Determination of the Cooper-pair mass in niobium

We have measured the ratio of Planck’s constant h to the observable mass m’ of a Cooper pair in a superconductor, using a rotating superconducting ring. The flux through such a ring of area S is zero for a set of rotation frequencies evenly spaced by Δν and h/2πm’=2SΔν. Our measurements of S and Δν enable us to quote a value of m’/2${\mathit{m}}_{\mathit{e}}$ for niobium of 1.000 084 (21). This result disagrees with predictions from currently available theories.