Opto-spintronics

Scientists from the University of Warsaw, Poland-based Military University of Technology, CNR Nanotec, the University of Southampton and the University of Iceland have designed a new photonic system with electrically tuned topological features, constructed of perovskites and liquid crystals. The new system can be used to create efficient light sources.

Perovskites are highly-studied materials that have the potential to revolutionize the solar energy fields, among others. These are durable and easy-to-produce materials, the special property of which is a high solar light absorption coefficient and they are therefore used to develop new, more efficient photovoltaic cells. In recent years, the emission properties of these materials, so far underestimated, have been used.

An international collaboration involving the Irradiated Solids Laboratory at EPFL has published a paper detailing the mechanisms for detecting circularly polarized light using spin-optoelectronic devices called spin photodiodes.

"In this work, we combined spintronics with optics. This is spin-optoelectronics," explains Henri-Jean Drouhin, co-author of the study published and head of the 'Physics and Chemistry of Nano-objects' group at the Irradiated Solids Laboratory (LSI). Light particles, photons, also have a spin. This spin manifests itself in the fact that light can be right- or left-handed circularly polarized (which means that the electric field of the light winds to the right or left like a helix in the direction of propagation of the photons). When this light hits the device designed by the researchers, photons can excite electrons in the material. The spin of these electrons then adopts a preferential direction that depends on the photon spin. Knowing how to selectively extract the electrons therefore makes it possible to obtain information on the polarization of the incident light, making these devices 'spin photodiodes', in contrast to conventional photodiodes that measure the intensity of the light.

A team of researchers from Sweden, Finland and Japan have designed a semiconductor component in which information can be efficiently exchanged between electron spin and light at room temperature and above.

Developments in spintronics in recent decades have been based on the use of metals, and these have been highly significant for the possibility of storing large amounts of data. There would, however, be several advantages in using spintronics based on semiconductors, in the same way that semiconductors form the backbone of today's electronics and photonics.

A team of researchers from the National Renewable Energy Laboratory (NREL) and the University of Utah has developed a new type of LEDs that utilizes spintronics without needing a magnetic field, magnetic materials or cryogenic temperatures.

“The companies that make LEDs or TV and computer displays don’t want to deal with magnetic fields and magnetic materials. It’s heavy and expensive to do it,” said Valy Vardeny, distinguished professor of physics and astronomy at the University of Utah. “Here, chiral molecules are self-assembled into standing arrays, like soldiers, that actively spin polarize the injected electrons, which subsequently lead to circularly polarized light emission. With no magnetic field, expensive ferromagnets and with no need for extremely low temperatures. Those are no-nos for the industry.”

Scientists at the U.S. Department of Energy's Ames Laboratory, along with collaborators at Brookhaven National Laboratory and the University of Alabama at Birmingham, have discovered a light-induced switch that twists the crystal lattice of the material, switching on a giant electron current that appears to be nearly dissipationless. The discovery was made in a category of topological materials that holds great promise for spintronics, topological effect transistors, and quantum computing.

Weyl and Dirac semimetals can host exotic, nearly dissipationless, electron conduction properties that take advantage of the unique state in the crystal lattice and electronic structure of the material that protects the electrons from doing so. These anomalous electron transport channels, protected by symmetry and topology, don't normally occur in conventional metals such as copper. After decades of being described only in the context of theoretical physics, there is growing interest in fabricating, exploring, refining, and controlling their topologically protected electronic properties for device applications. For example, wide-scale adoption of quantum computing requires building devices in which fragile quantum states are protected from impurities and noisy environments. One approach to achieve this is through the development of topological quantum computation, in which qubits are based on "symmetry-protected" dissipationless electric currents that are immune to noise.

Researchers from the Moscow Institute of Physics and Technology, in collaboration with researchers from Germany and the Netherlands have developed a new memory technology they call optically-assisted MRAM which is based on changing the spin state via THz pulses.

The researchers say that the new technique is extremely efficient (the power required to switch a "bit" will be a thousand times smaller compared to current MRAM devices) and fast.

Researchers from the University of Utah developed two spintronics devices based on perovskite materials. The researchers use these new devices to demonstrate the high potential of perovksites for spintronics systems. This is a followup to the exciting results announced in 2017 by the same group that showed advantages of perovskites for spintronics.

The researchers use an organic-inorganic hybrid perovskite material that has a heavy lead atom that features strong spin-orbit coupling and a long injected spin lifetime.The first device is a spintronic LED which works with a magnetic electrode instead of an electron-hole electrode. The perovskite LED lights up with circularly polarized electroluminescence.

Researchers from the NIST in the US and the University of Tokyo have discovered a metallic antiferromagnet (Mn₃Sn) that exhibits a large magneto-optic Kerr (MOKE) effect, despite a vanishingly small net magnetization at room temperature.

Compared to ferromagnetic materials, metallic antiferromagnets allow for faster dynamics and more densely packed spintronic devices due to the weak interactions between antiferromagnetic cells. The researchers believe that such materials hold promise for future antiferromagnetic spintronic devices, where the magnetic state could transduced optically and switched either optically or by applying current.

Reseaerchers from TU Delft developed a method to convert the spin information into light signals at room temperature. The researchers hope that this method could enable opto-spintronics devices.

The researchers used a device made from a thin silver thread and a 2D tungsten disulfide film on top. Using circularly polarised light, the researchers created excitons with a specific rotational direction (that could be intitialized using the rotational direction of the laser light). The excitons emit photons when they relax. And the emitted light contains the spin information.

Researcherrs from Likoping University in Sweden demonstrated a method to generate and manipulate the surface spin current in topological insulators.

The researchers used a combination of a topological insulator (Bismuth Telluride, Bi2Te3) and a regular GaAs semiconductor. The electrons were generated with the same spin in the GaAs using polarized light. The electrons were then transferred to the TI.