Researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Fritz Haber Institute of the Max Planck Society, and their collaborators in Berlin, Dresden, Jülich, and Eindhoven have made a significant discovery in quantum physics. Their research, published in Nature Physics, reveals how angular momentum behaves at the atomic level, particularly in relation to magnetism.
Unraveling Magnetism's Mysteries
In the realm of physics, fundamental quantities like energy and angular momentum are conserved, meaning they cannot be created or destroyed but can shift within a system. Angular momentum, which we often encounter through everyday spinning objects, is intrinsically linked to magnetism at the atomic scale.
Over a century ago, pioneers Albert Einstein and Wander Johannes de Haas illustrated that altering a material's magnetization could induce physical rotation. This foundational experiment established a connection between magnetic and mechanical angular momentum, prompting ongoing investigations into how angular momentum propagates through solid materials.
In a recent breakthrough, scientists have directly observed this phenomenon within a crystal structure.
Laser Technology Unveils Atomic Dynamics
The research team focused on how angular momentum transfers between the coordinated movements of atoms, known as lattice vibrations, within a crystal. Utilizing powerful terahertz laser pulses, they initiated one vibration into a circular motion, while a subsequent ultrafast laser pulse monitored the interaction with another coupled vibration.
Surprisingly, during the experiment, the researchers noted that as angular momentum transitioned from one vibration to another, the rotation direction reversed. This unexpected outcome is attributed to the rotational symmetry inherent in the crystal lattice, where certain rotational states are equivalent despite spinning in opposite directions. The findings provide a clear quantum mechanical signature of angular momentum conservation in solids.
A Fascinating Quantum Phenomenon
Using bismuth selenide as the experimental material, the researchers observed an extraordinary behavior where the angular momenta associated with its lattice vibrations merged, resulting in a new rotation at double the frequency but in the opposite direction. This phenomenon, likened to a "1 + 1 = −1" effect, resembles an Umklapp process, where motion is effectively reversed due to the crystal's symmetry. Although known in other areas of condensed matter physics, this is the first time such a process has been observed in the context of lattice angular momentum.
Olga Minakova, a doctoral researcher at the Fritz Haber Institute, remarked on the elegance of how natural symmetries dictate physical laws. Sebastian Maehrlein, head of the Institute of Radiation Physics at HZDR and study leader, expressed excitement over these groundbreaking results, anticipating their inclusion in future physics textbooks.
Implications for Quantum Technologies
This discovery not only addresses a long-standing question in physics but also holds potential for practical applications. The findings may enhance scientists' control over ultrafast processes in quantum materials, paving the way for advancements in information technology and next-generation memory devices.