Transistors

Researchers have found that lasers can generate stable patterns of electron spins in a thin layer of semiconductor material, a discovery that may help lead to advanced spin-based memory and computing. The scientists have revealed that lasers could generate complex stable patterns of electron spins called “spin textures” in thin films of semiconductors. These spin textures could help lead to what may be the holy grail of spintronics, a superefficient spin-based transistor.

The new findings are based on how light has momentum, just as a physical object moving through space does, even though light does not have mass. This means that light shining on an object can exert a force. Whereas the linear momentum of light supplies a push in the direction that light is moving, the angular momentum of light applies torque.

Scientists have long since been trying to use electric fields to control spin at room temperature but achieving effective control has thus far been elusive. In a recent research work, a team from Rensselaer Polytechnic Institute and the University of California at Santa Cruz took a step forward in addressing the issue.

An electron has a spin degree of freedom, meaning that it not only holds a charge but also acts like a little magnet. In spintronics, a key task is to use an electric field to control electron spin and rotate the north pole of the magnet in any given direction. The spintronic field effect transistor harnesses the so-called Rashba or Dresselhaus spin-orbit coupling effect, which suggests that one can control electron spin by electric field. Although the method holds promise for efficient and high-speed computing, certain challenges must be overcome before the technology reaches its true, miniature but powerful, and eco-friendly, potential.

University of Nebraska-Lincoln's scientist Christian Binek and University at Buffalo's Jonathan Bird and Keke He have teamed up to develop the first magneto-electric transistor.

Along with curbing the energy consumption of any microelectronics that incorporate it, the team's design could reduce the number of transistors needed to store certain data by as much as 75%, said Nebraska physicist Peter Dowben, leading to smaller devices. It could also lend those microelectronics steel-trap memory that remembers exactly where its users leave off, even after being shut down or abruptly losing power.

Researchers at CIC nanoGUNE BRTA in Spain and University of Regensburg in Germany have recently demonstrated spin precession at room temperature in the absence of a magnetic field in bilayer graphene. In their paper, the team used 2D materials to realize a spin field-effect transistor.

Sketch of the spin field-effect transistor. Image from article

Coherently manipulating electron spins at room temperature using electrical current is a major goal in spintronics research. This is particularly valuable as it would enable the development of numerous devices, including spin field-effect transistors. In experiments using conventional materials, engineers and physicists have so far only observed coherent spin precession in the ballistic regime and at very low temperatures. Two-dimensional (2D materials), however, have unique characteristics that could provide new control knobs to manipulate spin procession.

An international collaboration led by RMIT has achieved record-high electron doping in a layered ferromagnet, causing magnetic phase transition with significant promise for future electronics.

Control of magnetism (or spin directions) by electric voltage is vital for developing future, low-energy high-speed nano-electronic and spintronic devices, such as spin-orbit torque devices and spin field-effect transistors. Ultra-high-charge, doping-induced magnetic phase transition in a layered ferromagnet allows promising applications in antiferromagnetic spintronic devices.

An international research team, led by the University of Tsukuba, has used electron spin resonance (ESR) to monitor the number and location of unpaired spins going through a molybdenum disulfide transistor. ESR uses the same physical principle as the MRI machines that create medical images. The spins are subject to a very strong magnetic field, which creates an energy difference between electrons with spins aligned and anti-aligned with the field. The absorbance of photons that match this energy gap can be measured to determine the presence of unpaired electron spins.

15 1 Share Email Home Physics Condensed Matter MARCH 5, 2021 Taking 2-D materials for a spin by University of Tsukuba Schematic diagram of the MoS2 transistor in an ESR sample tube. Credit: University of Tsukuba

The experiment required the sample to be cooled to just four degrees above absolute zero, and the transistor to be in operation while the spins are being measured. "The ESR signals were measured simultaneously with the drain and gate currents," corresponding author Professor Kazuhiro Marumoto says. "Theoretical calculations further identified the origins of the spins," coauthor Professor Małgorzata Wierzbowska says. Molybdenum disulfide was used because its atoms naturally form a nearly flat two-dimensional structure. The molybdenum atoms form a plane with a layer of sulfide ions above and below.

A new theory by Rice University scientists could boost the field of spintronics. Materials theorist Boris Yakobson and graduate student Sunny Gupta at Rice’s Brown School of Engineering describe the mechanism behind Rashba splitting, an effect seen in crystal compounds that can influence their electrons’ “up” or “down” spin states, analogous to “on” or “off” in common transistors.

The left shows the crystal structure of a MoTe2

The Rice model characterizes single layers to predict heteropairs — two-dimensional bilayers — that enable large Rashba splitting. These would make it possible to control the spin of enough electrons to make room-temperature spin transistors, a far more advanced version of common transistors that rely on electric current.

Researchers from the University of Groningen developed a two-dimensional spin transistor, in which spin currents were generated by an electric current through graphene. The device also include a monolayer transition metal dichalcogenide (TMD) that is placed on the graphene to induce charge-to-spin conversion.

Graphene is an excellent spin transporter, but spin-orbit coupling is required to create or manipulate spins. The interaction is weak in the graphene carbon atoms, but now the researchers have shown that adding the TMD layer increases the spin-orbit coupling.

Researchers from the University of Nebraska-Lincoln designed a spintronics transistor that is based on graphene. The researchers say that such a device could be highly efficient, run at room temperature (and above) and feature a nonvolatile on-off current ratio and electrically controllable spin polarization.

The device is based on the discovery that an external voltage can be used to control the magnetic properties of few-layer graphene interfaced with chromium oxide. This is a theoretical research at this stage but the new device structure is expected to feature a large electrically controllable spin current.

Intel's technology and manufacturing group leader, William Holt, says that if Intel wants to keep improving its chips, it will soon have to start using fundamentally new technologies. The company does not know which technology will be adopted, but there are two possible candidates at this stage - Spintronics, and tunneling transistors.

William says that the new technologies will have to be commercialized in four to five years (when Intel moves over to 7-nm production, which is thought to be the limit of silicon transistors), and will initially be used alongside silicon transistors. Intel says we'll need to stop expecting chips to be faster - as the new technologies will mostly benefit the energy efficiency rather than the speed of Intel's future chips.