Can electrons do the splits?

The electronic behaviour of some forms of matter doesn't match theory. Geoff Brumfiel meets Senthil Todadri, a man who wants to change our view of how electrons behave.

In a spin: Senthil Todadri is interested in whether electrons can split themselves up inside solids. Credit: S. NARASIMHAN

Right from the start, it was clear that Senthil Todadri was no ordinary graduate student, says Subir Sachdev, a professor of physics and Senthil's adviser at Yale University. On his first day, Senthil made several observations that forced Sachdev to rethink his work. “He himself didn't understand the depth to which he understood things,” Sachdev says. That first day's work was enough to win Senthil co-authorship on the group's next paper.

Senthil (who grew up with no last name, but adopted his father's name, Todadri, when he came to the United States) is the son of a banker in the Indian city of Chennai. “I was going to work in a bank just like my father, but got more and more sucked into maths and science,” he says. “My family considered it a bit bizarre when I decided to take up physics.” After completing his undergraduate degree at the Indian Institute of Technology in Kanpur, he began his graduate studies at Yale in 1992.

Since his first day on the job, Senthil, now 34, has continued to make waves in condensed-matter physics, a field whose theoretical underpinnings are in upheaval. Since the early 1980s, experimentalists have uncovered dozens of materials that defy the present theory of how electrons behave in solids — often referred to as the Fermi liquid model. Senthil is helping to build a new theoretical framework that could explain these exotic materials, the most alluring of which are superconductors (they have no electrical resistance) at temperatures that exceed those predicted by current models.

Deep divisions

One of the more unconventional ideas Senthil has pursued is that an electron added to a material can ‘split’ under the right circumstances, so that a fraction of its charge goes one way, and a bit of its ‘spin’ the other. “It's a pretty dramatic thing if you think about it because an electron is supposed to be a fundamental particle,” Senthil says. The electron loses its identity in the collective behaviour of other electrons in the solid, he explains. As the electron's fundamental characteristics of charge and spin are shared among the other electrons, it essentially splits into fractional particles of spin and charge.

The idea of electron splitting has been around since the 1980s, but in the context of high-temperature superconductors it has been taken seriously only in the past five years or so — thanks in part to work by Senthil and his colleague Matthew Fisher of the University of California, Santa Barbara. In 2001, Senthil and Fisher proposed a novel way that their ideas could be tested against the behaviour of certain high-temperature superconductors¹.

Not long after, an experimental group at Stanford set out to search for the fractional charges². Unfortunately, the team failed to find the exotic behaviour predicted by Senthil and Fisher, says Piers Coleman, a theoretical physicist at Rutgers University in New Jersey. “It was a nice idea that didn't work out, but that's okay,” says Coleman. “Good science has interesting proposals that can be tested. I think everyone regarded the work they did as extremely interesting and very stimulating.”

Sachdev, who has been a co-author on Senthil's work, agrees. “Since his paper appeared a few years ago, we've found that those ideas have turned out to be remarkably powerful.” Although the version of the theory proposed by Senthil and Fisher was proved wrong for high-temperature superconductors, they continue to explore ways in which the collective properties of a solid's electrons can shape its behaviour.

Most recently, Senthil and his collaborators have made impressive progress in describing quantum phase transitions — sudden shifts in a material's behaviour that are caused by the quantum fluctuations of its electrons at a temperature of absolute zero³. Once again, this theory depends on fractional parts of electrons appearing briefly at the point where the material changes from one state to another.

Homeward bound

We're just starting to glimpse an entirely new world inside solids — a lot of the stuff we teach in textbooks needs to be revisited.

This January, Senthil will leave his position at the Massachusetts Institute of Technology and return to India, where he will lead a theoretical group at the Indian Institute of Science in Bangalore. The reasons for the move are personal and professional, he says. It will allow him, his wife and his young daughter to be closer to the rest of their family.

“I don't know what it is really going to be like,” he says. But the quality of Indian physics has been steadily improving over the past few decades, he adds. One outstanding question is whether Senthil will be able to recruit the high-quality graduate students that form the backbone of any good theory group. “I'm hoping it will be possible to get good students and postdocs in India, but I don't have firsthand experience,” he says.

Despite some setbacks and uncertainties, Senthil remains confident that within a decade an entirely new set of theories will be developed that can explain even the most bizarre of materials. “We're just starting to glimpse an entirely new world inside solids,” he says. “It's a great time for condensed-matter physics because a lot of the stuff we teach in textbooks needs to be revisited.”