R.J. Bartlett

Photoelectron spectroscopy study of YBa2Cu3Ox with varied oxygen stoichiometry: Possible evidence for strong coupling superconductivity

Photoelectron spectroscopy measurements are reported for samples of YBa2Cu3Ox with oxygen content varied in the range 6.2<x<6.9. For the most oxygen rich samples, a prominent cutoff at the Fermi edge is observed. The electronic density of states at the Fermi level N(EF) falls as oxygen is depleted. The superconducting transition temperature TcαtN(EF) suggest ing that a strong coupling mechanism controls the superconductivity.

Evidence from photoemission of strongly hybridized states at the Fermi level of Bi2Sr2CaCu2O8

The nature of the states at the Fermi level of the high temperature superconductors is of crucial importance since it is these states which form Cooper pairs below T c which are responsible for the superconductivity. In particular it is important to discern whether these states are derived from the Cu 3d or O 2p orbitals. Recent resonant photoemission experiments claim that there is no Cu 3d character at the Fermi edge of the Bi 2 Sr 2 CaCu 2 O 8 materials. While we observe similar resonant effects in these materials, an investigation over a wide range of photon energies, hv = 15 to 100 eV, has led us to reevaluate these results and to conclude that there is comparable O 2p and Cu 3d character at the Fermi edge. Our findings give support to strongly hybridized one band or two band descriptions of these materials.

Electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interfaces as studied by photoemission spectroscopy

High‐resolution photoemission has been used to probe the electronic structure of the gold/Bi2Sr2CaCu2O8 and gold/EuBa2Cu3O7−δ interface formed by a low‐temperature (20 K) gold evaporation on cleaved high quality single crystals. We find that the metallicity of the EuBa2Cu3O7−δ substrate in the near surface region (∼5 A) is essentially destroyed by the gold deposition, while the near surface region of Bi2Sr2CaCu2O8 remains metallic. This has potentially wide ranging consequences for the applicability of the different types of superconductors in real devices.

Vibronic coupling and other many‐body effects in the 4σ−1g photoionization channel of CO2

Vibrational branching ratios and photoelectron angular distributions were measured for 4σ−1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B 2Σ+u state. This behavior suggests that vibronic coupling to the B 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C 2Σ+g state photoelectron spectrum. We also observe evidence for other many‐body effects—shape‐resonance‐induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data.

Pulsed soft x-ray response of InP:Fe photoconductors

We report measurements of the energy‐dependent pulse response of InP:Fe photoconductors to soft x rays in the energy range 200 eV–3 keV. The experiments were performed with x‐ray pulses from the Stanford Positron Electron Asymmetric Ring storage ring at the Stanford Synchrotron Radiation Laboratory. The detector had an optical impulse response of 180 ps (full width at half‐maximum) and an active area of 1×1 mm. An array of filters (carbon, magnesium, and beryllium) was used to unfold the energy response of the detector to the soft x rays. Direct measurements of the time structure of the ring bunches were limited by the bandwidth of the real time recording system (400 MHz), but inidividual ring pulses were easily recorded with a detector pulse amplitude of 65 mV for the unfiltered beam. The detector response was constant over the energy range examined at 2.7×10−3 A/W.

Characteristics and performance of the Los Alamos VUV beamline at the NSLS

Abstract We describe the design and performance of the Los Alamos VUV synchrotron radiation beamline, U3C, on the VUV ring of the National Synchrotron Light Source at Brookhaven National Laboratory. The beamline uses separate function optics to collect and focus the horizontally and vertically diverging beam. The monochromator is a grazing incidence Rowland circle instrument of the extended grasshopper design (ERG). A postmonochromator refocusing mirror is used to focus or collimate the diverging beam from the monochromator. The beamline control and diagnostics systems are also discussed. Particular emphasis in the design has been placed on the reduction of stray and harmonic light. Higher order light is reduced by a grazing angle mirror low pass filter installed immediately downstream from the monochromator while stray light is reduced through the use of baffles and thin film filters. Also included in the line is a differential pumping section that permits gas phase and other experiments requiring pressures in the 10 −5 to 10 −4 Torr range to be coupled to the beamline.

Layered synthetic microstructures: Measurements and applications

Abstract Advances in thin film fabrication techniques have made metal multilayer diffracting optics an important new technology. In this paper we present the results of characterization measurements on a variety of state-of-the-art metal multilayer samples and we describe their possible use in several monochromator applications.

Temperature dependence of Jc in superconducting Nb3Ge

Measurements are presented of the temperature variation of the critical current density Jc for superconducting Nb3Ge in the field range 0–65 kG. Samples prepared either by sputtering or chemical vapor deposition (CVD) show a behavior similar to that exhibited by Nb3Sn. The data can be approximated by the relationship Jc∼[Hc2(t)]n. The n values range from 1 for self‐field Jc to 5 at a field intensity of 50 kG.

Evidence from photoemission of comparable oxygen-2p and copper-3d character in the states at the Fermi level of YBa2Cu3O6.9

Abstract Only the states at the Fermi level of superconductors from Cooper pairs responsible for superconductivity. An understanding of the nature of these states is therefore essential for a proper description of the superconductivity. Previous resonant photoemission experiments claim that there is only O 2p character in the states at the Fermi level of the Bi 2 CaSr 2 Cu 2 O 8 superconductors. We have extended these studies to the YBa 2 Cu 3 O 7- x material employing a wider range of photon energies and conclude that there is comparable O 2p and Cu 3d character in the density of states at the Fermi level in these materials. This supports strongly hybridized models of the high temperature superconductors and casts doubt on simplified one band models.

High‐field properties and scaling in CVD‐prepared Nb3Ge

The upper critical field Hc2 and the field‐dependent critical current density Jc(H) have been determined for high‐Tc Nb3Ge prepared by the chemical vapor deposition (CVD) process. Measurements were performed on short samples taken from long lengths (∼10 m) of Nb3Ge deposited under various conditions onto metallic substrates. Using the theory of Hohenberg and Werthamer for dirty type‐II superconductors, values of Hc2(0) were extrapolated from resistive measurements of Hc2 near Tc. Values of Hc2(0) ranged from 32.2 to 33.1 T, the highest upper critical field reported for bulk Nb3Ge. From measurements of Jc(H), pinning force densities Fp were calculated and compared to the Kramer model of flux pinning which predicts certain scaling laws for Fp. We have tested for these scaling laws and have found qualitative agreement with the model of Kramer. This represents the first demonstration of such scaling in bulk high‐Tc Nb3Ge. In addition, values of the flux line lattice shearing parameter Ks have been extracted f...