System Bits: Oct. 21

Simplified superconducting circuits
Computer chips with superconducting circuits, which means they have no electrical resistance, are said to be 50 to 100 times as energy-efficient as today’s technology. Superconducting chips are also said to have greater processing power: Superconducting circuits that use so-called Josephson junctions have been clocked at 770 gigahertz, or 500 times the speed of the chip in the iPhone 6, according to MIT researchers.

But chips with Josephson junctions are big and hard to make, and use such minute currents that the results of their computations are difficult to detect. As such, they’ve mostly been relegated to a few custom-engineered signal-detection applications.

Now, MIT researchers said they’ve devised a new design that could make simple superconducting devices much cheaper to manufacture. While the circuits’ speed probably wouldn’t top that of today’s chips, they could solve the problem of reading out the results of calculations performed with Josephson junctions.

They call their device the nanocryotron, after the cryotron, an experimental computing circuit developed in the 1950s by MIT professor Dudley Buck. The cryotron was briefly the object of a great deal of interest — and federal funding — as the possible basis for a new generation of computers, but it was eclipsed by the integrated circuit.

Shown here is a square-centimeter chip containing the nTron adder, which performed the first computation using the researchers’ new superconducting circuit.
(Source: MIT)

The most promising application for the nanocryotron could be in making calculations performed by Josephson junctions accessible to the outside world.

Revolutionizing high-speed transistors
Graphene is a 2D material with extraordinary properties, according to researchers at UCLA and Tohoku University who expect it to revolutionize high-speed transistors in the near future given its thickness of just one carbon atom, and ability to conductor heat and charge hundreds of times faster silicon.

They explained that graphene’s exotic electronic and magnetic properties can be tailored by cutting large sheets of the material down to ribbons of specific lengths and edge configurations. They have theorized that nanoribbons with zigzag edges are the most magnetic, making them suitable for spintronics applications.

However, this top down fabrication approach is not yet practical, because current lithographic techniques for tailoring the ribbons always produce defects.

The scientists have discovered a new self-assembly method for producing defect-free graphene nanoribbons with periodic zigzag-edge regions, which use a copper substrate’s unique properties to change the way the precursor molecules react to one another as they assemble into graphene nanoribbons.

This allows the scientists to control the nanoribbons’ length, edge configuration and location on the substrate.

Graphene nanoribbons imaged by scanning tunneling microscopy. The zigzag edges are highlighted by the red structure.
(Source: UCLA and Tohoku University)

The researchers believe this method of graphene fabrication by self-assembly is a stepping stone toward the production of self-assembled graphene devices that will vastly improve the performance of data storage circuits, batteries and electronics.