A bridge where pedestrians and cyclists flow together without getting in each other's way. That's roughly how hybrid systems of superconductors and semiconductors work: the former carry current without loss, the latter control that current. But to build such a bridge, you need a superconductor that isn't afraid of strong magnetic fields and doesn't damage the fragile semiconductor substrate during fabrication.
The authors found a suitable material — an alloy of platinum, iridium, germanium, and silicon (PtIrSiGe). It remains superconducting even in a magnetic field of 1.9 tesla — about 40,000 times stronger than Earth's magnetic field. What's more, it can be deposited onto a germanium wafer heated to 500 degrees, and the thin conducting layer inside the wafer retains its exceptional mobility—1.3 million square centimeters per volt-second. For comparison, silicon used in ordinary processors has mobility hundreds of times lower.
The researchers didn't just create the material; they put it to the test by building a Josephson junction — a key element of superconducting electronics. Such a junction acts as a highly precise switch that responds to magnetic fields. In effect, they assembled a simple quantum circuit — a SQUID (superconducting quantum interference device) capable of detecting extremely weak magnetic signals.
This superconducting bridge connects the world of high-mobility germanium with the world of strong magnetic fields. In doing so, it opens the door to new quantum devices — from topological qubits to interfaces between superconducting and spin systems.
Germanium was the first material on which a transistor worked in 1947, but it was later largely supplanted by silicon. Now, thanks to its ability to form ultra-clean layers with record mobility, germanium is making a comeback in big science: this time for quantum technologies.
In Peter Watts's novel Blindsight, superconducting SQUIDs are used to detect magnetic fields in the brain — a technology that comes closer to reality thanks to materials like the one described in this paper.