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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 -
KAIST Directly Visualizes the Hidden Spatial Order of Electrons in a Quantum Material
<(Back row, from left) Yeongkwan Kim, SungBin Lee, Heejun Yang, Yongsoo Yang_(Front row, from left) Jemin Park, Seokjo Hong, Jaewhan Oh>
· Cryogenic 4D-STEM reveals how charge density waves form, fragment, and persist across a phase transition
· First direct measurement of electronic amplitude correlations uncovers strain-driven inhomogeneity and localized order above the transition temperature
Electronic order in quantum materials often emerges not uniformly, but through subtle and complex patterns that vary from place to place. One prominent example is the charge density wave (CDW), an ordered state in which electrons arrange themselves into periodic patterns at low temperatures. Although CDWs have been studied for decades, how their strength and spatial coherence evolve across a phase transition has remained largely inaccessible experimentally.
Now, a team led by Professor Yongsoo Yang of the Department of Physics at KAIST (Korea Advanced Institute of Science and Technology), together with Professors SungBin Lee, Heejun Yang, and Yeongkwan Kim, and in collaboration with Stanford University, has for the first time directly visualized the spatial evolution of charge density wave amplitude order inside a quantum material.
A New Way to See Electronic Order at the Nanoscale
Using a liquid-helium-cooled electron microscope setup combined with four-dimensional scanning transmission electron microscopy (4D-STEM), the researchers mapped how CDW order develops, weakens, and fragments as temperature changes. This approach allowed them to reconstruct nanoscale maps of the CDW amplitude, revealing not just whether the order exists, but how strong it is and how it is spatially connected.
This study is similar to filming the growth of ice crystals as water freezes using an ultra-high-magnification camera. In this case, however, the researchers observed electrons arranging themselves at cryogenic temperatures of around –253°C, and used an electron microscope capable of resolving features one hundred-thousandth the width of a human hair instead of a conventional camera. The results showed that the electronic patterns do not appear uniformly across the material. In some regions, clear patterns are visible, while in neighboring areas they are entirely absent, much like a lake that does not freeze all at once, with patches of ice interspersed with liquid water.
How Electronic Order Breaks Apart in Real Space
The team further demonstrated that this spatial inhomogeneity is closely linked to local strain inside the crystal. Even extremely small distortions that are far below optical resolution strongly suppress the CDW amplitude. This clear anticorrelation between strain and electronic order provides direct evidence that local lattice distortions play a decisive role in shaping CDW patterns.
Unexpectedly, the researchers also observed that localized regions of CDW order can persist even above the transition temperature, where long-range order is generally thought to disappear. These isolated pockets of electronic order suggest that the CDW transition is not a simple, uniform melting process, but instead involves gradual loss of spatial coherence.
A key advance of this work is the world’s first direct measurement of CDW amplitude correlations. By quantifying how the strength of electronic order at one location is related to that at another, the study reveals how CDW coherence collapses across the transition, while local amplitude remains finite. Such information could not be obtained with conventional diffraction or scanning probe techniques.
Toward a New Framework for Studying Electronic Order
Charge density waves are a central feature of many quantum materials and often coexist or compete with other electronic states. By directly accessing their spatial structure and correlations, this study provides a new experimental framework for understanding how collective electronic order forms and evolves in real materials.
Dr. Yongsoo Yang, who led the research, explained the significance of the results: “Until now, the spatial coherence of charge density waves was largely inferred indirectly. Our approach allows us to directly visualize how electronic order varies across space and temperature, and to identify the factors that locally stabilize or suppress it.”
[Figure 1] Schematic illustration of an experiment employing 4D-STEM to probe the spatial variations of charge density waves in the prototypical quantum material NbSe2 under a liquid-helium cryogenic environment (AI-generated image).
This research, with Seokjo Hong, Jaewhan Oh and Jemin Park of KAIST as co-first authors, was published online in Physical Review Letters on January 6th (Title: Spatial correlations of charge density wave order across the transition in 2H-NbSe2).
The study was mainly supported by the National Research Foundation of Korea (NRF) Grants (Individual Basic Research Program, Basic Research Laboratory Program, Nanomaterial Technology Development Program) funded by the Korean Government (MSIT).
2026.01.20 View 3080 -
Defining the Hund Physics Landscape of Two-Orbital Systems
Researchers identify exotic metals in unexpected quantum systems
Electrons are ubiquitous among atoms, subatomic tokens of energy that can independently change how a system behaves—but they also can change each other. An international research collaboration found that collectively measuring electrons revealed unique and unanticipated findings. The researchers published their results on May 17 in Physical Review Letters.
“It is not feasible to obtain the solution just by tracing the behavior of each individual electron,” said paper author Myung Joon Han, professor of physics at KAIST. “Instead, one should describe or track all the entangled electrons at once. This requires a clever way of treating this entanglement.”
Professor Han and the researchers used a recently developed “many-particle” theory to account for the entangled nature of electrons in solids, which approximates how electrons locally interact with one another to predict their global activity.
Through this approach, the researchers examined systems with two orbitals — the space in which electrons can inhabit. They found that the electrons locked into parallel arrangements within atom sites in solids. This phenomenon, known as Hund’s coupling, results in a Hund’s metal. This metallic phase, which can give rise to such properties as superconductivity, was thought only to exist in three-orbital systems.
“Our finding overturns a conventional viewpoint that at least three orbitals are needed for Hund’s metallicity to emerge,” Professor Han said, noting that two-orbital systems have not been a focus of attention for many physicists. “In addition to this finding of a Hund’s metal, we identified various metallic regimes that can naturally occur in generic, correlated electron materials.”
The researchers found four different correlated metals. One stems from the proximity to a Mott insulator, a state of a solid material that should be conductive but actually prevents conduction due to how the electrons interact. The other three metals form as electrons align their magnetic moments — or phases of producing a magnetic field — at various distances from the Mott insulator. Beyond identifying the metal phases, the researchers also suggested classification criteria to define each metal phase in other systems.
“This research will help scientists better characterize and understand the deeper nature of so-called ‘strongly correlated materials,’ in which the standard theory of solids breaks down due to the presence of strong Coulomb interactions between electrons,” Professor Han said, referring to the force with which the electrons attract or repel each other. These interactions are not typically present in solid materials but appear in materials with metallic phases.
The revelation of metals in two-orbital systems and the ability to determine whole system electron behavior could lead to even more discoveries, according to Professor Han.
“This will ultimately enable us to manipulate and control a variety of electron correlation phenomena,” Professor Han said.
Co-authors include Siheon Ryee from KAIST and Sangkook Choi from the Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory in the United States. Korea’s National Research Foundation and the U.S. Department of Energy’s (DOE) Office of Science, Basic Energy Sciences, supported this work.
-PublicationSiheon Ryee, Myung Joon Han, and SangKook Choi, 2021.Hund Physics Landscape of Two-Orbital Systems, Physical Review Letters, DOI: 10.1103/PhysRevLett.126.206401
-ProfileProfessor Myung Joon HanDepartment of PhysicsCollege of Natural ScienceKAIST
2021.06.17 View 15477