Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A Grand Unified Theory of Exotic Superconductivity?

18.10.2013
Scientists introduce a general theoretical approach that describes all known forms of high-temperature superconductivity and their "intertwined" phases

Years of experiments on various types of high-temperature (high-Tc) superconductors—materials that offer hope for energy-saving applications such as zero-loss electrical power lines—have turned up an amazing array of complex behaviors among the electrons that in some instances pair up to carry current with no resistance, and in others stop the flow of current in its tracks.

The variety of these exotic electronic phenomena is a key reason it has been so hard to identify unifying concepts to explain why high-Tc superconductivity occurs in these promising materials.

Now Séamus Davis, a physicist who's conducted experiments on many of these materials at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Cornell University, and Dung-Hai Lee, a theorist at DOE's Lawrence Berkeley National Laboratory and the University of California, Berkeley, postulate a set of key principles for understanding the superconductivity and the variety of "intertwined" electronic phenomena that applies to all the families of high-Tc superconductors. They describe these general concepts in a paper published in the Proceedings of the National Academy of Sciences October 10, 2013.

"If we are right, this is kind of the 'light at the end of the tunnel' point," said Davis. "After decades of wondering which are the key things we need to understand high-Tc superconductivity and which are the peripheral things, we think we have identified what the essential elements are."

Said Lee, "The next step is to be able to predict which other materials will have these essential elements that will drive high Tc superconductivity—and that ability is still under development."

The role of magnetism

In all known types of high-Tc superconductors—copper-based (cuprate), iron-based, and so-called heavy fermion compounds—superconductivity emerges from the "extinction" of antiferromagnetism, the ordered arrangement of electrons on adjacent atoms having anti-aligned spin directions. Electrons arrayed like tiny magnets in this alternating spin pattern are at their lowest energy state, but this antiferromagnetic order is not beneficial to superconductivity.

However if the interactions between electrons that cause antiferromagnetic order can be maintained while the actual order itself is prevented, then superconductivity can appear. "In this situation, whenever one electron approaches another electron, it tries to anti-align its magnetic state," Davis said. Even if the electrons never achieve antiferromagnetic order, these antiferromagnetic interactions exert the dominant influence on the behavior of the material. "This antiferromagnetic influence is universal across all these types of materials," Davis said.

Many scientists have proposed that these antiferromagnetic interactions play a role in the ability of electrons to eventually pair up with anti-aligned spins—a condition necessary for them to carry current with no resistance. The complicating factor has been the existence of many different types of "intertwined" electronic phases that also emerge in the different types of high-Tc superconductors—sometimes appearing to compete with superconductivity and sometimes coexisting with it.

Intertwined phases

In the cuprates, for example, regions of antiferromagnetic alignment can alternate with "holes" (vacancies formerly occupied by electrons), giving these materials a "striped" pattern of charge density waves. In some instances this striped phase can be disrupted by another phase that results in distortions of the stripes. In iron-based superconductors, Davis' experiments revealed a nematic liquid-crystal-like phase. And in the heavy fermion superconductors, other exotic electronic states occur.

"When so many intertwined phases were discovered in the cuprates, I was strongly discouraged because I thought, 'How are we going to understand all these phases?'" said Lee. But after the discovery of the iron-based superconductors about five years ago, and their similarities with the cuprates, Lee began to believe there must be some common factor. "Séamus was thinking along a similar line experimentally," he said.

In the current paper, Davis and Lee propose and demonstrate within a simple model that antiferromagnetic electron interactions can drive both superconductivity and the various intertwined phases across different families of high-Tc superconductors. These intertwined phases and the emergence of superconductivity, they say, can be explained by how the antiferromagnetic influence interacts with another variable in their theoretical description, namely the "Fermi surface topology."

"The Fermi surface is a property of all metals and provides a 'fingerprint' of the specific arrangements of electrons that are free to move that is characteristic of each compound," Davis said. "It is controlled by how many electrons are in the crystal, and by the symmetry of the crystal, among other things, so it is quite different in different materials."

The theory developed by Lee incorporates the overarching antiferromagnetic electron interactions and the known differences in Fermi surface from material to material. Using calculations to "dial up" the strength of the magnetic interactions or vary the Fermi surface characteristics, the theory can predict the types of electronic phases that should emerge up to and including the superconductivity for all those different conditions.

"The basic assumption of our theory is that when we rip away all the complicated intertwined phases, underneath there is an ordinary metal," said Lee. "It is the antiferromagnetic interactions in this metal that make the electrons want to form the various states. The complex behavior originates from the system fluctuating from one state to another, e.g., from superconductor to charge density waves to nematic order. It is the antiferromagnetic interaction acting on the underlying simple metal that causes all the complexity."

"So far this theory has correctly produced all the electronic phases that we have observed in each type of strongly correlated superconductor," Davis said.

The next step is to search through new materials and use the theory to identify which should operate in similar ways—and then put them to the test to see if they follow the predictions.

"It is one thing to say, 'If we have the key ingredients, then a material is likely to exhibit high Tc superconductivity.' It is quite another thing to know which materials will have these key characteristics,'" Lee said.

If the search pays off, it could lead to the identification or development of superconductors that can be used even more effectively than those that are known today—potentially transforming our energy landscape.

This research was funded by the DOE Office of Science, in part through the Center for Emergent Superconductivity, a DOE-funded Energy Frontier Research Center at Brookhaven National Laboratory.

DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov

Karen McNulty Walsh | EurekAlert!
Further information:
http://www.bnl.gov

More articles from Materials Sciences:

nachricht Glass's off-kilter harmonies
18.01.2017 | University of Texas at Austin, Texas Advanced Computing Center

nachricht Explaining how 2-D materials break at the atomic level
18.01.2017 | Institute for Basic Science

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

Im Focus: How to inflate a hardened concrete shell with a weight of 80 t

At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).

Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

Nothing will happen without batteries making it happen!

05.01.2017 | Event News

 
Latest News

A big nano boost for solar cells

18.01.2017 | Power and Electrical Engineering

Glass's off-kilter harmonies

18.01.2017 | Materials Sciences

Toward a 'smart' patch that automatically delivers insulin when needed

18.01.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>