Nonequilibrium Singlet-Triplet Kondo Effect in Carbon Nanotubes

TL;DR

This paper reports a novel nonequilibrium Kondo effect observed as a finite-bias conductance peak in a carbon nanotube quantum dot, demonstrating persistent many-body correlations beyond equilibrium conditions.

Contribution

It introduces the observation and theoretical explanation of a nonequilibrium singlet-triplet Kondo effect in carbon nanotubes, expanding understanding of Kondo physics in non-equilibrium states.

Findings

Finite-bias conductance peak observed in nanotube quantum dot.

Agreement between experimental data and theoretical calculations.

Identification of a nonequilibrium Kondo effect involving triplet excitations.

Abstract

The Kondo-effect is a many-body phenomenon arising due to conduction electrons scattering off a localized spin. Coherent spin-flip scattering off such a quantum impurity correlates the conduction electrons and at low temperature this leads to a zero-bias conductance anomaly. This has become a common signature in bias-spectroscopy of single-electron transistors, observed in GaAs quantum dots as well as in various single-molecule transistors. While the zero-bias Kondo effect is well established it remains uncertain to what extent Kondo correlations persist in non-equilibrium situations where inelastic processes induce decoherence. Here we report on a pronounced conductance peak observed at finite bias-voltage in a carbon nanotube quantum dot in the spin singlet ground state. We explain this finite-bias conductance anomaly by a nonequilibrium Kondo-effect involving excitations into a spin triplet state. Excellent agreement between calculated and measured nonlinear conductance is obtained, thus strongly supporting the correlated nature of this nonequilibrium resonance.