<i>T</i>_c Maximum in Solid Solution of Pyrite IrSe₂–RhSe₂ Induced by Destabilization of Anion Dimers

We have established a well-defined dome-shaped <i>T</i>_c curve in Ir_{0.94–<i>x</i>}Rh_<i>x</i>Se₂ superconductors. The maximum <i>T</i>_c^onset of 9.6 K was obtained at <i>x</i> = 0.36, at which the Se–Se separation in the dimer anion is the longest. Simultaneously, the destabilization of Se–Se dimers accompanied by partial electron transfer from the Ir/Rh to the chalcogenide ions resulted in the emergence of optimal <i>T</i>_c

Layered Compounds BaM₂Ge₄Ch₆ (M = Rh, Ir and Ch = S, Se) with Pyrite-Type Building Blocks and Ge–Ch Heteromolecule-Like Anions

The structures and chemical features of layered compounds BaM₂Ge₄Ch₆ (M = Rh, Ir; Ch = S, Se) synthesized by high-pressure and high-temperature methods have been systematically studied. These compounds crystallize in an orthorhombic phase with space group <i>Pbca</i> (No. 61). These compounds have the remarkable structural feature of M–Ge–Ch pyrite-type building units, stacking with Ba–Ch layers alternatively along the <i>c</i> axis. It is very rare and novel that pyrite-type subunits are the building blocks in layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit an unusual valence state (∼+1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously

Layered Compounds BaM₂Ge₄Ch₆ (M = Rh, Ir and Ch = S, Se) with Pyrite-Type Building Blocks and Ge–Ch Heteromolecule-Like Anions

The structures and chemical features of layered compounds BaM₂Ge₄Ch₆ (M = Rh, Ir; Ch = S, Se) synthesized by high-pressure and high-temperature methods have been systematically studied. These compounds crystallize in an orthorhombic phase with space group <i>Pbca</i> (No. 61). These compounds have the remarkable structural feature of M–Ge–Ch pyrite-type building units, stacking with Ba–Ch layers alternatively along the <i>c</i> axis. It is very rare and novel that pyrite-type subunits are the building blocks in layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit an unusual valence state (∼+1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously

Real-Space Observation of Unidirectional Charge Density Wave and Complex Structural Modulation in the Pnictide Superconductor Ba_1–<i>x</i>Sr_<i>x</i>Ni₂As₂

Here we use low-temperature and variable-temperature scanning tunneling microscopy to study the pnictide superconductor, Ba1–xSrxNi2As2. In the low-temperature phase (triclinic phase) of BaNi2As2, we observe the unidirectional charge density wave (CDW) with Q = 1/3 on both the Ba and NiAs surfaces. On the NiAs surface of the triclinic BaNi2As2, there are structural-modulation-induced chain-like superstructures with distinct periodicities. In the high-temperature phase (tetragonal phase) of BaNi2As2, the NiAs surface appears as the periodic 1 × 2 superstructure. Interestingly, in the triclinic phase of Ba0.5Sr0.5Ni2As2, the unidirectional CDW is suppressed on both the Ba/Sr and NiAs surfaces, and the Sr substitution stabilizes the periodic 1 × 2 superstructure on the NiAs surface, which enhance the superconductivity in Ba0.5Sr0.5Ni2As2. Our results provide important microscopic insights for the interplay among the unidirectional CDW, structural modulation, and superconductivity in this class of pnictide superconductors

Superconductivity in Alkaline Earth Metal-Filled Skutterudites Ba_<i>x</i>Ir₄X₁₂ (X = As, P)

We report superconductive iridium pnictides Ba_<i>x</i>Ir₄X₁₂ (X = As and P) with a filled skutterudite structure, demonstrating that Ba filling dramatically alters their electronic properties and induces a nonmetal-to-metal transition with increasing the Ba content <i>x</i>. The highest superconducting transition temperatures are 4.8 and 5.6 K observed for Ba_<i>x</i>Ir₄As₁₂ and Ba_<i>x</i>Ir₄P₁₂, respectively. The superconductivity in Ba_<i>x</i>Ir₄X₁₂ can be classified into the Bardeen–Cooper–Schrieffer type with intermediate coupling

Topological Type-II Dirac Fermions Approaching the Fermi Level in a Transition Metal Dichalcogenide NiTe₂

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe₂-class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe₂, PdTe₂, and PtTe₂ candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here, we report the synthesis of a new type-II Dirac semimetal NiTe₂ in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry’s phase associated with these Dirac fermions. Our first-principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe₂ may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers