For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such "optical lattice" simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.
"It's a very big improvement over previous results," says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. "It's very exciting to see steady progress."
An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.
Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.
Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to "solve" that nobody has been able to show whether it produces superconductivity.
Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.
Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn't have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.
Hulet's team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones "evaporated" like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn't quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, Hulet says. "We don't have a good sense of what the limit of this method is," he says. "We could get a factor of two lower, we could get a factor of 10 lower."
"It is indeed very promising," says Tin-Lun "Jason" Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ's Bloch cautions that it may take still other techniques to get that cold. "There are several cooling techniques that people are developing and interesting experiments coming up," he says.
Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: "It's an exciting time."