Scientists capture the first view of a hidden quantum phase in a 2D crystal

The development of high-speed stroboscopic flash photography in the 1960s by the late MIT professor Harold “Doc” Edgerton allowed us to visualize events too fast for the eye: a bullet piercing an apple or a drop hitting a puddle of water. milk.

Now, using a suite of advanced spectroscopic tools, scientists at MIT and the University of Texas at Austin have captured snapshots of a light-induced metastable for the first time. phase hidden from the universe of balance. By using single shot spectroscopy techniques on a 2D crystal with nanoscale electron density modulations, they were able to see this transition in real time.

“With this work, we are showing the birth and evolution of a hidden quantum phase induced by a ultrashort laser pulse in an electronically modulated crystal,” says Frank Gao Ph.D. ’22, co-senior author of a paper on the work who is currently a postdoc at UT Austin.

“Usually shining lasers on materials is the same as heating them, but not in this case,” adds Zhuquan Zhang, co-lead author and current MIT graduate student in chemistry. “Here, the irradiation of the crystal rearranges the electronic order, creating an entirely new phase different from the high-temperature one.”

An article about this research was published today in Progress of science. The project was jointly coordinated by Keith A. Nelson, Haslam and Dewey Professor of Chemistry at MIT, and Edoardo Baldini, Assistant Professor of Physics at UT-Austin.

laser shows

“Understanding the origin of such metastable quantum phases is important to address longstanding fundamental questions in nonequilibrium thermodynamics,” says Nelson.

“The key to this result was the development of a state-of-the-art laser method that can ‘make movies’ of irreversible processes in quantum materials with a temporal resolution of 100 femtoseconds”, adds Baldini.

The material, tantalum disulfide, consists of covalently bonded layers of tantalum and sulfur atoms stacked on top of each other. under a critical temperaturethe atoms and electrons of the material pattern into nanoscale “Star of David” structures, an unconventional distribution of electrons known as a “wave of charge density.”

The formation of this new phase makes the material an insulator, but a single pulse of intense shining light pushes the material toward a hidden metastable metal. “It’s a transient quantum state frozen in time,” says Baldini. “People have observed this light-induced hidden phase before, but the ultrafast quantum processes behind its genesis are still unknown.”

Adds Nelson: “One of the key challenges is that observing an ultrafast transformation from an electronic order to one that can persist indefinitely is not practical with conventional time-resolved techniques.”

insight pulses

The researchers developed a unique method that involved splitting a single probe laser pulse into several hundred separate probe pulses that arrived at the sample at different times before and after the switch was initiated by separate ultrafast excitation. legumes. By measuring the changes in each of these probe pulses after they were reflected or transmitted through the sample and then stringing together the measurement results as individual frames, they were able to construct a movie that provides microscopic insights into the mechanisms across the sample. which transformations occur.

By capturing the dynamics of this complex phase transformation in a single shot measurement, the authors showed that charge density wave merger and rearrangement lead to the formation of the hidden state. Theoretical calculations by Zhiyuan Sun, a postdoc at the Harvard Quantum Institute, confirmed this interpretation.

While this study was carried out with a specific material, the researchers say the same methodology can now be used to study other exotic phenomena in quantum materials. This discovery may also help with the development of optoelectronic devices with on-demand photoresponses.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.