Back in 1937, an Italian physicist predicted the existence of a single, stable particle that could be both matter and antimatter. Nearly 80 years later, a Princeton University research team has actually found it.
Ettore Majorana proposed these exotic particles back in the 1930s. Named after him, they're a unique exception to the standard relationship of matter and antimatter in that they don't annihilate each other when they come into contact. Majorana surmised that a fermion (a type of small and light elementary particle that serves as a building block of matter) with no electric charge (i.e., it's electrically neutral) would have a completely identical antiparticle. Though many forms of antimatter have been observed since the days of Majorana (including an unconvincing discovery of a supposed Majorana particle in 2012), this particular combination remained elusive.
Now, using a two-story-tall microscope floating in an ultra-low vibration lab, Princeton scientists have captured a glowing image of a particle they believe to be the vaunted Majorana fermion. Their imaging showed it perched at the end of an atomically thin wire — exactly where it had been predicted.
Credit: Ali Yazdani Lab.
To locate the quasiparticle, Ali Yazdani and his team used a very simple combination of lead and iron. A release from Princeton University explains:
The setup they created starts with an ultrapure crystal of lead, whose atoms naturally line up in alternating rows that leave atomically thin ridges on the crystal's surface. The researchers then deposited pure iron into one of these ridges to create a wire that is just one atom wide and about three atoms thick. Considering its narrow width, this wire is very long — if it were a pencil it would be five feet from tip to eraser.
The researchers then placed the lead and the embedded iron wire under the scanning-tunneling microscope and cooled the system to -272 degrees Celsius, just a degree above absolute zero. After about two years of painstaking work, they confirmed that superconductivity in the iron wire matched the conditions required for Majorana fermion to be created in their material.
Ultimately, the microscope was also able to detect an electrically neutral signal at the ends of the wires, similar to those seen in the Delft experiment. However, the setup also allowed the researchers to directly visualize how the signal changes along the wire, essentially mapping the quantum probability of finding the Majorana fermion along the wire and pinpointing that it appears at the ends of the wire.
"It shows that this signal lives only at the edge," Yazdani said. "That is the key signature. If you don't have that, then this signal can exist for many other reasons."
This discovery carries some fascinating implications. The Princeton physicists speculate that neutrinos — another weak and elusive subatomic particle — could be a type of Majorana (a neutrino and anti-neutrino being the same particle). Also, they regard Majoranas as possible candidates for dark matter, the mysterious substance thought to account for most matter in the universe. More practically, Majoranas could greatly assist in quantum computing.
Read the entire study at Science: "Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor". And check out the entire Princeton release.