Weird superconductor leads double life

March 21, 2018 by Glennda Chui, SLAC National Accelerator Laboratory
One unusual property of superconducting materials is that they expel magnetic fields and thus cause magnets to levitate, as shown here. A study at SLAC and Stanford of a particularly odd superconductor, strontium titanate, will aid understanding and development of these materials. Credit: ViktorCap/iStock

Until about 50 years ago, all known superconductors were metals. This made sense, because metals have the largest number of loosely bound "carrier" electrons that are free to pair up and flow as electrical current with no resistance and 100 percent efficiency – the hallmark of superconductivity.

Then an odd one came along – strontium titanate, the first oxide material and first semiconductor found to be superconducting. Even though it doesn't fit the classic profile of a superconductor – it has very few free-to-roam electrons – it becomes superconducting when conditions are right, although no one could explain why.

Now scientists have probed the superconducting behavior of its electrons in detail for the first time. They discovered it's even weirder than they thought. Yet that's good news, they said, because it gives them a new angle for thinking about what's known as "high temperature" superconductivity, a phenomenon that could be harnessed for a future generation of perfectly efficient power lines, levitating trains and other revolutionary technologies.

The research team, led by scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University, described their study in a paper published Jan. 30 in the Proceedings of the National Academy of Sciences.

"If conventional metal superconductors are at one end of a spectrum, strontium titanate is all the way down at the other end. It has the lowest density of available electrons of any superconductor we know about," said Adrian Swartz, a postdoctoral researcher at the Stanford Institute for Materials and Energy Science (SIMES) who led the experimental part of the research with Hisashi Inoue, a Stanford graduate student at the time.

"It's one of a large number of we call 'unconventional' superconductors because they can't be explained by current theories," Swartz said. "By studying its extreme behavior, we hope to gain insight into the ingredients that lead to superconductivity in these unconventional materials, including the ones that operate at higher temperatures."

Dueling Theories

According to the widely accepted theory known as BCS for the initials of its inventors, conventional superconductivity is triggered by natural vibrations that ripple through a material's atomic latticework. The vibrations cause carrier electrons to pair up and condense into a superfluid, which flows through the material with no resistance – a 100-percent-efficient electric current. In this picture, the ideal superconducting material contains a high density of fast-moving electrons, and even relatively weak lattice vibrations are enough to glue electron pairs together.

But outside the theory, in the realm of unconventional , no one knows what glues the electron pairs together, and none of the competing theories hold sway.

To find clues to what's going on inside strontium titanate, scientists had to figure out how to apply an important tool for studying superconducting behavior, known as tunneling spectroscopy, to this material. That took several years, said Harold Hwang, a professor at SLAC and Stanford and SIMES investigator.

"The desire to do this experiment has been there for decades, but it's been a technical challenge," he said. "This is, as far as I know, the first complete set of data coming out of a tunneling experiment on this material." Among other things, the team was able to observe how the material responded to doping, a commonly used process where electrons are added to a material to improve its electronic performance.

'Everything is Upside Down'

The tunneling measurements revealed that strontium titanate is the exact opposite of what you'd expect in a superconductor: Its are strong and its carrier electrons are few and slow.

"This is a system where everything is upside down," Hwang said.

On the other hand, details like the behavior and density of its electrons and the energy required to form the superconducting state match what you would expect from conventional BCS theory almost exactly, Swartz said.

"Thus, strontium titanate seems to be an unconventional superconductor that acts like a conventional one in some respects," he said. "This is quite a conundrum, and quite a surprise to us. We discovered something that was more confusing than we originally thought, which from a fundamental physics point of view is more profound."

He added, "If we can improve our understanding of superconductivity in this puzzling set of circumstances, we could potentially learn how to harvest the ingredients for realizing superconductivity at higher temperatures."

The next step, Swartz said, is to use tunneling spectroscopy to test a number of theoretical predictions about why acts the way it does.

Explore further: Propagating "charge density wave" fluctuations are seen in superconducting copper oxides for the first time

More information: Adrian G. Swartz et al. Polaronic behavior in a weak-coupling superconductor, Proceedings of the National Academy of Sciences (2018). DOI: 10.1073/pnas.1713916115

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7 comments

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dnatwork
2 / 5 (1) Mar 21, 2018
Maybe the crystals form domains where the few available electrons are concentrated on the edges, so they reach the required density. If the vibrations are strong, maybe at resonant frequencies (only attained or sustainable at ultra-cold temperatures) they form standing waves that generate or reinforce the domains.

They don't give the current density of strontium titanate compared to classical superconductors. If it's much lower, that would support the idea above, as current would only be able to flow through "stripes" in the material. I'm assuming that in a classical superconductor, current can flow through the bulk material.
Da Schneib
3 / 5 (2) Mar 21, 2018
Actually, in superconductors, it's critical current density that determines the current density, and this is only determined by the temperature, the state of any external magnetic fields that impinge upon the superconductor's outside surface, and the size of the wire. It's not affected by intrinsic properties of the superconducting material.

See here: http://www.open.e...tion-2.5
Mark_Goldes
not rated yet Mar 21, 2018
The current density of polymer ultraconductors is estimated at 1 Billion Amperes/cm2. They are in fact room temperature superconductors. We call them ultraconductors to avoid arguments. They function identically from a temperature of 3 Kelvin to 200 degrees C. Resistance measurements at the Ioffe Institute in St. Petersburg were approximately 1 ohm. The experiment involved a sandwich of Tin with the ultraconductors in the middle. The measurements were made when the Tin was superconducting at about 3 Kelvin. AESOP Energy is reviving the ultraconductor work. See that heading at aesopinstitute.org
Parsec
5 / 5 (2) Mar 21, 2018
@Mark - A resistance of 1 ohm means it's not a superconductor. In fact a resistance of anything except exactly zero means it's not a superconductor.

By definition.
Mark_Goldes
not rated yet Mar 21, 2018
This is why we call them ultraconductors. See the heading ULTRACONDUCTORS at aesopinstitute.org to learn more. Scroll down for technical information.
swordsman
not rated yet Mar 22, 2018
The electron movement in an atom or molecule is three dimensional. Therefore, there are at least two planes of movement. In addition, the electron movement can be analyzed in "eigenvector" form with exponents that can be nonlinear and time-variant. This is extremely complex and difficult to model. Experiments of this type can greatly help in develop an accurate model that may lead to predictions of the various actions and reactions. This is somewhat similar to my efforts over the past 20 years in the development of an atom model.
alexander2468
not rated yet Apr 03, 2018
An interesting academic exercise, the composition of super conductors is expelling the electrons to a thin circumferential layer increasing resistance with magnetic inductance reversing conductance. This was pointed out in Shrewsbury College by an electricity manger going round the colleges, in response to use of the application of this conductor generating electricity, a point he made, until solving currant and field expulsion super conductors are an interesting academic exercise, he said that in 1995, it still applies today.

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