In unconventional superconductors, electrons often exhibit a tendency towards spatial ordering within their atomic structure.
In high-temperature superconductors, this comes in the form of the electronic structure exhibiting a pronounced difference in the lattice-bound directions along which atoms are ordered.
Within these materials, this electronic activity in turn breaks the rotational symmetry of the crystal, a phase known as electronic "nematicity." This state co-exists with superconductivity and researchers have sought to better understand this novel electronic state.
Boston College Associate Professor of Physics Ilija Zeljkovic and an international team of researchers set out to reveal the atomic-scale signature of electronic nematic transition in Fe(Te,Se)鈥攁 class of materials known as iron chalcogenide superconductors鈥攊n a particularly formulated composition of the material where electronic nematicity may spatially change most rapidly or fluctuate over time.
Associate Professor of Physics Ilija Zeljkovic
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鈥攋ust before the material enters the nematic state鈥攅lectronic nematicity first appears in localized nanoscale regions, Zeljkovic and colleagues reported in the online edition of the journal聽Nature Physics.
In addition, the team discovered that in the same 45-percent selenium composition tiny amounts of 鈥渟train鈥濃�攁 stretching of the material along one direction鈥攐f 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 鈥減inned鈥� by structural disorder, which hinders superconductivity in particular regions of the material, said Zeljkovic.
鈥淚t 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,鈥� said Zeljkovic. 鈥淭his could be indicative of nematic 鈥榝luctuations鈥�, thought to enhance superconductivity near the nematic transition, becoming static and thus reducing superconducting properties locally.鈥�
Zeljkovic was joined on the project by 天美传媒app 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鈥檚 Zhejiang University.
Zeljkovic said the results indicate that a hidden quantum critical point鈥攁 sought-after benchmark at the transition between different states in matter at zero degrees Kelvin鈥攎ay 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