Significant improvement of the lower critical field in Y doped Nb: potential replacement of basic material for the radio-frequency superconducting cavity

The research of high energy and nuclear physics requires high power accelerators, and the superconducting radio-frequency (SRF) cavity is regarded as their engine. Up to now, the widely used practical and effective material for making the SRF cavity is pure Nb. The key parameter that governs the efficiency and the accelerating field (E_acc) of a SRF cavity is the lower critical field Hc1. Here, we report a significant improvement of Hc1 for a new type of alloy, Nb_{1-x}Y_x fabricated by the arc melting technique. Experimental investigations with multiple tools including x-ray diffraction, scanning electron microscopy, resistivity and magnetization are carried out, showing that the samples have good quality and a 30%-60% enhancement of Hc1. First principle calculations indicate that this improvement is induced by the delicate tuning of a Lifshitz transition of a Nb derivative band near the Fermi energy, which increases the Ginzburg-Landau parameter and Hc1. Our results may trigger a replacement of the basic material and thus a potential revolution for manufacturing the SRF cavity.