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Many-body interactions in quasi-freestanding graphene

Description: The Landau-Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming one of the fundamental theories of solids. Whether this picture still holds for a semimetal such as graphene at the neutrality point, i.e., when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present such a study in quasi-freestanding graphene by using high-resolution angle-resolved photoemission spectroscopy. We see the electron-electron and electron-phonon interactions go through substantial changes when the semimetallic regime is approached, including renormalizations due to strong electron-electron interactions with similarities to marginal Fermi liquid behavior. These findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions.
Date: June 3, 2011
Creator: Siegel, David; Park, Cheol-Hwan; Hwang, Choongyu; Deslippe, Jack; Fedorov, Alexei; Louie, Steven et al.
Partner: UNT Libraries Government Documents Department

The hierarchy of multiple many-body interaction scales in high-temperature superconductors

Description: To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher energy scale, we present improved experimental data from four families of high-T{sub c} superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low energy anomaly ('kink') of 0.03-0.09eV, a high energy anomaly of 0.3-0.5eV, and an anomalous enhancement of the width of the LDA-based CuO{sub 2} band extending to energies of {approx} 2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.
Date: May 3, 2010
Creator: Meevasana, W.
Partner: UNT Libraries Government Documents Department

Superconductivity-Induced Self-Energy Evolution of the Nodal Electron in Optimally-Doped Bi2212

Description: The temperature dependent evolution of the renormalization effect in optimally-doped Bi2212 along the nodal direction has been studied via angle-resolved photoemission spectroscopy. Fine structure is observed in the real part of the self-energy (Re{Sigma}), including a subkink and maximum, suggesting that electrons couple to a spectrum of bosonic modes, instead of just one mode. Upon cooling through the superconducting phase transition, the fine structures of the extracted Re{Sigma} exhibit a two-processes evolution demonstrating an interplay between kink renormalization and superconductivity. We show that this two-process evolution can be qualitatively explained by a simple Holstein model in which a spectrum of bosonic modes is considered.
Date: May 3, 2010
Creator: Lee, W.S.
Partner: UNT Libraries Government Documents Department

Analysis of the Spectral Function of Nd1.85Ce0.15CuO4, Obtained by Angle Resolved Photoemission Spectroscopy

Description: Samples of Nd{sub 2-x}Ce{sub x}CuO{sub 4}, an electron-doped high temperature superconducting cuprate (HTSC), near optimal doping at x = 0.155 were measured via angle resolved photoemission (ARPES). We report a renormalization feature in the self energy ('kink') in the band dispersion at {approx} 50-60 meV present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.
Date: May 3, 2010
Creator: Schmitt, F.
Partner: UNT Libraries Government Documents Department

Angle-resolved photoemission studies of lattice polaron formation in the cuprate Ca2CuO2Cl2

Description: To elucidate the nature of the single-particle excitations in the undoped parent cuprates, we have performed a detailed study of Ca{sub 2}CuO{sub 2}Cl{sub 2} using photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard band are found to be well-described by a polaron model. By comparing the lineshape and temperature dependence of the lower Hubbard band with additional O 2p and Ca 3p states, we conclude that the dominant broadening mechanism arises from the interaction between the photohole and the lattice. The strength of this interaction was observed to be strongly anisotropic and may have important implications for the momentum dependence of the first doped hole states.
Date: May 3, 2010
Creator: Shen, K.M.
Partner: UNT Libraries Government Documents Department

Direct observation of the mass renormalization in SrVO3 by angle resolved photoemission spectroscopy

Description: We have performed an angle-resolved photoemission study of the three-dimensional perovskite-type SrVO{sub 3}. Observed spectral weight distribution of the coherent part in the momentum space shows cylindrical Fermi surfaces consisting of the V 3d t{sub 2g} orbitals as predicted by local-density approximation (LDA) band-structure calculation. The observed energy dispersion shows a moderately enhanced effective mass compared to the LDA results, corresponding to the effective mass enhancement seen in the thermodynamic properties. Contributions from the bulk and surface electronic structures to the observed spectra are discussed based on model calculations.
Date: May 3, 2010
Creator: Yoshida, t.
Partner: UNT Libraries Government Documents Department

Doping Dependent Charge Transfer Gap and Realistic Electronic Model of n-type Cuprate Superconductors

Description: Based on the analysis of the measurement data of angle-resolved photoemission spectroscopy (ARPES) and optics, we show that the charge transfer gap is significantly smaller than the optical one and is reduced by doping in electron doped cuprate superconductors. This leads to a strong charge fluctuation between the Zhang-Rice singlet and the upper Hubbard bands. The basic model for describing this system is a hybridized two-band t-J model. In the symmetric limit where the corresponding intra- and inter-band hopping integrals are equal to each other, this two-band model is equivalent to the Hubbard model with an antiferromagnetic exchange interaction (i.e. the t-U-J model). The mean-field result of the t-U-J model gives a good account for the doping evolution of the Fermi surface and the staggered magnetization.
Date: May 3, 2010
Creator: Xiang, T.
Partner: UNT Libraries Government Documents Department

Doping Evolution of the Underlying Fermi Surface in La_2&#8722xSr_xCuO_4

Description: We have performed a systematic doping dependent study of La{sub 2-x}Sr{sub x}CuO{sub 4} (LSCO) (0.03 {le} x {le} 0.3) by angle-resolved photoemission spectroscopy. In the entire doping range, the underlying 'Fermi surface' determined from the low energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly-doped region. This is in strong contrast to the result on Ca{sub 2-x}Na{sub x}CuO{sub 2}Cl{sub 2} (Na-CCOC), which shows a strong deviation from Luttinger's theorem. The differences between LSCO and Na-CCOC are correlated with the different behaviors of the chemical potential shift and spectral weight transfer induced by hole doping.
Date: May 3, 2010
Creator: Yoshida, T.
Partner: UNT Libraries Government Documents Department

Bulk band gaps in divalent hexaborides: A soft x-ray emission study

Description: Boron K-edge soft x-ray emission and absorption are used to address the fundamental question of whether divalent hexaborides are intrinsic semimetals or defect-doped bandgap insulators. These bulk sensitive measurements, complementary and consistent with surface-sensitive angle-resolved photoemission experiments, confirm the existence of a bulk band gap and the location of the chemical potential at the bottom of the conduction band.
Date: October 3, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Allen, James W.; Bianchi, Andrea D. & Fisk, Zachary
Partner: UNT Libraries Government Documents Department

Final Report on the Construction of the HERS Endstation, September 1, 1996 - October 31, 1998

Description: The construction of the HERS endstation was successfully completed. The resolution of the photoemission apparatus is better than 7meV. The angular resolution is +0.15 degrees. The system has a high precision sample manipulator w/ five-degrees of freedom and a temperature range from 10 - 450 degrees K. The sample transfer system allows transfer of a sample from atmosphere onto the crystal @ 5x10 in less than 2 hrs.
Date: April 3, 2000
Creator: Kellar, S.A. Dr. & Shen, Z.X.
Partner: UNT Libraries Government Documents Department

Photoelectron diffraction of magnetic ultrathin films: Fe/Cu(001)

Description: The preliminary results of an ongoing investigation of Fe/Cu(001) are presented here. Energy dependent photoelectron diffraction, including the spin-dependent variant using the multiplet split Fe3s state, is being used to investigate the nanoscale structures formed by near-monolayer deposits of Fe onto Cu(001). Core-level photoemission from the Fe3p and Fe3s states has been generated using synchrotron radiation as the tunable excitation source. Tentatively, a comparison of the experimental Fe3p cross section measurements with multiple scattering calculations indicates that the Fe is in a fourfold hollow site with a spacing of 3.6{Angstrom} between it and the atom directly beneath it, in the third layer. This is consistent with an FCC structure. The possibility of utilizing spin-dependent photoelectron diffraction to investigate magnetic ultrathin films will be demonstrated, using our preliminary spectra of the multiplet-split Fe3s os near-monolayer Fe/Cu(001). 18 refs., 10 figs.
Date: January 3, 1991
Creator: Tobin, J.G. (Lawrence Livermore National Lab., CA (USA)); Wagner, M.K. (Wisconsin Univ., Madison, WI (USA). Dept. of Chemistry); Guo, X.Q. & Tong, S.Y. (Wisconsin Univ., Milwaukee, WI (USA). Dept. of Physics)
Partner: UNT Libraries Government Documents Department