Unveiling new topological phases of matter is one of the current objectives in condensed matter physics. Recent experimental discoveries of Dirac and Weyl semimetals prompt the search for other exotic phases of matter. Here we present a systematic angle-resolved photoemission spectroscopy study of ZrSiS, a prime topological nodal semimetal candidate. Our wider Brillouin zone (BZ) mapping shows multiple Fermi surface pockets such as the diamond-shaped Fermi surface, elliptical-shaped Fermi surface, and a small electron pocket encircling at the zone center $\left(\mathrm{\Gamma }\right)$ point, the $M$ point, and the $X$ point of the BZ, respectively. We experimentally establish the spinless nodal fermion semimetal phase in ZrSiS, which is supported by our first-principles calculations. Our findings evidence that the ZrSiS-type of material family is a new platform on which to explore exotic states of quantum matter; these materials are expected to provide an avenue for engineering two-dimensional topological insulator systems.

The discovery of a topological nodal-line (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. Here, we present a comparative electronic structure study of $\mathrm{ZrSi}X$ (where $X=\text{S}$, Se, Te) using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Our ARPES studies show that the overall electronic structure of $\mathrm{ZrSi}X$ materials comprises the diamond-shaped Fermi pocket, the nearly elliptical-shaped Fermi pocket, and a small electron pocket encircling the zone center ($\mathrm{\Gamma }$) point, the $M$ point, and the $X$ point of the Brillouin zone, respectively. We also observe a small Fermi surface pocket along the $M-\mathrm{\Gamma }-M$ direction in ZrSiTe, which is absent in both ZrSiS and ZrSiSe. Furthermore, our theoretical studies show a transition from nodal-line to nodeless gapped phase by tuning the chalcogenide from S to Te in these material systems. Our findings provide direct evidence for the tunability of the TNL phase in $\mathrm{ZrSi}X$ material systems by adjusting the spin-orbit coupling strength via the $X$ anion.

A group of ternary cerium compounds ${\mathrm{Ce}}_2$ ${\mathit{T}}_2$In (T=Ni, Cu, Rh, Pd, Pt, and Au) has been synthesized. As found from single-crystal and powder x-ray-diffraction studies, all these phases crystallize in a primitive tetragonal structure of the ${\mathrm{Mo}}_2$ ${\mathrm{FeB}}_2$ type. Magnetic measurements (magnetization, dc and ac susceptibility) have revealed the physical properties of these intermetallics to be mainly governed by the 4f-d hybridization. Depending on the filling of the transition metal d band the ground state in ${\mathrm{Ce}}_2$ ${\mathit{T}}_2$In changes from a nonmagnetic to a well localized magnetic regime. It was proved that ${\mathrm{Ce}}_2$ ${\mathrm{Ni}}_2$In and ${\mathrm{Ce}}_2$ ${\mathrm{Rh}}_2$In are intermediate-valence systems, ${\mathrm{Ce}}_2$ ${\mathrm{Pt}}_2$In is a strongly temperature-dependent paramagnet, whereas ${\mathrm{Ce}}_2$ ${\mathrm{Cu}}_2$In, ${\mathrm{Ce}}_2$ ${\mathrm{Pd}}_2$In, and ${\mathrm{Ce}}_2$ ${\mathrm{Au}}_2$In order magnetically at low temperatures. Measurements of the electrical resistivity have corroborated the intermediate-valence behavior in ${\mathrm{Ce}}_2$ ${\mathrm{Ni}}_2$In and ${\mathrm{Ce}}_2$ ${\mathrm{Rh}}_2$In. In turn, the resistivity of the remaining ternaries studied was found to be determined by an interplay of Kondo scattering and crystal-field effects. For ${\mathrm{Ce}}_2$ ${\mathrm{Pt}}_2$In which behaves like a spin-fluctuating system due to strong Kondo-type interactions a nonmagnetic heavy-fermion ground state probably occurs. © 1996 The American Physical Society.

Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. A majority of discovered topological semimetals are nonmagnetic and conserve time reversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodal-line semimetallic state in GdSbTe using angle-resolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the presence of multiple Fermi surface pockets including a diamond-shape, and small circular pockets around the zone center and high symmetry X points of the Brillouin zone (BZ), respectively. Furthermore, we observe the presence of a Dirac-like state at the X point of the BZ and the effect of magnetism along the nodal-line direction. Interestingly, our experimental data show a robust Dirac-like state both below and above the magnetic transition temperature (T_N = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter.

We present electronic transport and magnetic properties of single crystals of semimetallic half-Heusler phase LuPdBi, having theoretically predicted band inversion requisite for nontrivial topological properties. The compound exhibits superconductivity below a critical temperature T_c = 1.8 K, with a zero-temperature upper critical field B_c2 ≈ 2.3 T. Although superconducting state is clearly reflected in the electrical resistivity and magnetic susceptibility data, no corresponding anomaly can be seen in the specific heat. Temperature dependence of the electrical resistivity suggests existence of two parallel conduction channels: metallic and semiconducting, with the latter making negligible contribution at low temperatures. The magnetoresistance is huge and clearly shows a weak antilocalization effect in small magnetic fields. Above about 1.5 T, the magnetoresistance becomes linear and does not saturate in fields up to 9 T. The linear magnetoresistance is observed up to room temperature. Below 10 K, it is accompanied by Shubnikov-de Haas oscillations. Their analysis reveals charge carriers with effective mass of 0.06 m_e and a Berry phase very close to π, expected for Dirac-fermion surface states, thus corroborating topological nature of the material.