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

Abstract

Electron spin resonance (ESR) of diluted Nd 3+ ions in the topologically nontrivial semimetallic (TNSM) YBiPt compound is reported. The cubic YBiPt compound is a noncentrosymmetric half Heusler material which crystallizes in the F43m space group. The low temperature Nd 3+ ESR spectra showed a g-value of 2.66(4) corresponding to a Γ 6 cubic crystal field Kramers' doublet ground state. Remarkably, the observed metallic and diffusive (Dysonian) Nd 3+ lineshape presented an unusual dependence with grain size, microwave power, Nd 3+ concentration and temperature. Moreover, the spin dynamic of the localized Nd 3+ 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 Nd 3+ ions to the thermal bath throughout the skin depth (δ 15 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.

Diffusive-like effects and possible non trivial local topology on the half-Heusler YPdBi compound

“…For comparison, the microwave power which saturates the Nd 3 + ESR in Y 1x Nd x PtBi is ≈ 5 mW. 18 This result corroborates with the first observation that the YPdBi system has a more insulating character than the YPtBi system, albeit the skin depth of the two systems are roughly the same. Furthermore, the microwave power necessary to saturate the Nd 3 + ESR in YPdBi at T = 4 K is x-dependent, which indicates a phonon bottleneck process in this system.…”

“…4 c) with the microwave power until the system is saturated, which is the same behavior observed for Y 1x Nd x PtBi. 18 These two results (the evolution with temperature and with the microwave power) strongly suggests that the origin of the diffusive effects in the Nd 3 + ESR is the same in YPdBi and YPtBi, but the diffusive effects are much more subtle in the former. In fact, the Dysonian Nd 3 + ESR lineshape for the dilute samples and the evolution of the Nd 3 + ESR lineshape as a function of the particle size assures us that the Nd 3 + local moments are probing the presence of ce within the skin depth.…”

“…The ratio A/B starts to decrease with high power due to saturation effects in the ESR lineshape, which could be associated to lineshape distortion in the saturated regime. 17 The ratio A/B 2.6 for all the measurements of the ESR lineshape of the Nd 3 + is an indicative that the YPdBi has an insulating character more pronounced than in the YPtBi compound, even though the two systems have nearly the same skin depth (δ = 15 µm in YPtBi 18 ). The Nd 3 + g-value observed for those ESR were g = 2.641(1), which is consistent with a Γ 6 cubic CEF ground state.…”

“…Using ρ 4K = 0.6(1) mΩ-cm, we estimate δ ≈ 12(2) µm, which is similar with the YPtBi skin depth. 18 Fig . 2 a), b) and c) shows the ESR Nd 3 + , which is associated to the Γ 6 cubic CEF state, in Y 1x Nd x PdBi, for x = 0.1, as a function of the particle size.…”

“…As a consequence, these Dirac fermions would diffuse over the skin depth. 18 In order to verify this hypothesis in a counterpart compound in the same family, we report a systematic ESR study in Y 1x Nd x PdBi. The YPdBi is on the frontier of topological triviality and non-triviality [12][13][14] and could be a useful platform to contrast with the previous results reported for YPtBi.…”