Solid State Communications, Vol. 89, No. 1, pp. 77-80, 1994. Printed m Great Britain.

0038-1098/94 $6.00 + .00 Pergamon Press Ltd

ELECTRONIC STRUCTURAL CHANGES IN YI-xPrxBa2Cu307-6 SINGLE CRYSTALS DUE TO P r 4 f STATES

A. Hartmann, I G.J. Russell, I W. Frentrup 2 and K.N.R. Taylor l

1School of Physics, The University of New South Wales, P.O. Box 1, Kensington NSW 2033, Australia 2Humboldt-Umversitfit zu Berlin, Invalidenstr. 110, D-1040 Berlin, FRG

(Received 18 June 1993 by R.T. Phillips)

Electronic structural changes in both tetragonal and orthorhombic Y i - xPrxBa2Cu307_ 6 (x = 0, 0.5, 1) single crystals due to P r4 f states have been studied using the techniques of EELS and NEXAFS. The EELS data from cleaved surfaces for the energy range 4-37 eV show no extra peaks due to P r4 f states, while the positions of the Y4d and Pr5d bands above E F appear to be identical indicating a very similar DOS structure above EF. The Ols-NEXAFS data shows empty p-states above EF arising from unfilled Pr4fstates hybridized with O2p states. The large relative intensity of the O2p-Pr4fhybrid-state peaks at ~ 3 eV, 5.5-6.0 eV for the tetragonal crystals and at --~ 4 eV, 6.3 eV for the orthorhombic crystals indicates very strong hybridization as the relative density of the empty Pr4fstates is small. Intensity changes associated with a peak at 1.3eV above E F found for the tetragonal crystals, could indicate changes in 02p-Cu3d(x2_y2),(3z2_r2 ) hybndization with different Pr content.

UNDERSTANDING the electronic properties of PrBa2Cu3OT_6 (PBCO), which is the only non- superconducting member within the (123) family of high T~ materials, is an important issue in determin- ing the mechanism for superconductivity in the high T~ materials. Due to the presence of O2p-hole states at the Fermi level (EF), YBa2Cu307 is a metallic conductor which becomes a superconductor for temperatures below T~. Fink et al. [l] have also shown the existence of O2p-hole states at Er for PrBa2Cu307, but to explain the semiconducting behaviour of PrBa2Cu307 these hole states must be localized. A probable source for this localization are negative exchange interactions arising from strong Pr4f-O2p hybn&zation [1-3]. To derive more information concerning the Pr related electronic structure in PBCO we have undertaken surface studies of Yl-xPrxBa2Cu3OT-6 (x = 0, 0.5, 1) single crystals using both reflection electron energy loss (EELS) and near edge extended X-ray absorption fine structure (NEXAFS) spectroscopies. Comparison of the peaks in the EELS spectra show directly electronic structural differences for YBCO and PBCO while, according to the dipole selection rule, NEXAFS measurements of the Ols edge reflect empty p-states. These studies are therefore direct probes of changes in the (empty) O2p hybrid states caused by unfilled Pr states.

Details of the growth process for the production of tetragonal and orthorhombic Y1 - xPrxBa2Cu307 -~ single crystals are analogous to those described previously for YBCO crystals [4]. Using X-ray diffraction methods the surface oxygen content was found to be centred at 061 ~:01 for the tetragonal and at O69+01 for the orthorhombic crystals. The large and thin crystals (--, 8 x 8 x 0.2mm 3) analyzed had a smooth surface and the Pr containing orthorhombic crystals were found to be basically twin-free. Weak twins could be found only at the edges of the orthorhombic YBCO crystals, but here also the mare area was twin-free. The reflection EELS studies of cleaved tetragonal YBCO and PBCO single crystal surfaces were performed in a Kratos-XSAM 800 ultra high vacuum system (< 1 × 10-1°Torr) with the hemispherical electron analyser set at a pass-energy of 20 eV. The primary energy was 1000 eV and the full width at half maximum of the elastically scattered beam was found to be 1.0eV. In order to minimize surface damage due to electron irradiation, the energy loss data were obtained using the electron beam scanning mode within an area of approxtmately 0 . 5 x 0 . 5 m m 2. Crystal cleavage was performed parallel to the a-b-planes of the crystals. Each EELS spectrum was completed within 20mln after sample cleavage w~th surface contamination after each measurement being monitored by the detection

77

78 ELECTRONIC STRUCTURAL CHANGES

i 30 20 10

' ' ' I ' ' ' ' [ ' T i i [ , i

B 2 B~ , .

Energy loss (eV)

Fig. 1. Negative second derivative reflection electron energy loss spectra, taken at a primary energy of 1000eV for (a) a cleaved YBa2Cu3061 crystal surface and (b) a cleaved PrBa2Cu3061 crystal surface.

of the carbon Auger signal. In several cases it was found to be just above the limit of detection, which indicates that m the worst case only minor surface contamination had occurred.

A detailed analysis of the loss peaks of both cleaved and argon ion bombarded YBCO surfaces for the energy range 3-27 eV has been presented earher [5]. Here we will focus on the main differences in the loss spectra from cleaved YBCO and PBCO surfaces. The negative second derivatwe loss spectra, for the energy range 4-37eV, from cleaved a-b-surfaces of t-YBCO and t-PBCO crystals surfaces are shown in Fig. l(a) and (b), respectively. The loss features below ,,~ 26 eV are very similar for both types of materials, which indicates only mmor differences in the density of states (DOS). This would confirm the results of Kirchner et al. [6], who have reported optical spectra for Yl-xPrxBa2Cu307 (x = 0 ,0 .1 , . . . , 1.0) thin films for the energy range ,~ 1-10eV and also found only minor differences. However, above ,,~ 26eV large differences arise in the spectra, which are indicated by the vertical marks m F~g. 1. For t-YBCO we interpret peak A, which is centred at 29.0eV, as transmons from the Y4pl/2 and Y4p3/2 initial states (unresolved) to empty Cu3d I° states above EF. The relatwe intensity of this peak appears to be strongly reduced after ion bombardment of the surface [7] and this can be explained in terms of a change in the valence of the copper m the bombarded YBCO surface which is now mainly Cu I+ [8] that has a filled Cu3d band. The peaks Bi and B 2 are centred at 34.4eV and 35.0eV, respectively, and can be attributed to transitions from the Y4p~/2 and Y4p3/2 initial states to the empty Y4d band above EF. The origin of the peaks Cl and 6"2 for t-PBCO are transitions from the Pr5pl/2 and Pr53/2 mltml states to the Pr5d empty states above EF. These peaks, which are centred at 28.4eV and 29.1eV,

Vol. 89, No. 1

respectively, are followed by another and smaller Pr related peak at 32.0 eV, which will be discussed later Our XPS studies of cleaved t-YBCO and t-PBCO single crystal surfaces [7] have shown that the Pr5p and the Y4p inittal states are centred at a binding energy of ,~ 18eV and -,~ 24.5eV, respectwely, and the difference of these values is very close to the energy difference of the loss peaks BI.2 for t-YBCO and CI,2 for t-PBCO. Therefore, the Y4d and Pr5d final states of these loss peaks are centred at a very similar energy, that is at ,-~ 10.5eV above Er.

Calculations of the total DOS performed by Guo et al. for YBa2Cu307 and pr(3+)Ba2Cu307 for the range of 8eV below and 6eV above E~- [9] have shown that the total DOS above E~- is very similar for both types of material. This would confirm our data, as we could not find additional loss peaks due to the P r 4 f states and therefore we conclude that the relative intensity in the DOS arising from empty P r 4 f states is rather small. In addition, our data indicates very similar energy values for the empty Pr5d and Y4d bands and this would be an indication of a very similar DOS at a higher energy range (,,~ 10.5eV) above E~.

The Ois NEXAFS data for Yi -xPrxBa2Cu307- (x = I, 0.5, 0 and b = 1.0, 0, 9) crystal surfaces were collected using synchrotron radiation from the HE-PGM 2 plane grating monochromator at BESSY [10] and had a resolution of < 1 at an energy of 500eV. Energy calibration was performed by com- paring the Ols absorption data with EELS data from Fink et al. [I], Niicker et al. [11, 12] and Kuiper et al.

[13] and by setting the Cu2p3/2 absorption peak of bulk CuO at 931.2eV [14]. The mtensity variation of the incident photon beam was derived from the total yield emission of an Au surface. The NEXAFS data were obtamed from the fluorescence-yield and due to the relatwe large escape depth of the photons involved, this method provided data that was less dependent on surface effects and contamination, which reduces the stringent surface preparation techmques normally undertaken. The photon detector was placed at an angle of 90 ° with respect to the incoming photon beam. For all the NEXAFS measurements shown the angle of the polarisation vector E of the photon beam was 45 + I ° with respect to the e-axis and 0 + 10 ° with respect to the a- or b-axis of the crystals.

Figure 2 shows our OIs-NEXAFS results from the surfaces of orthorhomblc (a) YBCO, (b) Y05P0sBCO and (c) PBCO single crystals. Figure 3 shows the NEXAFS data for the same energy range of tetragonal (a) YBCO, (b) Y05P05BCO and (c) PBCO single crystals. The NEXAFS spectra were

Vol. 00, No. 0 ELECTRONIC STRUCTURAL CHANGES 79

/ l l l ' l J ' ' l l ' ' l l l l

' - 5 0 5 10 hv - E a (OIs) (eV)

Fig. 2. NEXAFS spectra of the Ols absorption edge for (a) YBa2Cu3069, (b) Y05Pr05Ba2Cu3069 and (c) PrBa2Cu3069 single crystals recorded with the polarization vector E of the photon beam having an angle of 45 ° with respect to the c axis of the different crystals. The Ols binding energy [Es(Ols)] has been subtracted from the photon beam energy.

shifted 528.2eV (Fig. 2) and 528.8eV (Fig. 3), respectively, as these energy values correspond to the different Ols binding energies for the ortho- rhombic and tetragonal crystal surfaces [7]. There- fore, the zero of the energy scales corresponds to EF and according to the dipole selection rule, these data reflect empty p-states within the range 0-10 eV above E F. The peaks centred at positions close to zero in the spectra from the orthorhombic crystal surfaces can be associated with the O2p hole states at EF [1, 11-13]. Their differences in shape and intensity are assumed to be related to slightly different surface oxygen contents and possibly different a, b-orientations of the three types of crystals, as we could not distinguish the a- or b-axis of the orthorhombic crystals before

' I ' ' ' ' i , , ' ' I ' ' ' ' I ,

I

' - 5 0 5 10 hv - EaiOIs) (eV)

Fig. 3. NEXAFS spectra of the Ols absorption edge for (a) YBa2Cu306 l, (b) Y05Pr05 Ba2Cu3061 and (c) PrBa2Cu3061 single crystals recorded with the polarization vector E of the photon beam having an angle of 45 ° with respect to the e axis of the different crystals. The Ols binding energy [Es(Ols)] has been subtracted from the photon beam energy.

the measurements. The relative intensity of these peaks is dependent on the symmetry of the O2p hole states [12] and therefore different a, b-orientations could be an origin for differences in their shape and intensity. However, we will focus on electronic structural changes related to Pr4f states, which we believe are less dependent on the a, b-orientations of the crystals, as the Pr-ions are well separated from the CuO2 chains along the b-axis of the orthorhombic crystals and, as shown below, our results for the lsotropy tetragonal crystals are comparable. The vertical marks in the spectra from the Pr containing crystal surfaces [Figs 2(b, c) and 3(b, c)] indicate increased intensity at ~ 4eV and ,-~ 6.3eV for the orthorhombic and at ,-~3eV and 5.5-6.0eV for the tetragonal surfaces. The Pr dependence of the intensity of these peaks indicates that they are Pr related. We have shown above, that the empty Pr5d states are centred at a higher energy and therefore empty Pr5d-O2p hybrid states are most probably not involved in these changes of relative intensities. However, with our charge exchange studies of t-PBCO single crystal surfaces [15] we found that a peak due to the dipole forbidden Pr4f -Pr4f transition is centred at a loss energy of ,,~ 3.3eV, which would indicate empty Pr4fstates centred at an energy < 3.3eV above E F. It is accepted that the valence of Pr in PBCO is close to 3 + [1-3, 6]. Both Pr 3+ and metallic Pr have a similar configuration for the 4 f valence band [16] and calculations of the Pr4f-DOS above EF and bremsstrahlung isochromat spectroscopy studies for metallic Pr have shown Pr4f states within a range of approximately 2-6 eV above Er [17]. Therefore, we interpret the rise of intensity at ,-,4eV, ,-,6.3eV and at ,,~ 3eV, 5.5-6.0eV in Figs 2(b, c) and 3(b, c) as being due to empty Pr4f states in hybridization with O2p states.

We have indicated above that the relative weight of the empty Pr4f band is small. However, Figs 2(a-c) and 3(a-c) show strong changes, which are a direct indication of empty O2p-Pr4f hybrid states and this would underline the fact that the O2p-Pr4f hybridization is enhanced.

Finally, the origin of the peak centred at ,,~ 1.3 eV in Fig. 3(a) should be discussed. In Fig. 3(b) this feature is significantly decreased and partly masked by the increasing Pr4f-O2p hybrid states peak and appears not to be resolved in Fig. 3(c). No peak at

1.3 eV could be detected in the spectra from the orthorhombic surfaces (Fig. 2), as the intensity arising from the O2p hole states at Ee is too large. It is most probable that intensity on the higher energy side of the peak at ,-~ 1.3 eV is due to empty Cu3d l° states hybridized with O2p states, as the charge

80 ELECTRONIC STRUCTURAL CHANGES Vol. 89, No. 1

transfer gap for YBa2Cu306 is 1.7 eV [18]. Low-lying Cu4p states, arising from the presence of Cu 1+, have been reported for t-YBCO [19, 20] and could lead to additional intensity in this area. Empty O2p states on the low energy side of this peak could possibly be caused by the incompletely filled O2p valence-band, as reported for CuO [13, 21]. Comparison of the three curves in Fig. 3 indicates that the intensity of the peak at ,~ 1.3 eV is decreasing with increasing Pr content. However, due to the complex origin of this peak it is not yet clear if there is a systematic con- nection between its intensity and the Pr content, as possible changes in the Cu3dx2_y2-O2px,y and Cu3daz2_r2-O2pz hybridization or other processes may be the origin of the observed changes.

In summary, we have used reflection EELS and Ols-NEXAFS studies of Yl_xPrxBa2Cu307_~ (x = 0, 0.5, 1 and 6 = 0.1, 0.9) single crystals to determine electronic structural changes due to P r4 f states. The EELS data mdicates a very similar DOS structure above EF and the positions of the Y4d and Pr5d bands above E F appear to be identical. Enhanced O2p-Pr4f hybridization has been suggested as the reason for the suppressed superconductivity in PBCO and in fact our Ols-NEXAFS results confirm a very strong hybridization associated with O2p-Pr4f states above E F.

REFERENCES

1. J. Fink, N. Niicker, H. Romberg, M. Alexander, M.B. Maple, J.J. Neumeier & J.W. Allen, Phys. Rev. 1142, 4823 (1900).

2. J.-S. Kang, J.W. Allen, Z.-X. Shen, W.P. Ellis, J.J. Yeh, B.W. Lee, M.B. Maple, W.E. Spicer & I. Lindau, J. Less-Common Met. 148, 121 (1989).

3. H.-D. Jostarndt, U. Walter, J. Harnischmacher, J. Kalenborn, A. Severing & E. Holland- Moritz, Phys. Rev. B46, 14872 (1992).

4. B.A. Hunter, S.L. Town, D.N. Matthews, G.J. Russell & K.N.R. Taylor, Aust. J. Phys. 44, 421 (1991).

5. A. Hartmann, G.J. Russell & K.N.R. Taylor, Accepted by Solid State Commun. (1993).

6. J. Kirchner, M. Cardona, S. Gopalan, H.-U. Habermeier & D. Fuchs, Phys. Rev. 1344, 2410 (1991).

7. A. Hartmann, G.J. Russell & K.N.R. Taylor, To be published (1993).

8. A. Hartmann, G.J. Russell & K.N.R. Taylor, Physica C205, 78 (1993).

9. G.Y. Guo & W.M. Temmermann, Phys. Rev. 1141, 6372 (1990).

10. H. Petersen, Opt. Commun. 40, 402 (1982). 11. N. Nficker, J. Fink, J.C. Fuggle, P.J. Durham &

W.M. Temmermann, Phys. Rev. B37, 5158 (1988).

12. N. Niicker, H. Romberg, X.X. Xi, J. Fink, B. Geigenheimer & Z.X. Zhao, Phys. Rev. B39, 6619 (1989).

13. P. Kuiper, G. Kruizinga, J. Ghijsen, M. Grioni, P.J.W. Weijs, F.M.F. de Groot, G.A. Sawatz- ky, H. Verweij, L.F. Feiner & H. Petersen, Phys. Rev. B38, 6483 (1988).

14. A.S. Koster, Mol. Phys. 26, 625 (1973). 15. A. Hartmann, G.J. Russell & K.N.R. Taylor,

To be published (1993); charge exchange studies of metallic Pr see [16].

16. F.D. Valle & S. Modesti, Phys. Rev. B40, 933 (1989).

17. J.K. Lang, Y. Baer & P.A. Cox, J. Phys. F." Metal Phys. 11, 121 (1981).

18. H. Romberg, N. Nuecker, J. Fmk, T. Wolf, X.X. Xi, B. Koch, H.P. Geserich, M. Duerrler, W. Assmus & B. Geigenheimer, Z. Phys. B78, 367 (1990).

19. M.K. Kelly, P. Barboux, J.-M. Tarascon, D.E. Aspnes, W.A. Bonner & P.A. Morris, Phys. Rev. B38, 870 (1988).

20. M.K. Kelly, P. Barboux, J.-M. Tarascon & D.E. Aspnes, Phys. Rev. 1140, 6797 (1989).

21. M. Grioni, M.T. Czyzyk, F.M.F. de Groot, J.C. Fuggle & B.E. Watts, Phys. Rev. B39, 4886 (1989).