We study polycrystalline samples of the hexagonal pnictides, CaAgP and CaAgAs, both of which are ideal candidates for line-node Dirac semimetals. The polycrystalline samples of CaAgP and CaAgAs obtained in this study are low-carrier metals, where hole carriers are dominant. By combining the hole carrier densities estimated from Hall coefficients and the electronic structures calculated by first principles calculations, both samples are found to have a ring-torus Fermi surface, derived from a ring-shaped Dirac line node. In the phosphide sample, the Fermi energy E F is located at around the end of the linear dispersion region of the electronic bands, while the E F in the arsenide sample exists in the middle of this region, suggesting that the arsenide is a more promising system for uncovering the physics of line-node Dirac semimetals.

We report the discovery of a very large thermoelectric power over 400 V K 1 in the whisker crystals of a one-dimensional telluride Ta 4 SiTe 4 , while maintaining a low electrical resistivity of  = 2 mcm, yielding a very large power factor of P = 80 W cm 1 K 2 at an optimum temperature of 130 K. This temperature is widely controlled from the cryogenic temperature of 50 K to room temperature by chemical doping, resulting in the largest P of 170 W cm 1 K 2 at 220-280 K. These P values far exceed those of the Bi 2 Te 3 -Sb 2 Te 3 alloys at around room temperature, offering an avenue for realizing the practical-level thermoelectric cooling at low temperatures. The coexistence of a one-dimensional electronic structure and a very small band gap appearing in the vicinity of the Dirac semimetals probably causes the very large power factors in Ta 4 SiTe 4 , indicating that the "one-dimensional Dirac semimetal" is a promising way to find high-performance thermoelectric materials for the low temperature applications.

We report the discovery of a bulk superconducting transition at 3.4 K in the ternary phosphide, ScIrP, which crystallizes in a hexagonal ZrNiAl-type structure without spatial inversion symmetry. On the basis of heat capacity data in a zero magnetic field, ScIrP is suggested to be a weakly-coupled Bardeen-Cooper-Schrieffer superconductor. Alternatively, experimental results under magnetic fields indicate that this material is a type-II superconductor with an upper critical field H c2 at magnetic fields above 5 T at zero temperature. This moderately high H c2 does not violate the Pauli limit, but it does imply that there is a significant effect from the strong spin-orbit interaction of Ir 5d electrons in the noncentrosymmetric crystal structure. Electronic structure calculations show an interesting feature of ScIrP, where both the Sc 3d and Ir 5d orbitals contribute to the electronic density of states at the Fermi level.Elemental Ca and Sc show superconductivity with a relatively high transition temperature T c of ~20 K at high pressures. Ca is not a superconductor at ambient pressure but becomes superconducting above ~50 GPa [1]. T c increases with increasing pressure and exceeds 20 K at ~100 GPa [2], which is the highest T c in all elemental metals. Sc exhibits a superconducting transition above ~60 GPa, which approaches 20 K at 107 GPa [3,4]. At higher pressures, however, Sc undergoes a structural phase transition to a nonsuperconducting phase (Sc-III). If this transition did not occur, Sc would exhibit a T c higher than 20 K. In simple-metal superconductors such as Al, Sn, and Pb, T c decreases with increasing pressure due to a decrease of the electronic density of states (DOS). The pressure effects on T c in Ca and Sc, opposite to those in simple metals, have been discussed in terms of the pressure-induced s−d electron transfer where 4s electrons are dominant for electron conduction at ambient pressure and are transferred to the narrower 3d band by applying pressure [4,5]. The electrons occupying the 3d band play an important role in relatively high T c superconductivity.

Multiple line nodes in energy-band gaps are found in semimetals preserving mirror-reflection symmetry. We classify possible configurations of multiple line nodes with crossing points (crossing line nodes) under point-group symmetry. Taking the spin-orbit interaction (SOI) into account, we also classify topological phase transitions from crossing-line-node Dirac semimetals to other topological phases, e.g., topological insulators and point-node semimetals. This study enables one to find crossing-line-node semimetal materials and their behavior in the presence of SOI from the band structure in the absence of SOI without detailed calculations. As an example, the theory applies to hexagonal rare-earth trihydrides with the HoD3 structure and clarifies that it is a crossing-line-node Dirac semimetal hosting three line nodes.

We report the synthesis and superconducting properties of the ternary phosphide ScRhP. The crystal structure of ScRhP is determined to be the ordered Fe2P type with the hexagonal P-62m space group by powder X-ray diffraction experiments. Resistivity, magnetization, and heat capacity data indicate that ScRhP is a bulk superconductor with a transition temperature Tc of 2 K. This Tc is lower than that of its 5d analogue, ScIrP (Tc = 3.4 K), although ScRhP is found to have larger electronic density of states at the Fermi energy and a higher Debye temperature than those of ScIrP.Comment: 6 pages, 6 figure