We report about the influence of nitrogen incorporation on the superconducting transition temperature T C of TiNbMoTaW high entropy alloy films deposited using high power impulse magnetron sputtering. By measuring the temperature dependence of resistivity of (TiNbMoTaW)N x nitrides, we observe a significant increase of T C, from 0.62 K for x = 0 up to 5.02 K for x = 0.74. With further increase of x, T C is decreasing and reaches 1.08 K for x = 0.97. The eightfold T C enhancement seems to be associated with the incorporation of light N atoms into the face-centered cubic lattice and with the x dependent enhancement of the electron-phonon interaction, which may be related to the high configuration entropy in high entropy alloys. Measurements in magnetic field show that the upper critical fields B c2 of (TiNbMoTaW)N x with x > 0.15 provide B c2/T C > 2 T/K ratios, which are above the weak-coupling pair breaking limit. Additional heat capacity and point contact spectroscopy measurements show that the superconductivity in the ∼1 μm thick films is bulk in nature, consistent with conventional weak-coupling phonon mediated superconductivity. The proposed strategy of nitrogen incorporation into high entropy alloys may pave a pathway towards tailoring their superconducting properties, especially their T C.
Bibliographische NotizFunding Information:
We wish to acknowledge the support by projects Mobility Slovakia-Austria (Slovak Academy of Sciences) , AT-SK 2023-03-15-003 , Slovak Research and Development Agency under the contract no. APVV-18-0358 , VEGA 2/0032/20 , VEGA 2/0058/20 and the European Microkelvin Platform . Liquid nitrogen for experiments was sponsored by U.S. Steel Košice, s.r.o . The thin film synthesis was supported by the Austrian Research Promotion Agency (FFG) under project number 871687 . The authors are grateful to Michael Tabelander (Montanuniversität Leoben) for assistance with thin film synthesis.