HAL Id: jpa-00227334

HAL Id: jpa-00227334

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Submitted on 1 Jan 1987

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Cu-L X-RAY EMISSION SPECTRA FROM HIGH-Tc SUPERCONDUCTORS Y-Ba-CU-O BELOW AND

ABOVE THE CRITICAL TEMPERATURE

R. Perera, B. Henke, P. Batson, J. Kerner, D. Berkeland

To cite this version:

R. Perera, B. Henke, P. Batson, J. Kerner, D. Berkeland. Cu-L X-RAY EMISSION SPEC- TRA FROM HIGH-Tc SUPERCONDUCTORS Y-Ba-CU-O BELOW AND ABOVE THE CRIT- ICAL TEMPERATURE. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1185-C9-1188.

�10.1051/jphyscol:19879213�. �jpa-00227334�

Cu-L X-RAY EMISSION SPECTRA FROM HIGH-T, SUPERCONDUCTORS Y-Ba-Cu-0 BELOW AND ABOVE THE CRITICAL TEMPERATURE

R.C.C. PERERA, B.L. HENKE, P.J. BATSON, J.A. KERNER and D. BERKELAND

Center for X-Ray Optics, Lawrence Berkeley Laboratory, Berkeley, C A 94720, U.S.A.

and Carl Nordling, Institute of Physics, University

of

Abstract

The copper La and Lp x-ray emission spectra from high-T, Y-Ba-Cu-0 superconductors, above and below the critical temperature, have been measured in a laboratory x-ray spectrograph. The Zn-La (1011 eV) photon energy was used to produce the C ~ - 2 p ~ ~ , , , ~ ~ primary vacancies (binding energies -950 and 930 eVs, respectively). Changes in Cu-L x-ray emission spectra were observed indicating small changes in population of 3d and 4s electrons in the valence band when superconducting. For comparison Cu-L emission spectra from metallic Cu and from polycrystalline CuzO (valence I) and CuO (valence 11) were recorded at room temperature.

In this report, laboratory measurements of Cu-L x-ray emission spectra from high-T, Y-Ba-Cu-0 superconductors above and below the critical temperature are presented. The relative intensities of these emission bands due to fillinng of the copper 2p vacancies, indicate the population of copper 3d and 4s electrons in the valence band. X-ray emission spectroscopy provides direct information about the population, energy levels and widths of outer electron levels, complementary to the data available from photoelectron and Auger electron spectroscopy on the structure of valence band, molecular orbitals and solid state bands. It is less surface sensitive than electron spectroscopy and hence provides bulk properties.

For high excitation efficiency, Zn-La (1011 eV) photon excitation from a demountable soft x-ray source [I] was used to produce the C ~ - 2 p ~ ~ ~ , ~ ~ ~ primary vacancies. The binding energies of 2p112,312 electrons in metallic copper are 952 eV and 932 eV, respectively. The

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879213

C9-1186 JOURNAL DE PHYSIQUE

x-ray tube was operated at 8 kV and 150 mA. A KAP (2d = 2.66 nm) was used as the analyzer. A pressure tuned constant flow proportional counter filled with propane at sub-atmospheric pressure was used as the detector.

The temperature of the sample was reduced to below the critical temperature (T, -95 K) using a closed cycle refrigeration system (a modified cryo-pump) capable of reaching 20 K at the cold head and equipped with a heater and shroud surrounding the cold head. To avoid any surface contamination of the sample, the temperature of the shroud was maintained at about 20 K while the sample was held either above or below the critical temperature, using a resistive heater. The

temperature of the sample was monitored using a thermocouple. The below and above T, spectral measurements were obtained in succession (few degrees above and below T,) while monitoring the resistance of the sample by the standard four-point probe.

The step-scanned spectra thus obtained are presented in Fig. 1.

~ l l of the spectra presented in this work are the sum of at least three repeated runs with the individual runs reproducible to within

statistical deviations. Over lo4 counts were collected at the peak intensity in all spectra. The spectrometer was calibrated using Cu-La line 121 from OFHC copper sample. In all of the spectra, the peak intensities were normalized and background was not subtracted from data.

As seen from Fig. 1 , very small changes are observed in Cu-L emisisons spectra below and above T, indicating that the population of the 3d and 4s electrons in the valence band changes slightly when the sample is superconducting.

above Tc

- ^{below Tc}

Fig. 1. Cu-L x-ray emission spectra from high-T, Y-Ba-Cu-0

superconductor few degrees above and below the transition temperature obtained in succession.

Bragg Angle,

8 -

7 I

970 960 950 940 930 920

-

i \

Zn-

(1011

C

uo

Bragg Angle,

8 -

970 960 950 940 930 920

Energy (eV)

CU-L X-Ray Emission Spectra hA

Under the same measurement conditions, Cu-L emission spectra from Cu and polycrystalline CuzO (valence I) and CuO (valence 11' are recorded and presented in Fig. 2. All samples were obtained

commercially with purities over 99%. From a comparison of Cu-L spectra presented in Figs. 1 and 2, the valence state of copper in

superconducting materials is not valence I but closer to valence 11.

For a proper comparison of energy positions and relative intensities, the spectra in Figs. 1 and 2 must be corrected for "self-absorption" by calculating the effective source thickness 1 3 1 , especially since the superconducting materials have a barium-MZB3 edge at about 1100 eV, and the Cu-L emission spectra is on the lower energy side of the barium absorption edge. All spectra must be corrected for self-absorption by all constituents, not only barium, for a correct comparison. This self-absorption would reduce the intensity of high energy peaks more than the intensity of low energy peaks.

C9-1188 JOURNAL DE PHYSIQUE

In summary, Cu-L measurements from high-T, superconducting materials below and above critical temperature indicates small changes in the composition of 3d, 4s electrons in their valence band when superconducting.

Without high resolution in this spectral region, it is not possible to derive any valuable information regarding changes in the band

structure of copper when materials are superconducting. Therefore, construction of a grazing incidence spectrometer optimized for this spectral range has been proposed. Such an instrument employs a variable groove spacing diffraction grating [4] for high spectral resolution combined with a fixed detector position perpendicular to the image beam enabling position sensitive detection to enhance the spectrometer efficiency. Combining this instrument with high flux from proposed 1-2 GeV storage rings would provide valuable valence band information on these superconducting materials.

REFERENCES

1. B.L. Henke and M.A. Tester, Advances in X-Rav Analyses (Plenum, New York, 1975). Vol. 18, p. 76.

2. J.A. Bearden, Rev. Mod. Phys.

s,

3. B.L. Henke and K. Taniguchi, J. Appl. Opt. 42, 1027 (1976).

4. M.C. Hettrick, Appl. Opt.

3,

ACKNOWLEDGEMENTS

This work was supported by the U.S. Department of Energy under contract No. DE-AC-3-76SF00098. We would like to thank M.L. Cohen and A. Zettl for stimulating discussions and preparations of samples, J.U. Underwood for his encouragement and continuing interest in this study,

R. Tackaberry and E. Gullikson for assisting in the measurements and Debra Nanod for her invaluable assistances in preparation of this report.