A metal needle descends onto a piece of superconductor. All of this happens inside a refrigerator that cools the sample to 30 thousandths of a degree above absolute zero — a hundred times colder than outer space. A weak current is passed through the needle, and by how electrons travel through the tiny contact point, physicists read what is happening inside the material.
Previously, such measurements at such low temperatures were nearly impossible: the needle would tremble from the slightest heat, and running wires to it without warming the sample was a headache in itself. The authors of the new setup solved both problems. The needle is driven by a piezoelectric actuator — a crystal that changes shape under an electric voltage, shifting the needle by fractions of a nanometer, as if turning a microscope knob in steps of a single atom. And all wires from the needle to the measuring device are pressed against cold plates to carry away excess heat, much like plumbing carries hot water away from a tap.
The instrument was tested on a superconductor made of titanium, tantalum, and selenium — a compound that loses its resistance at 2.3 kelvin. The spectra showed clear dips and peaks — signatures of electrons pairing up and flowing without loss. Upon heating or applying a magnetic field, these features weakened and disappeared, just as the theory of superconductivity predicts.
Now physicists have a tool to peer into the world of quantum materials at temperatures where almost no thermal noise remains. A needle that listens to the silence, and hears electrons singing.
30 millikelvin is a temperature at which even helium, if it were in contact with the sample rather than in a vacuum, would turn into a solid. Thermal motion of atoms is so weak that the needle can touch the sample without displacing a single atom.