Electronic properties of topological semimetal candidates ACd$_2$Pn$_2$ (A = Eu, Sr; Pn = As, Sb)

Abstract

Weyl semimetals have attracted significant attention due to their intriguing electronic properties arising from the presence of Weyl points. Among them, ideal magnetic Weyl semimetals, characterized by a single pair of Weyl points close to the Fermi level, have long been sought for exploring the intrinsic topological effects associated with Weyl fermions free from interference by other trivial energy bands. In the past few years, the antiferromagnetic layered compound EuCd$_2$Pn$_2$ (Pn = As, Sb) has been regarded as a potential platform to realize several different topological phases via tuning its magnetic order. Specifically, EuCd$_2$As$_2$ was proposed to be one of the leading candidates to realize an ideal magnetic Weyl semimetal. The earlier studies of magneto-transport and photoemission seem to agree with the existence of the Weyl semimetal phase, however, the direct observation of Weyl crossings of EuCd$_2$As$_2$ is lacking. In this study, through materials synthesis, materials characterization, and systematic magneto-transport and photoemission measurements, we not only reveal the trivial topological nature of ACd$_2$Pn$_2$ (A = Eu, Sr; Pn = As, Sb) but also distinguish the origin of the overestimation of the topological region in ACd$_2$Pn$_2$. In the first part of the study, we successfully synthesized highly insulating EuCd$_2$As$_2$ crystals with carrier density reaching as low as $2\times 10^{15}$ $\text{cm}^{-3}$. The magnetotransport measurements revealed a progressive decrease of the anomalous Hall conductivity by several orders of magnitude as the carrier density decreases. The behavior contradicts what is expected from the intrinsic anomalous Hall conductivity generated by the Weyl points, which is independent of carrier density as the Fermi level approaches the charge neutrality point. In contrast, the scaling relationship between AHC and longitudinal conductivity aligns with the characteristics of variable-range hopping insulators. Our results suggest that EuCd$_2$As$_2$ is a magnetic semiconductor rather than a topological Weyl semimetal. There is a significant discrepancy between DFT calculation prediction and experimental results. In a very recent work, it was found there are two possible origins of the underestimation of the bandgap in EuCd$_2$As$_2$, the choice of DFT functional and the size of the Hubbard potential \textbf{U}. To elucidate the origin of the underestimation, we performed a comparative study of EuCd$_2$Pn$_2$ and SrCd$_2$Pn$_2$ (Pn = As, Sb). transport and optical transmittance study of EuCd$_2$Pn$_2$ and SrCd$_2$Pn$_2$. EuCd$_2$Sb$_2$ is the sister compound of EuCd$_2$As$_2$ with identical crystal structure and antiferromagnetic ground state. It was predicted that in the field-induced FM phase, EuCd$_2$Sb$_2$ is a type-II Weyl semimetal with five pairs of Weyl nodes. SrCd$_2$Pn$_2$ is a non-magnetic analog of EuCd$_2$Pn$_2$. It has the same properties as EuCd$_2$Pn$_2$ but without Eu magnetic moments. Hence, SrCd$_2$Pn$_2$ is an ideal platform to separate the effect of 4f electrons on band structures. We successfully synthesized high-quality ACd$_2$Pn$_2$ (A = Eu, Sr; Pn = As, Sb) and conducted studies of electrical transport, optical transmittance, and DFT calculation of these samples. In the electrical transport measurement, we found that the temperature dependence of resistivity of RCd$_2$As$_2$ (R = Eu, Sr) shows an insulating behavior. We observed the Shubnikov-de Haas oscillation on the magneto- and Hall resistivity of RCd$_2$Sb$_2$ (R = Eu, Sr), and extracted the effective mass of carriers using the temperature dependence of the SdH oscillation. In the Fourier Transform Infrared spectroscopy (FTIR) measurement, we observed an electronic gap of $\sim$0.74 eV, $\sim$0.86 eV, $\sim$0.48 eV, and $\sim$0.54 eV for EuCd$_2$As$_2$, SrCd$_2$As$_2$, EuCd$_2$Sb$_2$, and SrCd$_2$Sb$_2$, respectively. Additionally, the extra drops on the transmission spectrum of EuCd$_2$Sb$_2$ and SrCd$_2$Sb$_2$ are in agreement with the estimated values of plasma frequencies from the measured effective mass and carrier density. This further confirmed that EuCd$_2$Sb$_2$ and SrCd$_2$Sb$_2$ are hole-doped semiconductors. Our results suggest that the use of local functionals strongly overestimates the topological region in the prediction and design of topological phases in ACd$_2$Pn$_2$.