin-lattice relaxation rate from single exponential fits to relaxation data in the film at 10 K. The curve in (b) is a Lorentzian centered at zero of width 0.2 T.