The caesium–caesium scattering length aBB is shown with a blue line, and the lithium–caesium scattering length aBF with a red line. The interspecies Feshbach resonance near B = 892 G is used for sample preparation and the effective trapping frequency measurements, indicated by the light green shaded area. The caesium–caesium zero crossing near B = 880 G is used for the effective scattering length measurements, indicated by the blue shaded area. In this region, the lithium–caesium scattering length is nearly constant at aBF = −60a0 (ref. 22).