Professor Choi Seok-ho's Research Team at Kyung Hee University Achieves 'World's First' Experimental Realization of Two-Dimensional Weyl Semimetal

First Discovery of 3D Dirac Semimetal to 2D Weyl Semimetal Phase Transition Phenomenon
International Joint Research Achievement by Kyung Hee University and 8 Other Institutions Published in International Journal

Professor Choi Seok-ho's research team from the Department of Applied Physics at Kyung Hee University (President Han Gyuntae) discovered a phenomenon where a three-dimensional Dirac semimetal, a topological semimetal, undergoes a phase transition to a two-dimensional Weyl semimetal through thickness variation, experimentally realizing the world's first two-dimensional Weyl semimetal. This research achievement was published on the 31st of last month in the international journal Advanced Functional Materials (IF=19.00), and domestic and international patents have been filed.

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Three-dimensional topological semimetals are well known both theoretically and experimentally. However, two-dimensional topological semimetals (both Dirac and Weyl semimetals) have only been theoretically predicted, and even these predictions are subject to debate within the academic community. In fact, they have not yet been theoretically established. Professor Choi's research team experimentally realized a two-dimensional Weyl semimetal for the first time through this study. This simple realization of a two-dimensional Weyl semimetal, which is of global interest due to its novel physical properties and potential for new device applications, is evaluated to have further enhanced the possibility of device applications of this material.

Typically, metals and semiconductors are distinguished based on their energy band structure. The energy band consists of a conduction band and a valence band, separated by an energy gap. In insulators, the valence band is fully occupied by electrons, preventing electron movement. Conversely, the conduction band is completely empty. In conductors, the valence band is fully occupied, and some electrons also occupy the conduction band, which are free electrons. Metals always have free electrons that move as charge carriers, enabling electrical conduction.

Semimetals have the conduction and valence bands touching at a single point. If the energy gap is completely eliminated, the material becomes a metal; if the gap closes only at a few discrete points, it becomes a Dirac semimetal. Semiconductors have properties intermediate between metals and insulators. External energy (heat, light, electron beams, etc.) must be supplied to overcome the energy gap to generate free electrons and holes, allowing current to flow. Two-dimensional materials like graphene, similar to metals, have no energy gap and thus always have free electrons, allowing current to flow well. However, unlike metals, free holes also exist, so graphene is generally called a semimetal.

Topology is a powerful property that describes the characteristics of an object. An egg and a soccer ball belong to the same topological class of "three-dimensional objects without holes." Rings or donuts have a topology with "one hole." Scientists have revealed that traditional material classifications are incomplete and that there exist "topological states" with singular values of topological invariants. This led to the discovery of new topological materials such as topological semimetals. Topological semimetals can be seen as three-dimensional semimetals corresponding to two-dimensional graphene semimetals. However, two-dimensional topological semimetals are not well known yet.

Various topological semimetals can exist depending on crystal symmetries, with Dirac and Weyl semimetals being representative examples. Crystal structure symmetries include inversion symmetry and time-reversal symmetry. If both symmetries are preserved, the material is a Dirac semimetal; if one symmetry is broken, it becomes a Weyl semimetal. Weyl semimetals are a new type of metal. While electrons in ordinary materials have mass, electrons in Weyl semimetals behave as if massless and are quantum materials extremely sensitive to the strength and direction of magnetic fields.

Utilizing Weyl semimetals is expected to enable the precise fabrication of magnetic sensors used in various fields such as smartphones and magnetic resonance imaging (MRI) devices. Additionally, topological materials can be applied in thermoelectrics, fluid dynamics, catalysis, solar power generation, circular photogalvanic effects, data storage, and quantum computing.

In single-crystal solids, atoms are arranged periodically. Electrons orbit the nucleus in orbitals within atoms. The wave function in quantum mechanics describes which orbital an electron occupies. In periodic lattice structures, the wave number or momentum of the wave function also exhibits periodicity. How the wave function twists as momentum changes determines the topological properties of the material.

In ordinary materials, the wave function has no special twisting structure. However, in topological materials, the wave function is twisted in momentum space like a M?bius strip. Simply put, if the wave function twists when it completes one loop in momentum space, the material is topological; otherwise, it is ordinary. A M?bius strip cannot be converted into a normal strip without cutting it. The electronic structure of topological materials is preserved unless the chemical structure of the material changes, making topological materials very stable.

The properties of two-dimensional topological semimetals are not yet firmly established. Experimentally, only nodal-line semimetals, a type of topological semimetal, have been reported in a few papers. From a device application perspective, three-dimensional materials naturally have limitations compared to two-dimensional materials. Also, in the case of nodal-line semimetals, semimetals are formed only on the surface layer by depositing atomic layers on single-crystal metal substrates, making it impossible to isolate the semimetal alone, limiting practical device development.

This research signifies that genuine two-dimensional topological semimetals can now be fabricated. Therefore, topological materials can be used more flexibly for next-generation device development based on two-dimensional topological semimetal nanostructures. The research team varied the thin film thickness of the Dirac semimetal Bi0.96Sb0.04 from 2 to 300 nm using molecular beam epitaxy (MBE). To investigate changes in topological properties with thickness, various techniques were employed, including magnetoresistance, second harmonic scattering, Raman spectroscopy, synchrotron X-ray diffraction, terahertz emission, Hall effect, and thermoelectric effect.

As a result, they discovered for the first time a phase transition from a three-dimensional Dirac semimetal to a two-dimensional Weyl semimetal as the thickness decreased below 10 nm. They also elucidated the mechanism of this phase transition: as the thickness thinned into the two-dimensional regime, strain strongly formed due to lattice mismatch between the substrate and the thin film broke inversion symmetry, resulting in the formation of a two-dimensional Weyl semimetal state.

Professor Choi stated, "The two-dimensional state of the Weyl semimetal has been identified for the first time, and its fabrication is expected to become active. Various nanoscale new devices utilizing two-dimensional topological semimetals, which are more advantageous for device fabrication than three-dimensional materials, are expected to be developed." He added, "This lays the foundation to further activate academic research on topological materials such as topological semimetals, which are actively studied as new materials following graphene, into the stage of device applications. In particular, this achievement is expected to contribute to deepening academic research on topological materials whose properties are not yet fully understood, while promoting research on device applications."

This research was conducted as an international collaborative project supported by the Ministry of Science and ICT and the National Research Foundation of Korea's mid-career research program. Participants included Professor Choi, Research Professor Jung Chan-wook, Researcher Kim Jin-hee, Master's student Jung Tae-jin, Professor Kim Sung, and Professor Lee Jong-soo from Kyung Hee University, as well as faculty, students, and researchers from Daegu University, Ulsan University, Gwangju Institute of Science and Technology, Pohang University of Science and Technology, Sogang University, Korea Atomic Energy Research Institute, Australian National University (ANU), and University of Wollongong, totaling nine research institutions.

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