Superconductivity with large upper critical field in noncentrosymmetric Cr-bearing high-entropy alloys

A series of new Cr5+xMo35xW12Re35Ru13C20 high-entropy alloys (HEAs) have been synthesized and charac-terized by x-ray diffraction, scanning electron microscopy, electrical resistivity, magnetic susceptibility and specific heat measurements. It is found that the HEAs adopt a noncentrosymmetric cubic fl-Mn type structure and exhibit bulk superconductivity for 0 <= x <= 9. With increasing x, the cubic lattice parameter decreases from 6.7940(3) angstrom to 6.7516(3) angstrom. Meanwhile, the superconducting transition temperature Tc is suppressed from 5.49 K to 3.35 K due to the magnetic pair breaking caused by Cr moments. For all these noncentrosymmetric HEAs, the zero-temperature upper critical field Bc2(0) is comparable to the Pauli paramagnetic limit BP(0) = 1.86Tc. In particular, the Bc2(0)/BP(0) ratio reaches a maximum of similar to 1.03 at x = 6, which is among the highest for fl-Mn type superconductors.