Magnetism meets topology on a superconductor ‘

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image: an illustration depicting a topological surface condition with a forbidden energy band (a range of energy where electrons are forbidden) between the vertices of the corresponding upper and lower cones (allowed energy bands, or the range of energies that electrons are allowed to have). A topological surface state is a unique electronic state, existing only on the surface of a material, which reflects strong interactions between the spin of an electron (red arrow) and its orbital motion around the nucleus of a atom. When the spins of the electrons line up parallel to each other, as is the case here, the material has a type of magnetism called ferromagnetism.
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Credit: Dan Nevola, Brookhaven National Laboratory

UPTON, NY – Electrons in a solid occupy distinct bands of energy separated by vacancies. The energetic forbidden bands are an electronic “no man’s land”, a range of energy where no electron is allowed. Now, scientists studying a compound containing iron, tellurium, and selenium have found that an energetic band gap opens at a point where two allowed energy bands intersect on the material’s surface. They observed this unexpected electronic behavior when they cooled the material and probed its electronic structure with laser light. Their findings, reported in the Proceedings of the National Academy of Sciences, could have implications for the future of quantum information science and electronics.

The particular compound belongs to the family of high-temperature iron-based superconductors, which were first discovered in 2008. These materials not only conduct electricity without resistance at relatively higher (but still very cold) temperatures than others. classes of superconductors, but also show magnetic properties.

“For a while, people thought that superconductivity and magnetism would work against each other,” said first author Nader Zaki, scientific associate in the Electron Spectroscopy group of the Condensed Matter Physics and Human Sciences division. Materials (CMPMS) from the US Department of Energy. (DOE) Brookhaven National Laboratory. “We explored a material where the two develop at the same time.”

In addition to superconductivity and magnetism, some iron-based superconductors have the right conditions to host “topological” surface states. The existence of these unique electronic states, located on the surface (they do not exist in the mass of the material), reflects strong interactions between the spin of an electron and its orbital motion around the nucleus of an atom.

“When you have a superconductor with topological surface properties, you are excited about the possibility of topological superconductivity,” said corresponding author Peter Johnson, head of the electron spectroscopy group. “Topological superconductivity has the potential to support Majorana fermions, which could serve as qubits, the information storage building blocks of quantum computers.”

Quantum computers promise tremendous acceleration for calculations that would take inconvenient time or be impossible on traditional computers. One of the challenges in achieving practical quantum computing is that qubits are very sensitive to their environment. Small interactions cause them to lose their quantum state and thus the stored information is lost. The theory predicts that Majorana fermions (sought-after quasiparticles) existing in superconducting topological surface states are immune to environmental disturbances, making it an ideal platform for robust qubits.

Viewing iron-based superconductors as a platform for a range of exotic and potentially important phenomena, Zaki, Johnson, and their colleagues set out to understand the roles of topology, superconductivity, and magnetism.

The senior physicist of the CMPMS division, Genda Gu, first cultivated high-quality single crystals of the iron-based compound. Next, Zaki mapped the electronic band structure of the material using laser photoemission spectroscopy. When light from a laser is focused on a small point on the material, electrons from the surface are “ejected” (ie photoemitted). The energy and momentum of these electrons can then be measured.

When they lowered the temperature, something surprising happened.

“The material became superconducting, as we expected, and we saw a superconducting gap associated with it,” Zaki said. “But what we weren’t expecting was that the topological surface state opened up a second space at the Dirac point. You can imagine the structure of the energy bands of this surface state in the form of d ‘an hourglass or two cones attached to their top. The intersection of these cones is called the point of Dirac. “

As Johnson and Zaki explained, when a gap opens at Dirac’s point, it is evidence that the time reversal symmetry has been broken. Time reversal symmetry means that the laws of physics are the same whether you are watching a system move forward or backward in time, which is like rewinding a video and seeing the same sequence of events unfold in it. ‘towards. But during a time inversion, the spins of the electrons change direction and break this symmetry. So, one of the ways to break the time inversion symmetry is to develop magnetism – specifically ferromagnetism, a type of magnetism where all the spins of electrons line up in parallel.

“The system goes superconducting and apparently magnetism is developing,” Johnson said. “We have to assume that magnetism is in the region of the surface because in this form it cannot coexist in mass. This discovery is exciting because the material contains many different physics: superconductivity, topology and now the magnetism. I like to say it’s a one-stop-shop. Understanding how these phenomena arise in the material could provide a basis for many new and exciting technological directions. “

As previously indicated, the material’s superconductivity and its powerful spin-orbit effects could be exploited for quantum information technologies. Alternatively, the magnetism of the material and the strong spin-orbit interactions could allow transport without dissipation (no loss of energy) of the electric current in the electronics. This ability could be harnessed to develop electronic devices that consume little energy.

Co-authors Alexei Tsvelik, Principal Scientist and Group Leader of the CMPMS Division’s Condensed Matter Theory Group, and Congjun Wu, Professor of Physics at the University of California, San Diego, provided theoretical information on how the time reversal symmetry is broken and the magnetism originates in the superficial region.

“This discovery not only reveals deep links between topological superconducting states and spontaneous magnetization, but also provides important information about the nature of superconducting space functions in iron-based superconductors – an exceptional problem in the study of Strongly correlated unconventional superconductors, ”Wu said.

In a separate study with other collaborators from the CMPMS division, the experimental team examines how different concentrations of the three elements in the sample contribute to the observed phenomena. Apparently tellurium is needed for topological effects, too much iron kills superconductivity, and selenium improves superconductivity.

In follow-up experiments, the team hopes to verify the breaking of time inversion symmetry with other methods and explore how substitution of elements in the compound alters its electronic behavior.

“As materials scientists, we like to tweak the ingredients in the mix to see what happens,” Johnson said. “The goal is to understand how superconductivity, topology and magnetism interact in these complex materials.”

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This research was supported by the DOE Office of Science and the Air Force Office of Scientific Research.

The Brookhaven National Laboratory is supported by the Office of Science, US Department of Energy. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information visit https://energy.gov/science.

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