Scientists studying materials that exhibit quantum properties, such as superconductivity, would like to be able to manipulate those properties, both to better understand them and to create useful materials, including superconductors that work at temperatures well above absolute zero. They’ve been able to tweak the properties by applying pressure or magnetic fields to the materials or by doping them with other elements or electrons. Now a group of researchers using the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) have shown that severely deforming materials provides yet another way to control their quantum characteristics. Their results were published in the journal Nature Materials.
The group worked with electron-doped strontium titanate (STO), a ceramic that could prove useful in energy applications. Scientists have known for decades that STO can be superconducting, but do not really understand why. The team placed STO crystals in a pressure cell and irreversibly compressed them by up to a remarkable 8% at room temperature. This plastic deformation changed the atomic structure of the crystals. Evidence of the deformation was visible on the crystals’ surfaces, showing as streaks running perpendicular to the direction of the stress. The deformed material showed superconductivity at a higher temperature than the superconducting transition temperature, Tc, of STO that had not been deformed. In fact, the Tc of the deformed ceramic was as high as 0.8 K, nearly three times the 0.3 K of the non-deformed crystal. The scientists also found initial evidence of superconductivity at even higher temperatures up to about 50 K.
To understand how the changes in superconductivity related to the deformation of the material, the researchers performed x-ray diffuse scattering measurements at the X-ray Science Division Magnetic Materials Group’s x-ray beamline 6-ID-D of the APS, a DOE Office of Science user facility at Argonne National Laboratory. The results (Fig. 1) showed patterns of atomic dislocations in the crystalline structure of the material. Those dislocations, which form to accommodate the strain induced by the compression of the material, self-organize into dislocation walls, planar structures within the crystal. Those dislocation walls alter the local electronic property of the material, affecting not only superconductivity, but also ferroelectricity, an electrical polarization that can be reversed by applying an electric field.
Raman scattering experiments showed clear evidence of ferroelectricity induced by the deformation. The team also performed diffuse neutron scattering measurements using the CORELLI spectrometer of the Spallation Neutron Source at the DOE’s Oak Ridge National Laboratory. Neutrons tend to be more sensitive to the location of lighter atoms, such as oxygen, in the modified crystalline structure, while x-rays are better at discovering the location of heavier atoms, so the two experiments provide complementary information. In short, changes in the physical structure of the STO crystals were accompanied by changes in its electrical properties.
The researchers were able to easily deform the material at room temperature, but heating the crystals and applying pressure might lead to different changes in the structure. The scientists would also like to apply the method to other materials that show quantum properties to measure the effects of deformation in those. The researchers also plan to perform the deformation in situ, while the crystals are in the x-ray beam, in order to see in real time how the atomic structure changes.
This work opens a new range of parameters for researchers to explore in order to tune the properties of materials. Working with a single material, they can deform it in different ways, at different temperatures, and at different rates, and see how that creates different outcomes, both structurally and electronically. ― Neil Savage
See: S. Hameed1, D. Pelc1,2*, Z. W. Anderson1, A. Klein3, R. J. Spieker1, L. Yue4, B. Das1, J. Ramberger1, M. Lukas2, Y. Liu5, M. J. Krogstad6, R. Osborn6, Y. Li4, C. Leighton1, R. M. Fernandes1 and M. Greven1**, “Enhanced superconductivity and ferroelectric quantum criticality in plastically deformed strontium titanate,” Nat. Mater. 21, 54 (January 2022). DOI: 10.1038/s41563-021-01102-3
Author affiliations: 1University of Minnesota, 2University of Zagreb, 3Ariel University, 4Peking University, 5Oak Ridge National Laboratory, 6Argonne National Laboratory
Correspondence: * dpelc@phy.hr, ** greven@umn.edu
The authors thank D. Robinson and S. Rosenkranz for assistance with x-ray scattering experiments. The work at the University of Minnesota was funded by the U.S. Department of Energy (DOE) through the University of Minnesota Center for Quantum Materials, under grant number DE-SC-0016371. The work at Argonne was supported by the U.S. DOE of Science-Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research used resources at the Spallation Neutron Source, a U.S. DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. D.P. acknowledges support from the Croatian Science Foundation through grant number UIP-2020-02-9494. The work at Peking University was funded by the National Natural Science Foundation of China, under grant number 11874069. Sputtering and contacting of samples was conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network, award number NNCI-1542202. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory.
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