Mysterious Hidden Quantum Phase in a 2D Crystal Captured by Scientists for the First Time

Single-shot spectroscopic methods provide researchers a fresh perspective on a puzzling light-driven phenomenon.

High-speed strobe-flash photography, created in the 1960s by the late MIT professor Harold "Doc" Edgerton, enables us to see actions that would otherwise be invisible to the eye, such as a droplet hitting a pool of milk or a bullet penetrating an apple.

Using a variety of cutting-edge spectroscopic instruments, researchers from MIT and the University of Texas at Austin have now successfully acquired images of a light-induced metastable phase that is concealed from the equilibrium world. They used single-shot spectroscopic methods on a 2D crystal with nanoscale changes in electron density to witness this transition in real-time.

According to Frank Gao PhD '22, co-lead author of a publication describing the study and presently a postdoc at UT Austin, "With this work, we are revealing the genesis and development of a hidden quantum phase caused by an ultrashort laser pulse in an electronically controlled crystal."

Zhuquan Zhang, co-lead author and current MIT graduate student in chemistry, adds, "Usually, shining lasers on materials is the same as heating them, but not in this situation. Here, irradiating the crystal causes the electrical order to change, resulting in the creation of a whole new phase that is distinct from the high-temperature one.

On July 22, the journal Science Advances published a manuscript on this investigation. Keith A. Nelson, the Haslam and Dewey Professor of Chemistry at MIT, and Edoardo Baldini, an associate professor of physics at UT-Austin, collaborated on project coordination.

laser displays

Addressing long-standing basic concerns in nonequilibrium thermodynamics requires understanding the genesis of such metastable quantum phases, according to Nelson.

According to Baldini, the achievement of a cutting-edge laser technique that can "create movies" of irreversible events in quantum materials with a time precision of 100 femtoseconds was crucial to the achievement of this breakthrough.

Covalently bonded layers of tantalum and sulfur atoms are layered loosely on top of one another to form the substance known as tantalum disulfide. The material's atoms and electrons form tiny "Star of David" formations below a certain temperature; this unusual distribution of electrons is referred to as a "charge density wave."

The material becomes an insulator as a result of the development of this new phase, but emitting a single, powerful light pulse forces the material to transform into a metastable concealed metal. Baldini claims, "It is a momentary quantum state fixed in time." Although this light-induced hidden phase has previously been seen, its origins in the ultrafast quantum realm remain a mystery.

According to Nelson, one of the major difficulties is that using traditional time-resolved methods makes it impossible to see an ultrafast transition from one electrical order to another that may last eternally.

Rhythms of wisdom

The scientists created a novel technique that included dividing a single probe laser pulse into hundreds of discrete probe pulses that all arrived at the sample at various points both before and after switching was started by a different, ultrafast excitation pulse. They were able to create a video that offers tiny insights into the mechanisms through which transformations occur by monitoring changes in each of these probe pulses after they were reflected from or transmitted through the material.

The authors proved that the emergence of the hidden state is caused by the melting and reordering of the charge density wave by capturing the dynamics of this intricate phase shift in a single-shot measurement. This interpretation was supported by theoretical calculations made by Zhiyuan Sun, a postdoc at the Harvard Quantum Institute.

Although just one particular material was employed in this study, the researchers claim that the same technology may now be applied to investigate other unusual events in quantum materials. The creation of optoelectronic devices with on-demand photoresponses may benefit from this research as well.