Unusual diffusive effects on the ESR of Nd3+ ions in the tunable topologically nontrivial semimetal YBiPt

Electron spin resonance (ESR) of diluted Nd3+ ions in the topologically nontrivial semimetallic (TNSM) YBiPt compound is reported. The cubic YBiPt compound is a non-centrosymmetric half Heusler material which crystallizes in the F43m space group. The low temperature Nd3+ ESR spectra showed a g-value of 2.66(4) corresponding to a cubic crystal field Kramers' doublet ground state. Remarkably, the observed metallic and diffusive (Dysonian) Nd3+ lineshape presented an unusual dependence with grain size, microwave power, Nd3+ concentration and temperature. Moreover, the spin dynamic of the localized Nd3+ ions in YBiPt was found to be characteristic of a phonon-bottleneck regime. It is claimed that, in this regime for YBiPt, phonons are responsible for mediating the diffusion of the microwave energy absorbed at resonance by the Nd3+ ions to the thermal bath throughout the skin depth ( μm). We argue that this is only possible because of the existence of highly mobile conduction electrons inside the skin depth of YBiPt that are strongly coupled to the phonons by spin–orbit coupling. Therefore, our unexpected ESR results point to a coexistence of metallic and insulating behaviors within the skin depth of YBiPt. This scenario is discussed in the light of the TNSM properties of this compound.