By Udita Shukla
Phenomena and objects hypothesized decades ago engender a sense of utopia within a theorist, when demonstrated experimentally, or spotted observationally. Theoretical physics has been partaking a sizeable portion of scientific conversations over the past few months. Discovery of gravitational waves which were propounded by Einstein in 1913, and the discovery of the Higgs Boson, are some of the recent scientific milestones that recently created headlines.
A group of researchers at the University of Illinois at Urbana-Champaign, have proven the existence of an entirely new form of matter, termed as excitonium. While investigating pure crystals of the transition metal Dichalcogenide Titanium Diselenide (1T-TiSe2), the team was able to replicate their results on five different crystals.
What is excitonium?
Crudely speaking, on a micro level, everything around us is essentially a pool of electrons, protons and neutrons. With the right combination of temperature, pressure and number density, these particles can be confined to their lowest energy levels. Such a state of matter is known as a condensate. Condensates are found in many objects, like, neutron stars, superconductors, etc.
The enigma in the present discovery lies in the particles which form the condensate. While neutron stars are believed to be condensates of neutron stars and superconductors known to be condensates of paired electrons and phonons, this new form of matter has been demonstrated to be an excitonium condensatea particle theorised about fifty decades ago. It is supposed to be formed when an escaped electron pairs up with the vacancy that it left behind (known as a hole).
The strange quantum mechanical pairing occurs when an electron leaves the valence band to jump into the highest band of energy levels known as the conduction band, thereby, creating a hole in the valence band. The hole attracts the electron back as if it were an actual positively-charged particle. Consequently, the two oppositely charged particles form a composite particle known as an exciton.
The scientific technique
The scientists employed an innovative technique known as momentum-resolved electron energy-loss spectroscopy (M-EELS). A regular EEL spectrometer was retrofit with a goniometer, which allows a very accurate measurement of an electron’s momentum in real space. This enabled detection and measurement of collective excitations of the low-energy, paired electrons and holes, regardless of their momentum.
Significance of the discovery
The observation presents the first-ever proof of the existence of exciton condensation in a three-dimensional solid and the first-ever definitive evidence for the discovery of excitonium. The phase made its appearance as the test material approached its critical temperature of 190 Kelvin.
As per Professor of Physics, Peter Abbamonte and the team lead, Ever since the term ‘excitonium’ was coined in the 1960s by Harvard theoretical physicist Bert Halperin, physicists have sought to demonstrate its existence. Theorists have debated whether it would be an insulator, a perfect conductor, or a superfluidwith some convincing arguments on all sides. Since the 1970s, many experimentalists have published evidence of the existence of excitonium, but their findings weren’t definitive proof and could equally have been explained by a conventional structural phase transition. Clearly, the findings are of profound significance in the field of material science as well as the study of condensates.
Additionally, the applicability of M-EELS technique will also prove to be a major driving factor in advancing the study of materials and TiSe2 in particular. Moreover, the discovery may help in unravelling the intricate details of the phase reached during a metal-insulator transition in band solids, in which exciton condensation supposedly plays a role.
Discoveries of our universe
Exploration and discovery of physics at the ultra-microscopic level have always provided us with radical technologies. For example, the discovery of quantum tunnelling introduced field-effect transistors (FETs) to the world which consume about hundred times less power as compared to regular transistors. FETs are currently undergoing active research so they can be improved to be used in chips and integrated circuits.
Therefore, any new phenomenon on the scale of electrons, atoms or even molecules, holds the power to manipulate the properties and behaviour of the bulk material it is a part of. The commercial, industrial and medical fields are always in search of cost-effective and efficient materials, having versatile properties.
Which new material or technology will excitonium give us, remains to be seen!
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