Many properties of metals can remarkably be understood in terms of “free” electrons, even though electrons interact with each other through a Coulomb force, which can be both large and long ranged. It turns out that these interactions collectively result in the emergence of “quasiparticles” whose behavior is very similar to that of free, non-interacting electrons. This description forms the basis of the famous Landau Fermi liquid theory. In fact, the notion of finding new non-interacting quasiparticles to describe complicated interacting systems is a key paradigm in condensed matter physics.
Written by Illinois Physics
Illinois Physics Professor Fahad MahmoodMany properties of metals can remarkably be understood in terms of “free” electrons, even though electrons interact with each other through a Coulomb force, which can be both large and long ranged. It turns out that these interactions collectively result in the emergence of “quasiparticles” whose behavior is very similar to that of free, non-interacting electrons. This description forms the basis of the famous Landau Fermi liquid theory. In fact, the notion of finding new non-interacting quasiparticles to describe complicated interacting systems is a key paradigm in condensed matter physics.
A canonical example of such a complicated interacting system is phosphorus doped silicon near the insulating side of its metal-insulator transition. Here, a complex interplay between the effects of strong interactions and strong disorder results in observable properties markedly different from those of conventional metals or insulators. Perhaps such a system can also be described in terms of non-interacting quasiparticles like “free” electrons in a metal? This conjecture was first put forth by Phil Anderson in 1970 who dubbed such a system as a “Fermi-glass.” This description certainly holds in the absence of interactions as in the case of the celebrated “Anderson localization,” whereby strong disorder localizes electronic states. However, whether this description is possible in materials with both strong interactions and strong disorder has been an open question for nearly 50 years. By developing a unique spectroscopic technique based on strong terahertz frequency light (2D THz spectroscopy), we find the answer to this question is “No.” Interaction effects are always strong enough to preclude any description in terms of non-interacting quasiparticles.
Using 2D THz spectroscopy, we first observed distinct coherent phenomena, “THz photon echoes,” that allowed us to reliably measure the energy relaxation and decoherence times—typically known as T1 and T2times respectively—in a strongly interacting disordered material. The scaling of these times with temperature and disorder concentration was found to be at odds with Anderson’s conjecture. In our view, this new phenomenology can be best described as a “marginal Fermi glass.”
Our technique of 2D THz spectroscopy can be broadly applied to other complex materials, such as high-temperature superconductors and quantum spin liquids, to understand the fundamental nature of excitations and quasiparticles in these systems.
Research was performed by Fahad Mahmood of Illinois Physics; Dipanjan Chaudhuri and Peter Armitage of Johns Hopkins University; Sarang Gopalakrishnan of CUNY College of Staten Island; and Rahul Nandkishore of University of Colorado Boulder
Madeline Stover is a physics doctoral student at the University of Illinois Urbana-Champaign studying atmospheric dynamics applied to forest conservation. She interns as a science writer for Illinois Physics, where she also co-hosts the podcast Emergence along with fellow physics graduate student Mari Cieszynski. When Stover is not doing research or communications, she enjoys hosting her local radio show, singing with her band, and cooking with friends.
Daniel Inafuku graduated from Illinois Physics with a PhD and now works as a science writer. At Illinois, he conducted scientific research in mathematical biology and mathematical physics. In addition to his research interests, Daniel is a science video media creator.
Karmela Padavic-Callaghan, Ph. D. is a science writer and an educator. She teaches college and high school physics and mathematics courses, and her writing has been published in popular science outlets such as WIRED, Scientific American, Physics World, and New Scientist. She earned a Ph. D. in Physics from UIUC in 2019 and currently lives in Brooklyn, NY.
Jamie Hendrickson is a writer and content creator in higher education communications. They earned their M.A. in Russian, East European, and Eurasian Studies from the University of Illinois Urbana-Champaign in 2021. In addition to their communications work, they are a published area studies scholar and Russian-to-English translator.
Garrett R. Williams is an Illinois Physics Ph.D. Candidate and science writer. He has been recognized as the winner of the 2020 APS History of Physics Essay Competition and as a finalist in the 2021 AAAS Science and Human Rights Essay Competition. He was also an invited author in the 2021 #BlackinPhysics Week series published by Physics Today and Physics World.
Karmela Padavic-Callaghan, Ph. D. is a science writer and an educator. She teaches college and high school physics and mathematics courses, and her writing has been published in popular science outlets such as WIRED, Scientific American, Physics World, and New Scientist. She earned a Ph. D. in Physics from UIUC in 2019 and currently lives in Brooklyn, NY.