Spotty superconductivity – Boston College Chronicle

A focus of researchers trying to understand superconducting properties, iron chalcogenides are defined by their composition from varying percentages of sulfur, selenium, and tellerium. For their experiments, the team created compound samples containing between 35 to 50 percent selenium, ultimately finding that a 45-percent selenium construct revealed electronic nematicity that is spatially inhomogeneous, or failing to occur equally at each point in the material, Zeljkovic said.

Using low-temperature spectroscopic-imaging scanning tunneling microscopy (STM), the team found that at the transition point—just before the material enters the nematic state—electronic nematicity first appears in localized nanoscale regions, Zeljkovic and colleagues reported in the online edition of the journal Nature Physics.

In addition, the team discovered that in the same 45-percent selenium composition tiny amounts of “strain”—a stretching of the material along one direction—of just a fraction of a percent can lead to the appearance of local nematicity, which in turn suppresses superconductivity. This was not the case for Fe(Te,Se) samples constructed at a lower Se composition of 35 percent, which show negligible effects on superconductivity from the same amounts of strain.

The team found that in certain compositions of Fe(Te,Se) the nematic fluctuations can be “pinned” by structural disorder, which hinders superconductivity in particular regions of the material, said Zeljkovic.

“It was surprising that nematic regions appear to be not superconducting at all, despite the fact that the superconducting transition temperature should be the highest at the 45-percent composition,” said Zeljkovic. “This could be indicative of nematic ‘fluctuations’, thought to enhance superconductivity near the nematic transition, becoming static and thus reducing superconducting properties locally.”

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Zeljkovic was joined on the project by BC Professor of Physics Ziqiang Wang, and researchers He Zhao and Hong Li; as well as scientists from the University of California, Santa Barbara, Brookhaven National Laboratory, and China’s Zhejiang University.

Zeljkovic said the results indicate that a hidden quantum critical point—a sought-after benchmark at the transition between different states in matter at zero degrees Kelvin—may exist in Fe(Te,Se). He said further research into the material would be required to determine if that is the case.

Ed Hayward | University Communications | August 2021



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