Giant Rashba semiconductors show unconventional dynamics with potential applications

Germanium telluride is a strong candidate for use in functional spintronic devices due to its giant Rashba-effect. Now, scientists at HZB have discovered another intriguing phenomenon in GeTe by studying the electronic response to thermal excitation of the samples. To their surprise, the subsequent relaxation proceeded fundamentally different to that of conventional semimetals. By delicately controlling the fine details of the underlying electronic structure, new functionalities of this class of materials could be conceived. 

In recent decades, the complexity and functionality of silicon-based technologies has increased exponentially, commensurate with the ever-growing demand for smaller, more capable devices. However, the silicon age is coming to an end.  With increasing miniaturisation, undesirable quantum effects and thermal losses are becoming an ever-greater obstacle. Further progress requires new materials that harness quantum effects rather than avoid them. Spintronic devices, which use spins of electrons rather than their charge, promise more energy efficient devices with significantly enhanced switching times and with entirely new functionalities.

Spintronic devices are coming

Candidates for spintronic devices are semiconductor materials wherein the spins are coupled with the orbital motion of the electrons. This so-called Rashba effect occurs in a number of non-magnetic semiconductors and semi-metallic compounds and allows, among other things, to manipulate the spins in the material by an electric field.

First study in a non equilibrium state

Germanium telluride hosts one of the largest Rashba effects of all semiconducting systems. Until now, however, germanium telluride has only been 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. In the process, the physicists encountered a new and unexpected phenomenon.

First, the sample was excited with an infrared pulse and then measured with high time resolution using angle-resolved photoemission spectroscopy (tr-ARPES). “For the first time, we were able to observe and characterise all phases of excitation, thermalisation and relaxation on ultrashort time scales,” says Sánchez-Barriga. The most important result: “The data show that the thermal equilibrium between the system of electrons and the crystal lattice is restored in a highly unconventional and counterintuitive way”, explains one of the lead authors, Oliver Clark.

Read more on the HZB website

Image: Left: Electronic structure of GeTe taken with 11 eV photons at BESSY-II, showing the band dispersions of bulk (BS) and surface Rashba states (SS1, SS2) in equilibrium. Middle: Zoom-in on the region of the Rashba states measured with fs-laser 6 eV photons. Right: Corresponding out-of-equilibrium dispersions following excitation by the pump pulse.

Recycling alginate composites for thermal insulation

Thermal insulation materials represent one the most straightforward, yet effective, technologies for improving the energy efficiency of buildings (and not only) – one of the key strategies for reducing carbon emissions. Natural-based materials and downcycled industrial and agricultural waste, thanks to their potentially reduced environmental footprint, have already made their way up to the market with the aim of limiting the ever-growing waste stream generated by the industrial sector. Research efforts on the topic are currently mainly focused on developing new insulation solutions, in which waste is reconverted as a new valuable resource. Carbohydrates, such as alginate, cellulose or chitosanare currently extensively studied base materials for thermal insulation systems, in the form of aerogels or as low-impact binding agents in waste-filled panels. Unfortunately, little or no attention has been paid to the end-of-life fate of these recycled materials; disposal (or incineration) still represents the only available option. This unprofitable scenario is even more critical in the case of polysaccharide-based composites specifically developed to reuse industrial waste. 

This was the starting point of our work, mainly conducted at the laboratories of the Engineering and Architecture department of the University of Trieste, in collaboration with TomoLab at Elettra. We developed a recycling process for an alginate-based thermal insulation foam, in which the original material is fully recovered and the thermal and acoustic insulation performances are maintained. The original foam is produced via a patented process in which alginate is used as the host poly-anionic matrix for industrial fiberglass waste. 

Read more on the Elettra website

Image: SEM and μCT image of oCAF

Credit: Figure reprinted from Carbohydrate Polymers, 251, Matteo Cibinel, Giorgia Pugliese, Davide Porrelli, Lucia Marsich, Vanni Lughi, Recycling alginate composites for thermal insulation, 116995, Copyright 2021, with permission from Elsevier.