Unusual Al27 NMR shift in the Weyl-fermion systems LaAlGe and PrAlGe

Abstract

LaAlGe and PrAlGe are members of the $R\mathrm{AlGe}$ series of compounds (with $R$ a light rare-earth element), which have been shown to host Weyl fermions. By exchanging the rare-earth cation, $R\mathrm{AlGe}$ offers a remarkable tunability of its electronic properties thereby allowing to identify the electronic response of Weyl semimetals under varying temperature- and magnetic-field conditions. Through a comparative $^{27}\mathrm{Al}$ NMR study of LaAlGe and PrAlGe, we show that the rare-earth element can also be used to tune the NMR response of Weyl fermions in the host. In both cases, we observe unusual NMR shifts that deviate significantly from those in ordinary metals, such as Al. In LaAlGe, we report the observation of a small negative shift, strongly varying with temperature, which can be explained by a recent theory of NMR shifts invoking Weyl fermions. In PrAlGe, instead, we report a giant positive NMR shift of up to 20% at 6 K, most likely due to the transferred hyperfine field from the ${\mathrm{Pr}}^{3+}\phantom{\rule{0.28em}{0ex}}4f$-electron local moments. We discuss the implications of our findings on future studies of Weyl-fermion systems.