Lattice dynamics of 123 superconductors

Physica B 156 & 157 (1989) 897-901 North-Holland, Amsterdam

LATTICE DYNAMICS OF 123 SUPERCONDUCTORS W. REICHARDT”‘, D. EWERT’, E. GERING’, F. GOMPF’, L. PINTSCHOVIUS’, B. RENKER’, G. COLLIN3, A.J. DIANOUX4 and H. MUTKA4 ‘Kernforschungszentrum Karlsruhe, lnstitut fiir Nukleare Festkdrperphysik, P.O. B. 3640, D-7500 Karlsruhe, Fed. Rep. Germany ‘Laboratoire Lkon Brillouin, CEN-Saclay, 91191 Gif-sur-Yvette Cedex, France 3Laboratoire de Physique des Solides, Universiti Paris-Sud, 91405 Orsay Cedex, France ‘lnstitut Laue Langevin, 156X, 38042 Grenoble Cedex France

We report about systematic studies on the phonon spectra of 123 superconductors by means of inelastic neutron scattering using polycrystalline samples. We did not observe any unusual dependence on the temperature, in particular no changes were found when passing through the superconducting transition temperature. In contrast, very pronounced changes in the phonon spectra occur when superconductivity is destroyed by the removal of 0 or a partial replacement of Cu by Zn. Using small single crystals of YBa,Cu,O,_, we investigated the phonon dispersion curve up to energies -40 meV. The results were used to adjust the parameters of a lattice dynamical model.

The high superconducting transition temperatures (T,‘s) found among the cuprates are still far from being understood. Although most attempts to explain the high Tc’s assume a non-phononic mechanism, there is considerable experimental evidence of a strong electron phonon coupling in this class of materials. For a quantitative estimate of the phonon contribution to T, a detailed knowledge of the lattice vibrations is essential. This motivated our studies on the lattice dynamics of Y based 123 compounds. We report on measurements of the generalized phonon densities of states G(fio) of various compounds by inelastic neutron scattering which were undertaken with the aim to look for characteristic changes in the phonon spectra if the temperature is varied or if T, is reduced by changing the oxygen concentration or by replacing part of Cu by Zn. Furthermore we present first measurements of the phonon dispersion curves in YBa*Cu3O,.,,-

Generalized phonon densities of states were deduced from scattering law measurements on polycrystalline samples performed on three different time of flight (TOF) spectrometers, the Karlsruhe TOF spectrometer at the reactor Melusine and IN4 and IN6 at the ILL. In the

analysis the spectra were corrected for multiphonon contributions in a self-consistent manner. These corrections turned out to be very important even for sample temperatures of 6 K. In the experiments at Melusine an incident energy E, of 128meV was used in order to allow energy transfers up to 100 meV in down scattering. In a second experiment with E, = 32 meV good energy resolution at small energy transfers was obtained. The final spectra presented in this paper were obtained from a synthesis of the results of the two runs. Measurements of the phonon dispersion curves were carried out on the 2T triple axis spectrometer at the OrphCe reactor, using a Cu(ll1) and a PG(002) crystal as monochromator and analyzer, respectively. Both crystals were focussing horizontally to maximize the intensity while maintaining an adequate resolution. The structure and homogeneity of the polycrystalline samples prepared by the usual sintering method were characterized by X-ray analysis and in some cases by neutron diffraction. The Tc’s were determined by inductive and resistive methods. It is well known that Y based 123 compounds have a strong tendency to adsorb water. Due to the large scattering cross-section

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and the light mass of H even small amounts of water produce pronounced artefacts in measurements of the scattering law. Therefore great care was taken to avoid any contact of the samples with the moisture of the air. Single crystals were prepared by a flux method. By a subsequent heating in an oxygen flow the original oxygen content of O,,, was raised to 06,85. The oxygen concentrations were determined by neutron diffraction experiments. Fig. 1 summarizes the results of our search for temperature effects in YBa,Cu,O, (T, = 92 K). The comparison between the two spectra of fig. la measured at 6 K and 300 K, respectively, demonstrates that there is no anomalous temperature dependence, only the usual slight hardening of the spectrum at low temperatures is observed. Additional measurements were carried out on IN4 with E, = 67 meV to look for possible changes at T,. The two spectra in fig. lb obtained at 60 K and 130 K, respectively, are barely distinguishable indicating that the transition to the superconducting state has only a very small

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influence on the phonon spectrum. Recently Thomsen et al. [l] have observed that a Raman frequency at 42 meV softens by about 0.5 meV when YBa,Cu,O, is cooled below T,. As a similar effect in this range of energy is absent in our results we conclude that a softening occurs only in very few branches or is restricted to a small region in q-space. In fig. 2 we compare G(hw) of YBa,Cu,O, (T, = 92 K) with that of non-superconducting YBa,Cu,O,. The most spectacular effects are a strong reduction of the spectral weight between 40 and 65 meV and the appearance of a new peak at the upper end of the spectrum for the non-superconducting compound. In order to substantiate and extend these results we have performed a systematic study of the phonon spectra as a function of 0 content. These measurements were conducted on the IN6 spectrometer in the energy gain mode of operation. Therefore the structures above 50 meV are washed out due to insufficient energy resolution. Results were obtained for the following 0 concentrations: 6.07, 6.16, 6.40, 6.53, 6.81 and 6.90. For clarity we have plotted in fig. 3 only those spectra with concentrations of 6.07, 6.53 and 6.90 over the whole range of energies. The results for the intermediate concentrations 6.4 and 6.81 are marked by full lines between 40 and 70 meV. The distribution for 6.16 is very close to that of 6.07; therefore we have omitted it from the figure. It is seen that the spectra change monotoneously as a function of 0 concentration. This gives convinc-

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ElmeVI Fig. 1. G(&J) of YBa,Cu,O, measured at various temperatures. The spectra in (b) were obtained with E0 = 67 meV. Therefore the high frequency part of G(LJ) could not be determined.

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Fig. 3. G(ho) at 296K of YBa,Cu,O,_, as a function of oxygen deficiency 6. For clarity the spectra for 6 = 0.19 and 0.60 are indicated only between 35 and 70 meV as full lines.

ing evidence that the results reflect intrinsic properties of the system and not artefacts due to contamination with hydrogen. The replacement of 10% Cu by Zn in YBaCuO completely suppresses superconductivity even at high 0 concentrations [2]. Thus YBa,(Cu,Zn,)O,_, with small 6 represents another non-superconducting reference system to YBa,Cu,O, which has the advantage that the orthorhombic structure is retained. Unfortunately it is not possible to prepare samples with the full 0 concentration 0,. Therefore effects brought about by the Zn dopant cannot be fully separated from those due to the oxygen deficiency. In fig. 4 we show the phonon spectrum of a Zn doped sample with the maximum 0 concentration attainable in this system, i.e. Oh,*, and compare it with G(&J) of YBa,Cu,O,. Evidently, the replacement of 10% Cu by Zn has

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a similar effect on the high frequency part of the phonon spectrum as a drastic reduction of the 0 content. Measurements of the phonon dispersion curves are strongly hampered by the smallness of the available sin le crystals. We started with a P sample of 1 mm . At present we use a sample consisting of 4 coaligned single crystals with a total volume of 30 mm3 which is still much smaller than normally used for this type of measurements. The dispersion relation of YBa,Cu,O, is very complex as the unit cell contains 13 atoms and hence there are 39 phonon branches. A further difficulty arises from the fact that the crystals are twinned. This means that in the experiment only superpositions of phonons propagating in the 100 and 010 directions can be determined. Likewise it is not possible to distinguish between the transverse phonons in the 001 direction polarized along 100 and 010, respectively. Fig. 5 shows the results so far obtained. Due to the twinning of the crystals the points for the [ QIO]/[O[O] direction and the transverse branches along [OOl] represent frequency averages. For the interpretation of the data we performed lattice dynamical model calculations, using the same type of model which served to analyse the phonon dispersion curves of La,NiO,: a rigid ion model with anisotropic screening of the Coulomb forces and a breathing ““1 KoOl/lOt01

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term for the in-plane Cu-0 vibrations [3]. The parameters of the model were adjusted to reproduce the measured phonon frequencies and the existing Raman data [4]. For lack of experimental information about the high frequency modes similar effective ionic charges and the same breathing parameter as for La,NiO, were chosen. Fig. 6 gives a comparison between experiment and model calculation for the 100 direction. The model was used to calculate G(fio) which is compared to the experiment in fig. 7. The largest discrepancy is observed around 20 meV where the model predicts the density of states considerably too high. Thus some modes contributing to the peak at 20meV have to be shifted to higher energies. Although our model is still somewhat approximative it is helpful to interpret the results shown in figs. 2-4. There are three regions with strong changes when the 0 content is varied: below -15 meV, around 20 meV and above 35 meV. (i) The observed softening of the spectra at low energies for oxygen deficient samples can largely be explained by the fact that there are less bonds in 0, than in 0,. (ii) According to our model the peak near 20meV in YBa,Cu,O, has an

Fig. 6. Comparison of the observed phonon dispersion curves with the result of a model calculation for the 100 direction.

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appreciable contribution from transverse vibrations of O(1) in the Cu-0 chains. Thus the removal of 0( 1) leads to a strong reduction of this peak as is observed in the experiment. (iii) Above 35 meV G(hw) is almost exclusively due to 0 vibrations consisting of bond stretching and bond bending vibrations of O(2) and O(3) in the Cu-0 planes and of bond stretching vibrations of O(1) and O(4) ( in our labelling O(4) is located above Cu(1) on the c-axis). So far there is no direct experimental information about the energies of the Cu(l)-0( 1) stretching modes. Our model locates them near 60 meV. As their spectral weight is low (about 3% of the total area) the absence of 0( 1) in 0, can explain only a small part of the observed reduction in intensity between 40 and 70 meV It is now well established from our neutron investigations and from Raman measurements [l] that between 35 and 55 meV the spectrum is dominated by bond bending vibrations involving O(2) and O(3), whereas the corresponding bond stretching vibrations are located at about 70 meV Therefore we conclude that mainly the vibrations of the oxygens in the planes are strongly affected by the removal of

W. Reichardt et al. I Lattice dynamics of 123 superconductors

O(1) in the chains. This can only be understood by strong changes in the electronic system. It is tempting to correlate these changes with the reduction or complete suppression of T,. In conclusion we have observed important differences between the phonon spectra of superconducting YBa,Cu,O, and the non-superconducting reference compounds YBa,Cu,O, and which can be explained YBa2(Cu0.9Zn0.1)07, only partly as structural effects. In particular, the pronounced frequency changes of the 0 vibrations in the planes require large modifications of the interatomic forces. Therefore we speculate that the observed effects are a manifestation of a strong electron phonon coupling. For a better understanding of the lattice vibrations and their importance for superconductivity in YBa,Cu,O, further measurements of the phonon dispersion curves are necessary. Due to the smallness of our single crystal samples measurements so far were restricted to energies below 40 meV. When larger

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single crystals become available we shall extend our present investigations to higher phonon energies. In particular we intend to study the influence of the oxygen concentration on the phonon frequencies in order to answer the question: what are the changes in the phonon dispersion curves that lead to such drastic effects in the phonon densities of states?

References [l] C. Thomsen, M. Cardona, B. Gegenheimer, R. Liu and A. Simon, Phys. Rev. B 37 (1988) 9860. [2] Gang Xiao, F.H. Streitz, A. Gavrin, Y.W. Du and C.L. Chien, Phys. Rev. B 35 (1987) 8782. [3] L. Pintschovius, J.M. Bassat, P. Odier, P. Gervais, B. Hennion and W. Reichardt, Europhysics Lett. 5 (1988) 247. [4] R. Liu, C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. de Wette, J. Prade, A.D. Kulkarni and U. Schriider, Phys. Rev. B 37 (1988) 7971.