Abstract
Bose-Einstein condensates (BECs) composed of polarons would be an advance because they would combine coherently charge, spin, and a crystal lattice. Following our earlier report of unique structural and spectroscopic properties, we now identify potentially definitive evidence for polaronic BECs in photo- and chemically doped UO2(+x) on the basis of exceptional coherence in the ultrafast time dependent terahertz absorption and microwave spectroscopy results that show collective behavior including dissipation patterns whose precedents are condensate vortex and defect disorder and condensate excitations. That some of these signatures of coherence in an atom-based system extend to ambient temperature suggests a novel mechanism that could be a synchronized, dynamical, disproportionation excitation, possibly via the solid state analog of a Feshbach resonance that promotes the coherence. Such a mechanism would demonstrate that the use of ultra-low temperatures to establish the BEC energy distribution is a convenience rather than a necessity, with the actual requirement for the particles being in the same state that is not necessarily the ground state attainable by other means. A macroscopic quantum object created by chemical doping that can persist to ambient temperature and resides in a bulk solid would be revolutionary in a number of scientific and technological fields.
Highlights
Bose-Einstein condensates (BECs) composed of polarons would be an advance because they would combine coherently charge, spin, and a crystal lattice
We have previously described structural and both ultrafast optical pump-probe and static x-ray absorption spectroscopy experiments on the chemically and photo-doped, partly filled 5f Mott insulator UO2(+x) that we interpreted as the signatures of superfluid droplets of aggregated polarons that self-organized as coherent bosonic states[22]
This dissipation is like that in cuprate superconductors[23,24] where it was attributed to vortex reordering,[25,26,27] a phenomenon that occurs with BECs28–30, and the features are similar to the distinct excitations found in BECs but at radiofrequency energies consistent with their much lower stability[31,32,33,34,35]. These mesoscale condensates would be composed of composite bosons formed from phase-coherent fermionic polarons that are charge defects in the lattice and not quantum fluctuations[35,36]. The formation of these bosons by a dynamic charge transfer between the U ions and their condensation and display of superfluid properties are promoted by a Feshbach resonance, analogous to the formation of the Cooper pairs that give the pseudogap phase and its subsequent condensation in ultracold fermionic atom gases promoted by this type of resonance facilitating the conversion between the original atoms and bosonic diatomic molecules[37]
Summary
That the direction of the hysteresis is that of ferromagnets and not cuprates is consistent with the field interacting directly with the particles as in fermionic condensate formation instead of with excluded/expelled flux lines These UO2+x spectra are notable in not exhibiting any features coincident with the AFM ordering transition. The condensate emerges from individual or aggregated quasiparticles whose collective interactions are enabled by dynamically expanded wavefunctions extending into the UO2 host This interpretation of the unique oscillation observed in the IGS pump-TTDS probe experiment is corroborated by the presence of distinct mesoscale domains of a coherent quasiparticle fluid, as derived from our prior structural measurements and observed for other quasiparticle condensates[5,63]. To the extent that the proposed mechanism is correct, we note that the Feshbach resonance occurs in the crossover or pseudogap regime, which would suggest that the UO2(+x) condensate could be tuned in both the BEC and BCS directions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
