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Researchers Discover Excitonium - a Weird New Form of Matter
Excitonium, a strange form of matter that was first theorized almost 50 years ago, has now been discovered by researchers. What is excitonium? It is a rather exotic condensate that exhibits macroscopic quantum phenomena like a superconductor or a superfluid. It consists of excitons, particles formed from an unlikely pairing of an escaped electron and the hole it leaves behind. The hole actually behaves like a positively-charged particle itself. It attracts an electron and together they form the composite particle known as the exciton.
In their experiments on non-doped crystals of the transition metal dichalcogenide titanium diselenide (1T-TiSe2), the researchers were able to observe the material and its precursor soft plasmon phase, called "the smoking gun" that proves excitonium's existence. The precursor phase emerges as the material approaches its critical temperature. The scientists reproduced their results 5 times on different cleaved crystals during the testing, adding more confidence to the study.
What they achieved in particular is developing a new technique called momentum-resolved electron energy-loss spectroscopy (M-EELS) that is sensitive enough to distinguish the new material from Peierls phase, an unrelated substance that has the same symmetry.
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.
[...] Provided the interaction is attractive, an exciton can bind with other excitons to form a biexciton, analogous to a dihydrogen molecule. If a large density of excitons is created in a material, they can interact with one another to form an electron-hole liquid, a state observed in k-space indirect semiconductors.
Additionally, excitons are integer-spin particles obeying Bose statistics in the low-density limit. In some systems, where the interactions are repulsive, a Bose–Einstein condensed state, called excitonium, is predicted to be the ground state. Some evidence of excitonium has existed since the 1970s, but has often been difficult to discern from a Peierls phase. Exciton condensates have allegedly been seen in a double quantum well systems. In 2017 Kogar et al. found "compelling evidence" for observed excitons condensing in the three-dimensional semimetal 1T-TiSe2.
Also at Newsweek.
Signatures of exciton condensation in a transition metal dichalcogenide (DOI: 10.1126/science.aam6432) (DX)
(Score: 2) by turgid on Monday December 11 2017, @09:20PM (1 child)
And presumably knowing the amount of energy put in to put the electron into the excited state, one can calculate the probability of it falling back into the ground state emitting a packet of energy (photon?) in a give period of time?
I refuse to engage in a battle of wits with an unarmed opponent [wikipedia.org].
(Score: 3, Informative) by maxwell demon on Tuesday December 12 2017, @07:40AM
Actually you need to also know the "energy landscape" (band structure) of the material. Indeed, actually the excited electrons I've been speaking of are actually quasiparticles, too, consisting of an actual electron and the reaction of the material to the presence of that electron. Therefore it's energy-momentum relation (basically its kinetic energy) is different from that of a free electron; indeed, it can have its energy minimum at a non-zero momentum (note that the relation between momentum and speed is also affected by the material, so non-zero momentum doesn't necessarily mean non-zero speed). This is essentially what happens in the k-space indirect semiconductors mentioned in the summary: The electron and the hole have minima at different momenta, therefore in general the electron will have to get rid of some extra momentum in order to recombine with the hole. That certainly affects the probabilities.
Also note that there may be different states of same energy, but different physical properties (for example, in most materials, a spin up and a spin down quasiparticle have the same energy). The probabilities of de-excitation may be very different between those states.
The Tao of math: The numbers you can count are not the real numbers.