Progress in solid-state physics tends to occur in increments rather than leaps. Experimentalists perform measurements, and theorists refine their models based on the results. Eventually, their work could inform research into quantum computers and exotic materials like room-temperature superconductors. But every once in a while, an experimental finding forces researchers to question what they thought they knew and re-evaluate accepted theories.
In March, Sergey Kravchenko, a professor of physics at Northeastern University, in Boston, Massachusetts, and eight collaborators threw such a wrench in the works. In a paper published in Scientific Reports, they describe an experiment into the resistivity of a slim slice of silicon close to absolute zero. According to several theoretical models, a 2D system of strongly interacting electrons should rapidly transition from an insulator-like behavior (resistance grows with decreasing temperature) to a metallic behavior (resistance sharply drops with decreasing temperature) at a predictable temperature around 1 kelvin. In 2020, the team applied a theory—called dynamical mean-field theory—that quantitatively described the experimental results in a wide range of parameters.
The model largely held true, but then Kravchenko and his collaborators switched off the spin. All electrons have a spin associated with them, but applying a strong magnetic field parallel to the 2D plane effectively turns it off in 2D electron systems. ‘Ordinary’ spinless electron systems don’t exhibit the metallic behavior, but the electron system studied by the team — a layer of silicon sandwiched between two layers of silicon germanium — is unique in that the electrons have an additional (valley) degree of freedom that allows the metallic state to exist even when the spins are turned off.
When the team measured the resistivity of a spinless 2D electron system at ultralow temperatures, it behaved quite differently from theoretical predictions.
“The remarkable agreement between the experiment and theory collapsed when we eliminated spin,” Kravchenko says. Specifically, the team found that the temperature of the peak resistivity, which occurs just before the transition to the metallic behavior, did not follow predictions: instead of increasing by a factor of two, it decreased. That calls into question current thinking.
While the findings have theoretical physicists scratching their heads, Kravchenko is no stranger to disruptive experiments. In 1993, he and colleagues upended the existing theory describing 2D electron systems. In a heavily cited paper, they demonstrated that such systems switch from an insulator to a metal a few degrees from absolute zero. The result was so unexpected that the team had a hard time convincing others.
“It took us a year to publish the result because the belief that it could not happen was so ingrained,” Kravchenko recalls. It wasn’t until 1997 that other groups experimentally confirmed the effect. It has been listed on the American Physical Society timeline “A Century of Mesoscopic Physics (1899–1999)” among the 50 main discoveries in the field.
Whether Kravchenko’s most recent findings will remain unexplained for quite so long remains to be seen. They were able to observe this effect for the first time because their samples were ultraclean. “Our samples were very pure with very few scatterers, which is why we were able to achieve record high electron mobilities,” Kravchenko says.
As others puzzle over the latest finding, the team is already on to their next research project, exploring the insulation state in the same ultraclean samples.
To read more about the experiment and results, read the study in Scientific Reports.
Sergey Kravchenko is a professor of physics at Northeastern University (Boston, USA). He is a recipient of the Royal Society Fellowship (1992) and Sloan Research Fellowship (1998) and was elected a Fellow of the American Physical Society in 2008. Kravchenko studies the low-temperature properties of strongly correlated low-dimensional electron systems.