Possible Bose-condensate behavior in a quantum phase originating in a collective excitation in the chemically and optically doped Mott-Hubbard system UO2+x

Steven D. Conradson, Tomasz Durakiewicz, Francisco J. Espinosa-Faller, Yong Q. An, David A. Andersson, Alan R. Bishop, Kevin S. Boland, Joseph A. Bradley, Darrin D. Byler, David L. Clark, Dylan R. Conradson, Leilani L. Conradson, Alison L. Costello, Nancy J. Hess, Gerard H. Lander, Anna Llobet, Mary B. Martucci, Jose Mustre De Leon, Dennis Nordlund, Juan S. Lezama-PachecoThomas E. Proffen, George Rodriguez, Daniel E. Schwarz, Gerald T. Seidler, Antoinette J. Taylor, Stuart A. Trugman, Trevor A. Tyson, James A. Valdez

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38 Scopus citations


X-ray pair distribution function (pdf) and U L3 extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO2 show U(VI)-oxo moieties with x rays for mixed valence U4O9 and U3O7, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO2 are essentially identical at 30, 100, and 200 K, those of the UO2+x compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO2+x compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U4O9 does not behave as a linear combination of the UO2 and U3O7 fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump-1.57 eV probe reflectivity on UO2 that initially populates the U 6d-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-μsec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5f-dominated portion of the UHB shows an analogous 2.8-μsec peak but also a plateau bracketing this peak that ends in a cusp at 50-60 K and an abrupt change in the hardening rate of a novel 12-15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5-1.1 THz range with a decrease in temperature from ∼30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer-phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) ↠U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Original languageEnglish (US)
Article number115135
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number11
StatePublished - Sep 23 2013

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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