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Angular momentum is a fundamental quantity in physics that describes the rotational motion of objects. In quantum physics, it encompasses both the intrinsic spin of particles and their orbital motion around a point. These properties are essential for understanding a wide range of systems, from atoms and molecules to complex materials and high-energy particle interactions.

When a magnetic field is applied to a quantum system, particle spins typically align with or against the field. This well-known effect, known as spin polarization, leads to observable phenomena such as magnetization. Until now, it was widely believed that spin played the dominant role in how particles respond to magnetic fields. However, new research challenges this long-held view.

In this vein, Assistant Professor Kazuya Mameda of Tokyo University of Science, Japan, in collaboration with Professor Kenji Fukushima of School of Science, The University of Tokyo and Dr. Koichi Hattori of Zhejiang University, found that under strong magnetic fields, the orbital motion of magnetovortical matter becomes more significant than spin effects, leading to reversing the overall direction of angular momentum. The study will be in ÌÇÐÄÊÓƵical Review Letters on July 1, 2025.

"It was previously believed that most microscopic phenomena in a magnetic field were governed by spin angular momentum—a physical quantity characterizing the intrinsic rotational motion of microscopic particles," explains Dr. Mameda. "However, this study found that in a strong magnetic field, orbital motion can overwhelm spin effects, reversing the direction of rotational motion from what was previously believed."

The researchers studied fermionic systems—specifically Dirac fermions— subjected to both strong magnetic fields and rotation. By ensuring gauge invariance and thermodynamic stability in their theoretical framework, they demonstrated that orbital contributions to bulk properties can exceed spin contributions.

Unlike spin, which aligns with the magnetic field, the orbital angular momentum aligns according to Lenz's law—opposite to the direction of the magnetic field. As the magnetic field intensifies, the charge density from the orbital-rotation coupling and orbital angular momentum grow twice the magnitude of their spin counterparts, but with opposite sign.

This reversal in total angular momentum reshapes our understanding of magnetovortical matter and links its behavior to a broader class of quantum effects known as anomaly-induced transports. The findings also pave the way for simulations using lattice QCD—a powerful computational tool for studying strongly interacting particles such as quarks and gluons under extreme conditions.

The discovery that a strong magnetic field can reverse angular momentum in quantum systems challenges established theories. It highlights the previously underestimated role of orbital motion, showing it to be more influential than spin in certain regimes. This insight could spark advances in groundbreaking technologies, particularly in orbitronics, a field dedicated to manipulating the orbital motion of electrons.

"Total angular momentum reversal under strong magnetic fields has been overlooked across fields from materials science to astrophysics. Our findings redefine the foundational physics of modern physics and point to new frontiers in orbitronics—where controlling electron orbital motion could lead to innovative device applications," concludes Dr. Mameda.