Image courtesy of Sophia Zhao
If science aims to explain the structures of the natural world, picoscience refines the very structures themselves—at the atomic level. Engineering at the scale of picometers operates on a trillionth-of-a-meter scale, dealing with materials that can be roughly ten million times smaller than a human cell.
The research groups of Charles Ahn and Sohrab Ismail-Beigi at Yale have ventured into picoscience by manipulating bond configurations between individual atoms, thereby altering and engineering the element’s behavior in solid state. A recently published study—in collaboration with researchers from Flatiron Institute, Brookhaven National Laboratory, and Argonne National Laboratory—reported the successful incorporation of cobalt and titanium within an artificial crystal and the resulting perturbations in electronic behavior. This discovery heralds a future in which picoscience may be used to develop more efficient superconductors.
The Potential of Superconductors
If you have ever undergone a magnetic resonance imaging (MRI) test, or used a wireless charger, you have seen a superconductor at work. A superconductor is any material that has zero electrical resistance when it is cooled below a critical temperature, allowing electric current to pass through with no energy loss. In contrast, conventional conductors such as copper wire impose resistance on electrons travelling through them, leading to energy loss in the form of heat. Superconductors, with their perfect energy transfer efficiency, allow for cheaper and more efficient methods of electricity transport; the only catch is that large amounts of energy have to be invested into cooling them down in the first place. Early superconducting materials could only operate at temperatures near –269 °C, the boiling point of helium. Since liquid helium as a coolant is expensive to generate and store, the potential of superconductors was limited.
In 1986, materials derived from copper—known as cuprates—were discovered to possess superconductivity above -183 °C, a temperature that could be maintained by liquid nitrogen, a cheap and readily available coolant. The development of these high-temperature superconductors (HTS), which would go on to win a Nobel prize, ignited a rush to understand the exact mechanism behind superconductivity, in the hopes of developing superconducting materials that operate at even higher temperatures. Ahn’s research group aims to tackle both goals by designing materials that possess electronic properties similar to cuprate-based HTS. “We try to understand, from a fundamental viewpoint, why cuprates superconduct. To do that, we try to design from basic principles a new high temperature superconductor,” said Frederick Walker, a senior research scientist and co-author of the study.
Rather than focusing on the tried-and-true formula of cuprate materials, the researchers turned to cobaltate-based materials, which were anticipated to exhibit cuprate-like properties. “A lot of work was done by others and our group to realize something like cuprates—something very similar to copper but different enough to realize similar or better properties. Our group used to work with nickelates, but as a continuous effort, we moved onto cobalt as well,” said Sangjae Lee, first author of the study and graduate student in the Ahn group. The goal, therefore, was to artificially manipulate cobaltate-based materials to behave like that of cuprate. If this behavior was possible, then the researchers would be one step closer to realizing the properties responsible for cuprates’ exceptional superconductivity.
Altering the electronic structure of cobalt to mimic that of copper was no easy feat. An atom’s electronic structure is its defining feature, a sort of “atomic DNA.” Considering that atoms do not spontaneously convert between elements, it stands to reason that a large amount of energy—or ingenuity—would be required to alter the electronic properties of cobalt. “The orbital polarization seen in cuprate is thought to be a significant ingredient for superconductivity. We alter [cobalt’s] electronic structure, which determines how it behaves and superconducts. In some sense, it’s alchemy,” Walker said.
The researchers tackled this predicament by arranging atoms of one type alongside another species in a crystal lattice. When two different elements of different sizes are layered in a lattice structure, they compress and stretch, distorting the shape of the atomic orbitals and by extension, the distribution of electrons within them. The researchers aimed to manipulate the electronic properties of cobalt in this fashion—by engineering a heterostructure containing cobalt and a functionally different element, titanium. To grow the heterostructure, the researchers used a materials fabrication process called molecular beam epitaxy (MBE), which enabled them to precisely layer alternating one-atom-thick sheets of titanium oxide and cobalt oxide. The MBE process enables precise synthesis of the heterostructure by preventing intermixing of the layers with a high vacuum environment, temperature control, and computerized shutters that control the thickness of each layer.
After growing the cobaltate-titanate oxide heterostructure, the researchers characterized its structure at the atomic scale using a high-resolution synchrotron, which uses bright X-rays to visualize the atomic-scale structure and electronic properties of a material. Using spectroscopic and electron microscopy techniques, the researchers confirmed that they had assembled distinct layers of cobaltate and titanate with little intermixing. The collected data indicated unprecedented strong distortion of orbitals in the cobaltate layer, confirming the electronic structure of cobaltate within a heterostructure. In other words, they successfully made cobalt behave like copper.
This success was in large part supported and enabled by the work of theoretical physicists, who, throughout the process, had been using quantum computations to predict and corroborate experimental electronic properties. “The structure of these materials, like where the atoms are, how long the bonds are, and the distortions at the picometer scale, is something our theory, in principle, could calculate correctly. Most of what [the experimentalists did] agreed with the theory, but some of the theoretical predictions are hard to measure, and some of the experimental results we don’t understand,” said Sohrab Ismail-Beigi, the lead theorist of the study. Despite a few discrepancies between theory and experimental results, preliminary theoretical calculations provided a roadmap for designing the heterostructure. According to Ismail-Beigi, the theorists and experimentalists work together in a “closed loop” to generate ideas, build on previous data, and minimize deviations. “It doesn’t matter who suggests an idea first—sometimes an experimentalist creates a system and measures it, and sometimes, we’ll notice something in the theory and suggest it to the experimentalists,” Ismail-Beigi said.
Picoscience: The Final Frontier
Considered a forerunner of picoscience, nanoscience has led to tremendous progress and enabled technologies such as unmanned drones, 3D printers, and biomolecular imaging. Picoscience, which operates at a much smaller scale, is for now the final frontier for condensed matter physics. “All the properties of atoms stem from how electrons behave. Electronic behavior is sensitive to even very small [picoscale] distortions in the material, and how these kinds of distortions create or spur interesting physics needs to be understood in the regime of picoscience,” Lee said.
The successful design of the cobaltate-titanate oxide heterostructure serves as a case study for the potential of picotechnology in discovering techniques to alter properties of known elements. In particular, transition metal oxide (TMO) systems remain a primary area of interest for the researchers for their dynamic electronic properties. “TMOs have really interesting electronic motion between atoms, which makes them unusual superconductors. These very difficult properties to model also make them interesting,” Ismail-Beigi said. With the goal of realizing higher temperature superconductors in mind, the researchers hope to continue manipulating TMO systems at the picoscale level. “We have an idea as to how we can make the cobaltates superconduct and how we might build on this work. We’re [tackling] one of the premier problems in condensed matter physics: how do you make a superconductor at higher temperatures?”
Interviews with Sangjae Lee and Dr. Frederick J. Walker, November 5, 2019.
Interview with Dr. Sohrab Ismail-Beigi, November 8, 2019.
Sangjae Lee, Alex Taekyung Lee, Alexandru B. Georgescu, Gilberto Fabbris, Myung-Geun Han, Yimei Zhu, John W. Freeland, Ankit S. Disa, Yichen Jia, Mark P. M. Dean, Frederick J. Walker, Sohrab Ismail-Beigi, Charles H. Ahn. Strong Orbital Polarization in a Cobaltate-Titanate Oxide Heterostructure. Physical Review Letters, 2019; 123 (11)