Noise-canceling headphones work by generating sound waves that are the exact opposite of the noise surrounding you, canceling them out and leaving you in silence. Similarly, a “dark state” occurs in matter when quantum states are combined in a way that cancel their interaction with photons and render them undetectable by spectroscopic means. The dark state of electrons has been found in atoms and molecules, but it has widely been believed that the conditions needed for the dark state of electrons in the more complex case of solids may not exist.
Contrary to this belief, an international team led by Yonsei University studied a system with two pairs of sublattices and found that electrons in certain configurations exist in these dark states, undetectable because they do not absorb or emit photons. These results are published in Nature Physics and could provide crucial insights into complex phenomena in physics, such as high-temperature superconductivity and optoelectronics.
“The unique sublattice structure of the materials we studied created undetectable electrons,” said Keun Su Kim, the principal investigator of this work and professor of physics at Yonsei University. The three materials measured in this study (palladium diselenides, cuprate superconductors, and lead halide perovskites) share the common property of certain crystal symmetries that result in one type of detectable electron and three types of undetectable, dark state electrons. These “invisible” states occur because of the destructive interference – the cancellation of opposite, overlapping waves – from the sublattice structure.
The existence of the proposed dark states was identified by angle-resolved photoemission spectroscopy (ARPES) at Advanced Light Source (ALS) Beamline 7.0.2 (MAESTRO). While clear signals could be observed for the states expected to be detectable, little signal could be observed for the states expected to be undetectable. The materials were probed using different energies, polarizations, and geometries to confirm the dark states were the result of the electrons themselves and not the measurement technique.
The discovery of dark states in solids that are generated from sublattice interference may resolve previously unexplained quantum phenomena, opening up new approaches for using dark states in advanced applications.
“These results highlight that the sublattice degree of freedom, which has been overlooked so far, should be carefully taken into consideration when understanding the phenomena of complex materials,” said Kim. Looking forward, researchers can use this new understanding of dark electrons to solve problems related to high-temperature superconductivity, a long-standing mystery in physics.
Y. Chung, M. Kim, Y. Kim, S. Cha, J. W. Park, J. Park, Y. Yi, D. Song, J. H. Ryu, K. Lee, T. K. Kim, C. Cacho, J. Denlinger, C. Jozwiak, E. Rotenberg, A. Bostwick, and K. S. Kim, “Dark state of electrons in a quantum system with two pairs of sublattices,” Nature Physics 20, 1582-1588 (2024), doi: 10.1038/s41567-024-02586-x.