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.

Life in synchrotron radiation research

Including the day an earthquake interrupted my beam time!

Today’s #LightSourceSelfie is brought to you by Ro-Ya Liu, Assistant Research Scientist at NSRRC, operators of the Taiwan Light Source and the Taiwan Photon Source. Ro-Ya’s research area is focused on probing the electronic structure of novel materials by using angle resolved photoemission spectroscopy. She was inspired by her Master’s supervisor whose eyes shone as he presented his new data on the quantum well state of ultra-high silver thin film. Ro-Ya wanted to experience this spark and purpose in life. After a shaky first experiment (literally shaky due to an Earthquake!), Ro-Ya has done just that during a career that has already involved working at the Taiwan Light Source, the Photon Factory, Spring 8, HiSOR, Elettra, the Advanced Light Source and Diamond Light Source. Ro-Ya is still learning from colleagues including beamline engineers and users coming to conduct experiments at the Taiwan Light Source. Their deep knowledge helps Ro-Ya in her beamline manager role. She is looking to dig deep to acquire this knowledge and continue to find great purpose in her life in synchrotron radiation research.

Kuda’s #LightSourceSelfie

Kudakwashe Jakata is a Post-Doc in Materials Science at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.  He first experienced the ESRF as a user and reflects on the challenges of his early tomography experiments, what gets him up every day and a future where African scientists can conduct experiments at a light source based in Africa.