For the first time, scientists at MIT and the University of Texas at Austin report having immortalized a light-induced metastable quantum phase, using advanced spectroscopy techniques. Understanding the origin of this state will allow both to explore non-equilibrium thermodynamics and to develop optoelectronic devices with on-demand photoresponses.
Ultrafast light-matter interactions can trigger many so-called “exotic” phenomena in quantum materials, such as light-induced (or photo-induced) superconductivity. Research on photoinduced hidden phases, ie on thermally inaccessible non-equilibrium states, is increasingly flourishing.
The researchers of the new study, published in the journal Science Advances, report that some of these phases can persist indefinitely under appropriate environmental conditions, even though many hidden phases induced by laser pulses are short-lived. ” For these non-equilibrium metastable phases, there remain significant gaps in our understanding. “, they write. Indeed, mapping the ultrafast formation of a long-lived hidden phase remains a long-standing challenge, as the initial state is not recovered quickly.
« Understanding the origin of these metastable quantum phases is important for answering long-standing fundamental questions in non-equilibrium thermodynamics. “, said in a press release Keith A. Nelson, co-author of the study and professor of chemistry at MIT. Non-equilibrium thermodynamics is a field of research that studies irreversible phenomena that are not in thermodynamic equilibrium. He is interested in transport processes and rates of chemical reactions.
A state-of-the-art spectroscopy technique that produces several hundred laser pulses
The researchers therefore developed an advanced laser method to capture snapshots of these phenomena in quantum materials, with a temporal resolution of 100 femtoseconds. The material of the electronic crystal is tantalum disulfide (1T-TaS2), formed by layers of tantalum and sulfur atoms loosely stacked on top of each other.
« Usually, aiming lasers at materials amounts to heating them, but not in this case adds Zhuquan Zhang, co-author of the study and a chemistry graduate student at MIT. ” Here, the irradiation of the crystal rearranges the electronic order, creating an entirely new phase, different from that of the high temperature “. The atoms and electrons of the material then form a “star of David” structure, visible at the nanometric scale.
« It is a transient quantum state frozen in time says Edoardo Baldini, study co-author and assistant professor of physics at UT-Austin. ” People have observed this hidden light-induced phase before, but the ultrafast quantum processes behind its genesis were still unknown. ».
In detail, American scientists have developed a new method to produce several hundred separate laser pulses from a single probe laser pulse. These numerous pulses reach the sample at different times, and their changes following reflection/transmission by the sample have been measured.
This made it possible to reconstruct a film giving microscopic insight into the mechanisms by which the transformations occur. The authors finally demonstrated that the fusion and reorganization fluctuations of the charge density wave lead to the formation of the hidden state.
They say their results shed light on the origin of this elusive state and pave the way for the discovery of other exotic phases of matter — although the study was carried out with a particular material. In addition, they might allow the development of optoelectronic devices with on-demand photoresponses.
