June 3, 2010

Old question answered: 'Heavy fermions' aren't really heavy -- they just dawdle

For decades physicists have been fascinated and frustrated by "heavy fermions" -- electrons that move through a conductor as if their mass were up to 1,000 times what it should be.

Now for the first time scientists have produced images of heavy fermion behavior and resolved a theoretical question about its cause.

Using an incredibly sensitive scanning tunneling microscope (STM) and a technique called "spectroscopic imaging" that measures the energy levels of electrons under the STM probe, 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, determined that electrons moving through a particular uranium compound appear "heavy" because their motion is constantly interrupted by interaction with the uranium atoms.

"This is the first imaging of heavy electron waves by any machine anywhere in the world," Davis said.

The results appear in the June 3 edition of the journal Nature.

The heavy fermion phenomenon is found in a wide variety of materials -- mostly metals combined with rare-earth elements -- in which there is a periodic array of atoms that have a magnetic moment. Many heavy-fermion materials can become superconductors at very low temperatures, a puzzler because magnetism and superconductivity usually don't coexist.

Insight into how these materials work could be a step toward understanding the workings of superconductors in general. And because the ability of a material to absorb heat depends on the mass of its particles, the work could lead to advances in solid-state electronic refrigeration, Davis said.

Davis' team examined URu2Si2, composed of uranium, ruthenium and silicon, which has been a subject of much experimentation and debate since it was first synthesized 25 years ago. At about 55 kelvins (degrees above absolute zero), it begins to show heavy fermion behavior. At 17.5 kelvins it goes through a complex phase transition in which its conductivity, ability to absorb heat and other properties change. Theorists attribute this to a "hidden order" in the material's electrons, but what that might be remained a mystery.

Davis and Cornell graduate students Andrew Schmidt and Mohammad Hamidian varied the voltage between the STM probe and the surface to determine the amount of force needed to pull electrons free from the surface, and from this, the energy levels of the electrons. They scanned samples of URu2Si2 a few nanometers square at a range of temperatures from 17.5 K down.

They found that mobile electrons in the sample, rather than flitting lightly from atom to atom, were interacting strongly with the uranium atoms, in effect diving down into their lower energy levels for picoseconds. This confirms a theoretical explanation for the heavy fermion phenomenon that electrons, which have a tiny magnetic moment, interact with the magnetic moments of uranium atoms. They are not really "heavy," but move as if they were.

Imagine a crowd of frogs hopping across a pond on lily pads. If you know how much push a frog's legs can impart and measure the travel time across the pond, you could calculate the weight of the average frog. But suppose there's an attractive lady frog on every pad, and the frogs stop to chat. Measuring just the travel time, you might conclude that these frogs were all like Mark Twain's famous jumping frog, with bellies full of buckshot.

In addition to answering this question, the demonstration that the spectroscopic imaging STM can image the formation process of heavy electrons opens many more possibilities for further research on heavy-fermion materials, Davis said.

The research was funded by the U.S. Department of Energy, the Canadian Office of Science and the Canadian Institute for Advanced Research.