Magnetic-Field-Induced Crossover to a Nonuniversal Regime in a Kondo Dot

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We have measured the magnetic splitting, Δₖ, of a Kondo peak in the differential conductance of a Single-Electron Transistor while tuning the Kondo temperature, Tₖ, along two different paths in the parameter space: varying the dot-lead coupling at a constant dot energy, and vice versa. At a high magnetic field, B, the changes of Δₖ with Tₖ along the two paths have opposite signs, indicating that Δₖ is not a universal function of Tₖ. At low B, we observe a decrease in Δₖ with Tₖ along both paths, in agreement with theoretical predictions. Furthermore, we find Δₖ/Δ<1 at low B and Δₖ/Δ>1 at high B, where Δ is the Zeeman energy of the bare spin, in the same system.