Robust bulk superconductivity by giant proximity effect in Weyl semimetal-superconducting NbP/NbSe2 composites

We synthesize the Weyl semimetal/superconductor NbP/NbSe2 composite and observe stable bulk superconductivity at T-c = 7.2, 6.9, and 6.8 K for the NbSe2 crystal, NbP/NbSe2 (1:1), and NbP/NbSe2 (2:1) composites, respectively, despite large volume fraction of the non-superconducting NbP phase. From the Ginzburg-Landau theory, H-c2(0) is significantly enhanced in NbP/NbSe2 composites [22 T (1:1) and 18.5 T (2:1)] comparing with the pristine NbSe2 crystal (8 T). The bulk superconductivity in the Weyl semimetal/superconductor composite cannot be simply described by the de Gennes-Meissner theory in a proximity effect. From the electrical transport, magnetization, and heat capacity measurement, we obtain various superconducting parameters. The superconducting properties indicate that the NbP/NbSe2 composite is far from the conventional Bardeen-Cooper-Schrieffer superconductivity. It suggests that the Weyl semimetal/superconductor composite can have a giant proximity effect, resulting in the stable bulk superconductivity in a composite with a sizable volume fraction of non-superconducting Weyl semimetals. The giant proximity effect in the Weyl semimetal/superconductor interface can have a platform to investigate the proximity induced Weyl semimetallic superconducting states.