University Times

Single-electron transistor developed

A team led by Pitt researchers has created a single-electron transistor that provides a building block for new, more powerful computer memories, advanced electronic materials and the basic components of quantum computers.

The researchers report in Nature Nanotechnology that the transistor’s central component — an “island” 1.5 nanometers in diameter — operates with the addition of only one or two electrons. That capability would make the transistor important to a range of computational applications, from ultra-dense memories to quantum processors, powerful devices that promise to solve problems so complex that all of the world’s computers working together for billions of years could not crack them.

In addition, the tiny central island could be used as an artificial atom for developing new classes of artificial electronic materials, such as exotic superconductors with properties not found in natural materials, explained lead researcher Jeremy Levy, a faculty member in physics and astronomy.

Levy worked with graduate student Guanglei Cheng and researchers Feng Bi, Daniela Bogorin and Cheng Cen, all of physics and astronomy, in conjunction with researchers from the University of Wisconsin, HP Labs and the State University of Campinas in Brazil.

Levy and his colleagues named their device SketchSET (for sketch-based single-electron transistor) after a technique developed in Levy’s lab in 2008 that works like a microscopic Etch A Sketch, the drawing toy that inspired the idea.

Using the sharp conducting probe of an atomic force microscope, Levy can create such electronic devices as wires and transistors of nanometer dimensions at the interface of a crystal of strontium titanate and a 1.2-nanometer thick layer of lanthanum aluminate. The electronic devices then can be erased and the interface reused.

The SketchSET, which is the first single-electron transistor made entirely of oxide-based materials, consists of an island formation that can house up to two electrons. The number of electrons on the island results in distinct conductive properties, and wires extending from the transistor carry additional electrons across the island.

One virtue of a single-electron transistor is its extreme sensitivity to an electric charge, Levy said. Another property of these oxide materials is ferroelectricity, which allows the transistor to act as a solid-state memory. A computer memory based on this property would be able to retain information even when the processor itself is powered down, Levy said. The ferroelectric state also is expected to be sensitive to small pressure changes at nanometer scales, making this device potentially useful as a nanoscale charge and force sensor.

An atomic-scale depiction of the SketchSET is available at www.news.pitt.edu/news/Levy_SketchSET_NatureNano.

Causes of tinnitus probed

Tinnitus, a relentless and often life-changing ringing in the ears, is the result of under-inhibition of key neural pathways in the brain’s auditory center, report School of Medicine scientists in the Proceedings of the National Academy of Sciences.

The discovery, which used a new technique to image auditory circuits using slices of brain tissue in the lab, points the way to drug development and effective treatment for a condition that currently has no cure.

Prior research has shown that auditory circuits in the brain are more excitable in tinnitus sufferers, but until now it has not been clear whether that is due to hyperactivity of excitatory neural pathways, reduced activity of inhibitory ones or a bit of both, explained senior investigator Thanos Tzounopoulos, a faculty member in otolaryngology and neurobiology.

“This auditory imbalance leaves the patient hearing a constant ringing, buzzing or other irritating noise even when there is no actual sound,” he said. “Tinnitus drowns out music, television, co-workers, friends and family,and it profoundly changes how the patient perceives and interacts with the world.”

According to the American Tinnitus Association, tinnitus is the most common service-connected disability among veterans of the Iraq and Afghanistan conflicts. Of the 50 million people who have experienced it, 16 million have symptoms severe enough to seek medical attention and 2 million tinnitus sufferers are unable to carry out day-to-day activities.

To identify what goes wrong in the brain’s auditory circuits, Tzounopoulos’s team created tinnitus in a mouse model. While the rodent was sedated, one ear was exposed to 45 minutes of 116-decibel (dB) sound, equivalent to an ambulance siren. Several weeks later, the scientists confirmed the exposed mice had tinnitus by conducting startle experiments in which a continuous, 70dB tone was played for a period, then stopped briefly and then resumed before being interrupted with a much louder pulse.

Mice with normal hearing could perceive the gap and, because they were aware something had changed, were less startled than mice with tinnitus, whose ear ringing masked the moment of silence between the background tones.

The scientists then sought to determine what had gone wrong in the balance of excitation and inhibition of the auditory circuits in the affected mice. Experiments were performed in the dorsal cochlear nucleus (DCN), a specialized auditory brain center that is crucial in the triggering of tinnitus. Imaging showed that the tinnitus group had, as expected, a greater response than the control group to electrical stimulation. Most importantly, despite local stimulation, DCN responses spread farther in the affected mice.

After administering a variety of agents that block specific excitatory and inhibitory receptors and seeing how the brain center responded, Tzounopoulos’s team determined that blocking an inhibitory pathway that produces GABA, an inhibitory neurotransmitter, enhanced the response in the region surrounding the DCN in the control brain slices more so than it did in the tinnitus slices.

“That means the DCN circuits are already ‘disinhibited,’ or blocked, in tinnitus,” Tzounopoulos explained. “We couldn’t block inhibition anymore to elevate the evoked response, like we could in the normal brain. And, when we blocked another inhibitory circuit mediated by the neurotransmitter glycine, or when we blocked excitatory pathways, there was no difference in the responses between the groups.”

Tzounopoulos’s team now is trying to identify agents that increase GABA-mediated inhibition, which could be effective treatments for tinnitus.

Pitt co-authors of the paper included Jason W. Middleton and Courtney Pedersen.

The research was funded by the National Institutes of Health, the U.S. Department of Defense, the American Tinnitus Association and the Albert and Ellen Grass Faculty Award.

IS faculty member to conduct research in Norway

School of Information Sciences (SIS) faculty member Vladimir Zadorozhny was awarded a Leiv Eiriksson Fellowship by the Norwegian Research Council.

The Leiv Eiriksson Mobility Programme is designed to attract top-level researchers to Norway and to strengthen the long-term escalation of R&D collaboration between Norway and countries within North America. In the 2011 round of competition, 42 of approximately 150 project proposals were funded.

Zadorozhny will spend a portion of the summer at the University of Agder, conducting research on efficient data processing, privacy and security in large-scale information networks. One of the goals of this visit is to stimulate long-term research projects between Pitt and the University of Agder in information and communication technology (ICT).  Zadorozhny and his colleagues in Norway have an established track record of research and collaboration in ICT.

Under the Leiv Eiriksson program, Zadorozhny will also conduct research for a newly initiated project entitled “Security, Services, Networking and Performance of Next Generation IP-based Multimedia Wireless Networks,” which is supported by the European Commission. Within this project, Pitt, the University of Agder and 11 other universities will collaborate on research about next-generation mobile systems and wireless networks.

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The University Times Research Notes column reports on funding awarded to Pitt researchers as well as findings arising from University research.

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