Unconventional superconductivity in Cr-based nitride La₃Cr_10–xN₁₁

Magnetization, specific heat, and muon spin relaxation (μSR) experiments have been carried out on the Cr-based nitride superconductor La₃Cr_10−xN₁₁, which exhibits a number of unconventional superconducting properties. The susceptibility χ(T) shows nearly perfect superconducting diamagnetism (4πχ(T = 0) ≈ −1) and a remarkably high upper critical field μ₀H_c2 = 11.2 T. The specific heat displays activated exponential behavior exp(−Δ₀/k_BT), together with a large and field-dependent residual Sommerfeld coefficient. Transverse-field muon spin relaxation (μSR) measurements suggest s+s-wave or p-wave pairing symmetry, ruling out single s-wave pairing. Zero-field μSR yields no statistically significant evidence for time reversal symmetry breaking (TRSB), and places an upper bound of 1.5(1.3) ms⁻¹ on any TRSB-induced muon relaxation rate at T = 0. Our results suggest that the unconventional superconductivity in Ln₃Cr_10–xN₁₁, Ln = La and Pr, is mainly due to Cr 3d electrons and is similar in both compounds, whereas Pr 4f electrons are primarily responsible for the TRSB superconductivity observed in Pr₃Cr_10−xN₁₁.