The discovery of microorganisms. The discovery of the Higgs boson particle. The observation of two neutron stars colliding. Each of these discoveries shares a common element: new technology made them possible.
Of course, a highly dedicated, curious and skilled group of scientists was essential, but it was the creation of new scientific tools that made the difference. Discovering microorganisms was possible with the invention of the microscope and the Higgs Boson because of the creation of the Large Hadron Collider. Observing the collision of neutron stars was possible because of the LIGO observatory—the world’s largest laser interferometer. And now, the discovery of a new form of matter – excitonium – was made possible by a novel scientific instrument called momentum-resolved electron energy-loss spectroscopy, or M-EELS.
A team led by Peter Abbamonte, a professor in the department of physics at the University of Illinois at Urbana-Champaign, and an investigator in our Emergent Phenomena in Quantum Systems (EPiQS) Initiative, developed M-EELS, which confirmed the new form of matter. The University of Illinois at Urbana-Champaign's cover story reveals how the team was able to show that the elusive phenomenon predicted by the theory decades ago actually exists in nature. A paper explaining the discovery was published in the journal Science.
The term excitonium was first coined in 1968 by two theoretical physicists, Bert Halperin and Maurice Rice. Excitonium is a macroscopic quantum phenomenon, much like superconductivity. It is made up of quasiparticles called excitons, which are a combination of an electron escaped from an atom and the hole left behind from the electron. In a material, the hole behaves as a particle with positive charge, attracting the negatively charged electron and creating a bound state – an exciton. When large numbers of excitons are present in a material, they are expected to start behaving in a coordinated way, due to their quantum-mechanical, wave-like nature. Eventually, the whole enormous ensemble of excitons should start behaving as a single quantum-mechanical object: excitonium.
For physicists like Abbamonte, studying fundamental properties of materials can be exciting, and exasperating because of their enormous complexity. It can take years, even decades, to understand what large collectives of particles in materials do and why, so having patience is a necessity. Materials also often hide surprising effects, which makes studying them so much fun. In the case of Abbamonte and his colleagues, Anshul Kogar and Melinda Rak, the fun started when they were given the freedom to build an instrument to probe collective properties of electrons in a new way.
Image above: Artist’s depiction of the collective excitons of an excitonic solid. These excitations can be thought of as propagating domain walls (yellow) in an otherwise ordered solid exciton background (blue). Image courtesy of Peter Abbamonte, U. of I. Department of Physics and Frederick Seitz Materials Research Laboratory
Faith, fortitude and a nagging challenge
The idea of creating what is now the momentum-resolved electron energy-loss spectroscopy, was first seeded into Abbamonte’s mind 20 years ago by his mentor, Philip Moss Platzman, who was head of the scattering and low-energy physics research department at Bell Labs. Platzman, along with Peter Littlewood and Chandra Varma, challenged Abbamonte to find a way to measure how collective excitations of electrons in materials respond to external stimuli, such as electric or magnetic fields.
“During my time at Bell Labs I was told again and again ‘you’ve got to figure out how to measure the collective electronic charge excitations in materials’, said Abbamonte.” “I guess I never got it out of my head and it became a quest, and a difficult one at that,” he added.
For 15 years Abbamonte tried to develop such an instrument using X-ray scattering, but he was unsuccessful. Around 2010, he got the idea that it could be done using electrons instead. However, this wasn’t in his science wheelhouse. Abbamonte is known as a pioneer in X-ray scattering, and, as a result, he found that drumming up money for something outside his domain was tough. Yet he persevered, cobbling together what he could from various commercially available parts to produce a prototype. But, this early model of M-EELS did not work well enough to reveal excitonium and he needed more financial support to get it done right.
In 2014, Abbamonte secured the resources needed. The Moore Foundation was selecting experimental investigators in quantum materials to help accelerate scientific breakthroughs in the field through its Emergent Phenomena in Quantum Systems Initiative. The five-year, $34.2 million investigator portfolio supports physicists to pursue ambitious, high-risk research, including the development of new experimental techniques. Abbamonte fit the bill perfectly. With funding from the foundation of $1.8 million over five years, he set out to complete his (and his mentors’) vision.
“The financial support allowed me to do things the way that I wanted and with enough time to figure it out, which has resulted in some pretty amazing and unpredictable outcomes for science,” said Abbamonte.
Abbamonte made his prototype into a fully functional instrument to measure the momentum and energy of electrons with high accuracy. Once created, Abbamonte got more than he could have imagined from M-EELS. In the serendipitous nature of experimental science, this new instrument was the ticket to discovering excitonium. It turns out that the technology is sensitive and selective enough to study the collective excitations of the electronic system in materials.
“Science is a beautiful thing. For years, someone can envision creating something to serve a specific purpose and later discover that it leads to something entirely unanticipated."
Experimental investigations are essential to understanding emergent phenomena in materials, and yet they are not without risk. A clear outcome cannot always be defined and a path to discovery is not linear. But, breakthroughs happen when people are given the liberty to explore uncertain directions.
“Our Moore Investigator in Quantum Materials awards allow a group of experts in experimental research and materials synthesis to maximize their creativity,” said Dusan Pejakovic, Ph.D., program director of the Emergent Phenomena in Quantum Systems Initiative. “We enable them to pursue exploratory research with the potential to transform our understanding of how complex quantum matter organizes itself, and Abbamonte’s work is a great example of that.”