  Quantum dots and single-molecule transistors may exhibit level crossings
induced by tuning external parameters such as magnetic field or gate voltage.
For Coulomb blockaded devices, this shows up as an inelastic cotunneling
threshold in the differential conductance, which can be tuned to zero at the
crossing. Here we show that, in addition, level crossings can give rise to a
nearly vertical step-edge, ridge or even a Fano-like ridge-valley feature in
the differential conductance inside the relevant Coulomb diamond. We study a
gate-tunable quasidegeneracy between singlet and triplet ground states, and
demonstrate how these different shapes may result from a competition between
nonequilibrium occupations and weak (spin-orbit) mixing of the states. Our
results are shown to be in qualitative agreement with recent transport
measurements on a Mn complex [E. A. Osorio, et al., Nano Lett. 10, 105 (2010)].
The effect remains entirely general and should be observable in a wide range of
Coulomb blockaded devices.
