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

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

The development of high-speed strobe flash photography in the 1960s by the late MIT professor Harold “Doc” Edgerton allowed us to visualize events too quickly for the eye – a bullet piercing an apple or a droplet hitting a pool of milk.

Now, using a suite of advanced spectroscopic tools, scientists at MIT and the University of Texas at Austin have for the first time captured snapshots of a hidden light-induced metastable phase of the universe at balance. Using single-shot spectroscopy techniques on a 2D crystal with nanometric modulations of electron density, they were able to visualize 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 Frank Gao Ph.D. ’22, co-lead author of a paper on work that is currently postdoctoral at UT Austin.

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

An article on this research was published today in Scientists progress. The project was co-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 these metastable quantum phases is important for answering long-standing fundamental questions of non-equilibrium thermodynamics,” Nelson says.

“Key to this result was the development of a state-of-the-art laser method capable of ‘filming’ irreversible processes in quantum materials with a time resolution of 100 femtoseconds.” adds Baldini.

The material, tantalum disulfide, is made up of covalently bonded layers of tantalum and sulfur atoms stacked on top of each other. Below a critical temperature, the atoms and electrons in the pattern of the material form 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 a single intense light pulse pushes the material into a metastable hidden metal. “It’s a transient quantum state frozen in time,” Baldini explains. “People have observed this hidden light-induced phase before, but the ultrafast quantum processes behind its genesis were still unknown.”

Nelson adds: “One of the main challenges is that observing an ultrafast transformation from one electronic order to another that can persist indefinitely is impractical 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 that all arrived at the sample at different times before and after switching was initiated by a probe pulse. separate ultrafast excitation. By measuring the changes in each of these probe pulses after they have been 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 information about the mechanisms by which transformations occur.

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

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


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

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