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Title: Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in U O 2 + x

Abstract

Mixed valence O-doped UO 2+x and photoexcited UO 2 containing transitory U 3+ and U 5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Fröhlich-type, nonequilibrium, phonon-coupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO 2, U 4O 9, and U 3O 7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f 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 U5f 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 eVmore » excitation, show the unoccupied U5f states of UO 2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping 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 for a nonequilibrium 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, nonequilibrium, Fröhlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.« less

Authors:
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Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1406677
Report Number(s):
PNNL-SA-129630
Journal ID: ISSN 2469-9950; PRBMDO; KP1704020
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review B; Journal Volume: 96; Journal Issue: 12
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Conradson, Steven D., Andersson, David A., Boland, Kevin S., Bradley, Joseph A., Byler, Darrin D., Durakiewicz, Tomasz, Gilbertson, Steven M., Kozimor, Stosh A., Kvashnina, Kristina O., Nordlund, Dennis, Rodriguez, George, Seidler, Gerald T., Bagus, Paul S., Butorin, Sergei M., Conradson, Dylan R., Espinosa-Faller, Francisco J., Hess, Nancy J., Kas, Joshua J., Lezama-Pacheco, Juan S., Martin, Philippe, Martucci, Mary B., Rehr, John J., Valdez, James A., Bishop, Alan R., Baldinozzi, Gianguido, Clark, David L., and Tayal, Akhil. Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x. United States: N. p., 2017. Web. doi:10.1103/PhysRevB.96.125114.
Conradson, Steven D., Andersson, David A., Boland, Kevin S., Bradley, Joseph A., Byler, Darrin D., Durakiewicz, Tomasz, Gilbertson, Steven M., Kozimor, Stosh A., Kvashnina, Kristina O., Nordlund, Dennis, Rodriguez, George, Seidler, Gerald T., Bagus, Paul S., Butorin, Sergei M., Conradson, Dylan R., Espinosa-Faller, Francisco J., Hess, Nancy J., Kas, Joshua J., Lezama-Pacheco, Juan S., Martin, Philippe, Martucci, Mary B., Rehr, John J., Valdez, James A., Bishop, Alan R., Baldinozzi, Gianguido, Clark, David L., & Tayal, Akhil. Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x. United States. doi:10.1103/PhysRevB.96.125114.
Conradson, Steven D., Andersson, David A., Boland, Kevin S., Bradley, Joseph A., Byler, Darrin D., Durakiewicz, Tomasz, Gilbertson, Steven M., Kozimor, Stosh A., Kvashnina, Kristina O., Nordlund, Dennis, Rodriguez, George, Seidler, Gerald T., Bagus, Paul S., Butorin, Sergei M., Conradson, Dylan R., Espinosa-Faller, Francisco J., Hess, Nancy J., Kas, Joshua J., Lezama-Pacheco, Juan S., Martin, Philippe, Martucci, Mary B., Rehr, John J., Valdez, James A., Bishop, Alan R., Baldinozzi, Gianguido, Clark, David L., and Tayal, Akhil. Fri . "Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x". United States. doi:10.1103/PhysRevB.96.125114.
@article{osti_1406677,
title = {Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x},
author = {Conradson, Steven D. and Andersson, David A. and Boland, Kevin S. and Bradley, Joseph A. and Byler, Darrin D. and Durakiewicz, Tomasz and Gilbertson, Steven M. and Kozimor, Stosh A. and Kvashnina, Kristina O. and Nordlund, Dennis and Rodriguez, George and Seidler, Gerald T. and Bagus, Paul S. and Butorin, Sergei M. and Conradson, Dylan R. and Espinosa-Faller, Francisco J. and Hess, Nancy J. and Kas, Joshua J. and Lezama-Pacheco, Juan S. and Martin, Philippe and Martucci, Mary B. and Rehr, John J. and Valdez, James A. and Bishop, Alan R. and Baldinozzi, Gianguido and Clark, David L. and Tayal, Akhil},
abstractNote = {Mixed valence O-doped UO2+x and photoexcited UO2 containing transitory U3+ and U5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Fröhlich-type, nonequilibrium, phonon-coupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO2, U4O9, and U3O7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f 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 U5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO2 therefore fulfills the criteria for a nonequilibrium 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 UO2 excited state. This heterogeneous, nonequilibrium, Fröhlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.},
doi = {10.1103/PhysRevB.96.125114},
journal = {Physical Review B},
number = 12,
volume = 96,
place = {United States},
year = {Fri Sep 01 00:00:00 EDT 2017},
month = {Fri Sep 01 00:00:00 EDT 2017}
}