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Strange quantum effect found in an exotic superconductor

Researchers discovered a mysterious quantum effect that breaks a 60-year-old physics theorem.

Strange quantum effect found in an exotic superconductor

Red peaks show cobalt atoms added to an iron-based superconductor.

Princeton University
  • Princeton scientists lead an international team that discovered unusual behavior in iron-based superconductors.
  • The researchers observed how adding cobalt atoms disrupted superconductivity.
  • The experiment demonstrated unexpected quantum behavior.


An international team of researchers observed an unexpected quantum effect in an exotic superconductor. Their discovery can lead to the next generation of energy-saving technologies.

Traditional superconductors, used for conducting electricity without resistance, work at low temperatures. However, some iron-based superconductors discovered about a decade ago, work at high temperatures. How they do it has been unclear, especially as the magnetism of iron conflicts with the appearance of superconductivity, explains the press release from Princeton, whose scientists led the research.

Figuring out how these iron-based superconductors operate could open doors to new applications, prompting the focus of researchers. They probed these materials by adding impurities — atoms of cobalt — to see how superconductivity was created and dissipated. Introducing cobalt has been shown to make iron-based superconductors to lose the property of superconductivity. It starts acting like a regular metal, where electrical flow is met with resistance and loss of energy.

The team's leader, M. Zahid Hasan, Professor of Physics at Princeton University, likened their approach to throwing a stone in the water to see how water would react, pointing out "The way the superconducting properties react to the impurity reveals their secrets with quantum-level detail."

Their team employed a technique called scanning tunneling microscopy to image individual atoms in a superconductor made of lithium, iron and arsenic while they added in cobalt atoms. They were able to observe how introducing more cobalt atoms made superconductivity disappear.

What was surprising, the researchers found that the cobalt atoms were able to disrupt electron pairing while replacing iron atoms in the metal. This behavior, which resulted in a quantum phase transition, changing the state from superconducting to non-superconducting, also violated the well-established Anderson's theorem. Proposed in 1959 by the Nobel Prize-winning physicist Philip Anderson, it was the accepted explanation for what would happen if you added impurities to a superconductor. The new research clearly shows an exception to Anderson's theorem.

Another unusual find revealed that the cobalt impurities also transformed the nature and the shape of the so-called "energy gap" - a feature emblematic of superconductivity. The shape of the gap is indicative of the "order parameter" linked to the nature of the superconductivity. The effect on this property is mysterious and points to a sign change in the order parameter's phase.

From left to right: Graduate student Nana Shumiya, Professor M. Zahid Hasan, Postdoctoral Research Associate Jia-Xin Yin and Graduate Student Yuxiao Jiang. Photo by Zijia Cheng

Credit: Princeton University

Postdoctoral researcher Ilya Belopolski, the co-author of the study, explained the significance of the researchers' feat:

"Naively, distinguishing between conventional superconductivity and sign-changing superconductivity requires a phase-sensitive measurement of the superconducting order parameter, which can be extremely challenging," said Belopolski. "A beautiful aspect of our experiment is that by considering violations of Anderson's theorem, we can get around this requirement.

Check out the study published in Physical Review Letters.

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Quantum particles timed as they tunnel through a solid

A clever new study definitively measures how long it takes for quantum particles to pass through a barrier.

Image source: carlos castilla/Shutterstock
  • Quantum particles can tunnel through seemingly impassable barriers, popping up on the other side.
  • Quantum tunneling is not a new discovery, but there's a lot that's unknown about it.
  • By super-cooling rubidium particles, researchers use their spinning as a magnetic timer.

When it comes to weird behavior, there's nothing quite like the quantum world. On top of that world-class head scratcher entanglement, there's also quantum tunneling — the mysterious process in which particles somehow find their way through what should be impenetrable barriers.

Exactly why or even how quantum tunneling happens is unknown: Do particles just pop over to the other side instantaneously in the same way entangled particles interact? Or do they progressively tunnel through? Previous research has been conflicting.

That quantum tunneling occurs has not been a matter of debate since it was discovered in the 1920s. When IBM famously wrote their name on a nickel substrate using 35 xenon atoms, they used a scanning tunneling microscope to see what they were doing. And tunnel diodes are fast-switching semiconductors that derive their negative resistance from quantum tunneling.

Nonetheless, "Quantum tunneling is one of the most puzzling of quantum phenomena," says Aephraim Steinberg of the Quantum Information Science Program at Canadian Institute for Advanced Research in Toronto to Live Science. Speaking with Scientific American he explains, "It's as though the particle dug a tunnel under the hill and appeared on the other."

Steinberg is a co-author of a study just published in the journal Nature that presents a series of clever experiments that allowed researchers to measure the amount of time it takes tunneling particles to find their way through a barrier. "And it is fantastic that we're now able to actually study it in this way."

Frozen rubidium atoms

Image source: Viktoriia Debopre/Shutterstock/Big Think

One of the difficulties in ascertaining the time it takes for tunneling to occur is knowing precisely when it's begun and when it's finished. The authors of the new study solved this by devising a system based on particles' precession.

Subatomic particles all have magnetic qualities, and they spin, or "precess," like a top when they encounter an external magnetic field. With this in mind, the authors of the study decided to construct a barrier with a magnetic field, causing any particles passing through it to precess as they did so. They wouldn't precess before entering the field or after, so by observing and timing the duration of the particles' precession, the researchers could definitively identify the length of time it took them to tunnel through the barrier.

To construct their barrier, the scientists cooled about 8,000 rubidium atoms to a billionth of a degree above absolute zero. In this state, they form a Bose-Einstein condensate, AKA the fifth-known form of matter. When in this state, atoms slow down and can be clumped together rather than flying around independently at high speeds. (We've written before about a Bose-Einstein experiment in space.)

Using a laser, the researchers pusehd about 2,000 rubidium atoms together in a barrier about 1.3 micrometers thick, endowing it with a pseudo-magnetic field. Compared to a single rubidium atom, this is a very thick wall, comparable to a half a mile deep if you yourself were a foot thick.

With the wall prepared, a second laser nudged individual rubidium atoms toward it. Most of the atoms simply bounced off the barrier, but about 3% of them went right through as hoped. Precise measurement of their precession produced the result: It took them 0.61 milliseconds to get through.

Reactions to the study

Scientists not involved in the research find its results compelling.

"This is a beautiful experiment," according to Igor Litvinyuk of Griffith University in Australia. "Just to do it is a heroic effort." Drew Alton of Augustana University, in South Dakota tells Live Science, "The experiment is a breathtaking technical achievement."

What makes the researchers' results so exceptional is their unambiguity. Says Chad Orzel at Union College in New York, "Their experiment is ingeniously constructed to make it difficult to interpret as anything other than what they say." He calls the research, "one of the best examples you'll see of a thought experiment made real." Litvinyuk agrees: "I see no holes in this."

As for the researchers themselves, enhancements to their experimental apparatus are underway to help them learn more. "We're working on a new measurement where we make the barrier thicker," Steinberg said. In addition, there's also the interesting question of whether or not that 0.61-millisecond trip occurs at a steady rate: "It will be very interesting to see if the atoms' speed is constant or not."

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