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How Does Superconductivity Begin? Unveiling the Hidden Flow of Electrons Superconductivity, a phenomenon where electricity flows without resistance, is considered the core of quantum computers and next-generation power technologies. However, the exact states electrons undergo before superconductivity emerges have not yet been fully elucidated. KAIST researchers have provided experimental clues revealing the hidden order electrons form prior to superconductivity in a kagome metal, a material closely related to superconducting phenomena. The team confirmed that a loop-like circulating order of electrons (loop-current order) emerges earlier than the periodic clustering of electrons (charge density wave). KAIST (President Kwang Hyung Lee) announced on the 30th that a joint research team led by Professors Yeongkwan Kim, Myung Joon Han, and SungBin Lee from the Department of Physics discovered through circular dichroism angle-resolved photoemission spectroscopy (CD-ARPES) experiments and theoretical calculations that time-reversal symmetry breaking occurs at a higher temperature than the charge density wave formation in the kagome metal CsV3Sb5. Time-reversal symmetry is a property where physical phenomena appear identical even when time is reversed. The breaking of this symmetry implies that electrons within the material may have created a hidden flow with a specific directionality. A kagome metal is a material with a repeating triangular atomic arrangement, resembling the traditional Japanese basket weaving pattern 'kagome'. In this structure, electrons interact strongly with each other, giving rise to various quantum phenomena rarely seen in normal metals, such as charge density waves, superconductivity, and topological electronic states. In particular, CsV3Sb5 exhibits both charge density waves and superconductivity at low temperatures, drawing attention as a crucial platform for next-generation quantum materials research. However, there has been an ongoing debate over whether another hidden electronic order exists between the charge density wave and superconductivity in this material. Although several experiments have reported signals suggesting broken time-reversal symmetry, it was unclear whether this phenomenon was a consequence of the charge density wave formation or an independent electronic order that emerges prior to it. To resolve this debate, the research team alternately irradiated high-quality CsV3Sb5 single crystals with left- and right-circularly polarized light and precisely measured the difference in the intensity of the emitted electrons. They then eliminated spurious signals potentially caused by the experimental setup's geometry, isolating only the intrinsic signals originating from the symmetry breaking of the material itself. As a result, they confirmed that the signal of time-reversal symmetry breaking already appears around 140~145 K, which is significantly higher than the charge density wave formation temperature of about 94 K. This supports the interpretation that electrons form a loop-current order—a microscopic loop-like circulation—before creating the charge density wave pattern. The loop-current order is an electronic order where electrons behave as if flowing along small loops within the atomic lattice; it was theoretically proposed long ago but has been difficult to verify experimentally. The team also tracked how the electronic structure changed as the temperature was lowered. At high temperatures, a normal metallic state appeared; at lower intermediate temperatures, the loop-current order formed first. As the temperature decreased further, a complex state evolved where the charge density wave intertwined with the loop-current order, eventually leading to the superconducting state. This research proposes a hierarchical structure of phase transitions in CsV3Sb5, progressing from 'loop-current order → charge density wave → superconductivity'. This achievement provides a crucial clue for understanding the fundamental principles of superconductivity. It is not yet fully understood what kind of order electrons form before superconductivity occurs, or which electronic orders compete or cooperate with superconductivity. By demonstrating the existence of an electronic state with broken time-reversal symmetry prior to the superconducting state, this study offers an important lead in understanding unconventional superconductivity, which operates differently from standard mechanisms. Furthermore, this research is expected to help understand hidden electronic orders in other superconducting materials beyond kagome metals. In particular, it could serve as a reference for explaining the peculiar electronic state (pseudogap) prior to superconductivity, which has long been discussed in cuprate high-temperature superconductors. Professor Yeongkwan Kim stated, "This research is the result of directly tracking the time-reversal symmetry breaking of a kagome metal within its electronic structure, which had previously only been discussed through indirect signals. By showing the sequence in which electrons form order before reaching superconductivity, we have presented a new reference point for research on unconventional superconductivity and strongly correlated quantum materials.“ Professor Myung Joon Han added, "The key point is that the circular dichroism signal observed in the experiment aligns perfectly with the electrons' orbital motion pattern (orbital angular momentum pattern) expected from the loop-current order. This is a case where we uncovered the microscopic origin of the hidden electronic order by combining experiment and theory.“ KAIST Department of Physics researchers Jaehun Cha, Hyunggeun Lee, and Sangjun Sim participated as co-first authors in this study. The research findings were published online in the international physics journal Nature Physics on June 15, 2026. Paper Title: Evidence of time-reversal symmetry breaking above the charge density wave order in a kagome metal DOI: https://doi.org/10.1038/s41567-026-03331-2 This research was supported by the Mid-Career Researcher Program and the Accelerator Manpower Training Program (Ministry of Science and ICT, National Research Foundation of Korea), the Korea Research Institute of Standards and Science (KRISS), the Air Force Office of Scientific Research (AFOSR), and the US Department of Energy's Basic Energy Sciences (DOE BES).
2026.07.01 View 2385