Quasiparticles help physicists make sense of the world

Steven Cundiff wasn’t sure what would happen when he fired a laser at a target last year.

The condensed matter physicist wanted to see how the electrons inside a material used in solar panels and DVD players would behave when hit with an energy boost.

To deliver the energy, Cundiff and his team at JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder, fired a quick pulse of red laser light at a strip of gallium arsenide, a material similar to silicon. Trillionths of a second later, they followed up with a weaker pulse. Once through the target, the second laser beam struck a detector, which helped the researchers determine what kinds of particles had absorbed the light from the first pulse.

The laser barrage seemed to cause strange clusters of electrons to form inside the strip. The electrons weren’t bunched rigidly as they would be in a solid, nor were they jetting around randomly as they would in a gas. Rather, the particles clumped together as if they were nanosized droplets of water.

Cundiff and his team had discovered the dropleton, or quantum droplet (SN Online: 2/26/14). They published their finding February in Nature.

A dropleton looks like a particle and acts like a particle. But it’s not really a particle. Each dropleton exists for just a few trillionths of a second after the laser-energy injection, and it can’t be extracted from the material and isolated as an electron or an atom could be.

As unreal as it is, the dropleton offers an unprecedented probe of the inner workings of a commercially important material. The dropleton is a quasiparticle, a theoretical construct that helps physicists make sense of the jungle of particles and forces within the materials we use every day.

Instead of trying to calculate all the complex interactions taking place inside a given material, physicists can simplify the problem by envisioning these made-up particles moving through.

“Quasiparticles can be the key to understanding a particular material or system,” says Ross McKenzie, a condensed matter physicist at the University of Queensland in Australia.

Over the last 75 years, physicists have been making sense of incomprehensible things by identifying and exploiting more than a dozen quasiparticles (see partial list, right). And now, scientists have the engineering skills to tinker with materials at the nanoscale and introduce quasiparticle-inspired changes that lead to useful devices.

After studying the characteristics of quasiparticles in a variety of materials and conditions, scientists are designing new material combinations that efficiently transform sunlight into electricity or convert electricity into laser light. Plus, quasiparticles may eventually enable long-promised technology that revolutionizes the way the world delivers and uses electricity.

A horse and its dust cloud

For physicists, understanding the behavior of a single electron floating in space is easy. But most electrons in everyday life are not dancing around freely. Take a coil of copper wire, like the ones that carry electricity through buildings. It contains roughly as many electrons as there are stars in the observable universe. Until the 1920s, physicists thought of each of those electrons as minuscule charged pinballs. Negatively charged electrons would repel their fellow electrons and attract positively charged copper nuclei as they bounced around inside the wire.

The introduction of quantum theory revealed that this already complicated picture was way too simple. In 1924, Louis-Victor de Broglie theorized that matter can behave as both particles and waves. As a wave, an electron can interfere with other matter. Then in 1927, Werner Heisenberg proposed with his uncertainty principle that it is impossible to determine both a particle’s position and its velocity at a given moment.

Suddenly those electrons in the wire transformed from pinballs to fuzzy part-particle, part-wave entities that have no definite location or velocity and that can subtly influence each other with their mutual negative charge. Making sense of even a couple of those electrons, let alone a trillion trillion, “very quickly becomes an intractable problem,” Cundiff says. In the 1992 edition of his book A Guide to Feynman Diagrams in the Many-Body Problem, physicist Richard Mattuck compares the dilemma to trying to describe a galloping horse and all the grains of dust that it kicks up.

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In the late 1930s, Soviet physicist Lev Landau, just released from prison for holding anti-Stalinist views, found a way to simplify the problem. Instead of trying to sum up the complex interactions of each particle with its neighbors, physicists could combine a particle and its interactions into one composite quasiparticle. In other words, instead of trying to describe the horse and account for all its effects on its surroundings, Landau proposed the formulation of a quasi­horse: a single entity that includes the horse and its accom­panying dust cloud.

“It was a brilliant idea by Landau,” McKenzie says. “It works amazingly well.”

A simple example of Landau’s proposal is an electron quasi­particle, the most common quasiparticle, according to Cundiff. Because of interactions with its neighbors, an electron moving through a semiconductor, a type of material used in most electronic circuits, behaves differently than an electron moving through free space. Following Landau’s approach, physicists found that an electron in a semiconductor such as silicon or gallium arsenide actually does behave as if it were a noninteracting free electron — just one with a larger mass. That adjusted-mass electron is called an electron quasiparticle.

A hole is another common quasiparticle. When an electron absorbs energy and jumps to a new position, it leaves behind a positively charged vacancy. That hole behaves as if it were a positively charged electron even though there is no physical particle there. Further, the positively charged hole often gets attracted to the energized electron, leading the electron and hole to bind and form another quasiparticle called an exciton.

In April, researchers led by physicist Vladimir Bulovic´  at MIT’s Center for Excitonics reported in Nature Communications that they had imaged excitons traveling through an organic crystal called tetracene in real time. Bulovic´ ’s team fired a laser to inject energy into the crystal, which caused electrons to jump and then bond to their vacated holes to form excitons. The excitons appeared to race outward from the spot where the laser deposited energy.

In reality, an electron and a hole don’t move through the material; rather, electrons are simply passing on energy to their neighbors, and the energy cascades outward. But for Bulovic´  and other researchers, it’s far simpler to think of those energy cascades as quasiparticles with well-defined properties such as mass and charge. “You can’t directly see what’s going on at the nanoscale,” Bulovic´  says. “But you can infer what’s going on at the nanoscale with quasiparticles.”

Although they are, in a sense, just made-up entities for the sake of physicists’ sanity, quasiparticles describe reality well enough to lead to technological advances. Excitons figure prominently in the development of improved solar cells. When light strikes certain materials, it frees up electrons, which can be siphoned off as electricity. Einstein discovered this phenomenon, called the photoelectric effect, in 1905. But to truly understand solar cells and improve their capacity to convert sunlight into electricity, physicists apply the Landau treatment.

At the Center for Excitonics, Bulovic´ and colleagues are working to control the propagation of excitons in certain types of solar cells. When a photon strikes a material such as silicon, an electron jumps up in energy and binds to its hole to form an exciton. The problem, Bulovic´  says, is that most of the time the exciton doesn’t get very far before it dissipates and gives back its energy. To produce more effective solar cells, excitons have to travel far enough to reach a second layer of material, which splits up the exciton and creates an electric field that drives the flow of electricity. Bulovic´  and other scientists are crafting materials that maximize the lifetime of excitons. The aim is to increase the rate that cells convert solar energy into electrical energy from the single digits to nearly 20 percent.

See-through power source

Studying the properties of excitons and how they move is also leading to solar cells with unique properties. Bulovic´ ’s group has teamed with a group from Michigan State University to develop transparent solar cells. Conventional solar cells are opaque so they can absorb visible light to generate electricity. But these new transparent cells trap only infrared and ultraviolet light to trigger the formation of excitons; visible sunlight passes right through. Bulovic´  envisions future generations of the device placed over an e-reader screen to keep it charged indefinitely, or on eyeglasses, for example, to charge
a hearing aid.

Other quasiparticle-inspired technologies may soon find their way into electronics and medical devices. In June, a team led by electrical engineer Pallab Bhattacharya of the University of Michigan in Ann Arbor reported the development of a laser that requires a mere 0.4 percent of the electricity of a conventional laser. Its light is produced by the decay of quasiparticles (SN: 7/12/14, p. 20).

Commercially available lasers require a lot of electricity to energize atoms, which then emit laser light when the atoms drop back to lower energies, Bhattacharya explains. But his team’s device, built with the semiconductor gallium nitride, runs differently. A small jolt of energy creates excitons, which then absorb photons to form light-matter hybrid quasi­particles called polaritons. When the short-lived polaritons break apart, they release a beam of ultraviolet laser light made of photons that all have the same color and direction.

Game-changing materials

But not all materials can be so easily understood with quasiparticles. Superconductors are one example. These compounds can shuttle electricity around with no resistance as long as they are frigid — close to the coldest possible temperature, absolute zero, or –273° Celsius. In 1986, IBM researchers Johannes Bednorz and Karl Müller discovered the first “high-temperature” superconductor: a compound containing copper and oxygen that maintained zero electrical resistance at temperatures as high as −238° C. In this case, high temperature is still awfully cold. A flurry of discoveries of similar materials with even higher superconducting temperatures followed, helping Bednorz and Müller snag the 1987 Nobel Prize in physics. Although superconducting electromagnets are used in hospital MRI scanners, for example, superconductors have been limited in their applications by their requirement for very low temperatures.

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The Nobel Prize–winning discovery immediately raised hopes that physicists could develop superconducting materials at much higher temperatures, ideally room temperature, a technological breakthrough that would enable high-speed transport of electricity across the power grid with almost no loss of energy. The United States loses about 6 percent of its generated electricity during transmission, according to the U.S. Energy Information Administration. Those losses translate to a roughly $20 billion annual hit to the economy.

In its 1987 Nobel Prize announcement, the Royal Swedish Academy of Sciences noted that the “details of how superconductivity arises in the new materials are still unknown.” Nearly 30 years later, the same holds true. The interactions within these compounds are so complicated that they defy simplification even by quasiparticles.

Drawn in by the quest for a new, game-changing material, many physicists are trying to understand the inner workings of high-temperature superconductors by looking for patterns in the movement and interactions of electrons. “For each electron moving through the material, you have to understand what’s the influence on its neighbor and vice versa,” says J. C. Séamus Davis, director of the Center for Emergent Superconductivity at Brookhaven National Laboratory in Upton, N.Y. In a sense, he is looking for quasihorses.

Davis and his team recently used a specially designed microscope to analyze a superconductor made of cerium, cobalt and indium. They determined that magnetism is the crucial force that steers electrons through the material in its superconducting state, a finding that starts to simplify the complex interactions at work. To simplify things further and start to work toward room-temperature superconductivity, physicists hope to find some structure in the migrating electrons in the form of a quasiparticle — much like the clusters known as dropletons. The goal has real-life ramifications: For example, room-temperature superconducting cables could create the very powerful magnets needed to make high-speed levitated trains a viable means of transportation.

For McKenzie, the importance of quasiparticles in physics leads to a more philosophical question: What does it take for something to qualify as a real particle? All physicists would agree that fundamental entities like electrons and protons are particles. But McKenzie argues that excitons, polaritons and dropletons should join the club. “I would say they’re just as real as an electron,” he says. If it looks like a particle and quacks like a particle, then it might as well be a particle.

McKenzie seems to be in the minority opinion. Yet nobody doubts the importance of quasiparticles, particularly if they aid the search for room-temperature superconductivity. For now, researchers just hope that a not-so-real particle is hiding in the insanity of trillions of trillions of electrons, waiting to bring clarity and resistance-free electricity.