The Kondo effect is a cooperative many-body phenomenon where electrons in a metal interact via spin-exchange with magnetic impurity atoms. The impurity increases the scattering of electrons at the Fermi level, causing an anomalous increase in resistance below a certain temperature. The Kondo effect is also seen in a single-electron quantum dot connected to metallic leads, but here it manifests as an increase in conductance and perfect transmission through the dot.
In a recent issue of Physical Review Letters, Alexander Hübel, Karsten Held, Jürgen Weis, and Klaus von Klitzing at the Max Planck Institute in Stuttgart report a Kondo state with a novel symmetry [SU(4)] in a double quantum dot system. In their device, the dots are connected to independent leads and capacitively coupled to each other. Gate voltages are adjusted so that a single electron can occupy four degenerate states, (0, ↑), (↑, 0), (0, ↓), and (↓, 0) [here, (0, ↑) refers to no electron on one dot and an electron with a spin up on the other, and so on] and conductances through the two dots can be monitored independently. When tunneling to the leads is weak, tuning the gate voltage for each dot can cause the occupancy of that dot to fluctuate between 0 and 1 to produce a finite conductance. However, when tunneling to the leads is sufficiently strong and similar for both dots, conductance is achieved through both dots simultaneously due to an interdot Kondo effect. In this case, the charge and spin between the two dots are invariably entangled to produce an SU(4) Fermi liquid. – Sarma Kancharla