System Bits: Feb. 11

Ballistic transport in graphene
Using electrons more like photons could provide the foundation for a new type of electronic device that would capitalize on the ability of graphene to carry electrons with almost no resistance even at room temperature in a process known as ballistic transport, according to researchers at Georgia Tech.

Ballistic transport is the process by which electrical resistance in nanoribbons of epitaxial graphene changes in discrete steps following quantum mechanical principles. The research shows that the graphene nanoribbons act more like optical waveguides or quantum dots, allowing electrons to flow smoothly along the edges of the material. In ordinary conductors such as copper, resistance increases in proportion to the length as electrons encounter more and more impurities while moving through the conductor.

The ballistic transport properties, similar to those observed in cylindrical carbon nanotubes, exceed theoretical conductance predictions for graphene by a factor of 10. The properties were measured in graphene nanoribbons approximately 40nm wide that had been grown on the edges of three-dimensional structures etched into silicon carbide wafers.

Ballistic Transport in Graphene Nanoribbons: Conceptual drawing of an electronic circuit comprised of interconnected graphene nanoribbons (black atoms) that are epitaxially grown on steps etched in silicon carbide (yellow atoms). Electrons (blue) travel ballistically along the ribbon and then from one ribbon to the next via the metal contacts. Electron flow is modulated by electrostatic gates. (Source: John Hankinson, Georgia Tech)

This work shows that graphene electrons can be controlled in very different ways because the properties are really exceptional, which could result in a new class of coherent electronic devices based on room temperature ballistic transport in graphene and would be very different from today’s silicon devices, the researchers asserted.

The key to spintronics technologies
EPFL scientists said the key to future spintronics technologies are topological insulators and have unraveled how these strange materials work, overcoming one of the biggest obstacles on the way to next-generation applications.

Spintronics is an emerging field of electronics, where instead of using the charge of electrons, devices work by manipulating electron spin. Already tested in hard drives, spintronics are poised to replace current information technology, providing increased data transfer speeds, processing power, memory density and storage capacity. Controlling electron spin can be achieved with topological insulators; a novel class of materials that behave as insulators on the inside, but are highly conductive on their surfaces. However, it has been unclear how exactly a normal material can become a topological insulator, and also how to implement them for real technological impact. The EPFL scientists offer solutions to both problems by studying the spin structure of few atoms-thick films of a common topological insulator.

The researchers reminded that future electronics will most likely utilize an intrinsic property of electrons called spin. This spin can take either of two possible states: “up” or “down”, which in a classical picture corresponds to a clockwise or counterclockwise rotation of the electron around its axis. The electron spin can also be viewed as an extremely small magnetic field surrounding the electron. The field of spintronics aims to exploit spin in order to develop a new era of technological applications. Information technology in particular stands to gain the most, as spintronics can offer considerably higher overall computing speeds and storage capacities at lower power consumption.

Just like conventional electronics requires switching between high and low current states, spintronics require the control of electron spin states and switch between “up” and “down”. Recent interest has focused on topological insulators, which can conduct spin-polarised electrons on their surface while their inner bulk acts as an insulator. However, manufacturing and implementing topological insulators has been limited because of technical limitations and because the formation of their unusual properties has not been entirely clear.

Now, these researchers have shown how spin-polarised electrons evolves on the surface of atomically flat bismuth-selenide topological insulator films no more than 30 atoms thick. They used a spectroscopic technique called SARPES, which allowed them to determine the different spin states of electrons travelling across the conducting surface of a topological insulator and found that the ability of the topological insulator to control the electron spin depends on its interface to the substrate and, surprisingly, not on the film thickness.

This shows that tuning the chemical make-up of a topological insulator can directly manipulate the spin of electrons flowing across its surface, which contributes to the understanding of the function of topological insulators, but also provides a fundamental means of designing spintronics devices in the future.