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

This illustration represents the light-induced collapse of the nano-charge arrangement in a 2D crystal of tantalum disulfide (star shapes) and the generation of an unstable metallic state (spheres). Credit: Frank Yi Zhao

The development of high-speed ultra-high-speed photography in the 1960s by the late Professor Harold “Doc” Edgerton at MIT 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 spectroscopy tools, scientists at MIT and the University of Texas at Austin have, for the first time, been able to capture snapshots of an unstable phase caused by light and hidden from the equilibrium realm. Using single-shot spectroscopy techniques on a two-dimensional crystal with nanoscale modifications of the electron density, they were able to display this transition in real time.

“Through this work, we demonstrate the birth and evolution of a subtle quantum phase caused by an ultrashort laser pulse in an electronically modified crystal,” says Frank Gau Ph.D. ’22, is a co-author of a paper on the work and is currently a postdoctoral researcher at UT Austin.

“Usually using a bright laser on materials is the same as heating them, but not in this case,” adds Zhuquan Zhang, co-lead author and current graduate student in chemistry at MIT. “Here, the crystal’s irradiation rearranges the electronic system, creating an entirely new phase that is different from the high-temperature phase.”

A paper on this research was published today in science 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 unstable quantum phases is important to addressing fundamental, long-standing questions in non-equilibrium thermodynamics,” says Nelson.

“Key to this result was the development of a modern laser method that can ‘make films’ of irreversible processes in quantum materials with a time resolution of 100 femtoseconds.” Baldini adds.

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

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

Adds Nelson, “One of the major challenges is that observing an ultra-fast transformation from a single electronic system to one that may persist indefinitely is not practical with traditional technologies being resolved over time.”

pulses of insight

The researchers developed a unique method that involved splitting a single probe laser pulse into several hundred distinct probe pulses that all arrived at the sample at different times before and after the switch via a separate ultrafast excitation pulse. By measuring changes in each of these probe pulses after being reflected from or transmitted through the sample and then stringing the measurement results together like individual frames, they can create a movie that provides microscopic insights into the mechanisms through which the transitions occur.

By capturing the dynamics of this complex phase shift in a one-shot measurement, the authors show that the melting and rearrangement of the charge density wave lead to the formation of the hidden state. Theoretical calculations by Zhiyuan Sun, a postdoctoral researcher at Harvard University’s Quantum Institute, confirmed this interpretation.

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

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

more information:
Frank Y Jao et al., Snapshots of a hidden light-induced phase driven by charge order collapse, science progress (2022). DOI: 10.1126 / sciadv.abp9076

Provided by the Massachusetts Institute of Technology

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