Since the introduction of high-transition-temperature superconductors, particularly cuprate superconductors, that were introduced into the market in 1986, Researchers have continually looked at this powerfully thermal-resistant material. It is a material with a wide variety of large scale power applications such as magnetic energy-storage devices, fault current limiters, motors and other relating devices.
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As three decades of research has looked towards developing a superconductor structure that is similar to that of cuprates, a group of Researchers led by John Mitchell from the U.S. Department of Energy’s (DOE) Argonne National Laboratory have successfully identified a nickel oxide compound that is a promising compound for such an endeavor.
Cuprate superconductors, comprised of copper oxides, have emerged as an intriguing electronic system as a result of its extremely high boiling temperature that exceeds that of liquid nitrogen, as well as its unique crystalline structure.
Equipped with a high-transition-temperature (high-Tc) superconductivity, cuprate superconductors exhibit a quasi-two dimensional (quasi-2D) square crystal lattice, strong antiferromagnetic correlations that appear as a result of doping electrons or holes into the CuO2 planes, and a strong p-d hybridization1.
As compared to traditional superconductors that have transition temperatures that are usually found to measure below 30 Kelvin (K), high-Tc superconductors can maintain their superconductive nature with temperatures as high as 138 K.
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The advantage of such high-temperature superconductors allow for a faster and more efficient mode of electricity to flow through devices, thereby reducing the amount of energy lost, which is particularly important for large-scale applications of these materials.
As a result of its proximity to copper on the periodic table, nickel-based oxides have been a topic of consideration for applications as such potential high-Tc superconductors. In fact, Researchers have investigated the potential of various nickel oxides such as LaNiO3 and LaMO3 to be incorporated as models for cuprate superconductors as early as June of 19982.
While this early work concluded that the Ni 3d and O 2p orbitals resulted in a weak energy connection to exist between the nickelates as compared to the cuprate counterparts, Mitchell’s team utilized R4Ni3O8, a tri-layer nickeleate where R represents La or Pr, which has been shortened to R438 for the purposes of this study. This tri-layer quasi-2D nickelate structure closely resembled the structure of cuprate superconductors, as a high degree of orbital polarization exists between the Ni 3d and O 2p states.
To create the unique structure of crystals present within this nickelate material, the Argonne lab utilized a high-pressure optical-image floating zone furnace capable of attaining pressures of up to 150 atmospheres, which is equivalent to pressures found at the 5,000 feet below sea level, as well as approximately 2,000 °C.
By combining X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations, the Argonne team confirmed the tri-layer quasi-2D structure of the nickelate material as three consecutive layers of corner-sharing NiO2, with alternating square planes of R2O2 fluorite-type layers found along the c axis3.
The use of lanthanum (La) in previous nickel oxide high-Tc superconductors created a non-metallic analog that creates a “charge-stripe” phase to the superconductor, thereby converting the material into an insulator which opposes the desired superconductor forces. The praseodymium (Pr) addition in the newly developed nickel oxide superconductor Pr4Ni3O8 avoids this charge-striped ordered phase.
Further XAS measurements showed that the metallic Pr4Ni3O8 superconductor exhibited a low-spin configuration with significant orbital polarization, which is comparable to that which is present in cuprate superconductors.
With its 3d electron count, the Pr4Ni3O8 superconductor was determined to be the closest nickelate analog to traditional cuprate superconductors, which therefore allows this material to be a promising candidate for potential high-Tc superconductors to be developed in the future.
References:
- “Cuprate Superconductors” – Stanford Shen Laboratory
- “Electronic structure of possible nickelate analogs to the cuprates” V. I. Anismov, D. Bukhvalov, et al. American Physical Society. (1998). DOI: 10.1103/PhysRevB.59.7901.
- “Large orbital polarization in a metallic square-planar nickelate” J. Zhang, A. S. Botana, et al. Nature Physics. (2017). DOI: 10.1038/nphys4149.
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