One of key challenges in current material research is to search for new topological materials with inverted bulk-band structure. In topological insulators, the band inversion caused by strong spin-orbit coupling leads to opening of a band gap in the entire Brillouin zone, whereas an additional crystal symmetry such as point-group and nonsymmorphic symmetries sometimes prohibits the gap opening at/on specific points or line in momentum space, giving rise to topological semimetals. Despite many theoretical predictions of topological insulators/semimetals associated with such crystal symmetries, the experimental realization is still relatively scarce. Here, using angle-resolved photoemission spectroscopy with bulk-sensitive soft x-ray photons, we experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new family of topological insulators characterized by the inverted band structure and the mirror reflection symmetry of crystal. We have established the bulk valence-band structure in three-dimensional Brillouin zone, and observed the Dirac-like energy band and ring-torus Fermi surface associated with the line node, where bulk valence and conducting bands cross on a line in the momentum space under negligible spin-orbit coupling. Intriguingly, we found that no other bands cross the Fermi level and therefore the low-energy excitations are solely characterized by the Dirac-like band. CaAgAs provides an excellent platform to study the interplay among low-energy electron dynamics, crystal symmetry, and exotic topological properties.

We report the electronic properties of single crystals of candidate nodal-line semimetal CaAgP. The transport properties of CaAgP are understood within the framework of a hole-doped nodalline semimetal. In contrast, Pd-doped CaAgP shows a drastic increase of magnetoresistance at low magnetic fields and a strong decrease of electrical resistivity at low temperatures probably due to weak antilocalization. Hall conductivity data indicated that the Pd-doped CaAgP has not only hole carriers induced by the Pd doping, but also high-mobility electron carriers in proximity of the Dirac point. Electrical resistivity of Pd-doped CaAgP also showed a superconducting transition with onset temperature of 1.7-1.8 K. measured [26,27]. In the former study, EF of the polycrystalline sample was located far below (0.4 eV) the Dirac point probably due to lattice defects, and the samples showed conventional metallic resistivity and magnetoresistance. The latter study, which combined the ARPES measurements and calculation, suggested that band inversion does not occur in CaAgP.

Solid-state thermoelectric cooling is expected to be widely used in various cryogenic applications such as local cooling of superconducting devices. At present, however, thermoelectric cooling using p-and n-type Bi2Te3-based materials has been put to practical use only at room temperature. Recently, M4SiTe4 (M = Ta, Nb) has been found to show excellent n-type thermoelectric properties down to 50 K. This paper reports on the synthesis of high-performance p-type M4SiTe4 by Ti doping, which can be combined with n-type M4SiTe4 in a cooling device at low temperatures. The thermoelectric power factor of p-type M4SiTe4 reaches a maximum value of approximately 60 W cm −1 K −2 at 210 K and exceeds the practical level in a wide temperature range of 130−270 K. A finite temperature drop by Peltier cooling was also achieved in a cooling device made of p-and ntype Ta4SiTe4 whisker crystals. These results clearly indicate that M4SiTe4 is promising to realize a practical thermoelectric cooler for use at low temperatures, which are not covered by Bi2Te3based materials.

We found that whisker crystals of Mo-doped Nb 4 SiTe 4 show high thermoelectric performances at low temperatures, indicated by the largest power factor of ~70 W cm 1 K 2 at 230-300 K, much larger than those of Bi 2 Te 3 -based practical materials. This power factor is smaller than the maximum value in the 5d analogue Ta 4 SiTe 4 , but is comparable to that with a similar doping level. First principles calculation results suggest that the difference in thermoelectric performances between Nb and Ta compounds is caused by the much smaller band gap in Nb 4 SiTe 4 than that in Ta 4 SiTe 4 , due to the weaker spin-orbit coupling in the former. We also demonstrated that the solid solution of Nb 4 SiTe 4 and Ta 4 SiTe 4 shows a large power factor, indicating that their combination is promising as a practical thermoelectric material, as in the case of Bi 2 Te 3 and Sb 2 Te 3 . These results advance our understanding of the mechanism of high thermoelectric performances in this one-dimensional telluride system, as well as indicating the high potential of this system as a practical thermoelectric material for low temperature applications.

We report experimental observations of chiral magnetic skyrmion phases in thin films of molybdenum nitride with a filled β-Mn-type structure. A series of Fe2−xPdxMo3N (x = 0.15, 0.32, and 0.54) thin films are grown epitaxially with the (110) orientation on c-plane sapphire substrates by reactive magnetron sputtering, and their structural, magnetic, and transport properties are investigated. Studies using the Topological Hall effect and Lorenz transmission electron microscopy imaging for films with x = 0.32 identified the existence of two types of skyrmion phases with a size as small as 60 nm; one is a dense skyrmion phase at temperatures below 100 K, and the other is an isolated skyrmion phase in a higher temperature range to well beyond room temperature. These epitaxial thin films in the family of molybdenum nitrides open the way for the study of skyrmions, manipulation of their properties, and the exploration and optimization for skyrmion-based applications.