Researchers watch how ’heavy fermions’ get less heavy

By "tuning" the behavior of "heavy-fermion" materials, we may learn more about how superconductivity works, according to a new Cornell study reported Oct. 17 in the Early Online Edition of the Proceedings of the National Academy of Sciences.

Using an incredibly sensitive scanning tunneling microscope (STM), a team led by J.C. Séamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences at Cornell and director of the Center for Emergent Superconductivity at Brookhaven National Laboratory, has mapped the states of electrons around impurities that disrupt the characteristic behavior of heavy-fermion materials, confirming theoretical predictions.

"There have been beautiful theories that predict the effects... but no one knew how to look at the behavior of the electrons, until now," Davis said.

Fermions are a class of subatomic particles that includes electrons. In so-called heavy-fermion materials, which are metals combined with other elements, the free electrons that conduct electricity move far more slowly than expected, as if their mass had somehow increased 1,000 times. Davis and colleagues last year showed that the electrons actually are slowed by interactions with the metal atoms, but "heavy" is still used by convention.

The metal atoms in heavy-fermion materials are magnetic, and the heavy-fermion behavior can be broken by doping the material with a few non-magnetic atoms. Heavy-fermion materials become superconductors at very low temperatures, and non-magnetic doping also destroys that superconductivity. Magnetic interactions may be the glue that sticks electrons together in "Cooper pairs" that move without resistance in superconductors.

An STM, scanning in steps a fraction of the diameter of an atom, can measure the "energy states" of electrons. Each electron in a substance must occupy one of a limited number of energy states, and there is a special state for the conduction electrons that hop from atom to atom to carry current through a conductor.

The Cornell researchers scanned samples of a compound of uranium, rubidium and silicon, doped with a small number of thorium atoms. Previous work showed that when a conduction electron hops onto a uranium atom, it goes into a "hybrid" state -- somewhere between a local electron state and a conduction electron state. As theory predicted, these hybrid states were not found at the non-magnetic thorium atoms, but also disappeared all across the sample. The "hole" creates oscillations that spread out in waves, destroying heavy-fermion behavior around neighboring uranium atoms, the researchers explained. "It takes a tiny number of these impurities to make a lot of disorder," Davis said.

It should be possible to "tune" heavy-fermion behavior by varying the amount of non-magnetic doping, said postdoctoral associate Mohammad Hamidian, lead author of the paper. "First, we have to understand how we can break heavy-fermion behavior," Hamidian said. "Then, how does breaking heavy-fermion behavior break superconductivity?"
S. Department of Energy. Heavy-fermion samples were prepared by Graeme Luke of McMaster University in Hamilton, Ont.