MIT physicists watch a quantum material’s electrons take two different paths back to order
Inside a sliver of a rare-earth material chilled to roughly minus 230 degrees Celsius (minus 382 degrees Fahrenheit), two rival patterns of electrons were smashed apart by a laser pulse, and then filmed rebuilding themselves. The surprise: the two patterns came back in completely different ways. One reassembled smoothly, wave by wave, across the whole sample. The other sprouted in isolated patches that slowly spread. The result, published in Nature Physics, offers a rare direct look at how competing electronic orders emerge in quantum materials.
The research comes from the lab of Nuh Gedik, who holds MIT’s Donner professorship in physics, and was reported by MIT News on August 7 and covered by Interesting Engineering on August 9.
A checkerboard hidden in a crystal
The material is erbium tritelluride, a rare-earth compound that can be grown in atomically thin sheets. Its electrons normally sit scattered and uniform, but cooling reorganizes them into repeating wave-like patterns known as charge density waves, in which regions of high electron concentration alternate with regions of low concentration.
Erbium tritelluride hosts two such waves. The first, described as the dominant order, forms around minus 8 degrees Celsius (18 degrees Fahrenheit) and extends in one direction. Cool the material further, to about minus 113 degrees Celsius (minus 171 degrees Fahrenheit), and a second, weaker wave develops perpendicular to the first. The two patterns together occupy the same crystal simultaneously, producing what the researchers call an electronic checkerboard.
For decades, physicists have debated how the weaker of the two orders actually comes into being. Does it appear everywhere at once, or does it start somewhere and grow?
Shake, then listen
To find out, the team cooled atomically thin samples, synthesized by collaborators at Stanford, to about minus 230 degrees Celsius (minus 382 degrees Fahrenheit), cold enough for both waves to coexist. Then they hit each sample with a carefully sequenced pair of laser pulses.
The first pulse, the shake, scrambled or briefly erased the checkerboard pattern. A second pulse, fired after a precisely timed delay, ejected electrons from the material; reading the energy and momentum of each ejected particle let the team assemble a frame-by-frame picture of the electronic order rebuilding itself.
The technique, which Gedik describes as shaking the system and then listening to its response, let the team watch the two phases recover separately, something that had previously been extremely difficult.
The recovery was anything but uniform. The dominant charge density wave returned gradually and evenly, rebuilding across the entire sample in the smooth, continuous manner that physics textbooks expect of an electronic phase transition, like liquid water warming uniformly into vapor.
The subdominant wave behaved entirely differently. Instead of emerging everywhere, it appeared first in localized pockets that then expanded outward, more like liquid water crystallizing into ice. That pattern, nucleation followed by growth, is the signature of a first-order transition, and it answers a question that had been open for years about how the weaker order establishes itself.
Why it matters
Charge density waves matter because they are a much simpler collective phenomenon than the exotic states physicists ultimately want to understand. Superconductivity, where electrons team up into pairs and glide without resistance, and the various forms of magnetism both involve multiple coexisting electronic phases interacting inside a single material. If the way those phases form and interact is the key to their properties, then watching a simpler two-phase system rebuild itself is a step toward decoding the harder ones.
The study’s first authors are Yifan Su and Bai-Qing Lv, with co-authors including Dongsung Choi, Doron Azoury, and Masataka Mogi. Alfred Zong, a co-leader of the project during his MIT years, now teaches at Stanford. Funding came from the US Department of Energy, the National Science Foundation, and the Moore Foundation’s EPiQS initiative.
The findings appear in Nature Physics, framed as the time-domain identification of distinct mechanisms behind the two competing density-wave orders in the material (DOI 10.1038/s41567-026-03382-5).