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Charge transfer redistribution in ErBa2Cu3O6+x determined by neutron crystal field spectroscopy
Physica B 180 & 181 (1992) North-Holland
PHYSICA El
405-407
Charge transfer redistribution in ErBa,Cu,O,+, neutron crystal field spectroscopy J. Mesot”, P. Allenspach”, S. Bennington”
determined
by
U. Staub”, A. Furrer”, H. Mutkab, R. Osborn’ and
“Laboratory for Neutron Scattering, ETH Zurich, CH-5232 Villigen PSI, Switzerland ‘lnstitut Laue-Langevin, F-38042 Grenoble Cedex. France ‘Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK
A new parametrisation model which takes into account oxygen 2p-4f rare earth hybridization energies is tested in order to understand the unusually large total crystal field splitting in the RBazCu,O,+, (R = rare earth) compounds. This model gives also a quantitatively good picture of the charge-transfer mechanism involved by doping with oxygen.
1. Introduction An important feature of the RBa,Cu,O,+, (R = rare earth, 0 < x < 1) high-T, superconductors is their strong variation of T, as a function of the parameter x. This behavior has been discussed in terms of charge transfer from the chains to the planes [l]. In the R123 compound the R ions are sandwiched between CuO, superconducting planes, thus using the crystal-field theory of the rare earth, an attempt is made to understand qualitatively and quantitatively, the changes of the electronic structure in the neighborhood of the R ions, by doping with oxygen. 2. The model In most metallic rare earth systems, a pure pointcharge model (PC) is sufficient to reproduce the lifting of the (2J + I)-fold degenerate ground-state multiplet observed by neutron spectroscopy. The total splitting is typically of the order of 30meV. In this case the crystalline field (CF) potential at the R site takes the following form:
nation factors representing the angular dependence of the CF parameters. The sum runs over all neighboring atoms. Typical values for the parameters are m4 = 1 and oh =2.5 [2]. The R123 compounds seem to behave in a different way. In this case the total splitting is twice as large as in the above mentioned compounds (-80meV for R = Er). A simple PC calculation would require u4 f 2.5 and a, =7 in order to reproduce this energy splitting. In view of these results one might expect additional contributions to the CF potential. Such problems have already been treated for the case of insulating garnet-systems (31. These compounds show some similarities with the R123 compounds. In both cases the R ions are surrounded by 8 oxygen ligands, and the total CF splitting has the same order of magnitude [4]. Kuz’min et al. [5] proposed a simplification of the superposition-model developed by Newman [3], based on the allowance of cr and IZ bonds in pairs of the type R”+-O’, using a two-center approximation. The crystal-field potential at the rare earth site then takes the following form:
VCF= Vh,, + v,, a where qie denotes the charge at the site ligand ion. Using the operator equivalents can construct the following Hamiltonian: H,,=~~APcC n m
nm nm’
n = (2,4,6),
R, of the ith method,
0G m c n ,
we
(2)
where V,,, is the hybridization contribution and V,, is the point charge contribution; see eq. (1). Using the formalism developed by Stevens the crystal-field parameters can be rewritten as A nm = A;Lb + A,Pm ,
(5)
where
where C,,, are tensor operators, We phenomenological parameters characterizing screening, shielding and anti-shielding effects and the y,,, geometrical coordi0921-4526/92/$05.00
(4)
0
1992 - Elsevier
(6) E,
and E, respectively.
being
Science Publishers B.V. All rights reserved
the energies
of the (T and Kl bonds,
406 3.
J. Mesot et al. I Charge transfer redistribution in ERBa,CuO,
/1
Results and analysis
Measurements were performed on the time-of-flight (TOF) spectrometer IN4 at the high flux reactor of the ILL and on the HET TOF spectrometer at the ISIS pulsed neutron source. The calculations have been made in the intermediate-coupling approximation including J-mixing. The five tetragonal parameters have been adjusted to fit seven ground-state transitions and five relative intensities. The orthorhombic parameters (m = 2, 6) were fixed at their nominal position A,, = A ,I0 y,,,,/y,,,. Figure 1 shows observed and calculated spectra for ErBa,Cu,O,. Assuming that the long range CF parameter A,,, cannot be understood in terms of this simple model, we have concentrated our attention on the leading tetragonal parameters A,,, A 14, A,,, and A,,. When the structural details are known, they can be directly related to the parameters We obtained the ca, a,, EC, and E,, (relations (3)-(6)). following values for the ErBaZCu,O, compound: cd = 0.48, ah = 1.4, EC, = 43.7 meV, E,, = 17.2 meV. In order to understand the big changes in the energy spectra as a function of x (fig. 2), we made the assumption that the parameters oh, Us, EC, and E,, are weakly structure dependent. We there fixed Eq, E,,, rr, and a, for the x range at the values obtained for the Erl237 compound and allowed the charge at the plane-O (eq. (3)) only to be varied in order to reproduce the changes of the leading CF parameters. The structural quantities appearing in eq. (3) were known
Fig. 2. Low energy part of the energy spectra taken for on IN4 (T=lOK, E,=17meV). ErBaJulOhtr
from powder diffraction experiments performed at the reactor Saphir of the Paul Scherrer Insitut of Wiirenlingen. The results are represented in fig. 3. In the range from 6.00 to 6.35, structural considerations alone are sufficient to explain the observed changes. No charge transfer is needed. In the range from 6.35 to 6.75, a charge of 0.11 (el per oxygen atom has been transferred into the planes. These results are in remarkably good agreement with the crystallographic data of Cava et al. [6]. They obtained a change of 0.08lel in the valence bond sum of the copper. The main difference arises from the fact that we have not
Energy transfer[mevl
Fig. 1. Observed (upper) and calculated (lower) energy spectra for ErBaZCu,O,. The calculated spectra is the convolution of Lorentzians (dashed line) with the resolution function. The low energy part has been taken on IN4 (T = 10 K. E, = 17 meV) and the high energy part on HET (T = 10 K, E, = 100 meV).
J. Mesot et al. I Charge transfer redistribution in ERBa,CuO,
t,
407
observed any sign of the two-plateau structure observed by these authors. In the range between 6.75 and 7.0 the charge transfer process seems to saturate. References [II R.J. Cava. B. Batlogg,
6
62
6.4
Oxygen Fig. 3. Charge transfer relative to ErBalCu,O,.
6.6
6.8
7
concentration
per oxygen
atom
as a function
of x
K.M. Rabe. E.A. Rietman, P.K. Gallagher and L.W. Rupp Jr., Physica C 156 (1988) 523. PI J.M. Dixon and R. Dupree, J. Phys. F 3 (1973) 118. and Betty Ng. Progr. Phys. 52 (1989) 699. PI D.J. Newman S. Huefner, E. Orlich and J. Schmitt, J. [41 P. Gruenberg, Appl. Phys. 40 (1969) 1501. M.D. Kuz’min. A.I. Popov and A.K. Zvezdin, private communication. [6] R.J. Cava, A.W. Hewat, E.A. Hewat. B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419.
PHYSICA El
405-407
Charge transfer redistribution in ErBa,Cu,O,+, neutron crystal field spectroscopy J. Mesot”, P. Allenspach”, S. Bennington”
determined
by
U. Staub”, A. Furrer”, H. Mutkab, R. Osborn’ and
“Laboratory for Neutron Scattering, ETH Zurich, CH-5232 Villigen PSI, Switzerland ‘lnstitut Laue-Langevin, F-38042 Grenoble Cedex. France ‘Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK
A new parametrisation model which takes into account oxygen 2p-4f rare earth hybridization energies is tested in order to understand the unusually large total crystal field splitting in the RBazCu,O,+, (R = rare earth) compounds. This model gives also a quantitatively good picture of the charge-transfer mechanism involved by doping with oxygen.
1. Introduction An important feature of the RBa,Cu,O,+, (R = rare earth, 0 < x < 1) high-T, superconductors is their strong variation of T, as a function of the parameter x. This behavior has been discussed in terms of charge transfer from the chains to the planes [l]. In the R123 compound the R ions are sandwiched between CuO, superconducting planes, thus using the crystal-field theory of the rare earth, an attempt is made to understand qualitatively and quantitatively, the changes of the electronic structure in the neighborhood of the R ions, by doping with oxygen. 2. The model In most metallic rare earth systems, a pure pointcharge model (PC) is sufficient to reproduce the lifting of the (2J + I)-fold degenerate ground-state multiplet observed by neutron spectroscopy. The total splitting is typically of the order of 30meV. In this case the crystalline field (CF) potential at the R site takes the following form:
nation factors representing the angular dependence of the CF parameters. The sum runs over all neighboring atoms. Typical values for the parameters are m4 = 1 and oh =2.5 [2]. The R123 compounds seem to behave in a different way. In this case the total splitting is twice as large as in the above mentioned compounds (-80meV for R = Er). A simple PC calculation would require u4 f 2.5 and a, =7 in order to reproduce this energy splitting. In view of these results one might expect additional contributions to the CF potential. Such problems have already been treated for the case of insulating garnet-systems (31. These compounds show some similarities with the R123 compounds. In both cases the R ions are surrounded by 8 oxygen ligands, and the total CF splitting has the same order of magnitude [4]. Kuz’min et al. [5] proposed a simplification of the superposition-model developed by Newman [3], based on the allowance of cr and IZ bonds in pairs of the type R”+-O’, using a two-center approximation. The crystal-field potential at the rare earth site then takes the following form:
VCF= Vh,, + v,, a where qie denotes the charge at the site ligand ion. Using the operator equivalents can construct the following Hamiltonian: H,,=~~APcC n m
nm nm’
n = (2,4,6),
R, of the ith method,
0G m c n ,
we
(2)
where V,,, is the hybridization contribution and V,, is the point charge contribution; see eq. (1). Using the formalism developed by Stevens the crystal-field parameters can be rewritten as A nm = A;Lb + A,Pm ,
(5)
where
where C,,, are tensor operators, We phenomenological parameters characterizing screening, shielding and anti-shielding effects and the y,,, geometrical coordi0921-4526/92/$05.00
(4)
0
1992 - Elsevier
(6) E,
and E, respectively.
being
Science Publishers B.V. All rights reserved
the energies
of the (T and Kl bonds,
406 3.
J. Mesot et al. I Charge transfer redistribution in ERBa,CuO,
/1
Results and analysis
Measurements were performed on the time-of-flight (TOF) spectrometer IN4 at the high flux reactor of the ILL and on the HET TOF spectrometer at the ISIS pulsed neutron source. The calculations have been made in the intermediate-coupling approximation including J-mixing. The five tetragonal parameters have been adjusted to fit seven ground-state transitions and five relative intensities. The orthorhombic parameters (m = 2, 6) were fixed at their nominal position A,, = A ,I0 y,,,,/y,,,. Figure 1 shows observed and calculated spectra for ErBa,Cu,O,. Assuming that the long range CF parameter A,,, cannot be understood in terms of this simple model, we have concentrated our attention on the leading tetragonal parameters A,,, A 14, A,,, and A,,. When the structural details are known, they can be directly related to the parameters We obtained the ca, a,, EC, and E,, (relations (3)-(6)). following values for the ErBaZCu,O, compound: cd = 0.48, ah = 1.4, EC, = 43.7 meV, E,, = 17.2 meV. In order to understand the big changes in the energy spectra as a function of x (fig. 2), we made the assumption that the parameters oh, Us, EC, and E,, are weakly structure dependent. We there fixed Eq, E,,, rr, and a, for the x range at the values obtained for the Erl237 compound and allowed the charge at the plane-O (eq. (3)) only to be varied in order to reproduce the changes of the leading CF parameters. The structural quantities appearing in eq. (3) were known
Fig. 2. Low energy part of the energy spectra taken for on IN4 (T=lOK, E,=17meV). ErBaJulOhtr
from powder diffraction experiments performed at the reactor Saphir of the Paul Scherrer Insitut of Wiirenlingen. The results are represented in fig. 3. In the range from 6.00 to 6.35, structural considerations alone are sufficient to explain the observed changes. No charge transfer is needed. In the range from 6.35 to 6.75, a charge of 0.11 (el per oxygen atom has been transferred into the planes. These results are in remarkably good agreement with the crystallographic data of Cava et al. [6]. They obtained a change of 0.08lel in the valence bond sum of the copper. The main difference arises from the fact that we have not
Energy transfer[mevl
Fig. 1. Observed (upper) and calculated (lower) energy spectra for ErBaZCu,O,. The calculated spectra is the convolution of Lorentzians (dashed line) with the resolution function. The low energy part has been taken on IN4 (T = 10 K. E, = 17 meV) and the high energy part on HET (T = 10 K, E, = 100 meV).
J. Mesot et al. I Charge transfer redistribution in ERBa,CuO,
t,
407
observed any sign of the two-plateau structure observed by these authors. In the range between 6.75 and 7.0 the charge transfer process seems to saturate. References [II R.J. Cava. B. Batlogg,
6
62
6.4
Oxygen Fig. 3. Charge transfer relative to ErBalCu,O,.
6.6
6.8
7
concentration
per oxygen
atom
as a function
of x
K.M. Rabe. E.A. Rietman, P.K. Gallagher and L.W. Rupp Jr., Physica C 156 (1988) 523. PI J.M. Dixon and R. Dupree, J. Phys. F 3 (1973) 118. and Betty Ng. Progr. Phys. 52 (1989) 699. PI D.J. Newman S. Huefner, E. Orlich and J. Schmitt, J. [41 P. Gruenberg, Appl. Phys. 40 (1969) 1501. M.D. Kuz’min. A.I. Popov and A.K. Zvezdin, private communication. [6] R.J. Cava, A.W. Hewat, E.A. Hewat. B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419.