Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x

Abstract

with O XAS and NIXS and U M 5 RIXS shows that O-doping raises the peak of the U 5f states of the valence band by ~0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs. the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2-2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U 5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO 2 coincident with the excitation pulse with 4.7 eV excitation show the unoccupied 5f states of UO 2 and no hot electrons. 3.1 eV excitation, however, complements the Odoping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO 2 therefore fulfills the criteria prescribed for a non-equilibrium condensate. The electron distributions resulting from both excitations persist for 5-10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: 1) the direct formation of both of these long lived (>3-10 ps) excited states without the short lived nonthermal intermediate; 2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and 3) the absence of hot electrons accompanying the insulating UO 2 excited state. This heterogeneous, non-equilibrium, Fröhlich BEC stabilized by a FanoFeshbach resonance therefore continues to exhibit unique properties.Closing the Mott gap in UO 2(+x) -2

Charge Localization and Magnetic Correlations in the Refined Structure of U3O7

“…38,46,47 Interestingly, a minority fraction of U(VI) has also been reported for the fluorite-derived compounds. 48,49 The associated uranium coordination chemistry is, however, not straightforward. In addition to simple cubic and octahedral coordination environments also pentagonal bipyramids, square antiprisms and various distortions of the former polyhedra can be identified.…”

Local structure of Sr 2 CuO 3.3 , a 95 K cuprate superconductor without CuO 2 planes

“…The tunneling polaron remains in the same location within a crystal, where it could be pinned because of stationary defects or special aspects of the atoms of its location. In addition, in contrast to the original, elementary, three-atom model of a Cu atom bracketed by two Oap, it may involve a large number of atoms and the two structures will be nondegenerate (36,37).…”