Surface induced modifications in the electronic structure of YBa2Cu3O7−δ

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Solid State Communications, Vol. 87, No. 4, PP. 277-280, 1993. Printed in Great Britain.

0038-1098/93 $6 00+ 00 Pergamon Press Ltd

SURFACE INDUCED MODIFICATIONS IN THE ELECTRONIC STRUCTURE OF YBa2CU3OT.~ N. Schroeder, R. B6ttner, S. Ratz, S. Marquardt, E. Dietz, and U. Gerhardt Physikalisches Institut der Universit~it, Robert-Mayer-Str. 2-4, D-6000 Frankfurt am Main and Th. Wolf Kernforschungszentrum Karlsruhe, Institut fiir Technische Physik, P.O. Box 3640, D-7500 Karlsruhe (Received 15 February 1993, in revised form 22 April 1993 by G. Gfintherodt)

(accepted 19 May 1993) We present angle-resolved photoelectron measurements of YBa2CU3OT.8 single crystals cleaved perpendicular to the c axis. The results reveal photon energy dependent features in the dispersion of the sharp 1 eV peak, surface induced modifications in the electronic structure and a high surface sensitivity to adsorption of atomic hydrogen.

Recent angle-resolved photoelectron spectroscopy (ARIES) studies t-7 of the superconducting compound YBa2Cu3OT.~ (YBaCuO) revealed some bandstructure features as well as indications for the opening of the superconducting gap1,2: Due to the surface sensitivity of ARPES the interpretation of the data on the basis of calculated bulk electronic structure is difficult for several reasons. First, the escape depth of the emitted electrons is in the same order of magnitude as the lattice parameter c = 11.68 A. Second, from both experimental results and theoretical arguments surface reconstructions might take place for some of the six different possible cleavage planes. Third, it is expected that the surface under investigation is a composition of different crystallographic layers within the dimension of the light spot. Therefore a large variation of spectral features is expected for different surfaces and also found experimentally I. Besides the question of the existence of the superconducting gap (probably one of the most important) recent studies 1-4 dealt extensively with the so called 1 eV structure. While Tobin3 et al. consider it to arise from a bulk state and also to be a fingerprint of good sample and surface quality, Claessen 4 et al. first assigned this spectral feature to a surface derived state. The dispersion and the spectral weight of this state is independent of crystal twinninga. In this paper we analyse sample dependent ARPES features as well as the dispersion of the 1 eV peak and its sensitivity to adsorption of hydrogen in order to reveal some aspects of the nature of this structure. We further discuss the surface sensitivity of the states near F_~ which probably has some implications for the non-observation of the superconducting gap for all samples showing a pronounced 1 eV structure. The ARPES data were taken on the 2m Seya beamline at the Berliner Elektronenspeicherring fiir Synchrotronstrahlung (BESSY) using an eight channel angle resolving electron analyser with an angular resolution of 2 ° full width at half maximum (FWHM). The

combined energy resolution is about 80 meV and 70 meV FWHM for the spectra showing the 1 eV peak and the structures at the Fermi energy EF, respectively. A more detailed description of the experimental setup is given in Ref. 8. The cleaving perpendicular to the c axis and the ARPES measurements were both performed at a temperature of about 10 K. The polarisation vector of the incident light coincides with the plane of observation for all measurements. After the photoemission work Low Energy Electron Diffraction (LEED) measurements were performed in order to check the orientation and quality of the (001) cleavage planes. The twinned single crystals were grown in an A1203 crucible in air by the slow cooling method9. Due to the crucible corrosion the crystals contain various amounts of AI on the Cu(1) sites leading to reduced T c values of 89 K (sample: Y23F92; Pair = 1 bar), 86 K (sample: Y51F92, Y61H92, Y63H92; Pair = 1 bar) and 81 K (sample: Y41F92; Pair = 0.5 bar), respectively. In Fig. 1 spectra of three different samples are plotted for two different photon energies hu and two different emission angles O. The tangential component k t of the six uppermost spectra coincides with the X(Y) point of the BriUouin zone while kt has moved towards I' for the three spectra at the bottom. For h~ = 23 eV only a single peak is clearly visible whereas a splitting into two components occurs for hu = 18.5 eV. Furthermore it is noticeable that the splitting is most pronounced for sample Y41F92 which has the lowest intensity for hu = 23eV. The dispersion of the emission maxima along the I'X(Y) direction is given in Fig. 2 for the Y41F92 sample. A splitting of the 1 eV peak is observable around the X(Y) point (right hand side: h~ = 18.5 eV) and the distinct structures are smeared out moving away from it. The experimental E(k) dependency shown in Fig. 2 is derived from a fit of the spectra using a linear background and two peaks (if clearly separated). Only one peak is used for all spectra without a clear separation of the con277

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tributing lines. Setting the photon energy to 23 eV and taking similar spectra around the X(Y) point results in the dispersion shown in the left hand side in Fig. 2. None of the hi, = 23 eV spectra corresponding to the points in the left hand side of Fig. 2 shows a splitting of the structure. In a third series of experiments with hJ, = 15 eV the line shape of the spectra as well as the dispersion is similar to the results at h~, = 23 eV. No splitting into distinct components for any angle of observation is observable. We emphasize that the dispersion shown in Fig. 2 is only based on the measurements of a single cleavage surface of sample Y41F92. Therefore the observed differences are only caused by the different photon energies, they are not caused by different cleavages which is otherwise a source of line shape modifications in ARPES of YBaCuO as recently demonstrated in Ref. 1 and also evident in Fig. 1. A comparison of our data with the results of other groups 3,4 reveals strong similarities concerning the dispersion at h~ = 23 eV and hi, = 15 eV but does not give any coincidence with the observed splitting for photon energies around 18.5 eV which is interesting for several reasons. Calandra 1° et al. calculated the bandstructure of YBaCuO within the LDA approximation for the different possible crystal terminations in order to elaborate on the differences between the bulk derived and the surface induced electronic states. The atomic positions at the surface are assumed to be those of the bulk. According to this calculations the chemical bond in YBaCuO has a significant three-dimensional character resulting in surface induced modifications of the electronic structure after crystal truncation. From these calculations the authors found two surface states with a dispersion very similar to that in the h~, = 18.5 eV spectra for a crystal cleavage

between the C u O 2 arid BaO layers resulting in a CuO2 surface. Assuming the splitted structure to be connected to a distinct crystal termination the observed intensity variations between various samples in Fig. 1 can be explained by the domination of a particular surface layer within the light focus, with a higher contribution of the CuO 2 plane in case of sample Y41F92. A further conclusion from this model is that the 1 eV structure in the topmost spectra of Fig. 1 (h;, = 23 eV) is composed of different states which is also supported by the comparison of the experimental dispersion for various photon energies (Fig. 2). In order to obtain further information about the origin of the 1 eV structure and the possible contribution of different states we adapted an idea of Manzke n e t al. and performed adsorption experiments with hydrogen. Since molecular hydrogen is not suited for such an experiment we produced atomic hydrogen by dissociation of 1-12. The required energy for this process is provided by the incident photons with an energy in the range of 18 eV to 23 eV. After floating the vacuum chamber with molecular hydrogen to a fixed pressure of approximately 5"10 -1° mbar (operating pressure < 1"10-I° mbar) without irradiation we started (stopped) the process of dissociation by opening (closing) a beam shutter. From the pressure and the time of irradiation we calculated the exposure in the usual way. We checked that comparable rates of molecular hydrogen do not result in any spectral modifications at all. Hence, the time needed for adjusting the pressure can be neglected in the calculation of the adsorption rate. For that reason the relative values are very reliable, although the absolute values used in the following are only a rough estimate. The results are demonstrated in Fig. 3 and Fig. 4 for two different samples (Y61H92 and Y63H92, respect-

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ively). On the left hand side of each figure the near Fermi edge spectra are plotted while on the fight hand side we show the 1 eV structure in a small energy range. The filled circles denote a reference spectrum measured immediately before starting the first adsorption expefiment. The spectra plotted with open circles are taken after successive adsorption, resulting for example in a total exposure of 60 mL for sample Y63H92 in Fig. 4 after exposing it four times to 15 mL. For both samples the structure at a binding energy of 1 eV in the right hand panel of each figure is clearly reduced before the intensity at the Fermi edge is influenced. Nevertheless, there are remarkable differences between the two samples in the evolution of the line shape with increasing exposure. While in the case of sample Y63H92 no change is observable after the first adsorption of 15 mL, the result for sample Y61H92 reveals a higher sensitivity. Furthermore, in the case of sample Y63H92 the development of the line shape around 1 eV can be explained by a preferable reduction of one contributing line located at approximately 0.92 eV binding energy. On the other hand for sample Y61H92 the reduction of intensity appears symmetrically. A comparison between the two samples Y61H92 and Y63H92 is shown in Fig. 5 for the near Fermi edge and 1 eV region. The spectra are taken prior to exposure (same as filled circles of Fig. 3 and Fig 4). Even though the intensity at F_~ (left panel) is similar there are remarkable variations for the 1 eV structure (fight panel). Again, this differences can be understood as a result of different contributions to the 1 eV structure within each spectrum, reflecting our statement that after cleavage different crystal truncations with different percentage contributions to the surface within the light spot have to be considered in the interpretation of ARPES of this HTSC compound. The results of Fig. 3 and Fig. 4 clearly reveal a high sensitivity of both the intensity at EF and the 1 eV structure. Nevertheless, the influence is most pronounced for the 1 eV structure. For sample Y63H92 (Y61H92) the

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279

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Vol. 87, No, 4

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280

SURFACE INDUCED MODIFICATIONS

intensity at 1 eV binding energy is clearly reduced after 30 mL (20 mL) whereas the intensity at EF is nearly unchanged. These experimental results are consistent with the calculations of Calandra l° et al. since they (i) identified the 1 eV structure as a surface derived state and (ii) also found remarkable changes in surface DOS around the Fermi level for different crystal terminations. Therefore the observation of a sharp 1 eV structure in ARPES of YBaCuO reflects surface induced modifications of the electronic structure which probably explain the failure in measuring the superconducting gap on cleavage surfaces showing a pronounced intensity at 1 eV binding energy. In summary, we have demonstrated that the 1 eV structure must be viewed as a composition of different states. For photon energies around 18.5 eVa splitting into two components is observable. Their dispersion is similar to a calculated E(k) dependence of surface states for the CuO 2 crystal termination (resulting from a cleavage between the CuO2 and BaO planes). Sample dependent differences, like the splitting at hu = 18.5 eV and the line

Vol. 87, No. 4

shape, are best explained by the assumption that crystal cleavage always results in an unknown mixture of different basal planes within the area of the light spot. Adsorption of hydrogen reveals a high surface sensitivity of both the states around 1 eV binding energy and the intensity at the Fermi energy showing the cleavage to be a strong disturbance of bulk electronic structure. Even though the intensity at E F is very sensitive to adsorption our results demonstrate that the influence is most prominent for the 1 eV structure. - We would like to thank the staff of the Berliner Elektronenspeicherring-Gesellsehaft ffir Synchrotronstrahlung (BESSY)for their professional work in general and for helping us out on many occasions in particular. In addition we acknowledge the financial support by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 252 Darmstadt/Frankfurt/Mainz and by the Bundesminister for Forsehung und Technologic under Contract No. 05446AAB and 055RFAAI. Acknowledgement

REFERENCES

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2.

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5.

N. Schroeder, R. B6ttner, S. Ratz, E. Dietz, U. Gerhardt, and Th. Wolf, Phys. Rev. B 47, (1 March 1993), in print S. Ratz, N. Schroeder, R. B6ttner, E. Dietz, U. Gerhardt, and Th. Wolf, Solid State Communications 82, 245 (1992) J . G . Tobin, C. G. Olson, C. Gu, J. Z. Liu, F. R. Solal, M. J. Fluss, R. H. Howell, J. C. O'Brien, H. B. Radousky, and P. A. Sterne, Phys. Rev, B 45, 5583 (1992) R. Claessen, G. Mante, A. Huss, R. Manzke, M. Skibowski, Th. Wolf, and J. Fink, Phys. Rev. B 44, 2399 (1991); R. Manzke, G. Mante, R. Claessen, M. Skibowski, Surf. Science 269/270, 1066 (1992) Rong Liu, B. W, Veal, A. P. Paulikas, J. W. Downey, H. Shi, C. G. Olson, C. Gu, A. J. Arko, and J. J. Joyce, Phys. Rev. B 45, 5614 (1992)

6.

J. C. Campuzano, G. Jennings, M. Faiz, L. Beaulaigue, B. W. Veal, J. Z. Liu, A. P. Paulikas, K. Vandervoort, H. Claus, R. S. List, A. J. Arko, and R. J. Bartlett, Phys. Rev. Lett. 64, 2308 (1990) 7. G. Mante, R. Claessen, A. HuB, R. Manzke, M. Skihowski, Th. Wolf, M. Knupfer, and J. Fink, Phys. Rev. B 44, 9500, (1991) 8. R. B6ttner, N Schroeder, E. Dietz, U. Gerhardt, W. Assmus, and J. Kowalewski, Phys. Rev. B 41, 8679 (1990) 9. Th. Wolf, W. Goldacker, B. Obst, G. Roth, and R. Fl0kiger, J. Cryst. Growth 96, 1010 (1989) 10. C. Calandra, F. Manghi, and T. Minerva, Phys. Rev. B 46, 3600 (1992) 11. R. Manzke, 25. NRW-Seminar fiber Hoch-Tc Supraleitung, Dortmund, April 27, 1992