Spin effects in semiconductor quantum dot structures

The spin effects on the electronic properties are studied for two different quantum dots: a quantum dot well isolated from the leads and that strongly coupled to the leads. For the first quantum dot, we use a magnetic field to adjust the single-particle state degeneracy, and observe a singlet–triplet transition at zero and non-zero magnetic fields, favored by direct Coulomb and exchange interactions. The spin configurations in an arbitrary magnetic field are well explained in terms of two-interacting electron model. For the second quantum dot we adjust the single-particle state degeneracy as a function of magnetic field and observe a novel Kondo effect associated with the singlet–triplet degeneracy. This Kondo anomaly appears when the quantum dot holds an even number of electrons, and the characteristic energy scale is much larger than in the ordinary half-spin case.