Probing boundary between worlds

Physicist Subir Sachdev still remembers the excitement that accompanied the discovery of high-temperature superconductors while he was working as a postdoctoral fellow at AT&T Bell Laboratories in 1986.

Superconductivity – the property of some materials to have no resistance to electrical flow – had been discovered in the early 1900s. Materials with this amazing property could sustain an electrical current indefinitely once the electrons making up the current were set in motion. But one characteristic of superconductivity had been that it occurred only at very low temperatures: just a few degrees above absolute zero.

In 1986, however, scientists discovered new materials that became superconducting at dramatically higher temperatures, as much as 100 degrees above absolute zero, opening an era of new materials, new theories, and new possibilities.

“It was a once-in-a-lifetime event,” Sachdev said. “[High-temperature] superconductivity was discovered in 1986 at about 40 degrees Kelvin [K] and within a few months the critical temperature went from 40 degrees K to 100 degrees K.”

To Sachdev, a condensed matter physicist who came to Harvard as a professor of physics in July, the excitement that pervaded the field with that discovery has not entirely dissipated, as it has resulted in a steady stream of new materials, new technology, and new findings that keep things interesting.

“New experiments are finding fascinating new effects. That keeps the field fresh,” Sachdev said. “With the advances in experimental technology, the discovery of new materials, it’s a wonderful place for a theorist to be.”

Sachdev’s research seeks to illuminate the boundary between the everyday world we live in – in which many but not all phenomena can be explained through classical physics – and the arcane subatomic world of quantum physics, a world whose rules make routine such apparent contradictions as electrons that spin two directions at once.

Sachdev believes those two worlds come together most apparently during an event called a “quantum phase transition.” By studying these transitions, he believes he can help craft theories that can explain both worlds.

Though “quantum phase transitions” may be an off-putting term, Sachdev uses the familiar event of ice melting to water to explain what is going on. In the melting of ice, heat is added and causes a phase transition from solid to a liquid.

Another analogy used by Sachdev, perhaps less familiar, is the change caused by heating a magnet to a high temperature, causing it to lose its magnetism. At the atomic level, what is happening in the magnet is that the heating causes electrons, which had all been spinning in the same direction, to begin spinning in different directions.

Similarly, with quantum phase transitions, adding some external perturbation to a superconductor – such as applying pressure or putting it in a magnetic field – can cause its properties to change, sometimes dramatically. In a quantum phase transition, the role of heat is played by the fluctuations demanded by Heisenberg’s uncertainty principle.

In one example, Sachdev said, putting a high-temperature superconductor into a weak magnetic field prompts it to switch from being a superconductor, with no resistance to an electrical current, to an insulator, with infinite resistance to an electrical current.

In that example, he said, it takes an understanding of quantum mechanics to explain what’s going on at both the atomic level and with the larger material.

“My theoretical focus is to make the connection between the macroscopic and the microscopic. Quantum phase transitions are where this happens most directly,” Sachdev said.

Sachdev has published well over 100 articles on his research and, in 1999, published a book, “Quantum Phase Transitions.” He will teach a class in statistical thermodynamics this fall and a more advanced class in the spring that will serve as an introduction to his research.

Sachdev, who moved to the United States from India as a teenager, received a bachelor’s degree in physics from the Massachusetts Institute of Technology in 1982. He received a master’s degree in physics from Harvard in 1983 and a doctorate from Harvard in 1985.

He worked at AT&T Bell Laboratories from 1985 to 1987, when he became an assistant professor of physics and applied physics at Yale. He became an associate professor there in 1989 and professor of physics in 1995. He moved to Harvard this summer.