Physicists Observe Strange Quantum Rotation Effect That Defies Intuition
Like on a Ferris wheel, a powerful terahertz laser drives the atoms of a crystal along precise circular paths. The resulting collective oscillation of the crystal lattice was traced stroboscopically using ultrashort laser pulses; the blue lines show the measured data. Surprisingly, the oscillation rotates in the opposite direction. Credit: O. Minakova/ S.F. Maehrlein/ B. Schröder/ HZDRResearchers discovered that atomic rotations inside a crystal can unexpectedly flip direction while still obeying the laws of angular momentum conservation.An international team of researchers, including scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Fritz Haber Institute of the Max Planck Society, has directly observed for the first time how angular momentum moves and remains conserved inside a crystal lattice. By using powerful terahertz laser pulses, the team was able to precisely manipulate these motions and discovered an unexpected effect: during the transfer process, the direction of rotation flips because of the material’s rotational symmetry.The study, published in Nature Physics, offers new insight into the origins of magnetism and could help researchers develop more precise ways to control quantum materials.Understanding Angular Momentum in SolidsQuantities such as energy, momentum, and angular momentum are governed by conservation laws, meaning they cannot be created or destroyed in a closed system. Instead, they can only be transferred or converted into other forms. Although angular momentum is commonly associated with spinning objects like bicycles or carousels, it is also fundamental to quantum physics and plays a key role in magnetism.More than a century ago, Albert Einstein and Wander Johannes de Haas showed that altering a material’s magnetization could produce measurable mechanical rotation. Their experiment demonstrated that magnetic and mechanical angular momentum are closely connected. Since then, scientists have tried to determine exactly
