Deep inside a crystal cooled to nearly -230°C, MIT physicists have caught something rarely seen in quantum materials: two populations of electrons rebuilding themselves into wave-like order after being scattered apart by laser light, and doing so in two completely different ways. Working with erbium tritelluride, a rare-earth material known for hosting a pair of overlapping "charge density wave" phases, the team fired one pulse to break apart the material's electronic order, then a second to capture how it recovered. One phase crept back smoothly and evenly, spreading across the whole sample at once. The other behaved nothing like it, forming first in scattered pockets that slowly grew and merged, closer to ice crystallising within water than to any textbook electronic transition. The results settle a long-running debate over how this second phase actually forms and offer physicists a new way of watching competing quantum orders unfold in real time.
A charge density wave forms when electrons in a material spontaneously abandon their usual uniform scattering and instead organise into a rippling pattern, with crests of high electron density and troughs where few electrons are found. This kind of collective electron behaviour typically only appears at extremely low temperatures, and it has been studied by physicists for decades as a simplified stand-in for understanding more complicated forms of quantum order. According to MIT, Lead author Yifan Su noted that charge density waves matter to physicists precisely because they are a simpler cousin of superconductivity, a phenomenon in which electrons pair up and travel through a material without resistance. Studying the comparatively straightforward physics of charge density waves, the reasoning goes, offers a more tractable "playground" for working out the fundamental rules that govern collective electron behaviour, rules that may also apply to messier, more valuable phenomena further down the line.
Previous work had already established that erbium tritelluride hosts its first, dominant charge density wave once cooled to around -8 degrees Celsius, with a second, weaker wave appearing perpendicular to the first at roughly -113 degrees Celsius, producing a checkerboard of overlapping electronic order. For this study, the MIT-led team cooled thin samples of the material, grown by collaborators at Stanford, down to about -230 degrees Celsius, a temperature at which both waves coexist simultaneously. According to the study published in Nature, titled ‘Time-domain identification of distinct mechanisms for competing charge density waves in a rare-earth tritelluride’, the method involved "shaking" and then "listening" to the system. A first laser pulse disturbed or destroyed the checkerboard pattern, with its intensity tuned to vary how thoroughly the order was disrupted. A second, higher-energy pulse then knocked electrons out of the material at carefully staggered intervals afterwards, allowing the team to measure their energy and momentum and effectively take snapshots of the material as it recovered.
The results showed a clear split in behaviour between the two phases. The dominant charge density wave reappeared gradually and uniformly regardless of how forcefully it had been disrupted, consistent with a classic "second-order" phase transition, the same smooth pattern seen when a magnet steadily loses its magnetism as it warms. The subdominant phase told a different story. Rather than reforming evenly across the sample, it re-emerged in scattered pockets that expanded over time until they merged, a "first-order" transition more reminiscent of ice crystallising within liquid water. This abrupt, patchy mechanism had long been debated among physicists, and the team's laser-based approach finally allowed them to capture it directly in the time domain.
Understanding why one electronic phase transitions smoothly while another does so through nucleation and growth is more than a curiosity; it speaks to a much larger question in condensed-matter physics about how multiple electronic phases coexist and interact within the same material. Gedik has suggested that untangling these dynamics in a relatively simple system such as erbium tritelluride could serve as a template for probing far more complicated materials, including high-temperature superconductors, where magnetism, superconductivity and charge density waves are all thought to be intertwined. The research team believes their time-domain framework, which combined time- and angle-resolved photoemission spectroscopy with theoretical modelling, could be applied more broadly to other quantum materials where competing orders are suspected but not yet fully understood. As engineers continue searching for materials that might eventually replace silicon in next-generation electronics, work of this kind, probing exactly how and why coexisting electronic phases form, could prove essential to designing devices that exploit those exotic properties deliberately, rather than by accident.
