Because of its huge Rashba effect, germanium telluride makes a great candidate for functional spintronic devices. HZB scientists have now discovered an intriguing phenomenon in GeTe. They studied the electronic response of samples to thermal excitation. Surprised, the relaxation that followed was fundamentally different from conventional semimetals. New functionalities could be created by carefully controlling the electronic structure. The results were published in Advanced Materials.

The complexity and functionality of silicon-based technology has increased exponentially over the past decade, in line with growing demand for smaller, better-performing devices. The silicon age is ending. As miniaturization increases, unwanted quantum effects and thermal loss are becoming a greater obstacle. New materials will be required to harness quantum effects and not avoid them. Spintronic devices that use spins of electrons instead of their charge promise to be more efficient and have significantly improved switching times as well as completely new functionalities.

Spintronic devices are on the horizon

Materials that combine the orbital motions of electrons and spins make them suitable candidates for spintronics devices. This is known as the Rashba effect and can be found in many non-magnetic semi-metallic compounds. It allows for, among other things to manipulate the spins of the material using an electric field.

First study in an un-equilibrium state

Germanium telluride has one of the most powerful Rashba effects among all semiconducting system. However, germanium telluride was only studied in thermal equilibrium. Now, for the first time, a team led by HZB physicist Jaime-Sanchez-Barriga has specifically accessed a non-equilibrium state in GeTe samples at BESSY II and investigated in detail how equilibrium is restored in the material on ultrafast (<10-12 seconds) timescales. The physicists discovered a surprising phenomenon during this process.

The sample was first excited with an infrared pulse. Next, it was measured with high resolution using angle-resolved photoscopy (tr ARPES). Sanchez-Barriga says that this is the first time that we have been able to characterize and observe all phases of thermalization, excitation and relaxation on extremely short time scales. Oliver Clark, one of the principal authors of the study, stated that the most important result was “The data demonstrate that the thermal equilibrium between electrons and crystal lattices is restored in an extremely unconventional and counterintuitive manner.”

Equilibrium restored: The faster the cooler

Thermal equilibrium in simple metallic systems is primarily established by the interaction of electrons with one another and between electrons, lattice vibrations, and crystals (phonons). Lower temperatures cause this process to slow down. However, researchers found an opposite result in germanium telluride. The lower the sample’s lattice temperature, the quicker the thermal equilibrium can be established following excitation with a heat pulse. Sanchez-Barriga says, “That was very remarkable.”

Collaborations from Nanyang Technological University performed theoretical calculations in the Boltzmann framework. They were able to discern three types of thermalization processes. These included interactions between electrons within the same bands, electrons with phonons, and interactions between electrons.

The interaction between electrons seems to dominate the dynamics. This is evident when the lattice temperature decreases. This can be explained by Rashba splitting and the effect of fundamental electronic interactions on strength. This behavior is common to all Rashba silicons,” Sanchez-Barriga says: “The current results are important for future Rashba transistor applications and their excitations at ultrafast spintronics.”