Dark electrons discovered in solids for 1st time in superconductor breakthrough

The mystery of dark electrons

Scientists are able to detect regular electrons using various spectroscopy techniques because they can absorb a photon’s energy, allowing atoms and molecules to absorb light. 

Dark electrons, on the other side, are fundamental particles that form dark matter. They don’t interact with photons and any other electromagnetic forces, and therefore scientists can’t detect their presence using spectroscopy methods.

Since dark electrons don’t interact with light particles, dark matter which is around 27 percent of the universe, is also unable to absorb light and therefore remains invisible. 

While some scientists consider dark electrons to be hypothetical particles, many others believe that materials around us also contain electrons in a dark state that is hidden from us. For the first time, scientists have solid evidence suggesting that the latter is true.

Finding the hidden electrons 

Many previous studies that aimed to find dark electrons used materials such as graphenes that had only one pair of sublattices, a specific repeating arrangement of a subset of atoms within the larger crystal structure of a material.

However, the researchers chose materials with two pairs of sublattices for their study. This is because they could deduce the number of total electrons (including both regular and dark) in the sublattices of such materials. 

“We studied three materials, palladium diselenides (PdSe2), cuprate superconductors (Bi2Sr2CaCu2O8+δ or Bi-2212), and lead halide perovskites (CsPbBr3),” Keun Su Kim, one of the study authors and a physics professor at Yonsei University, said.

The study authors knew that materials with two pairs of sublattices in their quantum state have four types of electrons. However, when they used angle-resolved photoemission spectroscopy (ARPES), a technique used for studying electrons in solid materials, they obtained a different result.

ARPES involves photons striking a material’s surface and causing the emission of electrons (photoelectric effect). It revealed the presence of only one type of electron. This meant that the remaining three types of electrons didn’t interact with the photos, suggesting they were in a dark state. 

“We could see experimental signals only for electrons expected to be detectable (bright states) and could not see any experimental signals for electrons expected to be undetectable (dark states),” Kim said.

Although further research is required to confirm these findings, this experiment does highlight the significance of sublattices when it comes to studying dark electrons. 

“Our results suggest that the sublattice degree of freedom, which has been overlooked so far, should be considered in the study of dark electron-related phenomenon and optoelectronic characteristics,” the study authors added. 

The study is published in the journal Nature Physics.