Finite-bias conductance anomalies at a singlet-triplet crossing

TL;DR

This paper investigates conductance anomalies at singlet-triplet level crossings in quantum dots and single-molecule transistors, revealing how these features arise from nonequilibrium effects and state mixing, with implications for various Coulomb blockaded devices.

Contribution

It demonstrates that level crossings can produce distinctive conductance features, such as step-edges and Fano-like lineshapes, due to competition between nonequilibrium occupations and spin-orbit mixing.

Findings

Conductance features depend on nonequilibrium occupations.

Weak spin-orbit mixing influences conductance line shapes.

Results agree qualitatively with experimental data.

Abstract

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.