Significance: Among topological materials, experimental study of topological semimetals that host Dirac/Weyl fermions has just begun, even though these topological concepts were proposed nearly a century ago by Dirac and Weyl. Our work shows magnetic semimetal BaMnSb ₂ exhibits nearly zero-mass fermions with high mobility and a nontrivial Berry phase. What is unique is the magnetic ordering, indicating the system is Weyl type due to time-reversal symmetry breaking. Theory shows that the spin order is very fragile, so it is expected that the application of magnetic field or uniaxial pressure could drive the material to be type II Weyl semimetal. In this material, Mn mainly controls the magnetic structure, whereas Sb in only one lattice site is responsible for the Dirac behavior.

Eu‐based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd ₂ As ₂ . The authors investigate the structure and physical properties of EuZn ₂ As ₂ through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Their data show that EuZn ₂ As ₂ orders antiferromagnetically with an A‐type spin configuration below T _N = 19 K. Surprisingly, there is strong evidence for dominant ferromagnetic fluctuations above T _N , as reflected by positive Curie–Weiss temperature and extremely large negative magnetoresistance (MR) between T _N and T _fl ≈200 K. Furthermore, the angle dependence of the MR _ab indicates field‐induced spin reorientation from the ab ‐plane to a direction ≈45° from the ab plane. Compared to EuCd ₂ As ₂ , the doubled T _N and T _fl make EuZn ₂ As ₂ a better platform for exploring nontrivial magnetic and electronic properties in both magnetic fluctuation ( T _N < T < T _fl ) and ordered ( T < T _N ) regimes.

Transition-metal dipnictide PtBi₂ exhibits rich structural and physical properties with topological semimetallic behavior and extremely large magnetoresistance (XMR) at low temperatures. We have investigated the electrical and magnetic properties of trigonal-phase PtBi_2-x single crystals with x ~ 0.4. Profound de Haas–van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations are observed. Through fast Fourier transformation (FFT) analyses, four oscillation frequencies are extracted, which result from α, β, γ, and δ bands. By constructing the Landau fan diagram for each band, the Berry phase is extracted demonstrating the non-trivial nature of the α, β, and δ bands. Despite Bi deficiency, we observe the Zeeman splitting in dHvA and SdH oscillations under moderate magnetic field and the moderate Landé g factor (4.97–6.48) for the α band. Quantitative analysis of the non-monotonic field dependence including the sign change of the Hall resistivity suggests that electrons and holes in our system are not perfectly compensated thus not responsible for the XMR effect.

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below $T_c\sim 4.5\mathrm{K}$. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc. These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

We report temperature and thermal-cycling dependence of surface and bulk structures of double-layered perovskite Sr3Ru2O7 single crystals. The surface and bulk structures were investigated using low-energy electron diffraction (LEED) and single-crystal x-ray diffraction (XRD) techniques, respectively. Single-crystal XRD data is in good agreement with previous reports for the bulk structure with RuO6 octahedral rotation, which increases with decreasing temperature [$\sim${}6.7(6)\ifmmode^\circ\else\textdegree\fi{} at 300 K and $\sim${}8.1(2)\ifmmode^\circ\else\textdegree\fi{} at 90 K]. LEED results reveal that the octahedra at the surface are much more distorted with a higher rotation angle ($\sim${}12\ifmmode^\circ\else\textdegree\fi{} between 300 and 80 K) and a slight tilt [(4.5\ifmmode\pm\else\textpm\fi{}2.5)\ifmmode^\circ\else\textdegree\fi{} at 300 K and (2.5\ifmmode\pm\else\textpm\fi{}1.7)\ifmmode^\circ\else\textdegree\fi{} at 80 K]. While XRD data confirms temperature dependence of the unit-cell height/width ratio (i.e., lattice parameter c divided by the average of parameters a and b) found in a prior neutron-powder-diffraction investigation, both bulk and surface structures display little change with thermal cycles between 300 and 80 K.

Among one-dimensional transition-metal trichalcogenides, TaSe₃ is unconventional in many respects. One is its strong topological semimetallicity as predicted by first-principles calculations. We report the experimental investigations of the electronic properties of one-dimensional-like TaSe₃ single crystals. While the b-axis electrical resistivity shows good metallicity with a high residual resistivity ratio greater than 100, an extremely large magnetoresistance is observed reaching ≈7 × 10³% at 1.9 K for 14 T. Interestingly, the magnetoresistance follows the Kohler’s rule with nearly quadratic magnetic field dependence, consistent with the electron–hole compensation scenario as confirmed by our Hall conductivity data. Both the longitudinal and Hall conductivities show Shubnikov-de Haas oscillations with two frequencies: F_α ≈ 97 T and F_β ≈ 186 T. Quantitative analysis indicates that F_α results from the two-dimensional-like electron band with the non-trivial Berry phase [1.1π], and F_β from the hole band with the trivial Berry phase [0(3D) − 0.16π(2D)]. Our experimental findings are consistent with the predictions based on first-principles calculations.