New type of transistor one step closer
In order to make transistors that operate using the spin of electrons, rather than their charge, it is necessary to find a way of switching spin currents on and off. Furthermore, the lifetime of the spins should at least be equal to the time taken for these electrons to travel through a circuit.
University of Groningen scientists have now taken an important step forward by creating a device that meets both of these requirements, based on a double layer of graphene on top of a layer of tungsten disulfide. Their results have been published on 16 October in the journal Physical Review B.
Graphene, a two-dimensional form of carbon, is an excellent conductor of electron spins. However, it is difficult to manipulate spin currents in this material. Spin is a quantum mechanical property of electrons, which makes them behave like tiny magnets.
The Physics of Nanodevices group at the University of Groningen, led by Professor Bart van Wees, is working on this problem. They have previously shown that it is possible to control spin currents if the graphene is placed on top of a layer of tungsten disulphide (another 2D material).
'However, this approach reduces the lifetime of the spins', explains Siddhartha Omar, a postdoc in the Van Wees group. Tungsten is a metal, and its atoms influence the electrons passing through the graphene, dissipating the spin currents. This led Omar to use a double layer of graphene on the tungsten disulphide, based on the theory that electrons passing through the upper layer should 'feel' less of the metal atoms' influence.
Omar also used another new technique, in which two different types of spin current are passed through the graphene. Spin is a magnetic moment that has a given direction. In normal materials, the spins are not aligned. However, the magnetic moment of spin currents - like that of magnets - has a preferential alignment. Relative to the material through which the electrons are passing, their spins can either have an in-plane orientation or an out-of-plane orientation.
'We found that, as the electrons pass through the outer graphene layer, the in-plane spins are dissipated very quickly - in mere picoseconds. However, the lifetime of the out-of-plane spins is about one hundred times longer.' This means that, even in the presence of tungsten disulphide, one component of spin currents (spins with an out-of-plane orientation) can travel far enough to be used in devices such as transistors.
The energy level of the spin currents observed by Omar caused them to pass through the upper layer of graphene. This energy level can be boosted by applying an electric field, pushing the spin currents into the lower layer. 'Down there, the spins will feel the full effect of the metal atoms and the spin currents will quickly dissipate', Omar explains. This ability to switch the spin current off using an electric field is important, as it could be used to 'gate' transistors based on this technology.
'Unfortunately, certain technical limitations of the substrate on which we built these devices prevent us from creating electric fields that are strong enough to produce this gating effect', says Omar. 'However, we have shown that it is possible to send spin currents through a heterostructure made of graphene and tungsten disulphide. That is an important step towards the creation of a spin transistor.'
Electrons have a negative charge, but they also behave like tiny magnets. This property of electrons, called spin, can be used to transport or store information in electronic circuits. Scientists are looking for ways to create such spin-based electronics, as this is probably more energy efficient than normal electronics. University of Groningen physicist Siddhartha Omar discovered a way to transport spins over long enough distances to make such devices feasible. Furthermore, the material he used enabled him to control these spin currents.
Reference: S. Omar, B.N. Madhushankar, and B. J. van Wees: Large spin-relaxation anisotropy in bilayer-graphene/WS2 heterostructures. Phys. Rev. B 100, 16 October 2019.
Rene Fransen | EurekAlert!
The measurements of the expansion of the universe don't add up
19.11.2019 | FECYT - Spanish Foundation for Science and Technology
How LISA pathfinder detected dozens of 'comet crumbs'
19.11.2019 | NASA/Goddard Space Flight Center
Nanooptical traps are a promising building block for quantum technologies. Austrian and German scientists have now removed an important obstacle to their practical use. They were able to show that a special form of mechanical vibration heats trapped particles in a very short time and knocks them out of the trap.
By controlling individual atoms, quantum properties can be investigated and made usable for technological applications. For about ten years, physicists have...
An international team of scientists, including three researchers from New Jersey Institute of Technology (NJIT), has shed new light on one of the central mysteries of solar physics: how energy from the Sun is transferred to the star's upper atmosphere, heating it to 1 million degrees Fahrenheit and higher in some regions, temperatures that are vastly hotter than the Sun's surface.
With new images from NJIT's Big Bear Solar Observatory (BBSO), the researchers have revealed in groundbreaking, granular detail what appears to be a likely...
The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Dresden has succeeded in using Selective Electron Beam Melting (SEBM) to...
Carbon nanotubes (CNTs) are valuable for a wide variety of applications. Made of graphene sheets rolled into tubes 10,000 times smaller than a human hair, CNTs have an exceptional strength-to-mass ratio and excellent thermal and electrical properties. These features make them ideal for a range of applications, including supercapacitors, interconnects, adhesives, particle trapping and structural color.
New research reveals even more potential for CNTs: as a coating, they can both repel and hold water in place, a useful property for applications like printing,...
If you've ever tried to put several really strong, small cube magnets right next to each other on a magnetic board, you'll know that you just can't do it. What happens is that the magnets always arrange themselves in a column sticking out vertically from the magnetic board. Moreover, it's almost impossible to join several rows of these magnets together to form a flat surface. That's because magnets are dipolar. Equal poles repel each other, with the north pole of one magnet always attaching itself to the south pole of another and vice versa. This explains why they form a column with all the magnets aligned the same way.
Now, scientists at ETH Zurich have managed to create magnetic building blocks in the shape of cubes that - for the first time ever - can be joined together to...
15.11.2019 | Event News
15.11.2019 | Event News
05.11.2019 | Event News
19.11.2019 | Life Sciences
19.11.2019 | Physics and Astronomy
19.11.2019 | Health and Medicine