Study reveals dark states of condensed matter in quantum system with two pairs of sublattices

Dark states are quantum states in which a system does not interact with external fields, such as light (i.e., photons) or electromagnetic fields. These states, which typically arise due to interference between the pathways by which a system interacts with an external field, are undetectable using spectroscopic techniques.

Researchers at Yonsei University in South Korea and other institutes have recently discovered undetectable dark states of condensed matter in palladium diselenide, a quantum system with two pairs of sublattices in its primitive cell.

Their observations, presented in an article published in Physics of naturecould have interesting implications for the study of materials, quantum states and correlated phenomena.

“Angle-resolved photoemission spectroscopy is a powerful experimental technique that allows physicists to understand how electrons behave in solids,” Keun Su Kim, a professor of physics at Yonsei University and co-author of the paper, told Phys.org.

“Experience has taught us that not all electrons are detected by angle-resolved photoemission spectroscopy. In other words, some electrons are detectable, but others are not.”

For a long time, physicists assumed that the inability to detect some electrons using spectroscopic techniques was related to the methods used to conduct the experiments, rather than to the intrinsic properties of the materials.

However, in previous studies examining simple elementary materials with a pair of sublattices, such as graphene and black phosphorus, Kim and his colleagues have shown that this elusiveness is actually closely related to the intrinsic properties of the materials.

“We investigated this problem to extend it to two-pair sublattice materials and found that some electrons cannot be detected under any experimental conditions,” Kim said. “Simply put, we could only see experimental signals for electrons that were supposed to be detectable (bright states), and we could not see any experimental signals for electrons that were supposed to be undetectable (dark states).”

To carry out their experiments, the researchers used a technique called angle-resolved photoemission spectroscopy. This widely used experimental technique relies on the photoelectric effect discovered by Albert Einstein to gather information about the electronic structure of materials.

Basically, Kim and his colleagues irradiated their samples with a beam of high-energy photons. This beam of energy ejected some of the electrons from the sample, allowing them to gather information about the energy and momentum they exhibited while they were still in the sample.

“In this work, we studied three materials, palladium diselenides (PdSe₂), cuprate superconductors (Bi₂Mr.₂CaCu₂O₈+δ or Bi-2212) and lead halide perovskites (CsPbBr₃),” Kim explains. “An important common property of these three materials is that they exhibit certain crystal symmetries (multiple sliding mirror symmetries) that make all electrons in solid samples characterizable as one of the four types.”

The researchers essentially found that electrons in two-pair sublattice quantum systems can be classified into four different categories. One of these types of electrons could be detected using angle-resolved photoemission spectroscopy, while the other three types were undetectable because they were in dark states.

“For now, this is just a possibility, but our result offers a new way to explain one of the long-standing problems in the study of high-temperature superconductivity, called the “Fermi arc,” Kim said. “Our nature is too complex to include everything in the theoretical model, and one often has to make a choice about what to include and what to exclude for the approximation. Strictly speaking, there are sublattices in the unit structure of cuprate superconductors, but these sublattices have been neglected so far.”

Recent work by this team demonstrates the existence of dark states in various two-pair sublattice quantum systems, including palladium diselenides, cuprate superconductors, and lead halide perovskites. In the future, this work could have important implications for the study of these materials, potentially expanding the understanding of their underlying physics.

“Our results raise the question of whether it is really acceptable to leave out the sublattices in the unit structure of cuprate superconductors when interpreting angle-resolved photoemission spectroscopy data collected from these materials,” Kim added. “Our future research plan is to further investigate the Fermi arc problem of cuprate superconductors in the same context. We already have promising results and are working on the next paper.”

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