We have investigated the superconducting state of the noncentrosymmetric compound ${\mathrm{Re}}_{6}\mathrm{Zr}$ using magnetization, heat capacity, and muon-spin relaxation or rotation ($$\mu${}\mathrm{SR}$) measurements. ${\mathrm{Re}}_{6}\mathrm{Zr}$ has a superconducting transition temperature, ${T}_{c}=6.75\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{ }\mathrm{K}$. Transverse-field $$\mu${}\mathrm{SR}$ experiments, used to probe the superfluid density, suggest an $s$-wave character for the superconducting gap. However, zero and longitudinal-field $$\mu${}\mathrm{SR}$ data reveal the presence of spontaneous static magnetic fields below ${T}_{c}$ indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking.

The superconductivity of the noncentrosymmetric compound ${\mathrm{La}}_7{\mathrm{Ir}}_3$ is investigated using muon spin rotation and relaxation. Zero-field measurements reveal the presence of spontaneous static or quasistatic magnetic fields below the superconducting transition temperature $T_{\mathit{c}}=2.25\mathrm{K}$—a clear indication that the superconducting state breaks time-reversal symmetry. Furthermore, transverse-field rotation measurements suggest that the superconducting gap is isotropic and that the pairing symmetry of the superconducting electrons is predominantly $s$ wave with an enhanced binding strength. The results indicate that the superconductivity in ${\mathrm{La}}_7{\mathrm{Ir}}_3$ may be unconventional and paves the way for further studies of this family of materials.

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation ($\mu \mathrm{SR}$) studies of superconducting noncentrosymmetric ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ ($T_c=8.8\mathrm{K}$) and centrosymmetric Re ($T_c=2.7\mathrm{K}$). In ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$, the low–temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $\mu \mathrm{SR}$ below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric $\mathrm{Re}T$ ($T=\text{transition}$ metal), yet its preservation in the isostructural noncentrosymmetric superconductors ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ and ${\mathrm{Nb}}_{0.5}{\mathrm{Os}}_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and $\mathrm{Re}T$. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.

High-entropy alloys (HEAs) are a new class of materials with an attractive combination of tunable mechanical and physicochemical properties. They crystallize mainly in cubic structures, however, for practical applications, HEAs with hexagonal-close-packed (hcp) structure are highly desirable in connection with their, in general, high hardness. Herein, we report the synthesis, structure, and detailed superconducting properties of ${\mathrm{Re}}_{0.56}{\mathrm{Nb}}_{0.11}{\mathrm{Ti}}_{0.11}{\mathrm{Zr}}_{0.11}{\mathrm{Hf}}_{0.11}$—the first hexagonal superconducting HEA composed of five randomly distributed transition metals. A combination of room-temperature precession electron diffraction, precession electron diffraction tomography, and powder x-ray diffraction is utilized to determine the room-temperature crystal structure. Transport, magnetic, and heat capacity measurements show that the material is a type-II superconductor with the bulk superconducting transition at $T_c=4.4$ K, lower critical field $H_{c1}\left(0\right)=2.3$ mT, and upper critical field $H_{c2}\left(0\right)=3.6$ T. Low-temperature specific-heat measurement indicates that ${\mathrm{Re}}_{0.56}{\mathrm{Nb}}_{0.11}{\mathrm{Ti}}_{0.11}{\mathrm{Zr}}_{0.11}{\mathrm{Hf}}_{0.11}$ is a phonon-mediated superconductor in the weak electron-phonon coupling limit with a normalized specific-heat jump $\frac{\mathrm{\Delta }{C}_{el}}{{\gamma }_n{T}_c}=1.32$. Further, hexagonal to cubic structural transition is observed by lowering the valence electron counts and $T_c$ follows crystallinelike behavior.

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero field, time-reversal symmetry breaking has been observed in ${\mathrm{Re}}_6\mathrm{Hf}$. Moreover, the temperature dependence of the superfluid density exhibits $s$-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural ${\mathrm{Re}}_6\mathrm{Zr}$, shows that the ${\mathrm{Re}}_{6}X$ family is indeed a new and important group of unconventional superconductors.