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John O | July 2017

Researchers use magnetic tremors to reveal superconductivity in 2-D material


Researchers from the University of Maryland (UMD), University of California at Irvine (UCI) and Fudan University (China) have demonstrated that tiny magnetic tremors can reveal superconductivity in a 2-D material composed of metallic nano-layers, in this case nickel-bismuth.

 


This research could advance the production of quantum computers. (Wikimedia Commons)

 

A report from the University of Maryland’s Joint Quantum Institute explained, “In quantum materials, breaking the symmetry between the past and the future often signifies unconventional phases of matter. The nickel-bismuth (Ni-Bi) sample studied here is the first example of a 2D material where this type of superconductivity is intrinsic, meaning that it happens without the help of external agents, such as a nearby superconductor.”

 

This research indicates that nickel-bismuth is a potential choice for the quantum computers of the future and the scientists hope that this will lead to further research into the intrinsic nature of 2-D materials.

 

The article continued, “Bismuth alone is not a superconductor, except under extraordinarily low temperatures and high pressure—conditions that are not easy to achieve. Nickel is magnetic and not a superconductor. In fact, strong magnets are known to suppress the effect. This means that too much nickel destroys the superconductivity, but a small amount induces it.”

 

UMD researchers proposed that it was nickel’s magnetism that was causing tiny tremors that caused the electrons to form pairs and thus provide superconductivity. But, there has to be the right amount of each material because too much magnetism reduces the effect of the tremors and too much bismuth then the top layer (where the superconductivity occurs) is too far away from the source of the tremors.

 

“The goldilocks zone occurs when a 20-nanometer-thick bismuth layer is grown on top of two nanometers of nickel,” the article said. “For this layer combination, superconductivity happens at around four degrees above absolute zero. While this is about as cold as deep space, it is actually quite lab-friendly and reachable using standard cryogenic equipment.”

 

Using a novel apparatus at UCI, researchers were able to observe broken time symmetry in the material.

 

“In order to measure this rotation for Ni-Bi, light waves are first injected into one end of a single special-purpose optical fiber,” the article continued. “The two waves travel through the fiber, as if on independent paths. They hit the sample and then retrace their paths. Upon return, the waves are combined and form a pattern. Rotations of the light waves—from, say, symmetry breaking—will show up in the analyzed pattern as small translations.”

 

Researchers observed nearly 100 nanoradians of rotation, which confirmed the broken symmetry, and also observed the broken symmetry at the same time as the material gained superconductivity suggesting a link between the two.

 

The research was recently published in Science Advances. The abstract stated:

 

“Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of three-dimensional (3D) crystals with bulk inversion symmetry. We report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel.

 

“The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer.

 

“Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the  orbital symmetry between the electrons in Bi.

 

“In this model, the order parameter spontaneously breaks the TRS and has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.”

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