Scientists gain new insight into mysterious electronic phenomenon

Apr 14, 2014 by Jared Sagoff
A levitating magnet floats above a superconducting surface.

(Phys.org) —For more than a quarter of a century, high-temperature superconductors – materials that can transmit electric current without any resistance – have perplexed scientists who seek to understand the physical phenomena responsible for their unique properties.

Thanks to a new study by the U.S. Department of Energy's (DOE) Argonne National Laboratory, researchers have identified and solved at least one paradox in the behavior of . The riddle involves a phenomenon called the "pseudogap," a region of energy levels in which relatively few electrons are allowed to exist.

Despite their name, high-temperature superconductors are actually quite cold – roughly 250 degrees to 350 degrees below zero Fahrenheit. Conventional superconductors, like those used in MRI machines or particle accelerators, are even colder. Even though they are still quite cold, high-temperature superconductors are of special interest to researchers because, at least in theory, they are much easier to keep sufficiently cold and are thus potentially more useful.

In general, superconductors can be thought of like an army that prefers the cold. At room temperature, the soldiers – electrons – are oriented every which way, and they all "march" in different directions. As the superconducting material is cooled past a point called the , the electrons begin to couple with each other and "march" in formation, a process known as Cooper pairing.

One particular class of copper-containing offers exceptional promise because they have the highest superconducting transition temperatures, said Argonne Materials Science Division Director Michael Norman, who led the project. The researchers took a magnetic insulator and "doped" it by adjusting the quantity of oxygen present in the material, causing it to switch into a high-temperature superconductor. "With a sufficient quantity of charge carriers added into the system, you can maximize the transition temperature," Norman said.

However, the pseudogap – the mysterious region of suppressed charge density – interferes with this process. "You can dope in carriers, but the presence of the pseudogap means that fewer of them will contribute to the conductance of the material," Norman said.

When comparing theoretical calculations of the pseudogap effect with the observed behavior of the material, Norman and his colleagues made a surprising and perplexing discovery. Based on well-established mathematical models, they had anticipated that the pseudogap would cause the superconductivity of the material to vanish, but for some reason the material demonstrated higher superconductivity than predicted.

"These models assumed that the pseudogap has nothing to do with Cooper pairing, and that pair formation and superconductivity arise simultaneously, but now we know that this isn't necessarily the case," Norman said. "By demonstrating that what we predicted wasn't the case, we've shown our understanding of the pseudogap isn't as complete as many have thought. In particular, the pseudogap must have something to do with pairing."

Explore further: Pseudogap theory puts physicists closer to high temperature superconductors

More information: "Effect of the pseudogap on the transition temperature in the cuprates and implications for its origin." Vivek Mishra,et al. Nature Physics (2014) DOI: 10.1038/nphys2926 . Received 04 October 2013 Accepted 17 February 2014 Published online 30 March 2014

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johanfprins
1 / 5 (1) Apr 15, 2014
Pairing has nothing to do with superconduction! For God's sake WAKE UP!!
Technophebe
not rated yet Apr 15, 2014
Why on earth does this article refer to temperatures in fahrenheit and not celsius? Silly.
aau93940
not rated yet Apr 15, 2014
we've shown our understanding of the pseudogap isn't as complete as many have thought. In particular, the pseudogap must have something to do with pairing
Pseudogap is a "mystery", which is "solved" regularly each year. Some physicists say, that is has nothing do do with pairing, the others that is hasn't. But if you take a look at the superconductor, you'll realize, that the truth is exactly inbetween: the electrons are paired as a "teams", i.e. whole groups of electrons by their spin. The pseudogap refers to the superconductor state, where the islands of these "teams" are already created, but they still don't form a continuous phase across bulk of superconductor, so that they cannot contribute to the superconductive current. The material already exhibits a superconductive energy gap, but it still remains nonconductive - so it's called a "pseudogap".
aau93940
not rated yet Apr 15, 2014
The superconductors come in two main types: the "classical" low-temperature ones (niobium, lead, etc.) and these ones found in 1986, high-temperature ones (cuprates, pnictides). The electron pairing is dominant in the I-type superconductors, in the II-type SC is only weakly present. Therefore the tendency of electrons to pairing it the lower, the higher critical Tc temperature of pseudogap is, which is exactly, what the above study studied and observed (see the graph: "non-pairing pseudogap supresses the Tc").
johanfprins
not rated yet Apr 15, 2014
@aau93940

Hi Zephyr, the pseudo-gap just like super-conduction itself has nothing to do with pairing.

At least you got one aspect nearly right: The pseudo-phase is a nascent SC phase that cannot super-conduct since the density of suitable charge-carriers are too low: However, these charge-carriers are not pairs and never have been pairs, and never will be pairs.
FastEddy
not rated yet Apr 21, 2014
Pairing has nothing to do with superconduction! For God's sake WAKE UP!!


... neither does magnetic levitation. Room temp. "superconductors" would be better described as "perfect conductors" of zero resistance. Suspending choo choo trains on super magnetics may be a laudable as an experiment, but for real energy savings in the world power grid is a much more practical goal. Zero resistance conductors = hundreds of trainloads of coal not burned.
FastEddy
not rated yet Apr 21, 2014
Why on earth does this article refer to temperatures in fahrenheit and not celsius? Silly.


Those who can not interpolate C. to F. and back in your head ... are doomed to repeated mistakes.

The French system of measurements does have its uses, but water still freeze at a certain point, no matter the system of measurement.
johanfprins
not rated yet Apr 21, 2014
Pairing has nothing to do with superconduction! For God's sake WAKE UP!!


... Room temp. "superconductors" would be better described as "perfect conductors" of zero resistance.


Please explain zero resistance: If you only talk about an infinite scattering length like the two foolish London Bros. did, you still need to explain the absence of acceleration within the material and thus scattering within the contacts.

To model super-conduction one must explain why the charge-carriers move with a constant speed WITHOUT BEING ACCELERATED AND WITHOUT BEING SCATTERED. In a conductor the constant drift speed is caused by scattering. Within a superconductor the charge-carriers have a drift speed while not scattering. The latter CANNOT be explained by Ohm's law since Ohm's law is ONLY valid when there is scattering.