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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Article
  • Peer Reviewed
UC Irvine Previously Published Works (1996)
Although the heavy fermion problem is nearly twenty years old, progress is still being made, especially as experimental techniques and theoretical ideas developed for the cuprates are applied to the heavy fermion problem. Recent advances in heavy fermion physics have been made in four specific areas: superconductivity, Fermi surface measurements, marginal Fermi liquid behavior, and Kondo insulators. © 1996 Current Science Ltd.
    Cover page: Heavy fermionsCreative Commons 'BY' version 4.0 license
    • Article
    LBL Publications (2001)

    A dramatic temperature dependent enhancement of U 5f spectral weight at EF is observed in angle-resolved photoemission measurements of URu_2Si_2 at the center of an X-point hole-pocket. Comparison of this temperature dependent behavior for excitation both at and below the U 5d->5f resonant threshold is presented.

      Cover page: Temperature dependent 5f-states in URu2Si2
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2004)
      Theoretical work on Kondo systems predicts universality in the scaling of observable quantities with the Kondo temperature, T(K). Here we report infrared-frequency optical response measurements of the correlated system YbIn(1-x)AgxCu4. We observe that x-dependent variations in the frequency and strength of a low-energy excitation are related to the x-dependent Kondo temperature. Comparison of the inferred trends with existing theory and a model calculation provides a framework in which to view these experimental results as scaling phenomena arising from local-moment/conduction electron hybridization.
        Cover page: Kondo Scaling in the Optical Response of YbIn1-xAgxCu4Creative Commons 'BY' version 4.0 license
        • Article
        Lawrence Berkeley National Laboratory (2001)

        Surface-related effects in angle-resolved valence spectra of CeRu_2Si_2 at the Ce 4d->$5f resonance threshold for different cleaved surfaces are presented and compared to angle-integrated valence spectra and to bulk-sensitive Ce 3d-edge valence spectra. Evidence that Ce 4d-edge photoemission spectroscopy on ideal cleaved surfaces is dominated by bulk Ce 4f states is presented.

          Cover page: Possibility of minimal surface contributions to low photon energy angle-resolved photoemission of CeRu2Si2
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2006)
          We present an infrared/optical study of the dynamics of the strongly correlated electron system YbIn1-x Agx Cu4 as a function of composition and temperature for x ranging from 0 to 1, and T between 20 and 300 K. YbIn1-x Agx Cu4 evolves from a mixed-valent system at low x to a heavy-Fermion system at high x, and exhibits an unusual electronic phase transition in the low x region. This paper reveals information about the unusual phase transition as well as the phases themselves. Scaling relations emerge from the data and are investigated in detail using a periodic Anderson model based calculation. We also explore the temperature dependence of σ1 (ω) and the infrared sum rule behavior, and provide a picture in which to view both the low- and high-energy x -dependent features of the infrared data. © 2006 The American Physical Society.
            Cover page: Infrared dynamics of YbIn1−xAgxCu4: Kondo scaling, sum rules, and temperature dependenceCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2005)
            We report the infrared charge dynamics of YbIn1-xAgxCu4 in a series of x- and T-dependent reflectivity measurements. Within the low-temperature coherent phase, characteristics of a low energy (0.25 eV) excitation appear to be controlled by the same Kondo scale, TK, which is relevant to the magnetic response (Phys. Rev. B 56 (1997) 7993). We show that the observed TK dependence of the frequency and strength of this excitation is consistent with an interpretation in terms of a coherent-to-incoherent quasiparticle transition between the correlation-induced bands of the periodic Anderson model (Phys. Rev. Lett. 92 (2004) 186405). Additionally, we consider an interpretation of x-dependence in the higher frequency optical response in terms of a rigid band structure picture where the filling is depleted by Ag doping, augmented by a Kondo resonance which tracks the x-dependent Fermi level. In addition to the properties of the low-temperature phase, our results also encompass the anomalous temperature dependent behavior in the vicinity of the electronic phase transition at low doping (x<0.3). © 2005 Elsevier B.V. All rights reserved.
              Cover page: Optically probing the Kondo resonance in YbIn1-xAgxCu4Creative Commons 'BY' version 4.0 license
              • Article
              Lawrence Berkeley National Laboratory (2001)

              Angle-resolved photoemission measurements of divalent hexaborides reveals a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. While the global ARPES band structure and gap size observed are consistent with the results of bulk-sensitive soft x-ray absorption and emission boron K-edge spectroscopy, the surface-sensitive photoemission measurements also show a variation with cation, surface and time of the position of the surface chemical potential in the band structure.

                Cover page: Absence of X-point band overlap in divalent hexaborides and variability of the surface chemical potential
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2000)
                Infrared, visible, and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in (Formula presented) which exhibits an isostructural phase-transition into a mixed-valent phase below (Formula presented) In addition to a gradual loss of spectral weight with decreasing temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the mixed-valent phase. This feature can be described in terms of excitations into the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of resistivity, provides a direct reflection of the onset of coherence in this strongly correlated electron system. © 2000 The American Physical Society.
                  Cover page: Optical study of the electronic phase transition of strongly correlated YbInCu4Creative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  LBL Publications (2020)
                    • Article
                    • Peer Reviewed
                    LBL Publications (2016)
                    We report a study of high pressure x-ray absorption (XAS) performed in the partial fluorescence yield mode (PFY) at the U L3 edge (0–28.2 GPa) and single crystal x-ray diffraction (SXD) (0–20 GPa) on the UCd11 heavy fermion compound at room temperature. Under compression, the PFY-XAS results show that the white line is shifted by +4.1(3) eV at the highest applied pressure of 28.2 GPa indicating delocalization of the 5f electrons. The increase in full width at half maxima and decrease in relative amplitude of the white line with respect to the edge jump point towards 6d band broadening under high pressure. A bulk modulus of K0 = 62(1) GPa and its pressure derivative, K0 = 4.9(2) was determined from high pressure SXD results. Both the PFY-XAS and diffraction results do not show any sign of a structural phase transition in the applied pressure range.
                      Cover page: High pressure effects on U L3 x-ray absorption in partial fluorescence yield mode and single crystal x-ray diffraction in the heavy fermion compound UCd11Creative Commons 'BY-NC-ND' version 4.0 license
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