Angle-resolved photoemission of Y-doped Bi2Sr2CaCu2O8+δ

surface science ELSEVIER

Surface Science 352-354(1996) 788-792

Angle-resolved photoemission of Y-doped Bi2 Sr2CaCu208 ÷ W.R. Flavell a,*, A.G. Thomas a, S. SqLlire a, M. Mian a, Md.M. Sarker a, J.F. Howlett a, Z. Hashim a, H.R. Aghabozorg a, P.L. Wincott b, S. Downes c, L. Leonyuk d a Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK b lRCSS,/Department of Chemistry, University of Manchester, Manchester M13 9PL, UK c CLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK Department of Geology, Moscow State University, 119899 Moscow, Russian Federation

Received 5 September 1995; accepted for publication 31 October 1995

Abstract An angle-resolved photoemission study of a single crystal of the Y-doped BCSCO 2212 phase, of composition x has been undertaken using l~ne 6.2 at the CLRC Daresbury Laboratory. This superconducting material undergoes a poorly understood metal-to-non-metal transition when roughly half the Ca is replaced by Y. The dispersion of the valence band and Fermi level states in the (001) (ab) plane of the structure has been studied. Results for the high symmetry directions in the 2D Brillouin zone, FX, F Y and FZ, are presented. We compare the ab-plane band structure of this material with band-structure calculations, and with photoemission results from the well-studied undoped 2212 phase. The effect of Y-doping on the band structure close to the metal-to-non-metal transition is discussed. (Bil.87Pbo.13)(Sr1.92Yo.o8)(Cao.52Yo.41)(Cul.97Alo.o2Bio.O1)O

Keywords: Angle resolved photoemission; Complex cuprate; Copper oxides; Low index single crystal surfaces; Superconducting surfaces;

Superconductivity 1. I n t r o d u c t i o n The high temperature superconducting cuprate Bi2Sr2CaCu2Os+ 8 ( B S C C O 2212 phase) has been studied extensively by angle-resolved photoemission, particularly in an attempt to address questions concerning possible anisotropy o f the superconducting energy gap [1-9]. In c o m m o n with many other systems, superconductivity in this material manifests

* Corresponding author. Fax: +44 161 236 7677; e-mail: [email protected].

itself quite close to a metal-to-non-metal transition. A transition to the non-metallic state is induced in Bi2Sr2Cal_xYxCU208+ 8 (Y-BCSCO) by Y-doping for Ca [10-12] to around x = 0.5. Substitution of Y for Ca leads to a formal reduction of the C u - O 2 planes, and the fully Y-substituted is essentially a Cu H compound. A number of angle-integrated photoemission studies o f this material have been carried out, leading to controversy surrounding the evolution o f the valence band and Fermi level states on doping through the transition [13-18]. There have been very few angle-resolved studies o f this material [19]. Here we complement existing data by carrying out a detailed study of the ab-plane band structure o f a

0039-6028,/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0039-6028(95)01229-X

789

Wa~. Flavell et a l . / Surface Science 352-354 (1996) 788-792

single crystal of composition close to the metal-tonon-metal transition.

2. Experimental procedure Angle-resolved photoemission measurements employed the toroidal grating monochromator (15 _< h u < 90 eV) and VG ADES 400 analyser on station 6.2 at the Synchrotron Radiation Source, CLRC Daresbury Laboratory. The combined (monochromator and analyser) energy resolution for valence band scans was 0.16 eV, and an analyser entrance aperture was used to fix the angular acceptance at ___2 °. For scans of the states close to the Fermi level, a resolution of 0.125 eV was used. (The figures for resolution are determined by adding the calculated analyser resolution and fixed photon resolution, in quadrature.) A plate-like single crystal of Y-doped BSCCO 2212phase of composition (Bil.87Pb0.13)(SrL92Y0.08)(Ca0.52Y0.41)(CUl.97A10.o2Bi0.01)O x was grown from the melt (as described previously [20]) and characterised by EPMA and XRD [20]. Four-probe resistivity measurements showed the crystal to be non-superconducting, as expected for this composition, which is very close to the metal-to-non-metal transi, tion. A large ( ~ 6 × 4 m m 2) single face of the crystal was shown by Laue back reflection to be of (001) orientation. The crystal was cleaved in UHV at room temperature in a direction parallel to this large (001) face using a tab technique. The base pressure of the spectrometer was maintained at ~ 6.5 × 10- l mbar. Sample cleanliness was monitored using the valence band photoemission, particularly in the region of 9 eV binding energy, where a new feature may appear in the spectrum as the surface degrades [21]. Under these conditions, the surfaces produced were found to be extremely stable for periods of days, and no surface degradation was detected. Following completion of angle-resolved measurements, the crystal orientation was confirmed using LEED. The pattem observed was that characteristic of the (001) face, with superstructure ordering leading to small reciprocal lattice spacing in the b* direction [22], and with no evidence of any sample twinning. After removal from the spectrometer, Lane back reflection was again used to reconfirm the crystal orientation.

3. Results and discussion Fig. 1 summarises the valence band EDC data obtained. Polar angles with respect to the surface normal and azimuthal angles with respect to the a-axis are shown. For all azimuths, the major features of the spectra are similar, and show a good correspondence with those of BSCCO [1,2,8]. The Y-doping level of the crystal indicates that this sample should be metallic (although close to the metalto-non-metal transition), and this is confirmed by the presence of a clear DOS at E F. The major feature of the valence band at around 3.5 eV binding energy shows very little dispersion, but stronger dispersion (of around 0.5 eV) is seen in the higher binding energy valence band states in the range 5 - 7 eV. The dominant feature at around 3.5 eV binding energy is shifted by around 0.4-0.5 eV to higher binding energy from its position in undoped BSCCO [8], in

==

E z

8.0 4.0

EF 8.0 4.0 EF 8.0 Binding Energy (eV)

4.0

EF

Fig. 1. Angle-resolvedEDCs at 33 eV photon energy showing the valenceband regions of Y-BSCCO(001). Photoelectron emission angles relative to the surface normal (0) and the crystal a-axis (th) are shown. Spectra axe normalised to the beam monitor reading. The corresponding k-spacetraverses are shown in Fig. 2.

790

W3L Flavell et aL / Surface Science 352-354 (1996) 788-792

FX a

I-Y b

FZ c

Fig. 2. Projection of the experimental photoelectron Collection positions (for electrons at EF) onto the kxky plane. The experimental data points are shown by squares, the solid square in each case tagging the polar angle 0 = 30° for each of the data sets shown in Fig. 3. Open squares show analyser positions where an enhanced Fermi level DOS is seen, while grey circles indicate the experimental error.

line with angle-integrated results for the Y-BSCCO system which show a shift of the valence band states to higher binding energy as the metal-to-non-metal transition is approached [13,14,16,17]. This shift is replicated in electronic structure calculations for doped and undoped compositions [23]. Fig. 2 shows the directions in k-space probed by the different experimental measurements. In obtaining this analysis, we have followed the practice of treating the structure (and hence the band structure) of BSCCO as effectively two-dimensional [3], giving a well-defined measurement in the k x k y plane. The tracks shown in the diagram are calculated for the states at E F and, for reference, are plotted against the calculated Fermi surface of BSCCO [24]. The experiment essentially probes the FX, F Y and FZ directions. In previous angle-resolved measurements

of the Y-doped BSCCO material, the F X and FY directions were not distinguished [19]; the a and b unit cell parameters are very similar, and this in turn means that the two are difficult to distinguish in Lane back reflection. However, the b-axis superstructure ordering observed in LEED gives a method of unambiguously distinguishing the two directions. Comparison between the data obtained along the aand b -axes (respectively F X and F Y in Fig. 1) shows that the spectra are very similar at low polar angles. However, there is a difference in the spectral shape to the high binding energy of the main valence band for high polar angles. Along the b-axis (FY), the intensity of the states between 4 and 7 eV binding energy dominates the valence band for very high polar angles. This phenomenon is in excellent agreement with that recently reported for undoped BSCCO by Yokoya et al., and is attributed by these authors to the effect of the incommensurate superstructure in the BiO plane, which runs only along the b-axis [8]. As in the case of undoped BSCCO, a comparison of the measured valence band density-of-states function with band-structure calculations [23,24] reveals a number of discrepancies. Firstly, calculations for the 50% Y-substituted material predict the major valence band density of states to lie in the range of 2 - 3 eV binding energy, considerably lower in binding energy than the 3.5 eV observed. This is a very common discrepancy for oxide materials, where in order to align observed photoemission profiles with calculated LMTO or LAPW density-of-states profiles, it is often found necessary to introduce a rigid downward shift of the complete band structure by an energy of the order of 1 - 2 eV [25]. This has generally been attributed to the effects of correlation, which are not explicitly included in the L M T O / L A P W treatments. Secondly, it is clear that the dispersions observed are on the whole significantly smaller than those calculated, again in common with results for the undoped material [1,2,8]. For example, we observe a maximum disPersion along F Z of around 0.5 eV to lower binding energy for the state at around 6 eV binding energy (Fig. 1) compared with a calculated dispersion of around 2 eV [23]. However, calculations for doped and undoped materials do predict smaller dispersion for the lower binding energy valence band states, as ob-

W.R. Flavell et aL / Surface Science 352-354 (1996) 788-792

791

around 0 = 9°-12 ° for FX, F Y and F Z azimuths, which may be suggestive of a Fermi level crossing. These positions are in good agreement with crossings predicted by band-structure calculation [24] (see Fig. 2) and with experimental data for the undoped material [3-5,7-9]. In particular the suggestion of a crossing along F Z may indicate the presence of a saddle point singularity in the density of states in the Y-doped material similar to that recently characterised in undoped BSCCO [9].

==

tD

g¢J 4. Conclusions o

z

0.4

E F -0.4

0.4 E F -0.4 0.4 E F -0.4 Binding Energy (eV)

Fig. 3. Angle-resolved EDCs at 33 eV photon energy showing the Fermi level regions of Y-BSCCO (001). Photoelectron emission angles relative to the surface normal ( 0 ) and the crystal a-axis (~b) are shown. Spectra are normalised to the beam monitor reading, q~e corresponding k-space traverses are shown in Fig. 2.

served experimentally. The observed dispersion of the valence band states for the Y-doped material are very similar in size to those seen in undoped BSCCO [1,2]. Thus apart from a shift to lower binding energy, the valence band states are not significantly affected by Y-doping. Representative Fermi edge scans, recorded at combined (analyser and monochromator) resolution of 125 meV are shown in Fig. 3. The resolution of the experiment is not sufficient to allow us to make detailed comparison with data for the undoped material, where the states close to E F have been studied extensively, typically at overall resolutions in the range 2 5 - 6 0 meV. It is nevertheless clear that the dispersion of these states is not so marked as for undoped BSCCO, where distinct Fermi level crossings have been observed along F Y [3], F X [8] and F Z [4,9], and measured dispersions are typically in the range 0.1-0.3 eV [9]. However, we note small dispersions and enhancements of the DOS at E F at

In common with undoped BSCCO, the experimental valence band density of states of the Y-doped material shows differences from the calculated band structure which are typical for highly correlated oxide materials. The maximum in intensity of the observed valence band is significantly down-shifted relative to calculations [23], and the dispersion of the bands is less than predicted. Comparison with valence band EDCs for the undoped material [1,2,8] shows that the valence band in the Y-doped material is shifted by 0.4-0.5 eV to higher binding energy, consistent with angle-integrated measurements [13,14,16,17]. This is consistent with a model where Y-doping removes holes in the CuO 2 planes, so that the Fermi energy moves upward relative to the valence band density of states. Overall dispersion of the valence band states is similar to that observed experimentally in the undoped material [1,2,8]. However, the dispersion of the Fermi level states appears to be smaller than that measured for the undoped material, although the resolution of the data allows limited comparison with previous work [3-5,7-9]. Nevertheless, there appears to be a slight suggestion of the predicted crossings along FX, F Y and FZ, with the latter possibly indicative of a saddle-point singularity along this direction, as recently characterised in undoped BSCCO [9]. Further measurements of the Fermi level density of states are planned, using improved resolution, in order to confirm this observation.

Acknowledgement This work was supported by EPSRC (UK).

792

W.R. Flaoell et aL / Surface Science 352-354 (1996) 788-792

References [1] T. Takahashi, H. Matsuyama, H. Katayama-Yoshida, Y. Okabe, S. Hosoya, K. Seki, H. Fujimoto, M. Sato and H. Inokuchi, Phys. Rev. B 39 (1989) 6636. [2] R. B~ttner, N. Schroeder, E. Dietz, U. Gerhardt, W. Assmus and J. Kowalewski, Phys. Rev. B 41 (1990) 8679. [3] C.G. Olson, R. Liu, D.W. Lynch, R.S. List, A.J. Arko, B.W. Veal, Y.C. Chang, P.Z. Jiang and A.P. Paulikas, Phys. Rev. B 42 (1990) 381. [4] D.S. Dessau, Z.-X. Shen, D.M. King, D.S. Marshall, L.W. Lombardo, P.H. Dickinson, A.G. Leeser, J. DiCarlo, C.-H. Park, A. Kapitulnik and W.E. Spicer, Phys. Rev. Lett. 71 (1993) 2781. [5] R.J. Kelley, J. Ma, C. Quitmarm, G. Margaritondo and M. Onellion, Phys. Rev. B 50 (1994) 590. [6] P. Aebi, J. Osterwalder, P. Schwaller, L. Schlapbach, M. Shimoda, T. Mochiku and K. Kadowaki, Phys. Rev. Lett. 72 (1994) 2757. [7] H. Ding, J.C. Campuzano, K. Gofron, C. Gu, R. Liu, B.W. Veal and G. Jennings, Phys. Rev. B 50 (1994) 1333. [8] T. Yokoya, T. Takahashi, T. Mochiku and K. Kadowaki, Phys. Rev. B 51 (1995) 3945. [9] J. Ma, C. Quitmann, R.J. Kelley, P. Alm6ras, H. Berger, G. Margaritolado and M. Onellion, Phys. Rev. B 51 (1995) 3832. [10] A. Manthiram and J.B. Goodeuough, Appl. Phys. Lett. 53 (1988) 420. [11] T. Tamegai, K. Koga, K. Suzuki, M. Ichihara, F. Sakai and Y. lye, Jpn. J. Appl. Phys. 28 (1989) Ll12.

[12] W.A. Groen, D.M. de Leeuw and L.F. Feiner, Physica C 165 (1990) 55. [13] M.A. van Veenendaal, G.A. Sawatzky and W.A. Green, Phys. Rev. B 49 (1994) 1407. [14] M.A. van Veenendaal, R. Schlatmann, G.A. Sawatzky and W.A. Groen, Phys. Rev. B 47 (1993) 446. [15] R. Itti, F. Munakata, K. Ikeda, H. Yamauchi, N. Koshizuka and S. Tanaka, Phys. Rev. B 43 (1991) 6249. [16] M.S. Golden, S.J. Golden, R.G. Egdell and W.R. Flavell, J. Mater. Chem. 1 (1991) 63. [17] G. Mante, Th. Schmalz, R. Manzke, M. Skibowski, M. Alexander and J. Fink, Surf. Sci. 269/270 (1992) 1071. [18] W.R. Flavell, J. Hollingworth, J.F. Howlett, A.G. Thomas, Md.M. Sarker, S. Squire, Z. Hashim, M. Mian, P.L. Wincott, D. Teehan, S. Downes and F.E. Hancock, J. Synchrotron Radiat. 2 (1995) 264. [19] T. Kusunoki, T. Takahashi, S. Sato and H. KatayamaYoshida, Physica C 185-189 (1991) 1045. [20] L. Leonyuk, G.-J. Babonas, V. Maltsev and A. Vetkin, Supercond. Sci. Tectmol. 8 (1995) 53. [21] W.R. Flavell, J.H. Laverty, D.S.-L. Law, R. Lindsay, C.A. Muryn, C.F.J. Flipse, G.N. Raiker, P.L. Wincott and G. Thornton, Phys. Rev. B 41 (1990) 11623. [22] R. Claessen, R. Manzke, H. Carstensen, B. Burandt, T. Buslaps, M. Skibowski and J. Fink, Phys. Rev. B 39 (1989) 7316. [23] B. Szpunar and V.H. Smith, Phys. Rev. B 45 (1992) 10616. [24] S. Massidda, J. Yu and A.J. Freeman, Physica C 152 (1988) 251. [25] See e.g.R.G. Egdell, W.R. Flavell and M.S. Golden, Supercond. Sci. Technol. 3 (1990) 8.