Ground State of the Easy-Axis Rare-Earth Kagome Langasite ${\mathrm{Pr}}_3{\mathrm{Ga}}_5{\mathrm{SiO}}_{14}$

We report muon spin relaxation and ${}^{69,71}\mathrm{Ga}$ nuclear quadrupolar resonance local-probe investigations of the kagome compound ${\mathrm{Pr}}_3{\mathrm{Ga}}_5{\mathrm{SiO}}_{14}$. Small quasistatic random internal fields develop below 40 K and persist down to our base temperature of 21 mK. They originate from hyperfine-enhanced ${}^{141}\mathrm{Pr}$ nuclear magnetism which requires a nonmagnetic ${\mathrm{Pr}}^{3+}$ crystal-field (CF) ground state. In addition, we observe a broad maximum of the relaxation rate at $\simeq 10\mathrm{K}$ which we attribute to the population of the first excited magnetic CF level. Our results yield a Van Vleck paramagnet picture, at variance with the formerly proposed spin-liquid ground state.

Figure 1

(a) $T$ dependence of zero-field (ZF) ${\mu }^{+}$ depolarization. Solid lines are fits to the model $G_{\mathrm{VKT}}\exp [-(\lambda t{)}^{\alpha }]$ with $\beta =1.3$, while dashed lines correspond to the Gaussian Kubo-Toyabe model. (b) Longitudinal-field decoupling (see text) at 1.6 K. Solid lines show the agreement with the LF Voigtian Kubo-Toyabe model for the same $\beta =1.3$. The data have been corrected for muons stopping in the sample holder.

Figure 2

Temperature evolution of the width of the static-random-internal-field distribution. The inset proves its linear scaling with bulk susceptibility measured in 10 G.

Figure 3

$T$-dependent ${\mu }^{+}$ depolarization in a 500 G longitudinal field.

Figure 4

${\mu }^{+}$ relaxation rate $\lambda$ in LF 500 G (squares) and NQR spin-lattice relaxation rate $1/T_1$ measured at 27 MHz (circles). Solid line corresponds to the fit to the spin-gap model [${\Delta }_{\mathrm{CF}}=18(3)\mathrm{K}$] with residual zero-temperatures relaxation. The inset shows comparison of the NQR spectra recorded in ${\mathrm{Pr}}_3{\mathrm{Ga}}_5{\mathrm{SiO}}_{14}$ (PGS) and ${\mathrm{Nd}}_3{\mathrm{Ga}}_5{\mathrm{SiO}}_{14}$ (NGS) at 80 K.