Observation of a magnetic-field dependent g-factor ratio

A new type of magnetic shift has been observed in high-precision measurements on the hyperfine structure of ground-state rubidium atoms. In the range of magnetic fields used, 4.6 7.8 T, the measurements in both Rb85 and Rb87 are consistent with a fractional shift of -1.68(15)×10-9 T-2 in the nuclear-to-electronic g-factor ratio, gI/gJ. This effect is much larger than predicted by a second-order perturbation-theory analysis which gives other magnetic shifts in agreement with the measurements.