Optically- and thermally-driven huge lattice orbital and spin angular momenta from spinning fullerenes

TL;DR

This paper proposes a new method to generate and control large lattice and spin angular momenta in fullerenes using light and heat, enabling potential applications in phononic control of electronic spins and energy harvesting.

Contribution

It introduces a theoretical approach to induce significant phonon and spin angular momentum in C60 fullerenes via optical and thermal means, which was not previously demonstrated.

Findings

Laser pulses can inject helicity-dependent angular momentum into C60.

Switching light polarization reverses the direction of phonon angular momentum.

Thermal excitation at room temperature produces angular momentum of several hundred ħ.

Abstract

Lattice vibration in solids may carry angular momentum. But unlike the intrinsic spin of electrons, the lattice vibration is rarely rotational. To induce angular momentum, one needs to find a material that can accommodate a twisted normal mode, two orthogonal modes or excitation of magnons. If excitation is too strong, one may exceed the Lindemann limit, so the material melts. Therefore these methods are not ideal. Here, we theoretically propose a new route to phonon angular momentum in a molecular crystal $\rm C_{60}$. We find that a single laser pulse is able to inject a significant amount of angular momentum to $\rm C_{60}$, and the momentum transfer is helicity-dependent. Changing from right-circularly polarized light to left-circularly polarized light switches the direction of phonon angular momentum. On the ultrafast time scale, the orbital angular momentum change closely resembles the displacive excitation of coherent phonons, with a cosine-function dependence on time, different from the spin counterpart. Atomic displacements, even under strong laser excitation, remain far below the Lindemann criterion. Under thermal excitation, spinning $\rm C_{60}$ even at room temperature generates a huge angular momentum close to several hundred $\hbar$. Our finding opens the door to a large group of fullerenes, from $\rm C_{60}$, C$_{70}$ to their endohedral derivatives, where angular momentum can be generated through light or temperature. This paves the way to the phononic control electronic spin and harvesting thermal energy through phonon angular momentum.