Cold atoms in an optical lattice simulate graphene
ZURICH – The creation of a tunable system of ultracold atoms within a honeycomblike structure similar to that found in graphene may help identify the electronic properties of materials that have yet to be discovered.
Professor Tilman Esslinger of the Institute of Quantum Electronics at ETH Zurich and his team loaded ultracold potassium atoms into a special lattice structure made of laser light. Using a set of orthogonal and precisely positioned laser beams, they created a variety of two-dimensional light field geometries, including graphene’s honeycomb structure.
They cooled several hundred thousand potassium atoms inside a vacuum chamber to temperatures just above absolute zero, which brought the atoms to rest, then placed the optical lattice over the cloud of atoms.
“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,” Esslinger said.
Shortly after the discovery of graphene, scientists raised the question as to what would happen if the lattice structure of graphene could be modified. Researchers have tried to simulate graphene in experiments, but until now had been unsuccessful.
The behavior 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 conduction band of graphene touch in a linear crossing, where electrons behave like massless particles traveling at the effective speed of light.
The density distribution of the potassium atoms measured after acceleration through Dirac points (left and center) and without Dirac points (right). The upper row shows the corresponding regions of the calculated band structure. Courtesy of Tilman Esslinger’s Research Group/ETH Zurich.
Esslinger and his team reproduced graphene’s distinctive Dirac points in a 2-D honeycomb lattice by criss-crossing the laser beams. The lattice contained potassium atoms, which played the role of electrons in graphene.
Once the potassium atoms were trapped in an optical lattice, they began to act like electrons in the crystal structure of graphene. Upon accelerating the atoms with a magnetic field gradient, the researchers could identify Dirac points in the optical lattice. They observed that the atoms behaved like massless particles near the Dirac points, just as the electrons did in graphene, and that they can move from the valence to the conduction band, since the bandgap vanishes.
It is this transition to the higher band that the researchers observed in time-of-flight measurements. Once they switched off the laser beams, the optical honeycomb lattice disappeared, and the atoms flew through the vacuum.
A short time afterward, an absorption image of the atomic distribution was taken, which reconstructed the atomic trajectories.
Using the flexibility of the optical lattice setup, Esslinger’s team played with the Dirac points, moving and merging them until they suddenly vanished. They also observed that a slight change to the lattice symmetry restored the mass to the atoms.
“Using this method, it may become possible to simulate the electronic properties of materials long before they can be physically realized,” he said.
The work was described in Nature (doi: 10.1038/nature10871).
What is left to be answered, however, 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.
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