Abstract
"God does not play dice."-This famous remark by Albert Einstein critiqued the probabilistic nature of quantum mechanics. Paradoxically, his theory of relativity has become an essential tool for understanding the behavior of electrons, the primary subjects of quantum mechanics. Electrons are so minuscule that their behavior must be analyzed through quantum mechanics, yet they also move at speeds that require relativistic considerations. Due to the fundamentally different starting points of these two theories, achieving a unified, consistent description has posed significant challenges. Now, a groundbreaking study published in Physical Review Letters offers a novel approach that bridges this divide, potentially reshaping the way we understand electron dynamics in solids.
A team of researchers led by Professor Noejung Park in the Department of Physics at UNIST and Professor Kyoung-Whan Kim of Yonsei University has introduced a new theoretical framework that enables more accurate descriptions of electron spin within solid materials.
Electrons possess two types of angular momentum: spin and orbital angular momentum. To draw an analogy, spin can be likened to Earth's rotation, while orbital angular momentum resembles the Earth's revolution around the Sun. These two forms of angular momentum influence each other through 'spin-orbit coupling,' which plays a vital role in determining a material's magnetic and conductive properties.
However, the spin-orbit interaction arises predominantly from relativistic effects at high energies, whereas in solid-state systems such as semiconductors, quantum mechanical phenomena at low energies dominate. Traditionally, this disparity has limited the ability to comprehensively model spin-orbit effects within a unified framework. For instance, precisely defining orbital angular momentum within a crystalline lattice has been notably challenging.
In response, the research team proposed an innovative theoretical approach that describes spin-orbit coupling without relying on the orbital angular momentum operator. Instead, they introduced the concept of *spin-lattice interaction*, a relativistic effect that can be directly incorporated into the quantum mechanical description of electrons in solids.
The team validated their new method by applying it to a variety of physical systems, including one-dimensional conductors (such as platinum chains), two-dimensional insulators (like hexagonal boron nitride), and three-dimensional semiconductors (such as gallium arsenide). Their results demonstrated improved accuracy and efficiency in predicting spin distributions, spin currents, and magnetic responses compared to conventional models.
The joint research team commented, "Our approach resolves the longstanding computational inconsistencies stemming from the gap between quantum mechanics and relativity." They further noted, "It provides a robust foundation for future research in spintronics and next-generation memory devices."
This advancement, which was spearheaded by Dr. Bumseop Kim-currently a postdoctoral researcher at the University of Pennsylvania-paves the way for more precise modeling of spin-based phenomena and could serve as a foundational theory for designing advanced spintronic devices and quantum information technologies.
The research was supported by the Ministry of Science and ICT (MSIT), the National Research Foundation of Korea (NRF), UNIST-Samsung Electronics Semiconductor Industry-Academic Collaboration, Yonsei University, and the SRC Center for Quantum Angular Momentum Dynamics.
Journal Reference
Bumseop Kim, Noejung Park, and Kyoung-Whan Kim, "Relativistic Spin-Lattice Interaction Compatible with Discrete Translation Symmetry in Solids," Phys. Rev. Lett., (2025).
