A photoemission study of Bi3.6Pb0.4Sr3Ca2.4Er0.7Cu4O16 in normal and superconducting states

PHYSlCA ELSEVIER

Physica C 222 (1994) 105-110

A photoemission study of Bi3.6Pbo.4Sr3Ca2.4Ero.7Cu4016in normal and superconducting states O. Cohen, T.R. Cummins, G.C. Georgiadis Centrefor High TemperatureSuperconductivity, Department of Chemistry, Imperial College, London SW7 2AZ, UK

C.S. Rastomjee, F.H. Potter, R.G. Egdell * Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK

W.R. Havell Department of Chemistry, UMIST, P.O. Box 88, ManchesterM60 1QD, UK

D.S.-L. Law SERC Daresbury Laboratory, Warrington WA4 4AD, UK

L. Leonyuk Department of Geology, MoscowState University, 19899Moscow, Russian Federation Received 19 July 1993; revised manuscript received 13 January 1994

Abstract

Photoemission spectra of single crystalline Bi3.rPbo.4Sr3Ca2.4Ero.7Cu4Ot6have been measured in the photon energy range 20 eV < hv < 100 eV both above and below the superconducting transition. The variation with photon energy in the intensity of structure close to the Fermi energy suggests that these states are of dominant O 2p atomic character both below and above To. The symmetry of the states is investigated by varying the polarisation of the incident radiation relative to the sample surface. Again there are no indications of changes at To.

1. Introduction

A n issue of c o n t i n u i n g interest in relation to the electronic structure of high-temperature oxide superconductors concerns the atomic nature a n d symmetry of electronic states close to the F e r m i energy. The cross-sections for ionisation o f C u 3d a n d O 2p states * Corresponding author.

show different variations with photon energy, so that by studying the changes in photoemission intensity close to the F e r m i energy as the photon energy is varied it should be possible to infer something about the atomic nature of the states. Early work in this area concentrated on attempts to observe resonances in the Fermi-level intensity close to the Cu 3p core level threshold at photon energies around h v = 74 eV. Shen et al. suggested that there was resonant e n h a n c e m e n t

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of states within 0.5 eV of the Fermi energy in photoemission from YBa2Cu307 and La2_xSrxCuO4 [ 1 ]. However, for Bi2Sr2CaCu2Os+a there is now a general consensus that there is no specific enhancement at the Fermi energy on sweeping the photon energy through the Cu 3p core threshold [ 2-4 ]. However, it has been emphasized several times that the lack of resonant enhancement does not in itself establish that there is no Cu 3d contribution to the states at EF [ 35 ]. In contrast to the behaviour at the Cu threshold, Takahashi et al. [ 2 ] claimed to observe resonant enhancement at the O 2s threshold around h v = 18 eV and took this as an indication of the predominant O 2p character in the Fermi-level states. However, both these experiments and related work by Wells et al. [ 6 ], which found a similar enhancement around h v = 18 eV, suffered from possible interference from the excitation of shallow core levels by second-order synchrotron radiation [7,8]. More recently Olson et al. have emphasized the advantages of studying intensity variations in photoemission over a wide energy range, not necessarily restricted to the vicinity of resonances. In the energy regime up to 100 eV, the Cu 3d ionisation cross-section increases dramatically relative to the O 2p cross-section [ 9 ], so that by comparing the behaviour at EF with that in the valence band as a whole it is possible to deduce the relative O 2p and Cu 3d contributions. For both YBa2Cu307 [ 10,11 ] and Bi2Sr2CaCu208 [ 3 ] the interference is that the states are of dominant O 2p atomic character, but with significant Cu 3d admixture. These measurements were all carried out on single crystals cleaved at low temperature in UHV and investigated only the superconducting state.

The early photoemission literature on oxide superconductors contained several reports of major changes in low-resolution photoemission profiles on cooling through Tc [ 12-14 ]. Of course this is physically implausible as one would expect changes only very close to EF on an energy scale of order kT¢. We therefore felt it worthwhile to critically re-examine the issue as to whether large-scale changes in photoemission do occur at Tc (admittedly expecting a negative outcome), focussing on intensities and polarisation close to EF. In the current communication we present for the first time synchrotron-radiation excited photoemission spectra of Bi3.6Pbo.aSr3Ca2.4Ero.TCU4016 measured over a range of photon energies extending

up to h v = 100 eV in both the normal and superconducting states. Er doping on Ca sites in the two-layer BSCCO phase Bi2Sr2CaCu2Oa+a is known to lead to suppression of the concentration of mobile holes, but this is compensated to some extent by the Pb doping. The Er doping thus allows for a greater occupation of the Bi sites by Pb. The Pb doping in turn makes growth of single crystals somewhat easier and in the present study we were able to work with a large crystal which was cleaved several times during the course of our experiments. The states at the Fermi energy are shown to have predominant O 2p atomic character in both the normal and superconducting states. In addition we exploit the plane polarisation of the incident synchrotron radiation to examine the symmetry of the states close to EF, again carrying out measurements both above and below To.

2. Experimental Single crystals of Pb and Er doped Bi-Sr-Ca-Cu oxides were grown by slow cooling ( 1 ° C / h ) the corresponding oxide from the melt [ 15,16 ]. The crystal used in the present work had face dimensions of about 15 mm × 7 mm. The composition was measured by electron probe microanalysis. Using an RF inductance method it was established that the superconducting onset was at 83 K. Photoemission measurements were carried out on beamline 6.2 of the SRS, Daresbury Laboratory. This incorporates a monochromator with two sets of toroidal gratings covering the photon energy ranges 15 eV-60 eV (710 lines/mm) and 40 eV-140 eV ( 1800 lines/mm). The radiation is polarised with a degree of plane polarisation that decreases from about 80% at 20 eV to below 60% at 75 eV. The energy analyser has 150 ° spherical sector deflection elements of mean radius 50 mm. It was operated at 10 eV pass energy with 1 mm slits and 3.3 ° entrance half angle. The overall experimental resolution was 0.15 eV FWHM. The position of the Fermi energy was established from the Fermi-Dirac cutoff of the photoemission profile in the normal state. The crystal was glued to the copper cold finger of a liquid-helium cooled cryostat using silver loaded epoxy resin. A small loop of stainless steel wire was glued to the top of the crystal. After cooling the sample to 20 K in a base pressure of 10-1o

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mbar, the crystal was cleaved with the aid o f a h o o k a t t a c h e d to a wobble stick. Spectra were collected at 20 K, then at r o o m t e m p e r a t u r e a n d finally for a seco n d t i m e at 20 K. The crystal was thick enough to allow for subsequent cleavages with the same procedure. Sharp b u t weak features at 11.0 a n d 13.8 eV b i n d i n g energy were observed in spectra m e a s u r e d at 20 K. Offline experiments using a H e ( I I ) discharge source ( b y = 4 0 . 8 eV) c o n f i r m e d that these a d d i tional features were due to a d s o r b e d CO a n d could be d r a m a t i c a l l y e n h a n c e d by sub-Langmuir exposure to CO. The surface coverage in the low-temperature synchrotron experiments was e s t i m a t e d to be always less than 0.5 o f a m o n o l a y e r a n d the a d s o r b a t e peaks d i d not interfere with the analysis o f the d o m i n a n t valence-band features.

Bi 5d h~ = 90eV O 2s

VB

Pbsd

A

I

hv : 76eV

3. Results and discussion :30

Wide-scan p h o t o e m i s s i o n profiles extending bey o n d 30 eV b i n d i n g energy are shown in Fig. l, b o t h on ( b y = 7 6 eV) a n d off ( b y = 9 0 eV) the Cu 3p core threshold. This region encompasses: ( 1 ) the m a i n valence b a n d which is c o m p o s e d o f states o f strongly m i x e d Cu 3d a n d O 2p a t o m i c character a n d peaks at 4 eV b i n d i n g energy; ( 2 ) a b r o a d low-intensity b a n d peaking at a r o u n d 11 eV b i n d i n g energy assigned to states with substantial Bi 6s character a n d ( 3 ) a series o f shallow core levels, including Sr 4p, Ca 3p a n d Bi a n d Pb 5d. In a d d i t i o n the spectrum rec o r d e d at b y = 7 6 eV shows resonantly e n h a n c e d Cu 3d 8 satellite peaks at 10.0 eV a n d 12.5 eV b i n d i n g energy. Fig. 2 shows energy-distribution curves in the binding-energy range up to 15 eV taken at p h o t o n energies across the Cu 3p core threshold at b y = 7 4 eV: the Bi 6s b a n d a n d the Cu 3d s satellites are seen much m o r e clearly in this figure. S i m i l a r spectra were acquired over the c o m p l e t e range o f p h o t o n energies from 20 eV to 90 eV but these spectra are qualitatively similar to those off resonance in Fig. 2. The spectra o f Fig. 2 were m e a s u r e d at r o o m temperature, b u t d a t a m e a s u r e d at 20 K are essentially identical: our energy resolution is too p o o r to allow us to see evidence o f the o p e n i n g o f a gap at the F e r m i energy in the superconducting states [ 17-19 ]. There are four

20

~0

0

Binding energy/eV

Fig. 1. Valence-band and shallow core level photoemission spectra of Bi3.6Pbo.,SraCa2.4Ero.TCu,Om6excited at hv= 76 eV and at hv = 90 eV in normal emission and near-normal incidence at room temperature. Note the resonance enhancement of satellite peak at 12 eV binding energy in the former spectrum.

i

15

10

i

5

Binding energy (eV)

Fig. 2. Valence region room-temperature photoemission spectra of Bi3.6Pbo.,SraCa2.,Ero.7Cu,Ot6excited in normal emission and near-normal incidence at the photon energies indicated.

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distinct features in the valence-band region: a weak peak A at the Fermi energy with a cutoff whose sharpness is largely governed by the experimental resolution; and valence-band shoulders B and D with a valence-band m a x i m u m C in between them. There is an obvious resonance m a x i m u m in the intensity of the satellites (S) at h ~,= 74 eV. By contrast structure at the Fermi energy shows little sign of enhancement in intensity at this energy. This qualitative pattern o f behaviour conforms with the previous work alluded to in the introduction [ 1,3,4 ]. To further quantify the behaviour at EF, the height of the Fermi edge cutoff was measured relative to the integrated area of the main valence band. This ratio has an arbitrary absolute value in dimensions of height/area. Using the cross-sections for O 2p and Cu 3d atomic levels calculated by Yeh and Lindau [ 9 ] assuming a Hartree-Fock-Slater one-electron central potential model it is possible to define the dimensionless ratios a ( C u 3d 1) / [ 2 a ( C u 3d 9 ) 4- 80"(0 2p 6) ] and a ( O 2p t ) / [ 2 6 ( C u 3d 9) + 8 6 ( O 2p 6 ) ] . Assuming 2 x C u ions with configuration 3d 9 and 8 X O ions with configuration 2p 6 per unit cell, the variations of these ratios describe the variations in Fermi-energy intensity that would be expected for states of pure Cu 3d and pure O 2p atomic character, respectively, ignoring resonance effects (Fig. 3). The experimental data can be compared with these theoretical variations by scaling the experimental data, as described in outline by Olson et al. [3,10,11 ]. At photon energies in the range around 50 e V - 6 0 eV there are large and erratic variations in the apparent Fermi-energy intensity, as noted previously by Olson et al. and ascribed by them to modulations in the density of final states. At lower photon energies the spectra suffer from contamination of the Fermi energy region by shallow core levels excited with second-order radiation. For these reasons the experimental intensities have been scaled by least squares fitting the data above h v = 60 eV to a linear variation with photon energy and then defining the scale factor so that the experimental relative cross-sections coincide with the theoretical value at the point where the O 2p and Cu 3d relative cross-sections are equal. The

0.04

c 0.03 o k~

Cu 5¢

g o 0,02 o

0

0

[]

v

Cl 0

5~

v

0

v

0

D

v

0.01

o 0

2p

0.00 40

60 80 Photon Energy / eV

100

Fig. 3. Variations of the ratios a(Cu 3d~) / [2a(Cu3dg)+8a(O2p6)] (labelled Cu3d) and a(O2pl)/ [ 2a ( Cu 3d9) + 8a( O 2p6) ] (labelled O 2p ) with varying photon energy compared with the experimental variation of the intensity of the Fermi energycutoff at both room temperature 300 K (open circles) and 20 K (open triangles) relative to the integrated valence-band intensity for Bi3.rPbo.4Sr3Ca2.4Ero.vCu4Ol6.The open squares show similar data for Bi2Sr2CaCu2Os+,taken from ref. [3]. results of the analysis are shown in Fig. 3. Despite the rather large scatter on our data points it is obvious that the intensity at the Fermi energy shows an overall decline in the photon-energy range between 60 eV and 100 eV, in a way expected for states of dominant O 2p atomic character. In addition we also show the data for Bi2Sr2CaCu208+ a itself reported by List et al. [3 ]. In comparison with their data we have tentative evidence for greater O 2p atomic character in our doped and compensated E r - P b material. It is further clear from Fig. 3 that these is no significant variation in the intensity profile between normal and superconducting states. Thus we are able to offer evidence that there is no significant change in the atomic character o f states close to the Fermi energy on passing through the normal metal-to-superconducting transition. Turning next to the symmetry of the states at EF, we have employed an approach applied by Wells et al. to photoemission in Bi2.oSrl.sCao,sLao.3Cu2.1Os+a [6]. The (001) cleavage surfaces o f the 2212 BISCCO phase has approximate four-fold symmetry. At the zone centre wavefunctions and operators may be classified according to irreducible representations

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of the point group C4v. In normal emission, the condition ofno-node-at-the-detector requires that the final state wavefunction ~ufbelongs to the totally symmetric irreducible representation A~ [20-22]. The transition matrix element ( ~b¢i I ~-/1~bcf) between initial (i) and final ( f ) states must also be totally symmetric, a condition which is only satisfied if the dipole moment operator/t and the initial state wavefunction belong to the same irreducible representation. For normal incidence of the synchrotron radiation, the electric vector is in the surface plane and transforms as the E irreducible representation, whereas for grazing incidence, the vector may be chosen to lie along the direction of the surface normal and to transform as A~. We therefore investigated differences between nearnormal and more grazing incidence normal photoemission spectra at a range of photon energies in both normal and superconducting states. The size of the analyser and the lateral spread of the synchrotron beam relative to the size of the sample restricts the lower and upper incidence angles which can be investigated: in these measurements the range of emission angles was between 20 ° (near normal) and 60 ° (near grazing) relative to the surface normal. The most pronounced spectral changes were found at low photon energies, as expected from the greater degree of plane polarisation at low energy. Some typical normal-state data are shown in Fig. 4(a). Note that the spectra have been normalized to have the same intensity at the valence-band maximum C. It is clear from Fig. 4(a) that the states close to EF (band A) and the states giving the valence band shoulder B are stronger in near-normal incidence than in grazing incidence spectra. We may therefore infer they are predominantly composed of orbitals of E symmetry. The same polarisation effects are observed in both normal (Fig. 4 ( a ) ) and superconducting (Fig. 4 ( b ) ) states. As was perhaps to be expected we therefore infer that the symmetry of the states close to EF is not affected by the transition into the superconducting state. In conjuction with the intensity variations observed as the photon energy is varied, this implies states of dominant O 2px.y character both above and below To. In summary we have demonstrated for the first time that the atomic character and symmetry of states close to the Fermi energy in an oxide superconductor show

45eV; 300K . ~ !~': ,\ ,

.... --

grazing normal a}

i

-/3-7-6-5-4-3-2-1

0

1

Binding energy (eV) 45eV ; 2OK,

.............. g r a z i n g --

normal

{b)

i

-5

i

0

Binding energy (eV) Fig, 4. Normal emission valence region photoelectron spectra of Bi3.6Pbo.4Sr3Caz4Ero.TCU40~6 excited near normal incidence (20 ° to surface normal) and at more grazing incidence ( 60 ° to surface normal) at both room temperature (300 K) and 20 K at hu=45 eV.

no pronounced changes on passing through the transition from normal to superconducting states. This conclusion accords with simple physical expectations.

References

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[3] R.S. List, A.J. Arko, R.J. Bartlett, C.G. Olson, A.B. Yang, R. Liu, C. Gu, B.W. Veal, Y. Chang, P.Z. Jiang, K. Vandervoort, A.P. Paulikas and J.C. Campuzano, Physica C 159 (1989) 439. [4] C.G.J. Flipse, W.R. Flavell, R. Lindsay, G.N. Raiker, P.L. Wincott and G. Thornton, Physica C 193 (1992) 309. [5] O. Gunnarsson, J.W. Allen, O. Jepsen, T. Fujiwara, O.K. Andersen, C.G. Olsen, M.B. Maple, J.S. Kang, L.Z. Liu, J.H. Park, R.O. Anderson, W.P. Ellis, R. Liu, J.T. Markert, Y. Dalichaouch, Z.X. Shen, P.A.P. Lindberg, B.O. Wells, D.S. Dessau, A. Borg., I. Lindau and W.E. Spicer, Phys. Rev. B 41 (1989) 4811. [6] B.O. Wells, P.A.P. Lindberg, Z.X. Shen, D.S. Dessau, W.E. Spicer, I. Lindau, D.B. Mitzi and A. Kapitulnik, Phys. Rev. B40 (1989) 5259. [7] W.R. Flavell, J.R. 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 44 (1991) 878. [ 8 ] P.L. Wincott, A.G. Thomas, R. Lindsay, C.A. Muryn, C.F.J. Flipse, G. Thornton, T.R. Cummins, R.G. Egdell, O. Cohen, J.H. Laverty, P.W. Mitchell, G. Ross, D.S.L. Law, D.M.P. Holland, L. Leonyuk, B.C. Morris and W.R. Flavell, Physica C 185-189 (1991) 1047. [ 9 ] J.J. Yeh and I. Lindau, Atomic and Nuclear Data Tables 32 (1985) 1. [ 10] A.J. Arko, R.S. List, R.J. Bartlett, S.W. Cheong, Z. Fisk, J.D. Thompson, C.G. Olson, A.B. Yang, R. Liu, C. Gu, B.W. Veal, J.Z. Liu, A.P. Paulikas, K. Vandervoort, H. Claus, J.C.

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