Well insulated: Conducting experiments

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  • Published: Oct 1, 2012
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
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Extreme science

Extreme ultraviolet radiation has been used to shed light on the fundamental differences between different types of electrically insulating materials with femtosecond time-resolved photoelectron spectroscopy.  

Extreme ultraviolet radiation has been used to shed light on the fundamental differences between different types of electrically insulating materials with femtosecond time-resolved photoelectron spectroscopy.

Why are some materials conductors and others insulators of electricity? And, why do some insulators differ from others? One can talk of electron motility, band-gaps, current flow and other parameters, but getting a closer look at the detailed explanation has been an important aim since at least then 1960s when researchers began to propose different mechanisms for the classification of insulators. All theoretical, until now.

A team of physicists from Kiel University in Germany and the University of Colorado at Boulder in the USA has now developed a new experimental method to differentiate between the various classes of insulator definitively.

Writing in the journal Nature Communications, the team explains how insulators are the flip-side of the electrical coin. Conductors undertake the currency transfers, but insulators allow the electronic traffic to move along the various paths of least resistance and not into undesirable areas, whether we are discussing mundane electrical transmission or the high-speed computations taking place in the chip embedded in the latest smart phone. The ubiquity of insulators, of course, makes them a particularly hot research topic in solid-state science driven in part by the urge to comply with Moore's Law and develop faster and smaller electronic circuitry.

The theory of insulators stacks up well, but has not really been experimentally verified despite many attempts. "For many years, expert discussions went round and round without any final answer about the insulator class," explains team leader Kai Rossnagel of Kiel University. He and his team hoped to develop an objective approach to the experimental classification of insulating behaviour. Key to their success was the known phenomenon in which certain types of electrical conductors can be switched to insulators when they are cooled to very low temperature. Warming those neo-insulators causes the electric state to revert to the conducting phase. By investigating how this change occurs and how quickly, the team have developed a new way to distinguish between classes of insulators.

The team used femtosecond time-resolved photoelectron spectroscopy with extreme ultraviolet radiation to sequence the changes. "The electronic changes visible in [our] film, take about one to fifty femtoseconds for some materials and 100 to 200 femtoseconds for others," Rossnagel explains.

One material of particular interest is titanium diselenide, which the team can now class as an excitonic insulator based on experimental evidence. "We believe that our results may terminate the discussion about titanium diselenide after decades," adds Rossnagel. However, he concedes that, "Only after several years of cross-checking our results, we will know for sure if our method is as useful as we think now."

First application

The team developed and published their technique in March 2011 in Nature, but the new study is the first to apply the system to an actual scientific question. "One logical next step of our research will be to extend the approach to the best conductors there are: high-temperature superconductors," Rossnagel told SpectroscopyNOW. "These are also characterized by a gap in their electronic spectrum, more precisely by two gaps: the superconducting gap and the infamous pseudogap. Using femtosecond time- and angle-resolved photoemisson spectroscopy, we want to measure how and how fast these two gaps melt. The ultimate goal would be to clarify the nature of the pseudogap which is often regarded as a key to understanding the phenomenon of high-temperature superconductivity."

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Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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