Solid materials, including metals, semiconductors and insulators, are extremely complex to study because they are comprised of many billions of interacting particles. Scientists studying the basic properties of materials must take an integrated approach: synthesizing materials, probing materials using various experimental methods and developing theoretical approaches to elucidating the experimental results.
Materials discovery and refinement are key drivers of progress in quantum materials experimentation. The quality of a given experimental study depends strongly on a material’s purity, crystal size and nanostructure, or a combination of these characteristics.
An exciting new class of materials, called quantum materials, are attracting great interest from the scientific community. In these materials the collective behavior of electrons leads to many novel, and often unexpected, properties, such as transport of electrical current without resistance and the creation of emergent particles with properties different from any known elementary particle. New discoveries in quantum materials could lead to revolutionary applications in electronics, computing, energy technology and medical devices. Through our Emergent Phenomena in Quantum Systems Initiative (EPiQS), we are enabling scientists to advance knowledge about quantum materials and to develop new materials with unique properties.
“By supporting an integrated research program in materials synthesis, experimentation and theory, the Moore Foundation aims to help scientists answer long-standing questions about how complex quantum matter organizes and behaves,” said Dusan Pejakovic, Ph.D., director of the foundation’s Emergent Phenomena in Quantum Systems Initiative. “Investigations of quantum materials not only deepen our fundamental understanding of the world around us but may eventually lead to important, currently unanticipated, technological applications.”
Accelerating our understanding of superconductors
In an illustrative example of how experts in materials synthesis and experimental research working together can fast-track insight into quantum materials, EPiQS materials synthesis investigator Paul Canfield and experimental investigator J.C. Seamus Davis demonstrated evidence for a new type of electron pairing in an iron-selenium compound. Electron pairing is a low-temperature phenomenon that allows electrical current to flow unimpeded through materials, resulting in a ‘superconducting’ state.
Canfield and Davis’ findings, published in Science, provide new insights into the origin of high-temperature superconductivity in iron-based compounds that may help guide the search for materials that exhibit superconductivity at even higher temperatures.
Currently, most superconducting materials require cooling to extremely low temperatures to start behaving as superconductors. For example, the iron-based compound studied by Canfield and Davis has a superconducting transition temperature of about -443°F. Transition temperatures at or above room temperature (about 70°F) would allow superconductors to be used widely in technological applications.
Using high-quality samples from Canfield’s group at Iowa State University, Davis’s team at Cornell University was able to perform an extraordinarily detailed mapping of the electronic signatures in the material and compare them with theoretical calculations.
“For the first time, we can measure the binding energy and momentum of electrons in the ‘Cooper pairs’ responsible for superconductivity and identify which energy-momentum characteristics they have—which orbital they’re from,” said J.C. Seamus Davis, professor of physical sciences at Cornell University.
Using scanning tunneling microscopy, a technique that captures both subatomic-resolution images of the surface of a material and its electronic properties at each point in space, the experimental team led by Davis managed to identify which atomic orbitals contain electrons most likely to form Cooper pairs. Their state-of-the-art, custom-built microscope scans across a material’s surface in steps as small as a few trillionths of a meter, measuring the electron tunneling current at each location. Tunneling is a quantum mechanical effect in which electrons express “wave-like” behavior and leak through barriers in a system. Monitoring the tunneling current during a scanning tunneling microscopy scan helps yield an image of a material’s surface.
The team’s findings may guide the search for new superconductors that can operate at conditions closer to room temperature, and pave the path for real-world, energy-saving applications such as power lines or energy storage devices.
Bringing elusive particles to the surface
A research team led by scientists at Oak Ridge National Laboratory and University of Tennessee confirmed magnetic signatures likely related to elusive particles called Majorana fermions. First theorized in 1937 by physicist Ettore Majorana, these particles—unlike electrons or protons who have antiparticle counterparts with equal but opposite charges—are their own antiparticle and have no charge. And, unlike electrons and protons, they do not exist outside of the material.
Majorana fermions could be the basis for a quantum bit in quantum computers. By synthesizing a large, high-quality single crystal of the material alpha-ruthenium trichloride, Moore Foundation grantee David Mandrus and colleagues were able to observe directly the quantum nature of this system using neutron scattering experiments. Mandrus is an EPiQS materials synthesis investigator and a professor of materials science and engineering at University of Tennessee.
The findings, published in Science, reveal in unprecedented detail new insights into an unusual magnetic state called a quantum spin liquid. In this state, the magnetic moments, or spins, associated with electrons are disordered, like molecules in a liquid, but at the same time strongly interconnected—they share the same quantum state. One consequence of this behavior is the occurrence of ‘emergent’ Majorana fermions in the system.
“If we can understand these magnetic excitations in detail, then we will be one step closer to finding a material that would enable us to pursue the ultimate dream of quantum computation,” said lead author Arnab Banerjee, a postdoctoral researcher at Oak Ridge National Laboratory.
Changing behavior of electrons
In another study, EPiQS experimental investigator Pablo Jarillo-Herrero and colleagues at Massachusetts Institute of Technology found that a flake of graphene – a sheet of carbon only one atom thick – when brought in close proximity with two superconducting materials, can inherit some of those materials’ superconducting qualities. When graphene is sandwiched between superconductors, its electronic state changes dramatically.
Jarillo-Herrero and his colleagues found that electrons in graphene, which normally behave as individual, scattering particles, instead pair up with holes (the absence of electrons in an orbital) in so-called Andreev states when they are placed in proximity to superconducting materials. These states allow graphene to carry an electric current that flows without dissipating energy. Andreev states have been observed only in a handful of systems, such as silver wires, and never in a two-dimensional material such as graphene.
To study this effect, the team of researchers synthesized a hybrid nanostructure by exfoliating a very thin flake of graphene, just a few hundred nanometers wide, from a larger chunk of graphite. They placed this flake on a small platform made from a crystal of boron nitride overlaying a sheet of graphene. On either end of the graphene flake, they placed an electrode made from aluminum, which becomes a superconductor at low temperatures.
Their findings, published in Nature Physics, are the first investigation of Andreev states in a two-dimensional material. Down the road, the researchers’ graphene ‘sandwich’ platform may be another way to explore Majorana fermions, which are thought to sometimes arise from Andreev states.
“Our strategy is to provide support to a group of outstanding scientists to pursue discovery-driven research without many constraints,” added Pejakovic. “We aim to strengthen the bond between materials synthesis, theory and experimental efforts at leading academic institutions. These connections generate a synergistic effect that helps accelerate materials discovery for a wide range of potential applications.”