Power/Performance Bits: June 27

Superconducting nanowire memory cell; reconfigurable circuits; wireless charging.


Superconducting nanowire memory cell
Researchers at the University of Illinois at Urbana-Champaign and the State University of New York at Stony Brook developed a new nanoscale memory cell that provides stable memory at a smaller size than other proposed memory devices, and holds promise for successful integration with superconducting processors.

The device comprises two superconducting nanowires, attached to two unevenly spaced electrodes that were “written” using electron-beam lithography. The nanowires and electrodes form an asymmetric, closed superconducting loop, called a nanowire ‘SQUID’ (superconducting quantum interference device). The direction of current flowing through the loop, either clockwise or counterclockwise, equates to the “0” or “1” of binary code.

The memory state is written by applying an oscillating current of a particular magnitude, at a specific magnetic field. To read the memory state the scientists ramp up the current and detect the current value at which superconductivity gets destroyed. It turns out that such destruction or critical current is different for the two memory states, “0” or “1”. The scientists tested memory stability, delaying reading of the state, and found no instances of memory loss. The team performed these experiments on two nanowire SQUIDS, made of the superconductor Mo75Ge25, using a method called molecular templating.

Alexey Bezryadin, professor of physics at Illinois, comments, “This is very exciting. Such superconducting memory cells can be scaled down in size to the range of few tens of nanometers, and are not subject to the same performance issues as other proposed solutions.”

Superconducting nanoscale memory cell. Binary information is encoded in the direction of the electrical current in the loop. Clockwise indicates ‘0’, counter clockwise, ‘1’. The superconducting electrons flow indefinitely, so memory is nonvolatile. (a) Photo of device: A superconducting strip of Mo75Ge25 (yellow) with a pair of superconducting nanowires forming a closed loop (also yellow). (b) The critical current (maximum current that can be injected without destroying superconductivity) plotted as a function of magnetic field: To set the memory state ‘0’, a positive current is applied, targeting the shaded diamond. To set the memory to ‘1’, a negative current is applied. To read out the state, the current is ramped, as shown by the red rhombus, and the current value at which voltage occurs is measured. This measured value, the critical current, depends on the pre-set memory; its statistical distribution is shown in (c). (Source: Alexey Bezryadin and Andrew Murphey, U. of I. at Urbana-Champaign)

The researchers argue that this device can operate with a very low dissipation of energy, if the energies of two binary states are equal or near equal. The switching between the states of equal energy will be achieved either by quantum tunneling or by adiabatic processes composed of multiple jumps between the states.

In future work, the team plans to address the measurements of the switching time and to study larger arrays of the nanowire squids functioning as arrays of memory elements. They will also test superconductors with higher critical temperatures, with the goal of a memory circuit that would operate at 4 Kelvin. Rapid operations will be achieved by utilizing microwave pulses.

Reconfigurable circuits
Researchers at Queen’s University Belfast discovered a new way to create 2D electrically conducting sheets, called domain walls, in copper-chlorine boracite crystals. Just a few atomic layers thick, the sheets can appear, disappear or move around within the crystal, without permanently altering the crystal itself.

The researchers envision the creation of even smaller electronic devices, as electronic circuits could constantly reconfigure themselves to perform a number of tasks, rather than just having a sole function.

Professor Marty Gregg of Queen’s University notes, “As things currently stand, it will become impossible to make these devices any smaller – we will simply run out of space. This is a huge problem for the computing industry and new, radical, disruptive technologies are needed. One solution is to make electronic circuits more ‘flexible’ so that they can exist at one moment for one purpose, but can be completely reconfigured the next moment for another purpose.

“Our research suggests the possibility to “etch-a-sketch” nanoscale electrical connections, where patterns of electrically conducting wires can be drawn and then wiped away again as often as required. In this way, complete electronic circuits could be created and then dynamically reconfigured when needed to carry out a different role, overturning the paradigm that electronic circuits need be fixed components of hardware, typically designed with a dedicated purpose in mind.”

Domain walls. (Source: Queen’s University Belfast)

In creating these 2D sheets, long straight walls need to be created which can effectively conduct electricity and mimic the behavior of real metallic wires. It is also essential to be able to choose exactly where and when the domain walls appear and to reposition or delete them.

According to Dr Raymond McQuaid, a lecturer at Queen’s University, “Our team has demonstrated for the first time that copper-chlorine boracite crystals can have straight conducting walls that are hundreds of microns in length and yet only nanometers thick. The key is that, when a needle is pressed into the crystal surface, a jigsaw puzzle-like pattern of structural variants, called “domains”, develops around the contact point. The different pieces of the pattern fit together in a unique way with the result that the conducting walls are found along certain boundaries where they meet.

“We have also shown that these walls can then be moved using applied electric fields, therefore suggesting compatibility with more conventional voltage operated devices. Taken together, these two results are a promising sign for the potential use of conducting walls in reconfigurable nano-electronics.”

Wireless charging of moving things
Scientists at Stanford University developed a method for wirelessly transmitting electricity to a nearby moving object. So far, the group has been able to wirelessly power a moving LED bulb, but has hopes the system can be used to charge up electric vehicles as they travel on the road. While the range of new EVs has grown to upward of 200 miles, the batteries still take several hours to fully charge.

“In theory, one could drive for an unlimited amount of time without having to stop to recharge,” said Shanhui Fan, a professor of electrical engineering at Stanford. “The hope is that you’ll be able to charge your electric car while you’re driving down the highway. A coil in the bottom of the vehicle could receive electricity from a series of coils connected to an electric current embedded in the road.”

Mid-range wireless power transfer, as developed at Stanford and other research universities, is based on magnetic resonance coupling. Just as major power plants generate alternating currents by rotating coils of wire between magnets, electricity moving through wires creates an oscillating magnetic field. This field also causes electrons in a nearby coil of wires to oscillate, thereby transferring power wirelessly. The transfer efficiency is further enhanced if both coils are tuned to the same magnetic resonance frequency and are positioned at the correct angle.

However, the continuous flow of electricity can only be maintained if some aspects of the circuits, such as the frequency, are manually tuned as the object moves. So, either the energy transmitting coil and receiver coil must remain nearly stationary, or the device must be tuned automatically and continuously – a significantly complex process.

To address the challenge, the team eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor. This system automatically figures out the right frequency for different distances without the need for human interference.

The group used an off-the-shelf, general-purpose amplifier with a relatively low efficiency of about 10%. They say custom-made amplifiers can improve that efficiency to more than 90%.

“In addition to advancing the wireless charging of vehicles and personal devices like cellphones, our new technology may untether robotics in manufacturing, which also are on the move,” said Fan. “We still need to significantly increase the amount of electricity being transferred to charge electric cars, but we may not need to push the distance too much more.”

The group built on existing technology developed in 2007 at MIT for transmitting electricity wirelessly over a distance of a few feet to a stationary object. In the new work, the team transmitted electricity wirelessly to a moving LED lightbulb. That demonstration only involved a 1-milliwatt charge, whereas electric cars often require tens of kilowatts to operate. The team is now working on greatly increasing the amount of electricity that can be transferred, and tweaking the system to extend the transfer distance and improve efficiency.