What are Quantum Materials?
Quantum=cool. In the movie, Avengers: Endgame, entering the “Quantum Realm” was a major plot point that allows the basic laws of physics to be broken in largely fantastical ways. In the movie, “Ant-Man and the Wasp,” Scott Lang (Ant-Man) asks Dr. Foster “Do you guys just put the word ‘quantum’ in front of everything?” Does the word “quantum” mean anything nowadays and does it have any relevance to the real world today? Can we really make, use, and hold something quantum?
The reality is that quantum materials are in technologies that you have likely already encountered, such as hospital MRIs, which use superconductors, and hard disk drives, which use giant magnetoresistance sensors. However, quantum materials’ use in energy technologies remains scarce. The U.S. Department of Energy’s Office of Basic Science funds seven quantum materials-related Energy Frontier Research Centers (EFRCs) seeking to change that: the Institute for Quantum Matter (IQM), the Center for Programmable Quantum Materials (Pro-QM), Quantum Materials for Energy Efficient Neuromorphic Computing (Q-MEEN-C), the Center for Novel Pathways to Quantum Coherence in Materials (NPQC), the Center for the Advancement of Topological Semimetals (CATS), the Center for Molecular Magnetic Quantum Materials (M2QM), and the Spin and Heat in Nanoscale Electronic Systems (SHINES) Center. Tyrel McQueen of IQM and a professor at Johns Hopkins University says the potential impact of quantum materials on energy is enormous. “Imagine solar materials in which defects do not damage photovoltaic performance; catalysts whose surfaces retain activity even in the presence of strong disorder; or sensors immune to noise from the environment,” said McQueen. “Any one of these is a revolutionary achievement; the excitement is that quantum materials are known to contain the necessary ingredients to make each of these possible in a real device.”
Part of the promise of quantum materials for energy is that, although quantum mechanics govern the properties of all materials, quantum materials behave differently from normal materials. All solids are made of clumps of atoms. When the atoms are brought together to form solids, their electrons interact, which is where quantum mechanics comes into play. For most materials, it is possible to have a basic understanding of their behavior with minimal quantum mechanics. As Professor Joel Moore of NPQC and UC Berkeley explains, “There are some materials for which you can’t even make a rough picture of what is going on without quantum mechanics.” These are quantum materials.
Quantum states aren’t generally forever—but can be comparatively long-lasting in some materials
Schrodinger’s cat is a thought experiment, proposed by the Physicist Erwin Schrodinger, in which a cat is simultaneously alive and dead (disclaimer: no cats are harmed or even touched by this research). The cat is both alive, state 1, and dead, state 0, a superposition of both states. Superpositions like this can’t last forever in most materials due to interactions with the environment, and don’t generally extend over large distances. Anything visible to the naked eye is unlikely to exist as a superposition but, as a result of quantum mechanics, superpositions can become relevant on length scales smaller than ~100 nm (1/10,0000 of a millimeter). Quantum materials break this trend. In quantum materials, this superposition of states can last for a long time and over extended spaces. The term “coherence” refers to how long and how far this superposition extends—is Schrodinger’s cat both alive and dead for a nanosecond or a minute, and is it the entire cat or just a few atoms in the cat that is/are alive and/or dead?
As Moore explains, “Coherence means the superposition survives whereas decoherence means that it collapses into being either 0 or 1 like a classical [binary] bit. We’re interested in solids where, even in an environment of many billions of atoms, that coherence survives for milliseconds [or longer].” Quantum materials researchers are not literally studying whether things are alive or dead—we study the states of particles and particle-like things, like electrons and how they interact.
Electrons act as both particles and waves in materials
To understand quantum materials, it helps to start with electrons. Quantum mechanics tells us that an electron is both a particle and a wave. When you drop two stones in a pond, each creates small waves that will interfere with each other to make bigger waves in some places and smaller waves in others. Similarly, electrons behave as waves, too, which can result in some really unusual electrical properties.
According to Moore, in topological materials, a class of quantum materials, “the wavefunctions of the electrons are wrapped up like a complicated knot.” The word “topological” comes from math; “topology describes how easy it is to deform something and turn it into something else, without cutting,” according to Robert McQueeney of CATS. “In this sense, a donut has the same topology as a coffee cup because both have one hole,” said McQueeney, who is also a professor at Iowa State University.
That single hole or no hole property is an invariant, or constant inherent to the structure. In the case of topological materials, it is electron wavefunctions that deform; we are not physically stretching the materials from one shape to another. Technologies based on these topological properties should be more tolerant to defects and disorder because they are designed around these constants. In topological insulators, the material is an insulator (or semiconductor) but, no matter how you slice the material, the surface conducts due to the topology of the electron bands. Unlike the electrons in most materials and on most surfaces, the direction in which electrons on these surfaces travel is tied to the spin (see below) of the electron, potentially enabling new types of devices. Theorists have proposed a closely related class of materials called axion insulators, which are like topological insulators but are magnetic. “These materials should exhibit gigantic magnetoelectric coupling and be useful in energy conversion,” said McQueen.
Semimetals have no energy gap between electron states like a metal, but have fewer charge carriers (both electrons and holes) than metals. Dirac and Weyl semimetals are quantum materials, and many are topological like a topological insulator. Dirac semimetals (both 2-D and 3-D) have an hourglass of electron energy states due to their symmetry that create an unusually high electron mobility.
“In Dirac semimetals, electrons behave, in many ways, like photons of light,” said McQueeney. “If you apply a magnetic field or insert magnetic ions into a Dirac semimetal, the Dirac points [at the middle of the hourglass] can split into mirror opposites of each other to form a Weyl semimetal.”
Magnetic Weyl semimetals may be useful for switching, memory, and sensing because the spins of electrons in the Weyl semimetal are tied to their direction of travel, coupling their magnetic and electrical properties together. It’s not magic—it’s just cool physics.
Electrons have spin, which gives rise to magnetism
In addition to being like both a particle and a wave, all electrons have a property called spin, which is what makes magnetic materials magnetic. An electron can have a spin that is either up or down, and spins tend to interact following certain rules. A dizzying number of quantum materials, including spin liquids, spin glasses, and spin ices, exhibit strange electronic properties because of the ways these materials bend the rules of how unpaired electron spins align. These exhibit what is known as frustration, where, due to the geometry of the structure—for example, a triangle—the spins can’t all simultaneously obey the rules that govern how the spins should align.
“Frustration is a design principle that allows one to engineer in a large number of energy states that are very similar in energy,” explained McQueen. “By forcing these states to have the same energy, you design materials that have the potential to adopt more complicated spin structures (arrangements of spins) that require spins that are very far away from each other to be entangled with each other.”
Entangled particles become “linked” such that there is a correlation between properties of the individual particles, like the spins of two electrons that began in the same place. This is what Einstein famously referred to as “spooky action at a distance.” These types of materials can exhibit coherence over large regions and may be useful for data storage, memory, and quantum computation.
Electron spin is also important for soft, molecule-based compounds. “Soft materials can have different magnetic functionalities,” said Vivien Zapf, a senior scientist at the National High Magnetic Field Lab and member of M2QM. “For some d-shell magnetic atoms such as manganese or iron, the size of the atom can change significantly by moving electrons into different orbitals. This tends not to happen very often in hard materials because the size of the magnetic atom changes by as much as 10% during a spin crossover, and the lattices in hard materials are too hard to accommodate this at ordinary temperatures and pressures. For example, manganese can transition from spin-1 state, which has two aligned electrons to spin-2 state, which has four aligned electrons, changing how it bonds to the surrounding atoms. Materials with organic components are soft and flexible and can allow spin crossovers to happen.” This is useful because spin crossover can be the trigger that changes the electrical properties of the material.
“In the long run, if you want a magnetic sensor or device, you’d want to read out the magnetic state via its electrical properties,” said Zapf. “Many of these materials are insulating, so you replace currents with voltages and lower power consumption. With molecular magnets, a single molecule can be placed in a tiny circuit that is controlled electrically.” These materials might, in the future, be used in magnetic memory, sensors, and quantum computing.
2-D quantum materials can offer additional functionality compared to 3-D quantum materials
Materials comprised of assemblies of atomically thin, 2-D, plate-like layers can exhibit many of the remarkable properties of 3-D quantum materials, in addition to other unique capabilities.
“2-D materials are, in a sense, all interface,” explained Andrew Millis of Pro-QM and a professor at Columbia University. “To quote Herbert Kroemer, who won the Nobel Prize in physics for developing semiconductor devices, ‘The interface is the device.’” According to Millis, “An unprecedented range of 2-D materials can now be synthesized, allowing an incredible diversity of effects. For example, the atomically thin sheets comprising 2-D materials concentrate the electronic states, enhancing the light-matter interactions and opening new possibilities both for basic physics studies of quantum excitations such as electron-polaritons and for device applications including sensors.” Consequently, 2-D quantum materials are an active area of research alongside their 3-D counterparts.
Basic materials research is needed to enable the technologies of the future
Quantum materials may enable a paradigm shift in many technologies. According to Amanda Petford-Long of Q-MEEN-C, a researcher at Argonne National Laboratory and a professor at Northwestern University, quantum materials have many properties we know how to control, but we need to understand their basic behavior if we are to enable optimization for a specific purpose. “Discovering new quantum materials, and putting together new combinations of existing materials, allow us to manipulate spin and charge,” said Petford-Long. “This can lead to design of completely new types of devices that are not just incremental improvements. Significant energy efficiency improvements will likely be realized.”
Quantum materials are a promising and broad class of materials that should enable technologies of the future, just as they enable technologies like MRIs, biosensors, and disk drives today. EFRCs sponsored by DOE discover and design these strange materials and are inventing new devices that will transform the way that we work and live. To answer Ant-Man’s implied question, yes, quantum has real meaning!