The onset of superconductivity in nearly all high-temperature superconductors is a complex phenomenon that is linked to the appearance of certain electronic and magnetic orderings. More specifically, as the temperature of these materials is lowered, their electrons arrange into a nematic ordering reminiscent of that found in liquid crystal displays. Similarly, an antiferromagnetic ordering also typically arises at low temperatures. It appears that superconductivity in these compounds can be suppressed or enhanced by the strength of their nematic electron ordering and magnetism. In this study researchers set out to vary the nematic electron ordering in an iron-based superconductor by squeezing and stretching it, during experiments performed at low temperatures. The induced stresses caused distortions in the material's crystalline lattice that were precisely measured by x-ray diffraction experiments performed at the U.S. Department of Energy’s Advanced Photon Source (APS). The experiments demonstrated the capability to dramatically promote or suppress superconductivity by applying small amounts of strain. The ability to vary a material's superconductivity using purely mechanical means is an important advance in the effort to harness superconductivity for use in a wide variety of applications.

The discovery of superconductivity goes back to 1911, when the electrical resistance of mercury was found to disappear at a temperature around 4 kelvin (4 K). Eventually other elements and compounds were found to superconduct, almost always at temperatures below 30 K. In the 1950s, the Bardeen–Cooper–Schrieffer (BCS) theory was introduced, becoming the first successful quantum theory of superconductivity.

In 1986, the first high-temperature superconductor was discovered. This copper-based (cuprate) ceramic exhibits superconductivity at a temperature above 30 K. Scientists soon synthesized similar cuprates that superconduct at 90 K and above. Later, an entire class of iron-based, high-temperature superconductors was developed. Generally speaking, the behavior of these high-temperature superconductors cannot be described by the BCS theory, so deciphering how they superconduct is currently one of the outstanding challenges in physics and materials science.

Compounds that host high-temperature superconductivity often undergo symmetry breaking at low temperatures. To illustrate this effect, consider a cube with six identical faces. If the cube initially lies on one of its faces, it looks exactly the same after turning it onto a different face. This is called rotational symmetry. Now imagine that one face bulges outwards. Rotating the cube can point the bulge in a different direction. The cube's rotational symmetry has been lost.

This is analogous to what happens in a high-temperature superconductor as its temperature is lowered. The electrons begin to interact and align with one another, forming a nematic ordering that distorts the superconductor's crystalline lattice which eliminates rotational symmetry. Some scientists propose that a fluctuating (partially-formed) nematic order allows a material's superconductivity to occur at higher temperatures. Conversely, when the electrons align in a full nematic ordering the superconducting state is retarded, and superconductivity only occurs at much lower temperatures.

To test the relationship between nematic ordering and superconductivity, the researchers from the University of Washington and the University of Tennessee prepared crystalline samples of an iron-based superconductor composed of iron, barium, and arsenic. Some iron was replaced with cobalt to add more electrons to the compound, which facilitates the superconducting state. The exchange of cobalt for iron is called doping, and some samples were more highly doped (more cobalt was added) than others.

Samples were placed in a custom-built device (Fig. 1a) that could compress or expand them while the temperature was lowered towards absolute zero. The exact change in each sample's crystalline structure was determined via x-ray diffraction performed with colleagues from Argonne National Laboratory using the X-ray Science Division Magnetic Materials Group‘s 6-ID-B beamline of the APS, an Office of Science user facility at Argonne. The experiments revealed that both under-doped and optimally-doped samples placed under stress required dramatically lower temperatures to achieve superconductivity as compared to unstressed samples (Fig. 1b,c)

These results have important theoretical implications. Altering a sample's lattice by applying force effectively enhanced its nematic ordering, which required lower temperatures to achieve superconductivity. This suggests that the appearance of nematic fluctuations promotes the formation of the superconducting state, a finding that is vital to understanding high-temperature superconductivity. The results also demonstrate control of superconductivity via mechanical stress.

It should additionally be noted that even though the samples were small (1 to 2 millimeters in length) their volumes were nonetheless sufficient to constitute a truly three-dimensional system, in contrast to the two-dimensional thin films typically used for studying superconductivity. The capability to probe 3D samples will allow investigation of the thermodynamics and other aspects of this type of phase transition. ― Philip Koth

See: Paul Malinowski¹, Qianni Jiang¹, Joshua J. Sanchez¹, Joshua Mutch¹, Zhaoyu Liu¹, Preston Went¹, Jian Liu², Philip J. Ryan³, Jong-Woo Kim³, and Jiun-Haw Chu¹*, “Suppression of superconductivity by anisotropic strain near a nematic quantum critical point,” Nat. Phys. 16, 1189 (December 2020). DOI: 10.1038/s41567-020-0983-9

Author affiliations: ¹University of Washington, ²University of Tennessee, ³Argonne National Laboratory

Correspondence: * jhchu@uw.edu

This work was mainly supported by National Science Foundation (NSF) MRSEC at UW (DMR-1719797) and the Gordon and Betty Moore Foundation’s EPiQS Initiative, grant GBMF6759 to J.-H.C. The development of strain instrumentation is supported as part of Programmable Quantum Materials,  an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE)  Office of Science-Basic Energy Sciences under award DE-SC0019443. The integration of x-ray diffraction with in situ strain is supported by the Air Force Office of Scientific Research Young Investigator Program under grant FA9550-17-1-0217 and the Defense University Research Instrumentation Program Award FA9550-19-1-0180.  J.L. acknowledges support from the NSF under grant no. DMR- 1848269. J.-H.C. acknowledges the support of the David and Lucile Packard Foundation and the State of Washington-funded Clean Energy Institute. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

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