Snapshots of the electronic structure of antimony acquired with femtosecond time-resolution. Note the changing spectral weight above the Fermi energy (EF). Image: HZB/Nature Communication Physics (2021).
Snapshots of the electronic structure of antimony acquired with femtosecond time-resolution. Note the changing spectral weight above the Fermi energy (EF). Image: HZB/Nature Communication Physics (2021).

The laws of quantum physics rule the microcosm. They determine, for example, how easily electrons move through a crystal and thus whether the material is a metal, a semiconductor or an insulator.

Quantum physics can also lead to exotic properties in certain materials: in so-called topological insulators, only the electrons that can occupy certain specific quantum states are free to move like massless particles on the surface, while this mobility is completely off-limits for electrons in the bulk. What's more, the conduction electrons in the 'skin' of the material are necessarily spin polarized, and form robust, metallic surface states that could be utilized as channels in which to drive pure spin currents on femtosecond time scales.

These properties open up exciting opportunities to develop new information technologies based on topological materials, such as ultrafast spintronics, by exploiting the spin of the electrons on the surface rather than the charge. In particular, optical excitation by femtosecond laser pulses in these materials represents a promising alternative for realizing the highly efficient, lossless transfer of spin information. Spintronic devices utilizing these properties have the potential for superior performance, able to transport information up to 1000 times faster than in modern electronics.

However, many questions still need to be answered before spintronic devices can be developed. For example, the details of exactly how the bulk and surface electrons in a topological material respond to an external stimulus, such as the laser pulse, and the degree of overlap in their collective behaviors on ultrashort time scales.

A team led by physicist Jaime Sánchez-Barriga at Helmholtz-Zentrum Berlin (HZB) in Germany has now brought new insights into these mechanisms. The team, which also established a Helmholtz-RSF Joint Research Group in collaboration with colleagues from Lomonosov State University in Moscow, Russia, examined single crystals of elemental antimony (Sb), previously suggested to be a topological material. They report their findings in a paper in Communications Physics.

"It is a good strategy to study interesting physics in a simple system, because that's where we can hope to understand the fundamental principles," explains Sánchez-Barriga. "The experimental verification of the topological property of this material required us to directly observe its electronic structure in a highly excited state with time, spin, energy and momentum resolutions, and in this way we accessed an unusual electron dynamics."

The aim was to understand how fast excited electrons in the bulk and on the surface of Sb react to the external energy input, and to explore the mechanisms governing their response. "By controlling the time delay between the initial laser excitation and the second pulse that allows us to probe the electronic structure, we were able to build up a full time-resolved picture of how excited states leave and return to equilibrium on ultrafast time scales," says team member Oliver Clark at HZB. "The unique combination of time- and spin-resolved capabilities also allowed us to directly probe the spin-polarization of excited states far out-of-equilibrium."

The data show a 'kink' structure in the transiently occupied energy-momentum dispersion of surface states, which can be interpreted as an increase in effective electron mass. The team was able to show that this mass enhancement plays a decisive role in determining the complex interplay in the dynamical behaviors of electrons in the bulk and at the surface of Sb, which also depends on their spin, following the ultrafast optical excitation.

"Our research reveals which essential properties of this class of materials are the key to systematically control the relevant time scales in which lossless spin-polarized currents could be generated and manipulated," explains Sánchez-Barriga. These are important steps on the way to developing spintronic devices based on topological materials that possess advanced functionalities for ultrafast information processing.

This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.