Nav: Home

Research reveals new aspects of superconductivity and correlated phenomena

April 11, 2018

Discovered accidentally over a century ago, the phenomenon of superconductivity continues to inspire a technological revolution. In 1911, while studying the behavior of solid mercury supercooled to 4 K (-269 °C), Dutch physicist Heike Kamerlingh Onnes (1853-1926) observed for the first time that certain materials conducted electricity with neither resistance nor losses at temperatures in the vicinity of absolute zero.

Interest revived in the 1980s when superconductivity was experimentally observed at much higher temperatures in the range of 90 K (-183 °C). This level was later surpassed, and the expectation of superconductivity at room temperature currently motivates research at several front-ranking scientific institutions.

This information provides the background for a study recently conducted by the Solid State Physics Group at São Paulo State University (UNESP) in Rio Claro, Brazil. The principal investigator was Valdeci Pereira Mariano de Souza. In addition to other researchers affiliated with UNESP, the team included scientists from Paris South University (Orsay) in France.

In Rio Claro, the research team used equipment purchased with support from the São Paulo Research Foundation - FAPESP to obtain the results, which provided the basis for an article published in Physical Review B.

"In several materials, the superconductive phase is manifested in the proximity of what's known as the 'Mott insulating phase'. The Mott metal-insulator transition is a sudden change in electrical conductivity that occurs at a given temperature when the Coulomb repulsion between electrons becomes comparable to the free-electron kinetic energy," said Mariano.

"When the Coulomb repulsion becomes relevant, the electrons that were itinerant become localized, and this minimizes the system's total energy. This electron localization is the Mott insulating phase. In some cases, an even more exotic process unfolds. Because of the interactions between electrons occupying neighboring sites in the network, the electrons rearrange themselves in the network in a non-homogeneous manner, and a so-called 'charge ordering phase' occurs. Our study addressed this kind of phenomenon."

When the charge ordering phase occurs, the non-homogeneous charge distribution, which is sometimes accompanied by a distortion of the crystalline network, makes the material electrically polarized, and as a result, it behaves like a ferroelectric material. This stage is known as the "ferroelectric Mott-Hubbard phase" after two British physicists who studied the topic: Nevill Mott (1905-96), 1977 Nobel Laureate in Physics, and John Hubbard (1931-80).

To experimentally explore these exotic phases, the UNESP researchers chose a material called Fabre salts, which are formed from an organic molecule, tetramethyltetrathiafulvalene (TMTTF), with a symmetrical configuration comprising a central double carbon bond and two methyl radicals on either side. They used a cryostat, also acquired with FAPESP's support, to reach the coldest and most magnetic point available at UNESP, with a temperature of 1.4 K and a 12 Tesla field.

"With this experimental setup, we aimed not just to characterize materials, although that's important, but to investigate the fundamental properties of matter that manifest themselves under extreme conditions," Mariano said. "Fabre salts have extremely rich phase diagrams for those who undertake this kind of research. The molecular systems concerned had already been explored using nuclear magnetic resonance imaging, infrared spectroscopy and other techniques. What we essentially did was measure their dielectric constants in the low-frequency regime."

It is worth recalling that the dielectric constant varies from material to material and, while it is a macroscopic quantity, it tells us how polarizable a material is.

"Given that Fabre salts are highly anisotropic and therefore have strongly crystallographic-direction-dependent transport properties, when charge ordering occurs, we observe Mott-Hubbard electric polarization throughout the TMTTF stack. This polarization is considerable and was reported in the literature in 2001," said the FAPESP-supported researcher.

"The ionic contribution to the dielectric constant of these materials was measured for the first time in this study. We found that as the temperature decreases, the ionic contribution also decreases, which gives rise to the Mott-Hubbard phase. This was a new observation that had not yet been reported in the literature - a genuinely original contribution of ours. We also explored the effect of the disorder induced by irradiation in the Mott-Hubbard phase in detail."

This is important, he added, because of the proximity of the Mott-Hubbard ferroelectric phase to superconductivity.

"William Little, Emeritus Professor of Physics at Stanford University, stated that low-dimensional molecular conductors would be candidates for obtaining room temperature superconductivity. In his work, Little proposed that room temperature superconductivity would be achieved by means of 'spines', or conducting chains with highly polarizable side chains. The materials we're studying have precisely these elements," Mariano said.

The production of spines was a first step. The next step, which has already been conceived by the researchers in Rio Claro, is to stress Fabre salts to induce superconductivity in the Mott-Hubbard ferroelectric phase.
About São Paulo Research Foundation (FAPESP)

The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. For more information:

Fundação de Amparo à Pesquisa do Estado de São Paulo

Related Superconductivity Articles:

First report of superconductivity in a nickel oxide material
Scientists at SLAC and Stanford have made the first nickel oxide material that shows clear signs of superconductivity - the ability to transmit electrical current with no loss.
A hallmark of superconductivity, beyond superconductivity itself
Physicists have found 'electron pairing,' a hallmark feature of superconductivity, at temperatures and energies well above the critical threshold where superconductivity occurs.
Manipulating superconductivity using a 'mechanic' and an 'electrician'
Strongly correlated materials can change their resistivity from infinity to zero with minute changes in conditions.
Triplet superconductivity demonstrated under high pressure
Researchers in France and Japan have demonstrated a theoretical type of unconventional superconductivity in a uranium-based material, according to a study published in the journal Physical Review Letters.
The mechanism of high-temperature superconductivity is found
Russian physicist Viktor Lakhno from Keldysh Institute of Applied Mathematics, RAS considers symmetrical bipolarons as a basis of high-temperature superconductivity.
More Superconductivity News and Superconductivity Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...