For decades, physicists have appreciated the power of sandwiching layers of different substances to create materials with novel properties. Such sandwiches, called heterostructures because they are not edible, are essential components in a wide range of modern technologies. Heterostructure materials are used in various products relying on transistors, from supercomputers to cell phones, and are crucial materials for such devices as electronic sensors and solar cells.
Nature provides about 90 different atoms and a huge number of compounds made from them. But even the vast range of natural substances does not always offer the precise combination of properties needed for a specific technological task. Heterostructures’ usefulness stems from their ability to exhibit properties and perform feats that homogeneous substances — the elements and compounds provided by ordinary chemistry — cannot achieve on their own.
Magnetic, electric, optical and electronic properties of a substance generally depend on its arrangement of atoms and their electrons. Layering materials into sandwiches creates novel atomic positions and electron arrangements that engineers can exploit for technological purposes.
Traditionally, heterostructure materials have been built from layers of standard insulators, semiconductors and metals composed of such elements as silicon, gallium and aluminum. But in recent years physicists have turned to a new heterostructure strategy: building sandwiches that incorporate layers of “quantum matter.”
The quantum arena
Quantum matter heterostructures are opening a new arena of solid state physics, Hans Boschker and Jochen Mannhart write in this year’s Annual Review of Condensed Matter Physics. “Unprecedented effects” can be achieved by stacking layers of quantum matter, they write, and “the phenomena thus induced are unforeseeable in their breadth and complexity.”
“Quantum matter” might seem at first a redundant label — all matter is ultimately “quantum,” composed of particles that form atoms and molecules obeying the bizarre rules of quantum mechanics. But physicists restrict the term “quantum matter” to substances in which odd quantum effects are observable on large scales. One such quantum effect has been known since ancient times: magnetism. Elements such as iron and nickel are examples of quantum matter because their magnetic ability depends on quantum properties of the electrons orbiting their atomic nuclei.
In the twentieth century, scientists found other observable properties rooted in quantum effects, such as superconductivity and superfluidity. Superconductors transmit electric current without resistance; superfluids exhibit weird flowing abilities. Both are inexplicable without invoking quantum processes. Usually such phenomena are observed only at very low temperatures — near absolute zero — but some materials, such as cuprate oxide ceramics, superconduct in the relatively balmy realm of temperatures above the boiling point of liquid nitrogen (77 kelvins or — 321 degrees Fahrenheit). Such quantum matter materials provide attractive candidates for making quantum heterostructure sandwiches.
Enthusiasm for quantum matter heterostructures, Boschker and Mannhart point out, has been accompanied by progress in enlisting more of the elements in the periodic table.
“We discern a trend toward growing multilayers using elements from throughout the periodic table, opening up an ever increasing choice of material combinations and stacking sequences,” they write. “This expansion of the material space is a grandiose, singular undertaking.… It provides new degrees of freedom and a toolset to tailor and create materials, phases, effects, and functionalities that nature would not make on her own.”
Researchers expect quantum matter heterostructures to have numerous intriguing applications. No doubt many such applications will be surprises, unforeseeable today. But already some structures are being devised to build improved transistors for various electronic uses or to make more effective catalysts. Transistors incorporating quantum heterostructures could pack more power into ever smaller devices. Enhanced quantum matter catalysts could be useful for energy storage and conversion, such as splitting water molecules to make hydrogen fuel. Multilayered structures based on the high-temperature superconducting cuprates are being designed for use in electric power transmission cables.
Achieving the theoretical potential of quantum matter heterostructures will require advances in the technologies used to build them. Refined methods will also be needed to test the new materials and to predict what combinations of layers will be likely to possess particular properties.
Making the most of heterostructures will require improvements in precision manufacturing on the atomic scale. In some cases, the slightest defect would ruin a structure’s usefulness, and with the complexity of heterostructure components, many different types of defects can occur. Better tools are needed to identify defects, Boschker and Mannhart write, and to determine which of a material’s properties are intrinsic to its composition and which are side effects of defects. In some cases, it may be that the very presence of defects, such as a missing atom in a particular spot, is just what’s needed to make the material work. Of course, even the tools used to measure a heterostructure’s features might alter its properties in the measuring process.
“In our opinion, the effects of defects and analytical tools on sample properties pose considerable challenges to the development of the field,” Boschker and Mannhart write. “Real-life heterostructures usually do not match the idealized Lego-like structures envisioned during design.” In fact, another major challenge is improving the match between theorists’ predictions of how a heterostructure will behave and its actual experimental performance.
All in all, Boschker and Mannhart expect that efforts to build quantum matter heterostructures will benefit from a series of advances that are “likely to happen” — including a wider range of higher-quality substrates on which to deposit layers of atoms, along with better methods of controlling and monitoring the growth process as layers are added.
“Merely by the vast possibilities and surprising discoveries that will be made, the exploration of quantum matter heterostructures will continue to be a burgeoning, highly rewarding field of science for decades to come,” Boschker and Mannhart write. “The field is veritably exploding in width and depth. Science has just scratched the surface of an enormously fertile ground of great application potential and unimaginable limits.”