- Physicists discover a new form of matter called Excitonium, which was theorized 50 years ago.
- It’s made of excitons, particles formed in a strange quantum mechanical pairing.
- Momentum-resolved Electron Energy-Loss Spectroscopy (M-EELS) is used to discover it.
Physicists at University of California, Berkeley and University of Illinois, Urbana Champaign scrutinized non-doped crystals of the transition metal Dichalcogenide Titanium Diselenide (1T-TiSe2) and generated astonishing results 5 times on different cleaved crystals.
The new form of matter, called Excitonium, has gained a lot of attention from other scientists because it was first theorized by Harvard physicist Bert Halperin, nearly 50 years ago. Since then many researchers have tried to demonstrate its existence but none of them came close to a solid result. So what exactly US physicists have discovered and why it took 50 years to prove it, lets find out.
What is Excitonium?
It’s a condensate made up of excitons, which are particles formed in an odd quantum mechanical pairing, thus exhibiting macroscopic quantum phenomena like superfluid, superconductor, or insulating electronic crystal.
Artist’s drawing showing solid exciton background (blue) and domain walls (yellow)
The odd quantum mechanical pairing refers to an escaped electron leaving behind a hole. Let me explain in a simple terms, when an electron gets excited and jumps, it leaves behind a hole, which behaves as though it was a particle itself with a positive charge. Because of this positive charge, the hole can attract an electron and pair them to generate a composite particle, a boson — an exciton.
Why It Took More Than 50 Years to Discover It?
Until now, scientists haven’t had the experimental tools to clearly distinguish the complex matters, so they didn’t know how excitonium looks like. Because of lack of technology and equipments they couldn’t differentiate the matter from what’s called Peierls Phase.
Peierls phases and exciton condensation share the similar observables (superlattice and the opening of single particle energy gap) and same symmetry.
The EEL (Electron Energy Loss) Spectrometer we have now can only measure the trajectory of an electron, and provide details like how much momentum and energy it lost. Moreover, the goniometer only measures the momentum of an electron in real space.
The New Technique
To overcome this challenge, the research team developed a new technique called Momentum-resolved Electron Energy-Loss Spectroscopy (M-EELS), which captures the data of valence band excitations more efficiently as compared to other neutron scattering methodologies or inelastic X ray.
The relationship between momentum and energy of excitonium with M-EELS. Image credit: Peter Abbamonte
The M-EELS allowed researchers to calculate the collective excitation of low-energy bosonic particles, the paired holes and electrons, irrespective of their momentum. For the first time, they observed a soft plasmon phase, which evolved when the material approached to 190 Kelvin (critical temperature). It’s a first ever definitive evidence of exciton condensation in a 3D solid, or excitonium
The results obtained were replicated 5 times on different cleaved crystals, and each of them gave reproducible data for nearly 40 hours under high vacuum conditions.
M-EELS data versus previous studies of TiSe2
M-EELS measurement of Titanium diselenide are consistent with results obtained with other techniques. The figure A and B shows static M-EELS maps of momentum space captured at 6meV energy resolution.
Frequency dependent M-EELS spectra taken at zero momentum for a series of temperatures is shown in figure C. At 300 Kelvin, a highly damped electronic mode, the energy and line-width reduce when cooling through critical temperature, is observed at 82 meV. The mode energy rises again at low temperature (17 Kelvin), reaching 47 meV.
As compared to previous IR spectroscopy researches, these changes identify excitation as a free carrier plasmon because its energy is significantly higher than that of the highest optical phonon at 36 meV. Moreover, these changes in plasmon energy and line-width were considered decreasing in carrier density and Landau damping caused by opening an energy gap at the critical temperature.
The macroscopic quantum phenomena have shaped our understanding of quantum mechanics. This research looks very promising to further unlock the mysteries of quantum mechanical.
It could also explain the metal insulator transition in band solids, where exciton condensation plays a crucial role. Beyond that,excitoniu m has hundreds of pure speculative applications, and even numerous possible applications are yet to be identified.