Scientists have captured the first-ever view of a hidden quantum phase in a 2D crystal

This illustration represents the light-induced collapse of nanoscale charge order in a 2D tantalum disulfide crystal (star shapes) and the generation of a hidden metastable metallic state (spheres). Credit: Frank Yi Gao

The development of high-speed 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 milk.

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

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

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

A paper on this research was published today in Scientific progress. 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 for addressing long-standing fundamental questions in non-equilibrium 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 time resolution of 100 femtoseconds,” adds Baldini.

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

The formation of this new phase makes the material an insulator, but the emission of a single, intense light pulse pushes the material into a metastable hidden 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 the ultrafast transformation from a single electron order to one that can persist indefinitely is not practical with conventional time-resolved techniques.”

Impulses of insight

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

By capturing the dynamics of this complex phase transformation in a single measurement, the authors demonstrated that the melting and rearrangement of the charge density wave leads to the formation of a hidden state. Theoretical calculations by Zhiyuan Sun, a postdoctoral fellow at the Harvard Quantum Institute, confirmed this interpretation.

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

Physicists use extreme infrared laser pulses to reveal frozen electron waves in magnetite

More info:
Frank Y. Gao et al, Snapshots of a light-induced metastable hidden phase driven by charge order collapse, Scientific progress (2022). DOI: 10.1126/sciadv.abp9076

Courtesy of MIT

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