The electron is a tiny particle, but inside a crystal it behaves as if it had a different mass. Physicists call this the effective mass. To measure it, they make electrons run in circles in a magnetic field, like a speed skater on an ice rink who is kept from leaving the orbit.

In the new study, the team built a device that cools the sample to 145 millikelvins — colder than outer space, just 0.145 degrees above absolute zero. They also placed a magnet generating a field up to 7 tesla — hundreds of thousands of times stronger than Earth's magnetic field. The sample is a thin layer of gallium arsenide where electrons can move freely only in one plane.

Under the magnetic field, electrons in this layer circle along circular paths. The frequency of this circling depends on the effective mass. By measuring this frequency, the scientists calculated the mass: it turned out to be 0.073 times the mass of a free electron. This is slightly larger than the known value of 0.068 for a bulk crystal. The difference is small, but it shows that electrons in a thin layer experience their environment differently.

Such precise measurements help improve transistors and lasers and also test quantum theories. And, as often happens in science, a tiny discrepancy in the third decimal place can lead to new understanding.