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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

Absence of X-point band overlap in divalent hexaborides and variability of the surface chemical potential

Description: 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.
Date: November 4, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Mo, Sung-Kwan; Allen, James W.; Sarrao, John L.; Bianchi, Adrian D. et al.
Partner: UNT Libraries Government Documents Department


Description: High resolution angle-resolved photoemission spectroscopy of highly overdoped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} with a T{sub c} = 51K indicates that the basic transport processes in this material are fundamentally different from both the lesser doped cuprates as well as model metallic compounds. The overdoped sample has sharp ARPES peaks at the Fermi energy throughout the Brillouin zone even in the normal state, unlike the lesser-doped compounds. In particular, the spectra near ({pi},0) point show the presence of a sharp peak well above T{sub c}. The ARPES lineshapes, and thus the self energy, at a given energy are almost independent of k. Further, the quasiparticle scattering rate at the Fermi energy seems to be closely tied to direct resistivity measurements. This leads us to the conclusion that overdoped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is best described as a quasiparticle liquid. However, the energy dependence of the scattering rates is quite similar to that found in the lesser-doped compounds and quite different from that seen in a typical metal.
Date: July 23, 2001
Partner: UNT Libraries Government Documents Department

Studies of photo-emission and field emission in an RF photo-injector with a high quantum efficiency photo-cathode

Description: An 18 MeV electron photo-injector facility was constructed at Fermilab in collaboration with UCLA, INFN-Milano, University of Rochester, and DESY. The photoinjector is now being operated in partnership with the Northern Illinois Center for Accelerator and Detector Development. The facility consists of an RF gun with a cesium telluride (Cs{sub 2}Te) photo-cathode [3] excited by a pulsed ultra-violet (UV) laser [4], a 9-cell superconducting cavity, a magnetic chicane, and a beam line for experiments with the electron beam. The original goal was to produce a beam with high charge per bunch (8 nC) and low transverse emittance (< 20 mm mrad) in long bunch trains (800 bunches, 1 {micro}s apart). The facility at Fermilab was used to develop, fabricate, and commission an RF gun for the TeSLA Test Facility Linac. This gun has been in use at DESY since November 1998, and its behavior is under study [5]. A second RF gun was subsequently installed at Fermilab. Recent activities at Fermilab include studies of photo-injector dynamics [6,7] and various experiments with the electron beam: electro-optic sampling of wake fields [8], plasma wakefield acceleration, channeling radiation studies [9], and production of flat beams [10]. Another major goal for the photo-injector was high QE and low field emission (dark current) from the gun (the latter being a source of unwanted radiation and unwanted background signals). Hence, in between the work outlined above, studies of the photo-emission and field emission behavior of the gun were carried out. The preliminary results will be summarized in this paper.
Date: August 10, 2001
Creator: al., W. Hartung et
Partner: UNT Libraries Government Documents Department

X-ray photoemission electron microscopy, a tool for the investigation of complex magnetic structures.

Description: X-ray Photoemission Electron Microscopy unites the chemical specificity and magnetic sensitivity of soft x-ray absorption techniques with the high spatial resolution of electron microscopy. The discussed instrument possesses a spatial resolution of better than 50 nm and is located at a bending magnet beamline at the Advanced Light Source, providing linearly and circularly polarized radiation between 250 and 1300 eV. We will present examples which demonstrate the power of this technique applied to problems in the field of thin film magnetism. The chemical and elemental specificity is of particular importance for the study of magnetic exchange coupling because it allows separating the signal of the different layers and interfaces in complex multi-layered structures.
Date: August 30, 2001
Creator: Scholl, Andreas; Ohldag, Hendrik; Nolting, Frithjof; Stohr, Joachim & Padmore, Howard A.
Partner: UNT Libraries Government Documents Department

Electron-like Fermi surface and Remnant (pi,0) features in overdoped La(1.78)Sr(0.22)CuO4

Description: We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The data analysis using energy distribution curves, momentum distribution curves and intensity maps all show evidence of an electron-like Fermi surface, which is well explained by band structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around (pi,0), where the band is predicted to lie above EF.
Date: January 1, 2001
Creator: Yoshida, T.; Zhou, X.J.; Nakamura, M.; Kellar, S.A.; Bogdanov, P.V.; Lu, E.D. et al.
Partner: UNT Libraries Government Documents Department

Possibility of minimal surface contributions to low photon energy angle-resolved photoemission of CeRu{sub 2}Si{sub 2}

Description: Surface-related effects in angle-resolved valence spectra of CeRu{sub 2}Si{sub 2} at the Ce 4d {yields} 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.
Date: November 1, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Allen, James W. & Sarrao, John L.
Partner: UNT Libraries Government Documents Department

Temperature dependent 5f-states in URu2Si2

Description: A dramatic temperature dependent enhancement of U 5f spectral weight at EF is observed in angle-resolved photoemission measurements of URu{sub 2}Si{sub 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.
Date: November 1, 2001
Creator: Denlinger, Jonathan D.; Gweon, Gey-Hong; Allen, James W. & Sarrao, John L.
Partner: UNT Libraries Government Documents Department

In/Si(111): Self-assembled one and two-dimensional electrongases

Description: We present angle-resolved photoemission measurements forultrathin In films on Si(111). Depending on the coverage, this systemself-organizes into a metallic monolayer with either 4x1 or sqrt7 x sqrt3symmetry relative to the substrate. Electronically, they behave likeideal one- and two-dimensional electron gases (1DEG and 2DEG),respectively. The 4x1 system has atomic chains of In whose energy bandsdisperse only parallel to the chains, while for the sqrt7 x sqrt3 system,the dominant reciprocal space features (in both diffraction andbandstructure) resemble a pseudo-square lattice with only weakersecondary features relating to the sqrt7 x sqrt3 periodicity. In bothmaterials the electrons show coupling to the structure. The 1DEG couplesstrongly to phonons of momentum 2kF, leading to an 8x"2" Peierls-likeinsulating ground state. The 2DEG appears to be partially stabilized byelectron gap formation at the sqrt 7 x sqrt3 zone boundary.
Date: January 22, 2001
Creator: Rotenberg, Eli; Yeom, H. W.; Takeda, S.; Matsuda, I.; Horikoshi, K.; Schaefer, J. et al.
Partner: UNT Libraries Government Documents Department