Temperature dependence of an SU(4) to SU(2) Kondo crossover in a half-filling carbon nanotube quantum dot

In a carbon nanotube quantum dot, an SU(4) Kondo effect occurs owing to four-fold degenerate levels. An external magnetic field lifts the degeneracy and induces a new type of crossover from the SU(4) to SU(2) Kondo state which is observed at half-filling [1]. This crossover can be explained if two energy levels among the four still remain unlifted as a result of a matching of the spin and orbital Zeeman splittings [2]. We calculate the conductance G at finite temperatures T using the numerical renormalization group. The results of conductance nicely agree with the experimental results in a wide range of the gate voltages. Furthermore, at half-filling, the profile of G vs T relation varies from the SU(4) to the typical SU(2) form as magnetic field B increases. The Kondo temperature, at which G shows the log T dependence, also decreases as B increases. We also discuss how the spectral function evolves during the crossover.
[1] M. Ferrier, Y. Teratani, et al, Phys. Rev. Lett. 118, 196803 (2017).
[2] Y. Teratani, et al, J. Phys. Soc. Jpn. 85, 094718 (2016).