PHYSICAL REVIEW B VOLUME 35, NUMBER 16 1 JUNE 1987 Resonant photoemission study of superconducting Y-Ba-Cu-0 Richard L. Kurtz and Roger L. Stockbauer Surface Science DivisionN, ational Bureau of Standards, Gaithersburg, Maryland 20899 Donald Mueller, Arnold Shih, Louis E. Toth, * Michael Osofsky, and Stuart A. Wolf Naval Research Laboratory, 8'ashington, D. C. 20375 (Received 15 April 1987) Ultraviolet photoelectron spectra of a 93-K superconducting compound, YBa2Cu30q, have been obtained using photon energies ranging from 60 to 106 eV. Resonant photoemission is used to identify the chemical origin of the features in the valence-band electronic structure. Recent developments in the production of high-T, su- perconductors have spurred the rapid development of theories of the geometric and electronic structure of these compounds. A number of theoretical calculations of the band structures of La-Ba-Cu-0 have been produced and studied in an effort to understand the nature of the super- conducting process. ' However, these calculations have not been supported by direct measurements of the band structures. With the production of the series of Y-Ba- Cu-0 materials with critical temperatures above 77 K, measurements of the electronic structures below T, have become experimentally more tractable. An essential component of the interactions that produce the supercon- ductivity is the electronic structure, yet measurements of this structure have not been made. In this paper, we report the first measurements of the electronic structure of YBa2Cu307 using synchrotron ra- diation in the range 60-106 eV. In this photon energy range, we are able to use resonant photoemission in order to assist in the chemical identification of the Cu and Ba features observed in the valence-band spectra. Resonant photoemission is associated with an enhancement of valence photoelectron features resulting from the coupling of excitation and decay mechanisms at core-electron pho- toabsorption onsets. The samples of YBa2Cu307 were prepared by mixing oxides and carbonates of the metals in the appropriate atomic ratios, and calcining in air. The samples were pressed into pellets and sintered in air and 02. The ma- terial is single phase and the structure is that determined by Beech, Miraglia, Santora, and Roth using neutron powder diffraction at the National Bureau of Standards. Four-point ac resistance measurements show a sharp drop in the resistance at 93 K and zero resistance is attained at 91 K. The photoemission measurements were made using the National Bureau of Standards SURF-II synchrotron light source, a toroidal grating monochromator, and a double- pass cylindrical mirror analyzer (CMA). Base pressures were — 1 x10 ' Torr. The sample normal was at an an- gle of 45 to both the photon beam and the axis of the analyzer; the spectra obtained are angle integrated. Cal- culated photon resolution is 350 meV at a photon energy of 60 eV. The CMA was operated with a constant pass energy giving a constant resolution of 240 meV; the data are presented uncorrected for CMA transmission. Sam- ples were mounted on a liquid-nitrogen-cooled manipula- tor using Ta foil and held into an Al ring with In in order to produce good thermal contact. The temperatures were measured with a W-5% Re, W-26% Re thermocouple imbeded in the In. The surfaces were prepared in ul- trahigh vacuum by fracturing the microcrystalline needles with a stainless-steel blade. The Fermi level was deter- mined from photoemission spectra of a clean gold foil that was in electrical contact with the sample. Measurements were made on two separate batches of oxide superconduc- tor. Spectra from both samples showed the same features at identical binding energies; however, the second sample which had been sintered longer showed sharper peaks. Figure 1 shows a survey of ultraviolet photoelectron spectra (UPS) from the first sample in the photon energy range 60-106 eV in 2-eV increments. At a photon energy of 60 eV (top curve), we observe primarily two valence- band features centered at binding energies of 5 and 9. 4 eV. As the photon energy is increased, the 9. 4-eV peak is enhanced slightly and an additional feature at 12. 4 eV is observed. This peak rises to a maximum at a photon ener- gy near 74 eV and then decays in intensity, along with a portion of the 9. 4-eV feature. With increasing photon en- ergy, two additional peaks become apparent, centered at binding energies of 15 and 28. 8 eV. Over the photon ener- gy range spanned by Fig. 1, several low-energy Auger features are also observed; they appear at constant kinetic energy, i.e. , at 2 eV higher binding energy in successive curves. Figure 2 shows, in more detail, photoemission spectra obtained from the second sample with photon energies of 74 and 94 eV, the resonant energy for Cu and an off reso- nance energy, respectively. The Ba resonance occurs at photon energies near 104 eV. The long dashed curves in Fig. 2 represent an approximation to the secondary elec- tron contribution and the multiple features observed in these curves are fit using simple Gaussian line shapes. The detailed form of the secondary background does not, to first order, affect the locations or widths of the features but it can inAuence their intensities. Here, we are pri- marily interested in locating the energies of these features and this decomposition allows a better estimation. The primary photoelectron feature (valence band), lo- cated immediately below EF, contains at least two contri- 8818