Search

Arsenic doping in silicides has been much less studied compared with phosphorus. In this study, superconductivity is successfully induced by As doping in Mo$_5$Si$_3$. The superconducting transition temperature ($T_c$) reaches 7.7 K, which is higher than those in previously known W$_5$Si$_3$-type superconductors. Mo$_5$Si$_2$As is a type-II BCS superconductor with upper and lower critical fields of 6.65 T and 22.4 mT, respectively. In addition, As atoms are found to selectively take the 8$h$ sites in Mo$_5$Si$_2$As. The emergence of superconductivity is possibly due to the shift of Fermi level as a consequence of As doping, as revealed by the specific heat measurements and first-principles calculations. Our work provides not only another example of As doping, but also a practical strategy to achieve superconductivity in silicides through Fermi level engineering., Comment: Supporting Information available at the corresponding DOI

By performing P doping on the Si sites in the topological semimetal Mo$_5$Si$_3$, we discover strong-coupling superconductivity in Mo$_5$Si$_{3-x}$P$_x$ (0.5 $\le$ $x$ $\le$ 2.0). Mo$_5$Si$_3$ crystallizes in the W$_5$Si$_3$-type structure with space group of $I4/mcm$ (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter $a$ decreases while $c$ increases monotonously. Bulk superconductivity is revealed in Mo$_5$Si$_{3-x}$P$_x$ (0.5 $\le$ $x$ $\le$ 2.0) from resistivity, magnetization, and heat capacity measurements. $T_c$ in Mo$_5$Si$_{1.5}$P$_{1.5}$ reaches as high as 10.8 K, setting a new record among the W$_5$Si$_3$-type superconductors. The upper and lower critical fields for Mo$_5$Si$_{1.5}$P$_{1.5}$ are 14.56 T and 105 mT, respectively. Moreover, Mo$_5$Si$_{1.5}$P$_{1.5}$ is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo$_5$Si$_3$. The $T_c$ and upper critical field in Mo$_5$Si$_{3-x}$P$_x$ are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W$_5$Si$_3$-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family., Comment: 15 pages, 5 figures. Supplementary Information availabe at the corresponding DOI

We report the synthesis, crystal structure, and superconductivity of Ti$_4$Ir$_2$O. The title compound crystallizes in an $\eta$-carbide type structure of the space group $Fd\overline{3}m$ (No. 227), with lattice parameters $a=b=c=11.6194(1)$ \AA. The superconducting $T_c$ is found to be 5.1 $\sim$ 5.7 K. Most surprisingly, Ti$_4$Ir$_2$O hosts an upper critical field of 16.45 T, which is far beyond the Pauli paramagnetic limit. Strong coupled superconductivity with evidences for multigap is revealed by the measurements of heat capacity and upper critical field. First-principles calculations suggest that the density of states near the Fermi level originates from the hybridization of Ti-3$d$ and Ir-5$d$ orbitals, and the effect of spin-orbit coupling on the Fermi surfaces is prominent. Large values of the Wilson ratio ($R_W$ $\sim$ 3.9), the Kadowaki-Woods ratio ($A/\gamma^2$ $\sim$ 9.0 $\times$ 10$^{-6}$ $\mu\Omega$ cm/(mJ mol$^{-1}$ K$^{-1}$)$^2$), and the Sommerfeld coefficient ($\gamma$ = 33.74 mJ mol$^{-1}$ K$^{-2}$) all suggest strong electron correlations (similar to heavy fermion systems) in Ti$_4$Ir$_2$O. The violation of Pauli limit is possibly due to a combination of strong-coupled superconductivity, large spin-orbit scattering, and electron correlation. With these intriguing behaviors, Ti$_4$Ir$_2$O serves as a candidate for unconventional superconductor., Comment: 8 pages, 4 figures + 5 figures in the Supporting Information

We report systematic studies on superconducting properties of the Laves phase superconductor ZrIr2. It crystallizes in a C15-type (cubic MgCu2-type, space group F d 3 ¯ m ) structure in which the Ir atoms form a kagome lattice, with cell parameters a = b = c = 7.3596(1) Å. Resistivity and magnetic susceptibility measurements indicate that ZrIr2 is a type-II superconductor with a transition temperature of 4.0 K. The estimated lower and upper critical fields are 12.8 mT and 4.78 T, respectively. Heat capacity measurements confirm the bulk superconductivity in ZrIr2. ZrIr2 is found to possibly host strong-coupled s-wave superconductivity with the normalized specific heat change ΔC e/γ T c ∼ 1.86 and the coupling strength Δ0/k B T c ∼ 1.92. First-principles calculations suggest that ZrIr2 has three-dimensional Fermi surfaces with simple topologies, and the states at Fermi level mainly originate from the Ir-5d and Zr-4d orbitals. Similar to SrIr2 and ThIr2, spin–orbit coupling has dramatic influences on the band structure in ZrIr2.

${\mathrm{TaAs}}_{2}$ and ${\mathrm{NbAs}}_{2}$ are members of the ${MPn}_{2}$ family with weak topological invariants of ${Z}_{2}$ = [0;111]. Here, we report the tip-induced superconductivity achieved on both materials using the point-contact technique. Although the weak ${Z}_{2}$ number leads to strongly crystal-orientation-dependent surface states, we observed a universal relationship between the zero-temperature upper critical field ${H}_{c2}(0)$ and the critical temperature ${T}_{c}$ for different crystal orientations. Combined with first-principles calculations, the findings here could identify a bulk band, instead of the much speculated surface bands, playing the dominant role in the induced superconductivity. These results predict a great possibility to realize superconductivity in other members of the ${MPn}_{2}$ family possessing similar band structures and provide clues to understand the mechanism of tip-induced superconductivity.

Tip-induced superconductivity at point contacts on the nonsuperconducting Weyl semimetal TaAs has been reported recently. Meanwhile, both the main tetragonal phase and the intergrown hexagonal phase of TaAs were identified to be topologically nontrivial according to theoretical calculations. Here, we report the observation of two independent local superconducting states induced at the point contacts formed on TaAs, which exhibit distinct temperature dependencies of the critical magnetic field ${H}_{c2}(T)$. A universal relationship between ${H}_{c2}(0)$, ${T}_{c}$, and the Fermi velocities of bulk bands near Weyl points of the two TaAs phases was uncovered, demonstrating the dominant role of the bulk bands in the emergence of superconductivity, rather than the surface states. These results provide a direction to study the mechanism of the tip-induced superconductivity in topological semimetals.

We report a detailed study of tunneling spectra measured on 2H-Ta_{x}Nb_{1-x}Se_{2} (x=0∼0.1) single crystals using a low-temperature scanning tunneling microscope. The prominent gaplike feature, which has not been understood for a long time, was found to be accompanied by some "in-gap" fine structures. By investigating the second-derivative spectra and their temperature and magnetic field dependencies, we were able to prove that inelastic electron tunneling is the origin of these features and obtain the Eliashberg function of 2H-Ta_{x}Nb_{1-x}Se_{2} at an atomic scale, providing a potential way to study the local Eliashberg function and the phonon spectra of the related transition-metal dichalcogenides.

Here we report a novel quaternary selenide Cu6GeWSe8 synthesized directly from the constituent elements, which forms within the temperature range from 460 to 620 ​°C. It crystallizes in the space group P63mc with two formula units in the cell of dimensions a ​= ​7.8651 (5) A, c ​= ​12.9275 (6) A. In the unit cell composed of MSe4 (M ​= ​Cu, Ge and W) tetrahedrons, Cu atoms occupy two sets of 6c Wyckoff positions, Ge and W atoms respectively occupy 2a and 2b positions, while Se atoms are located at four inequivalent sites, respectively 2b (Se1), 6c (Se2), 6c (Se3) and 2a (Se4). The electrical resistivity and Vis-Nir absorption spectrum indicate that the compound exhibits a semiconducting behavior with an optical band gap of 1.586 ​eV Cu6GeWSe8 shows diamagnetism at room temperature and the emergence of a Curie-Weiss-like tail at low temperature. The band structure calculation demonstrates that the title compound has a direct band gap, and Cu and Se atoms dominantly contribute to the energy bands and density of states. Cu is mostly in 3d10 state (+1 valence) and Se in 4p6 state (−2 valence). On the basis of the experimental and calculation results, it is concluded that Cu6GeWSe8 is a diamagnetic direct-gap semiconductor.

We report the tip-induced superconductivity on the topological semimetal NbSb2, similar to the observation on TaAs2 and NbAs2. Belonging to the same family of MPn 2, all these materials possess similar band structures, indicating that the tip-induced superconductivity may be closely related to their topological nature and share a common mechanism. Further analysis suggests that a bulk band should play the dominant role in such local superconductivity most likely through interface coupling. In addition, the compatibility between the induced superconductivity and tips’ ferromagnetism gives an evidence for its unconventional nature. These results provide further clues to elucidate the mechanism of the tip-induced superconductivity observed in topological materials.

We report the observation of local superconductivity induced at the point contact formed between a normal metal tip and WC -- a triple point topological semimetal with super hardness. Remarkably, the maximum critical temperature is up to near 12 K but insensitive to the tip's magnetism. The lateral dimensions of the superconducting puddles were evaluated and the temperature dependencies of superconducting gap and upper critical field were also obtained. These results put constraints on the explanation of the induced superconductivity and pave a pathway for exploring topological superconductivity., Comment: 7 pages, 6 figures

We report scanning tunneling microscopy/spectroscopy (STM/STS) studies on iron-based superconductors of Ba1−xKxFe2As2 and nearly optimally doped Fe(Te,Se). Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observations in other samples and can be ascribed to the interaction between electrons and spin excitations. Furthermore, an almost linear relationship between the superconducting gaps and the superconducting transition temperatures was noted and should also be taken into account in understanding the mechanism of iron-based superconductors.