Brighter set represents a larger

A novel quantum couple

US and Japanese researchers have created and imaged a novel pair of quantum dots – tiny islands of confined electric charge that act like interacting artificial atoms.

They are the first, they say, to probe the interior of a coupled quantum dot system so deeply, imaging the distribution of electrons with atomic resolution – as you can see above.

To do so, they used the ultra-sharp tip of a scanning tunnelling microscope as if it were a stylus. Hovering the tip above a very cold sheet of graphene they briefly increased the voltage of the tip.

The electric field generated by the voltage pulse penetrated through the graphene into an underlying layer of boron nitride, where it stripped electrons from atomic impurities in the layer and created a pileup of electric charge. The pileup corralled freely floating electrons in the graphene, confining them to a tiny energy well.

But when the team applied a magnetic field of four to eight tesla (about 400 to 800 times the strength of a small bar magnet) it dramatically altered the shape and distribution of the orbits that the electrons could occupy. 

Rather than a single well, the electrons now resided within two sets of concentric, closely spaced rings within the original well separated by a small empty shell. The two sets of rings for the electrons now behaved as if they were weakly coupled quantum dots.

The work was led by researchers from the National Institute of Standards and Technology, in the US. 

The findings and their implications are discussed in a paper in the journal Physical Review B.