Avanzando hacia la electrónica de petahercios basada en materiales cuánticos

Físicos teóricos del Instituto Max Planck para la Estructura y Dinámica de la Materia (MPSD) han demostrado cómo el acoplamiento entre láseres intensos, el movimiento de los electrones y su espín influye en la emisión de luz en la escala de tiempo ultrarrápida.

Los electrones, presentes en todo tipo de materia, son partículas cargadas y por tanto reaccionan ante la aplicación de la luz. Cuando un campo de luz intenso golpea un sólido, los electrones experimentan una fuerza, llamada fuerza de Lorentz, que los anima e induce dinámicas exquisitas que reflejan las propiedades del material. Esto, a su vez, hace que los electrones emitan luz en diferentes longitudes de onda, un fenómeno bien conocido llamado generación de armónicos altos.

Exactamente cómo se mueven los electrones bajo la influencia del campo de luz depende de una combinación compleja de propiedades del sólido, incluidas sus simetrías, topología y estructura de banda, y la naturaleza del pulso de luz. Además, los electrones son como peonzas. Tienden a girar en sentido horario o antihorario, una propiedad llamada «giro» de los electrones en la mecánica cuántica.

En un estudio reciente, un equipo de MPSD asumió la difícil tarea de comprender cómo la luz y el espín de los electrones pueden interactuar en Na₃Bi, un material topológico conocido como semimetal de Dirac (el análogo tridimensional de[{» attribute=»»>graphene), via an effect known as spin-orbit coupling. This relativistic effect couples the particle’s spin to its motion inside a potential, a potential that intense light can modify on the ultrafast timescale.

A better understanding of how spin-orbit coupling influences the electron dynamics on these timescales is an important step towards understanding the electron dynamics in complex quantum materials, where this effect is often present. Indeed, it is the spin-orbit coupling that often makes quantum materials interesting for future technological applications. It is expected to lead to the next generation of electronic devices, namely topological electronic systems.

The authors show how spin-orbit coupling affects the velocity of the electrons within the electron bands of solids, effectively acting like a magnetic field that depends on the electrons’ spin.

They demonstrate how changes in the electron velocity can affect the electron dynamics in Na3Bi and that this effect can sometimes be detrimental to the generation of high-order harmonics. While this material is non-magnetic, the team has shown that the spin of the electrons is important for the dynamics, as it couples to the potential felt by the electrons, which is modified by the intense applied light-field.

A further important finding is that the spin-orbit coupling can modify the properties of the emitted high harmonics, for example, their timing. These changes contain crucial information of the internal electron dynamics. In particular, the authors show that the ultrafast spin dynamics, given by the spin current, get encoded in the property emitted light. Given that it is presently challenging to measure spin currents, the present work opens up interesting perspectives towards using intense light to perform high-harmonic spectroscopy of spin currents, as well as magnetization dynamics, or unusual spin textures that can be present in quantum materials.

This work serves as a platform for a better understanding of the link between spin-orbit coupling, spin current, topology, and electron dynamics in solids driven by strong fields – a crucial step towards the development of petahertz electronics based on quantum materials.

Reference: “Effect of spin-orbit coupling on the high harmonics from the topological Dirac semimetal Na₃Bi” by Nicolas Tancogne-Dejean, Florian G. Eich and Angel Rubio, 6 July 2022, npj Computational Materials.
DOI: 10.1038/s41524-022-00831-6