Classifying coherent peaks in nanoelectronic devices by the presence or absence of spin exchange

Coherent peaks appearing in the differential conductance of quantum-dot single-electron transistors (QDSETs) and quantum point contact (QPC) devices are classified into two categories according to the scaling function onto which the temperature-scaled differential-conductance maxima collapse and the underlying spin dynamics. The zero-bias peaks (ZBPs) observed in QPCs and in the triplet state of the even-particle sector of QDSETs belong to the same category, whereas the ZBP in the odd-particle sector of a QDSET belongs to a different category together with all finite-bias coherent peaks observed in QPCs and in the even-particle sector of QDSETs. The spin dynamics of the former category involve spin exchange, a hallmark of Kondo dynamics, whereas those of the latter category involve only cotunneling of an up--down spin pair. Furthermore, for the former type of ZBP, the scaling temperature coincides with one-half of the full width at half maximum (FWHM), which corresponds to the Kondo temperature. In contrast, for the latter type, the scaling temperature does not coincide with the (1/2)FWHM-derived energy scale. To support these findings, the gate-voltage-dependent differential-conductance line shapes measured in the odd-particle sector of a QDSET are theoretically reproduced. The results demonstrate that the observed ZBP is a merging of two coherent side peaks generated solely by the cotunneling of up--down spin pairs.