Physicists in Germany and the US are the first to see the superexchange interaction between atomic spins in an optical lattice. Superexchange leads to magnetism in a wide range of materials — including some that are high-temperature superconductors — and the team believes that their technique could shed light on the electronic and magnetic properties of these materials.
Superexchange is an interaction that normally occurs between electron spins in a crystalline material. Unlike the more familiar exchange interaction, which affects electrons that are close enough together to have overlapping quantum-mechanical wavefunctions, superexchange does not require an overlap. Instead, the interaction has its basis in the “virtual hopping” of electrons from one lattice site to another. This is a quantum mechanical process by which an electron can “tunnel” through the region separating neighbouring lattice sites and join its neighbour, only for the electron or its neighbour to hop back a moment later.
The likelihood of this happening is governed by the relative orientations of the spins of the electrons. As a result superexchange can either cause the spins of neighbouring electrons to point in the same direction, or opposite directions, depending on the exact composition of the material.
10,000 double potential wells
Now, Immanuel Bloch of the Johannes Gutenberg University in Germany along with researchers at Harvard University and Boston University in the US have observed superexchange in an optical lattice of ultracold rubidium atoms (Sciencexpress). The team crisscrossed several laser beams to create 10,000 identical double potential wells, each containing two atoms. The double wells were arranged in a line creating a 1D lattice (see figure “Superexchange in action”).
The lattice was set up such that the pairs of atoms had their spins pointing in opposite directions — something that would occur in an antiferromagnetic material. The team then adjusted the lasers to reduce the potential barrier between the pairs of atoms. This made it more likely that tunnelling between wells would occur, thereby increasing the strength of the exchange interaction.
The team then watched as the atomic spins responded to this change. While they were unable to monitor individual atoms, they could measure the average direction of the spins in the right and left sides of the double wells.
The directions of the spins were seen to oscillate back and forth between wells. For example, if the atoms in the left wells began as spin up and the right wells as spin down, about 25 ms later the left wells would contain spin down atoms and the right wells spin up. The team claims that this observation is in agreement with the theory of superexchange between pairs of atoms.
Bloch and colleagues were also able to change the superexchange coupling from antiferromagnetic to ferromagnetic — in which spins of neighbouring pairs point in the same direction — by raising one side of the double well relative to the other side.
Bloch told physicsworld.com that the team hope to extend their technique to create a 2D optical lattice. This could be used to study a wide range of exotic magnetic systems, including those with antiferromagnetic interactions along one axis and ferromagnetic interactions along the other.
Such optical lattices could also be used to gain insight into some high-temperature superconductors, which are known to have magnetic properties related to superexchange. These materials consist of stacked 2D layers, and therefore a 2D optical lattice could be very useful in understanding the interactions that bring about superconductivity.
Bloch also believes that the ability to fine-tune superexchange interactions between atoms in an optical lattice could be used to create logical components for quantum computers.