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We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er2O2Bi with anti-ThCr2Si2-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature Tc of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field Hc2(0) and electron-phonon coupling constant λe−ph in Er2O2Bi are similar to those in the previously reported non-magnetic superconductor Y2O2Bi with the same structure, indicating that the superconductivity in Er2O2 Bi may have the same origin as in Y2O2Bi. The first-principle calculations of Er2O2Bi show that the Fermi surface is mainly composed of the Bi 6p orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4f electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er2OxBi, Tc increases from 1 to 1.23 K and TN decreases slightly from 3 K to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of Tc and TN with different oxygen content. [ABSTRACT FROM AUTHOR]

Superconductivity frequently appears by doping compounds that show a collective phase transition. So far, however, this has not been observed in topological materials. Here we report the discovery of superconductivity induced by Ga doping in orthorhombic Re3Ge7, which undergoes a second-order metal–insulator-like transition at ~58 K and is predicted to have a nontrivial band topology. It is found that the substitution of Ga for Ge leads to hole doping in Re3Ge7−xGax. As a consequence, the phase transition is gradually suppressed and disappears above x = 0.2. At this x value, superconductivity emerges and Tc exhibits a dome-like doping dependence with a maximum value of 3.37 K at x = 0.25. First principles calculations suggest that the phase transition in Re3Ge7 is associated with an electronic instability driven by Fermi-surface nesting and the nontrival band topology is preserved after Ga doping. Our results indicate that Ga-doped Re3Ge7 provides a rare opportunity to study the interplay between superconductivity and competing electronic states in a topologically nontrivial system. [ABSTRACT FROM AUTHOR]

We report the superconducting properties of new hexagonal Nb 10 + 2 x Mo 35 − x Ru 35 − x Rh 10 Pd 10 high-entropy alloys (HEAs) (0 ≤ x ≤ 5). With increasing x , the superconducting transition temperature T c shows a maximum of 6.19 K at x = 2.5, while the zero-temperature upper critical field B c2 (0) increases monotonically, reaching 8.3 T at x = 5. For all x values, the specific heat jump deviates from the Bardeen-Cooper-Schreiffer behavior. In addition, we show that T c of these HEAs is not determined mainly by the density of states at the Fermi level and would be enhanced by lowering the valence electron concentration. [ABSTRACT FROM AUTHOR]

[Display omitted] The addition of carbon in medium-entropy and high-entropy alloys offers a convenient way to improve their mechanical as well as physical properties. However, no fundamental change in their crystal structures has been found so far. Here we report the observation of structural transformation by adding carbon in the hexagonal MoReRu medium-entropy alloy. It is found that a single face-centered cubic phase exists in MoReRuC x over a wide carbon content range of x = 0.9 to 1.2. Within this phase region, MoReRuC x exhibits both high hardness above 10 GPa and bulk Bardeen-Copper-Schrieffer-like superconductivity, with T c mainly governed by the electron-phonon coupling strength. We argue that the observed structural transformation is driven by valence electron concentration and local lattice distortion, which suggests that it might also occur in carbon-added high-entropy alloys isostructural to MoReRu. [ABSTRACT FROM AUTHOR]

• Hexagonal MoReRuC x medium-entropy alloys with 0 ≤ x ≤ 0.20 are synthesized. • The C atoms occupy the interstitial sites for x ≤ 0.10 while both substitutional and interstitial sites at higher x values. • T c shows a dome-like dependence on x with a maximum of 9.62 K. • The specific heat data imply an inhomogeneous superconducting state. • The pairing mechanism in MoReRuC x is essentially the same as that in ReC x. We report the results of structural and physical studies on the MoReRuC x medium-entropy alloys (MEAs) with x varying from 0 to 0.20. In the whole x range, a single hexagonal phase is formed. The variation of lattice parameters indicates that carbon atoms are located at the interstitial site for x ≤ 0.10 while occupy both the interstitial and substitutional sites at higher x values, which is corroborated by the x-ray photonelectron spectroscopy (XPS) results. Moreover, the resistivity, magnetic and thermodynamic measurements evidence that these MEAs are bulk fully gapped superconductors. In particular, the transition temperature T c displays a dome-like dependence on carbon content with a maximum of 9.62 K at x = 0.05, whereas a maximal zero-temperature upper critical field B c2 (0) of 10.5 T is achieved at x = 0.10. For all MEAs, the specific heat data reveal the existence of an inhomogeneous superconducting state, which is discussed in relation to the chemical inhomogeneity and strong atomic disorder. In addition, we show that the key factors that control superconductivity are essentially the same for both MoReRuC x and ReC x , despite their very different T c values. Our work not only provides the first series of interstitial MEAs with a hexagonal structure, but also demonstrates a feasible way to tune the superconductivity in multicomponent alloys. [ABSTRACT FROM AUTHOR]

• A series of ReC x samples with x up to 0.10 are studied. • The incorporation of C induces an anisotropic expansion of the lattice. • The C atoms occupy the interstitial sites surrounded by six Re atoms. • Both T c and B c2 (0) increase monotonically with C concentration. • The enhancement in T c is due to the increase in both N (E F) and λ ep. We report the effect of carbon doping on the structural and superconducting properties of elemental Re studied in a series of polycrystalline ReC x samples with x up to 0.10. Structural analyses indicate that the C atoms occupy the octahedral interstitial sites in the hexagonal close-packed Re lattice. This leads to an expansion of unit cell mainly along the a -axis and a shift of the binding energies of Re towards higher values. Furthermore, these ReC x samples are found to be weakly coupled, fully gapped, type-II superconductors. With increasing C content x , both T c and zero temperature upper critical field B c2 (0) increase monotonically. At x = 0.10, T c and B c2 (0) reach 2.65 K and 1.18 T, respectively, which are more than 50% and 60 times larger than the corresponding values of pure Re. Combined with theoretical calculations, we show that the enhancement in T c induced by C doping is due to the increase in both the density of states at the Fermi level and electron-phonon coupling. Our results suggest that the introduction of interstitial atoms provides a viable way to tune the superconductivity in transition metal elements. [ABSTRACT FROM AUTHOR]

Single crystals of Sn 4 Sb 3 with mm size have been obtained for the first time via a Sn flux method. Structural refinements confirm its trigonal structure with the centrosymmetric R 3 ̄ m space group. The unit cell consists of ordered [Sn–Sb–Sn–Sb–Sn–Sb–Sn] lamellae stacked along the c -axis, in which Sn is found to exhibit mixed valence states of 0 and 2+. Below 1.47 K, single crystalline Sn 4 Sb 3 becomes a weakly coupled, fully gapped superconductor. In contrast to polycrystalline samples, the resistive transition of Sn 4 Sb 3 crystals coincides well with the specific-heat jump, which supports that grain boundaries are responsible for the higher resistive T c in the former case. Theoretical calculations show that the density of states at the Fermi level are mainly contributed by Sn and Sb 5 p orbitals and almost unaffected by the spin-orbit interaction. Our study lays a foundation for experimental investigations of the predicted topological properties in this intermetallic compound. ∙ Sn4Sb3crystalswith mm size aregrown by a Sn flux method. ∙The structure consists of [Sn4Sb3]lamellae stacked along the c-axis. ∙Snexhibitsmixed valence states of 0 and 2+. ∙The crystals exhibitBCS-like superconductivitybelow 1.47 K. ∙The density of states at the Fermi level are dominated bySn andSb 5porbitals. [ABSTRACT FROM AUTHOR]

We report the structural and physical properties of the Ta 10 Mo 35− x Cr x Re 35 Ru 20 high-entropy alloys (HEAs) with 5 ≤ x ≤ 13. These HEAs adopt a single tetragonal structure with the P 4 2 / mnm space group and show bulk Bardeen-Cooper-Schreiffer (BCS)-like superconductivity with the transition temperature T c up to 4.79 K. In the normal-state, a paramagnetic behavior is observed at low temperature for all x values due to the magnetic moment of Cr, which is found to be about one-third that in Cr metal. The T c depression rate with increasing Cr content is considerably lower than those in typical elemental, intermetallic, cuprate and iron pnictide superconductors, pointing to weak pair breaking in HEA superconductors by the Cr-3 d electrons. [Display omitted] • New tetragonal Ta 10 Mo 35−x Cr x Re 35 Ru 20 (5 ≤ x ≤ 13) HEAs are synthesized. • These HEAs are BCS-like superconductors with T c up to 4.79 K. • The magnetic moment per Cr atom is one-third that in Cr metal. • The magnetic pairing breaking effect by the Cr-3d electrons is weak. [ABSTRACT FROM AUTHOR]

We report the results of structural, mechanical and physical property studies on two new carbides MoRe 2 C and WRe 2 C, which were not identified in previous Mo-Re-C and W-Re-C phase diagrams. These ternary phases adopt a face-centered cubic structure at ambient pressure likely with a disordered atomic arrangement. Due to the covalent bonds between transition metal and carbon atoms, MoRe 2 C and WRe 2 C exhibit high hardness values of 15.3 GPa and 13.7 GPa, respectively. Furthermore, these carbides are discovered to be bulk superconductors with very similar T c values about 4.50 K. The analysis of their specific-heat jumps points to a weakly coupled, fully gapped superconducting state. The electron-phonon coupling constant, upper critical field and coherence length are estimated to be 0.56, 5.3 T and 7.9 nm for MoRe 2 C, and 0.57, 6.3 T, 7.2 nm for WRe 2 C. Our results suggest that transition-metal-rich carbides provide a platform to study the link between configurational disorder, high hardness and superconductivity. • MoRe2C and WRe2C with a face-centered cubic structure are synthesized. • The disordered atomic distribution results in a medium configurational entropy. • MoRe2C and WRe2C have high hardnesses of 15.3 GPa and 13.7 GPa, respectively. • Both carbides are weakly coupled, fully gapped superconductors below 4.5 K. [ABSTRACT FROM AUTHOR]

We report the structural and superconducting properties of ternary transition metal borides MoReB and WReB. Both compounds crystallize in the tetragonal CuAl 2 -type structure, in which the Re and Mo/W atoms share the same crystallographic site and form one-dimensional zigzag chains along the c -axis. The results of resistivity, magnetic susceptibility, and specific heat measurements indicate that these borides are weakly coupled, fully gapped, type-II superconductors. The superconducting transition temperature T c , upper critical field, and Ginzburg-Laudau parameter are determined to be 5.25 K, 2.1 T, 15.6 for MoReB, and 5.30 K, 2.0 T, 15.7 for WReB. Despite their nearly identical T c values, the Sommerfeld coefficient of WReB is more than 20% smaller than that of MoReB, suggesting that the electronic density of states at the Fermi level is not the dominant factor in determining T c. In addition, the upper critical fields of both MoReB and WReB exhibit an anomalous linear temperature dependence and possible origins are discussed. • In both MoReB and WReB, the Mo/W and Re atoms share the same crystallographic site. • Both borides are found to be weakly coupled, fully gapped, type-II superconductors. • T c of these borides is not determined primarily by the density of states at the Fermi level. • A linear temperature dependence of upper critical field is observed in both cases, ascribing to Fermi surface anisotropy or multiband effects. [ABSTRACT FROM AUTHOR]