Power/Performance Bits: March 22

Superconducting memory; measuring organic transistors; moldy batteries.

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Superconducting memory

A group of scientists from the Moscow Institute of Physics and Technology and the Moscow State University developed a fundamentally new type of memory cell based on superconductors, which they believe will be able to work hundreds of times faster than memory devices commonly used today.

The basic memory cells are based on quantum effects in “sandwiches” of superconductor – dielectric (or other insulating material) – superconductor. Called Josephson junctions, these sandwiches are able to tunnel from one layer of a superconductor to another, passing through the dielectric like balls passing through a perforated wall.

Josephson junctions with ferromagnets used as the middle of the “sandwich” are currently of greatest practical interest. In memory elements that are based on ferromagnets the information is encoded in the direction of the magnetic field vector in the ferromagnet.

In order to switch the system from “zero” to “one” and back again, the scientists have suggested using injection currents flowing through one of the layers of the superconductor. They propose to read the status using the current that flows through the whole structure. These operations can be performed hundreds of times faster than measuring the magnetization or magnetization reversal of a ferromagnet.

Superconducting currents during reading various states of the memory cell are shown. The greater current the larger arrow. S - superconductor, I - insulating tunnel barrier, F - ferromagnet, N - normal metal, shaded area -- potential barrier arising in the superconducting zone. (Source: S. V. Bakurskiy, N. V. Klenov, I. I. Soloviev, M. Yu. Kupriyanov and A. A. Golubov/MIPT)

Superconducting currents during reading various states of the memory cell are shown. The greater current the larger arrow. S – superconductor, I – insulating tunnel barrier, F – ferromagnet, N – normal metal, shaded area — potential barrier arising in the superconducting zone. (Source: S. V. Bakurskiy, N. V. Klenov, I. I. Soloviev, M. Yu. Kupriyanov and A. A. Golubov/MIPT)

“With the operational function that we have proposed in these memory cells, there will be no need for time-consuming magnetization and demagnetization processes. This means that read and write operations will take only a few hundred picoseconds, depending on the materials and the geometry of the particular system, while conventional methods take hundreds or thousands of times longer than this,” said Alexander Golubov, head of MIPT’s Laboratory of Quantum Topological Phenomena in Superconducting Systems.

Plus, the method requires only one ferromagnetic layer, which according to Golubov means that it can be adapted to single flux quantum logic circuits, with no need to create an entirely new architecture for a processor.

Measuring organic transistors

If measurements fail to account for divergent behaviors in organic field-effect transistors (OFETs), the resulting estimates of how fast electrons or other charge carriers travel in the devices may be more than 10 times too high, report researchers from the National Institute of Standards and Technology, Wake Forest University and Penn State. The team’s measurements implicate an overlooked source of electrical resistance as the root of inaccuracies that can inflate estimates of organic semiconductor performance.

“Organic semiconductors are more prone to non-ideal behavior because the relatively weak intermolecular interactions that make them attractive for low-temperature processing also limit the ability to engineer efficient contacts as one would for state-of-the-art silicon devices,” said electrical engineer David Gundlach, who leads NIST’s Thin Film Electronics Project. “Since there are so many different organic materials under investigation for electronics applications, we decided to step back and do a measurement check on the conventional wisdom.”

A circuit made from organic thin-film transistors is fabricated on a flexible plastic substrate. (Source: Patrick Mansell/Penn State)

A circuit made from organic thin-film transistors is fabricated on a flexible plastic substrate. (Source: Patrick Mansell/Penn State)

Using what Gundlach describes as the semiconductor industry’s “workhorse” measurement methods, the team scrutinized an OFET made of single-crystal rubrene, an organic semiconductor with a molecule shaped a bit like a microscale insect. Their measurements revealed that electrical resistance at the source electrode—the contact point where current is injected into the OFET— significantly influences the subsequent flow of electrons in the transistor channel, and hence the mobility.

In effect, contact resistance at the source electrode creates the equivalent of a second valve that controls the entry of current into the transistor channel. Unaccounted for in the standard theory, this valve can overwhelm the gate and become the dominant influence on transistor behavior.

The researchers hope this knowledge can inform efforts to develop accurate, comprehensive measurement methods for benchmarking organic semiconductor performance, as well as guide efforts to optimize contact interfaces.

Moldy batteries

A naturally occurring red bread mold could produce more sustainable electrochemical materials for use in rechargeable batteries, found researchers at the University of Dundee. The fungus in question, Neurospora crassa, shows the ability to transform manganese into a mineral composite with favorable electrochemical properties.

Combining the fungus with urea and manganese chloride resulted in the long branching fungal filaments becoming biomineralised, a process where living organisms produce minerals. After heat treatment, they were left with the mixture of carbonised biomass and manganese oxides.

“We had the idea that the decomposition of such biomineralized carbonates into oxides might provide a novel source of metal oxides that have significant electrochemical properties,” said Professor Geoffrey Gadd, who heads the Geomicrobiology Group at the University of Dundee.

However, Gadd was surprised it performed so well. “In comparison to other reported manganese oxides in lithium-ion batteries, the carbonised fungal biomass-mineral composite showed an excellent cycling stability and more than 90% capacity was retained after 200 cycles.”

Gadd said the team will continue to explore the use of fungi in producing various potentially useful metal carbonates. They are also interested in investigating such processes for the biorecovery of valuable or scarce metal elements in other chemical forms.