The development of functional materials by thermal management is expected to be a crucial technology for solving modern societal problems, such as the efficient use of energy. Recently, chiral materials, in which atoms are arranged in a helical pattern, have been studied as new functional materials because they exhibit unique lattice vibrations with circular motion of atoms, called “chiral phonons.” When a temperature gradient is applied to chiral materials, the angular momentum caused by the chiral phonons induces macroscopic mechanical rotation by conserving the total angular momentum. This phenomenon, the thermal Einstein–de Haas (EdH) effect, has recently garnered attention as a novel functional property made possible by thermal management.

The conventional EdH effect, which was the subject of the only experiment Einstein performed, occurs when a magnetic field is applied to magnetic materials. This is well known as an important experiment that directly proved the conservation of angular momentum in materials. By contrast, the thermal EdH effect has not yet been clearly observed in experiments. The main reason for this is that no suitable chiral materials have been proposed for its observation.

Carbon nanotubes (CNTs), which are one-dimensional cylindrical structures, are broadly classified into three distinct structural types (zigzag, armchair, and chiral). The phonon properties of achiral CNTs, namely zigzag and armchair CNTs, have been investigated extensively. In this letter, it is demonstrated theoretically that chiral CNTs possess chiral phonons, such as transverse acoustic phonon modes and certain optical phonon modes, which are split into two branches carrying opposite signs of angular momentum. In addition, it is revealed that angular momentum is most efficiently generated in small-diameter nanotubes with intermediate chiral angles.

The crystal structure of chiral materials has a specific handedness based on its helical twist (right and left handedness). Generally, controlling this handedness in standard inorganic crystals is extremely difficult. However, recent research on CNTs has led to major advances in chiral separation technology. Methods for controlling the length and diameter of CNTs are also well established. Owing to such technological advances, experimental observation of the thermal EdH effect using chiral CNTs is expected.

(Written by Raimu Akimoto, Hiroyasu Matsuura, and Takahiro Yamamoto.)

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