Superconductivity in the Honeycomb-Lattice Pnictide SrPtAs

We report superconductivity in the honeycomb-lattice pnictide SrPtAs with Tc = 2.4 K. To our knowledge, SrPtAs is the first superconducting pnictide with a honeycomb lattice structure. Our finding opens up new playground to develop pnictide superconductors with honeycomb lattices, in addition to square lattices known in iron-based superconductors.Comment: 2 pages, 3 figure

Copper Doping of BaNi2_{2}As2_{2}: Giant Phonon Softening and Superconductivity Enhancement

The effects of copper doping on the structural and superconducting phase transitions of Ba(Ni$_{1-x}$Cu$_{x}$)$_{2}$As$_{2}$ were studied by examining the resistivity, magnetic susceptibility, and specific heat. We found an abrupt increase in the superconducting transition temperature $T_{\rm c}$ from 0.6 K in the triclinic phase with less copper ($x$ $\leq$ 0.16) to 2.5-3.2 K in the tetragonal phase with more copper ($x$ $>$ 0.16). The specific-heat data suggested that doping-induced phonon softening was responsible for the enhanced superconductivity in the tetragonal phase. All of these observations exhibited striking similarities to those observed in the phosphorus doping of BaNi$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ [K. Kudo et al., Phys. Rev. Lett. 109, 097002 (2012).], which markedly contrast the behavior of phosphorus and copper doping of the iron-based superconductor BaFe$_{2}$As$_{2}$.Comment: 5 pages, 4 figure

Suppression of Structural Phase Transition in IrTe2 by Isovalent Rh Doping

We report that the isovalent Rh doping of IrTe2 suppresses the structural/electronic phase transition and induces superconductivity at 2.6 K. The doping level of Rh that is necessary for suppressing the transition is three times higher than those of Pt and Pd. Ir1-xRhxTe2 might provide us with a unique opportunity to study the origin of the structural/electronic phase transition in IrTe2.Comment: 6 pages, 2 figure

Superconductivity in MgPtSi: An orthorhombic variant of MgB2

A ternary compound, MgPtSi, was synthesized by solid-state reaction. An examination of the compound by powder X-ray diffraction revealed that it crystallizes in the orthorhombic TiNiSi-type structure with the Pnma space group. The structure comprises alternately stacked layers of Mg and PtSi honeycomb network, which is reminiscent of MgB2, and the buckling of the honeycomb network causes orthorhombic distortion. Electrical and magnetic studies revealed that MgPtSi exhibited superconductivity with a transition temperature of 2.5 K. However, its isostructural compounds, namely, MgRhSi and MgIrSi, were not found to exhibit superconductivity.Comment: 6 pages, 7 figures, 1 tabl

Enhancing high-temperature thermoelectric properties of PtAs2 by Rh doping

The effects of Rh doping on the thermoelectric properties of Pt1-xRhxAs2 (x = 0, 0.005, and 0.01) with pyrite structure were studied by conducting measurements of electrical resistivity rho, Seebeck coefficient S, and thermal conductivity kappa. The sample with x = 0.005 exhibited large S and low rho, resulting in a maximum power factor (S^2/rho) of 65 muW/cmK^2 at 440 K. The peculiarly shaped "corrugated flat band" predicted for PtSb2 might explain the enhanced thermoelectric properties of doped PtAs2.Comment: 3 pages, 2 figure

Breakdown of Chemical Scaling for Pt-Doped CaFe2As2

We report a breakdown of the scaling of the antiferromagnetic transition temperature T_N on chemical doping for Pt-doped CaFe2As2. The substitution of Pt is ineffective in the reduction of antiferromagnetic ordering as well as in inducing superconductivity in Ca(Fe1-xPtx)2As2. The Pt-doped CaFe2As2 that does not exhibit superconductivity will, however, provide us an ideal opportunity to elucidate the role of chemical doping in the occurrence of superconductivity in iron-based materials.Comment: 7 pages, 2 figure

Large Seebeck effect in the electron-doped FeAs2_2 driven by quasi one dimensional pudding-mold type band

We investigate the thermoelectric propeties of the electron-doped FeAs$_2$ both experimentally and theoretically. Electrons are doped by partially substituting Se for As, which leads to a metallic behavior in the resistivity. A Seebeck coefficient of about $-$200 $\mu$V/K is reached at 300 K for 1% doping, and about $-$120 $\mu$V/K even at 5% doping. The origin of this large Seebeck coefficient despite the metallic conductivity is analyzed from a band structure point of view. The first-principles band calculation reveals the presence of a pudding-mold type band just above the band gap, somewhat similar to Na$_x$CoO$_2$, but with a quasi-one-dimensional nature. We calculate the Seebeck coefficient using a tightbinding model that correctly reproduces this band structure, and this gives results close to the experimental observations. The origin of this peculiar band shape is also discussed.Comment: 6 pages, 6 figure

Composition-induced structural instability and strong-coupling superconductivity in Au1-xPdxTe2

The physical properties and structural evolution of the MX$_2$-type solid solution Au$_{1-x}$Pd$_x$Te$_2$ are reported. The end member AuTe$_2$ is a normal metal with a monoclinic distorted CdI$_2$-type structure with preformed Te-Te dimers. A monoclinic--trigonal structural phase transition at a finite temperature occurs upon Pd substitution and is suppressed to zero temperature near $x$ = 0.55, and a superconducting phase with a maximum $T_{\rm c}$ = 4.65 K emerges. A clear indication of strong coupling superconductivity is observed near the composition of the structural instability. The competitive relationship between Te-Te dimers and superconductivity is proposed.Comment: 6 pages, 8 figure

Superconducting Transition Temperatures of up to 47 K from Simultaneous Rare-Earth Element and Antimony Doping of 112-Type CaFeAs2

The effects of simultaneous Sb doping on the superconductivity of 112-type Ca1-xRExFeAs2 (RE = La, Ce, Pr, and Nd) were studied through measurements of the magnetization and electrical resistivity. In Sb-free materials, the superconducting transition temperature Tc of the La-doped sample was 35 K, while those of the Pr- and Nd-doped samples were ~10 K; no superconductivity was observed in the Ce-doped sample. Sb doping increased the Tc of all RE-doped samples: Tc increased to 47, 43, 43, and 43 K for RE = La, Ce, Pr, and Nd, respectively. We also found that the enhanced superconductivity results from the increase in the lattice parameter b, which increases the As-Fe-As bond angle to be closer to the ideal tetrahedron value. These observations provide insight for further increasing the Tc of the 112 phase.Comment: 4 pages, 4 figure

Pressure-Induced Superconductivity in Mineral Calaverite AuTe2

The pressure dependences of resistivity and ac susceptibility have been measured in the mineral calaverite AuTe$_2$. Resistivity clearly shows a first-order phase transition into a high-pressure phase, consistent with the results of a previous structural analysis. We found zero resistivity and a diamagnetic shielding signal at low temperatures in the high-pressure phase, which clearly indicates the appearance of superconductivity. Our experimental results suggest that bulk superconductivity appears only in the high-pressure phase. For AuTe$_2$, the highest superconducting transition temperature under pressure is $T_{\rm c}$ = 2.3 K at 2.34 GPa; it was $T_{\rm c}$ = 4.0 K for Pt-doped (Au$_{0.65}$Pt$_{0.35}$)Te$_2$. The difference in $T_{\rm c}$ between the two systems is discussed on the basis of the results obtained using the band calculations and McMillan's formula.Comment: 5 pages, 6 figure