Cold Atoms Simulate Graphene

Photo: Density distribution of the potassium atoms

The Physicists Andre Geim and Konstantin Novoselov from Manchester University were the first to isolate and identify graphene in 2004. Graphene is a single layer of carbon atoms, which form a two-dimensional honeycomb structure. This makes graphene an exceptionally good conductor, of great interest for future electronic devices. The award of the 2010 Nobel Prize for Physics for the discovery of graphene also shows the importance of this material.

The behaviour of electrons in the vicinity of the so-called Dirac point is central to understanding the special properties of graphene. At the Dirac point, the valence and the conduction band of graphene touch in a linear crossing. There the electrons behave as massless particles travelling with an effective speed of light.

Soon after the discovery of this “magic” material, scientists raised the question what would happen if the lattice structure of graphene could be modified. As this is difficult to realize with real graphene, researchers attempted to simulate graphene in experiments. Two research groups have now independently succeeded in doing exactly this. Tilman Esslinger, Professor at the Institute of Quantum Electronics, ETH Zurich and at NCCR Quantum Science and Technology, leads one of the research groups.

Esslinger and his team loaded ultracold potassium atoms into a special lattice structure made of laser light: the researchers used a set of orthogonal and precisely positioned laser beams to create a variety of two-dimensional light field geometries, including the honeycomb structure of graphene.

In the experiment they cooled several hundred thousand potassium atoms inside a vacuum chamber to temperatures just above absolute zero, thereby bringing the atoms to rest. Then, they place the optical lattice over the cloud of atoms. A great challenge was to precisely control the laser beams. “Designing a structure like this with laser beams is similar to creating a beautifully regular pattern in a lake by simultaneously throwing several pebbles in at carefully chosen positions”, says Esslinger.

Once trapped in the optical lattice, the potassium atoms behave like electrons in the crystal structure of graphene. By accelerating the atoms with a magnetic field gradient, the researchers could identify Dirac points in the optical lattice. Near a Dirac point the atoms behave like massless particles – just as the electrons in graphene – and they can move from the valance to the conduction band since the band gap vanishes. It is this transition to the higher band that the researchers observed in time-of-flight measurements. They switched off the lasers beams and the optical honeycomb lattice disappeared so that the atoms flew through the vacuum. A little later, an absorption image of the atomic distribution was taken, allowing it to reconstruct the atomic trajectories (see figure).

Using the flexibility of the optical lattice set-up, the researchers could now play with the Dirac points. They moved and merged them until they suddenly vanished. They could also observe that a slight change in the lattice symmetry made the atoms get their mass back.

The newly created tool now offers new options in the search for useful materials. „Using this method, it may become possible to simulate the electronic properties of materials long before they can be physically realized “, hopes Esslinger. Another open question is what is going to happen if there are strong interactions between the atoms, a situation that has not yet been attained for the electrons in graphene.; Source: ETH Zurich