The late MIT scientist Harold Edgerton invented high-speed flash photography in the 1960s. It allowed us to see actions invisible to the human eye, such as a bullet piercing an apple or a droplet hitting a pool of milk.
Using a variety of sophisticated spectroscopic instruments, researchers from MIT and the University of Texas at Austin have for the first time captured images of a light-induced metastable phase hidden from the equilibrium universe. They observed this transition in real time on a 2D crystal with nanoscale electron density modulations using single-shot spectroscopic methods.
According to co-author and postdoctoral researcher Frank Gao, Ph.D. '22, UT Austin, in the study "we demonstrate the formation and evolution of a latent quantum phase caused by an ultra-short laser pulse in an electronically controlled crystal".
Zhuquan Zhang, now a graduate student in chemistry at MIT, adds: “Usually, lasering on materials is the same as heating them, but not in this case. Here, irradiating the crystal causes a change of electrical order and consequently in the creation of an entirely new stage different from high temperature.
The study was published in an article in the journal Science Advances.
Addressing longstanding fundamental concerns in disequilibrium thermodynamics requires understanding the origin of such metastable quantum phases, according to Nelson.
According to Baldini, the achievement of a cutting-edge laser technique that can "create films" of irreversible processes in quantum materials with a time resolution of 100 femtoseconds was crucial to achieving this breakthrough.
Covalently bonded layers of tantalum and sulfur atoms are loosely layered on top of each other to form the substance known as tantalum disulfide. The atoms and electrons of the material form small “Star of David” formations below a certain temperature; This unusual distribution of electrons is called a "charge density wave".
The material becomes an insulator as a result of the development of this new phase, but emits a single, powerful pulse of light, forcing the material to transform into a metastable latent metal. “It is an instantaneous quantum state frozen in time,” says Baldini.
While this light-induced latent phase has been seen before, its origins in the ultra-fast quantum realm remain a mystery.
According to Nelson, one of the biggest challenges is that using traditional time-resolved techniques makes it impossible to see an ultra-fast transition from one electrical pattern to another that could take forever.
The scientists developed a new technique that involves splitting a single probe laser pulse into hundreds of separate probe pulses, all of which reach the sample at various points before and after switching with a different, ultrafast excitation pulse.
By watching the changes in each of these probe pulses after they were reflected or transmitted from the sample, they were able to create a video that offered little insight into the mechanisms by which the transformations occurred.
The authors proved that the emergence of the latent state is due to the melting and rearrangement of the charge density wave, capturing the dynamics of this complex phase shift in a single measurement.
This interpretation was supported by theoretical calculations by Zhiyuan Sun, a postdoctoral fellow at the Harvard Quantum Institute.
Although only one particular material was used in this study, the researchers claim that the same technology could be applied to probe other unusual phenomena in quantum materials. The creation of optoelectronic devices with on-demand photoresponses could also benefit from this research.
source: physorg
📩 26/07/2022 14:09
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