CQE PI Feature – Riccardo Comin

What has always drawn me to my current field of research, the study of so-called “quantum materials”, is the thrill of the unexpected. The absolute best part of my work is witnessing those surprising results emerge from our experiments, like the signature of an unpredicted emergent phase of matter. In basic research, surprises are not just commonplace, they are the manifestation of interesting new physics, and the study of quantum materials has proven to be an especially fertile ground for such discoveries.

My path to this field wasn’t exactly linear. My initial research experiences, as an undergraduate, focused on the physics of liquids and glasses, which is determined by the motion and arrangement of atoms. As I considered a PhD, I was intrigued by “electronic” materials, with their many degrees of freedom and strong interactions promising a rich playground full of surprises. I eventually settled on studying copper oxide (cuprate) high-temperature superconductors, a problem central to condensed matter physics that has shaped our field and ushered in the era of quantum matter. The electronic phase diagrams of cuprates, akin to beautiful paintings, showcase the multiple phases of matter that coexist within them. Working on these materials was incredibly enjoyable and solidified my desire for an academic career. At the same time, I also felt drawn to topics beyond fundamental science and decided to pursue a postdoc on optoelectronic materials and devices. This experience was eye-opening, providing a valuable perspective on the connection between materials research and real-world challenges that new materials can address.

With this in mind, we have been building research efforts at MIT to discover and characterize (and improve) functional quantum materials. Transition metal-based compounds have long fascinated me, dating back to the early days of my PhD. Within this class of materials, I found that transition metal oxides are especially intriguing and exciting to work on because they display a rich interplay between all their microscopic degrees of freedom – charge, spin, and orbital – a characteristic which results from the strong interactions between electrons in these systems. This interplay – almost unique across all known solid-state systems – engenders a rich variety of exotic phases emerging out of electronic disorder. Some of these collective phases of matter (such as superconductivity) display quantum behavior even at the macroscale, hence the name “quantum materials”. Understanding the spatial structure, microscopic symmetry, and dynamics of these phases is crucial for harnessing them for technological applications including superconducting energy transport, high-magnetic field applications (MRI), data storage, and quantum computing.

In our lab, we combine synthesis, (nano)fabrication, and spectroscopy to gain a comprehensive understanding of these intriguing phenomena. I like to call this a “farm-to-table” approach, as we begin with raw ingredients (or, more precisely, high-purity chemical precursors for crystal growth) and conclude the discovery cycle with measurements on tabletop setups (optical and photoelectron spectroscopy). We also utilize large-scale research facilities equipped with ultrabright X-ray beams to conduct experiments such as resonant X-ray scattering and inelastic X-ray spectroscopy. These techniques help us uncover the emergent collective behavior of electrons in quantum matter.

Our latest and most exciting research focuses on two-dimensional transition metal-based compounds and novel “magnetoelectric” materials for spintronic and data storage applications. In magnetoelectric materials, the two fundamental properties of electrons, their spin and their charge, are intimately coupled at the microscopic level, enabling control of magnetic properties using electric fields, and vice versa. Despite their potential for creating highly energy-efficient memory devices, “natural” magnetoelectric materials are relatively scarce. Recently, we’ve been exploring ways to create a new type of synthetic magnetoelectric material at the interfaces between two-dimensional (2D) magnets. The control and tunability of 2D magnets make them particularly appealing for magnetoelectric memory devices. 2D magnets are known to respond to various tuning parameters, including pressure, strain, carrier doping, and electric fields. However, when these external stimuli are removed, the materials revert to their original state; in other words, this control is volatile. For this reason, we have been working to demonstrate how to incorporate ferroelectricity into 2D magnetic insulators to achieve robust, nonvolatile electrical control of magnetism. Ferroelectric order enables the switching of electronic spins using electric fields or voltages, paving the way for a new generation of low-power electronics for data storage and processing. In recent years, we have successfully demonstrated processable and switchable magnetoelectric states in ultrathin van der Waals material nickel iodide (NiI2). While the prospects are quite promising, this is a nascent field with significant challenges that must be overcome before practical applications can be realized. The most pressing issue is the low temperatures at which these magnetoelectric states exist. In our studies, these states were stable below 60 Kelvin, a temperature too low for most applications. Currently, the highest reported transition temperature at ambient pressure for a bulk magnetoelectric material is 230 Kelvin (-45 F), observed in another transition metal compound: cupric oxide (CuO). This opens new avenues for realization of these phenomena at room temperature, making these explorations very timely, not only in quantum materials research, but also in energy science. If (when) such a material with high-temperature magnetoelectric properties is discovered, a new class of magnetic memory devices could be integrated into current technology, enhancing both the computing power and energy efficiency of today’s electronics.