Stable Unpaired Electron States in the Lu-Lu Bond Leading to the Absence of Odd-Even Parity in the Kondo Effect of Lu@C Transistors.

Spin qubits constructed in endohedral fullerenes benefit from the protective shielding of the carbon cage, which effectively mitigates external decoherence and enables ultralong coherence times. However, endohedral fullerene spin qubits face the challenge of charge transfer in complex electrical environments, such as during qubit readout or large-scale integration, which can induce spin state modifications. In this study, we developed transistors based on the endohedral fullerene Lu@C and observed the absence of parity dependence in the Kondo effect; this result was contradictory to the typical behavior of the Kondo effect observed in C. Density functional theory calculations revealed that upon electron loss, a spin-1/2 electron predominantly from the -orbitals formed in the Lu-Lu bond and its orbital energy was significantly lower than that of the highest occupied molecular orbital. Based on these results, Lu@C held stable unpaired electron states across multiple charge states and has potential applications in spin quantum devices.