Cyclotron resonance in rare-earth monopnictides

Abstract

Recently, we have developed equipment which can be used for both cyclotron resonance (CR) and electron spin resonance (ESR) measurements for studying $f$-electron systems. Using this equipment and preparing high-quality single crystals with a residual resistivity ratio of about 500, we have successfully observed CR signals in rare-earth monopnictides $RX\phantom{\rule{0.3em}{0ex}}(R=\mathrm{La},\mathrm{Ce},\mathrm{Pr},\mathrm{Gd},X=\mathrm{Sb},\mathrm{Bi})$. The purpose of this paper is to describe the development of our measurement system in detail and to reexamine our recent results of CR measurements on the single crystals of $RX$. And then, we discuss the origin of mass enhancement in this system by comparing the case of a strongly correlated $f$-electron system CeSb with the case of a nonmagnetic simple semimetal LaSb. The CR measurements have been performed in the temperature range from $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in the frequency range from $50\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}190\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. We have observed CR signals in LaSb, LaBi, CeSb, PrSb, and GdSb. The cyclotron effective masses ${m}_{\mathit{CR}}^{*}$ determined by the CR measurements are compared with the masses ${m}_{\mathit{QO}}^{*}$ estimated by measurements of quantum oscillations and the masses ${m}_{\mathit{BC}}^{*}$ deduced from band structure calculations. The determined ${m}_{\mathit{CR}}^{*}$ of LaSb, LaBi, and GdSb is in the range of $(0.17--0.65){m}_{0}$. These values are reasonably consistent with the values of ${m}_{\mathit{QO}}^{*}$ and ${m}_{\mathit{BC}}^{*}$. This fact shows that both ${m}_{\mathit{CR}}^{*}$ and ${m}_{\mathit{QO}}^{*}$ mainly depend on the band structure if the interaction between conduction electrons and $f$ electrons is negligible. On the other hand, in the case of a strongly correlated $f$-electron system CeSb, ${m}_{\mathit{CR}}^{*}$ is in the range of $(0.26--1.5){m}_{0}$. These values are much larger than those of ${m}_{\mathit{BC}}^{*}$. This finding shows that ${m}_{\mathit{CR}}^{*}$, as well as ${m}_{\mathit{QO}}^{*}$, is considerably enhanced in CeSb. Our results indicate that the interband interaction is important for the mass enhancement of ${m}_{\mathit{CR}}^{*}$ in CeSb.