Crystal electric field excitations in REBa2Cu3O7−x compounds

Physica

C 153-155

North-Holland,

(1988) 184

185

Amsterdam

CRYSTAL ELECTRIC FIELD EXCITATIONS IN REBa2Cu307_X COMPOUNDS E. GERING, B. RENKER, F. GOMPF, D. EWERT, H. SCHMIDT, R. AHRENS, M. BONNET*, and A. DIANOUX** Kernforschungszentrum Karlsruhe, INFP and ITP, P.O.B. 3640, D-7500 Karlsruhe, FRG *CENG, B.P.85X, F-38041 Grenoble Cedex, France; **ILL,B.P.156X, F-38042 Grenoble Cedex, France By i n e l a s t i c neutron scattering (INS) experiments we have examined crystal f i e l d excitations (CEF) in RE Ba2Cu307-x (RE=Pr,Nd,Tm,Tb). The investigations are complemented by measurements of the spec i f i c heat. In high Tc Y-Ba-Cu-O superconductors the Y atom can be replaced by ions of the rare earth (RE) series with large local 4f moments without a f f e c t i n g the superconducting t r a n s i t i o n temperature Tc. This could be understood by the p a r t i cular perovskite structure where the central Y s i t e is well separated from the Cu-O plains which are important for c o n d u c t i v i t y . But, there are alsn exceptions as Pr and Tb which, i f subs t i t u t e d f o r Y, causes Tc to decrease. No superc o n d u c t i v i t y is found for PrBa2Cu307 whereas TbBa2Cu307 could not be synthesized in the correct I-2-3 structure. By i n e l a s t i c magnetic neutron scatterinq (INS) we have observed well deE n e r g y (meV) 20. O. - 1 0 . ) I I

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fined crystal e l e c t r i c f i e l d (CEF) t r a n s i t i o n s f o r RE-Ba-Cu-O superconductors which o f f e r the p o s s i b i l i t y f o r a detailed study of the coexistence of magnetism and superconductivity. In Fig. I typical INS spectra due to CEF exc i t a t i o n s in TmBa2Cu307_x f o r x=1 and 0 are shown. By measurements at 6 K, 30 K and 120 K we have i d e n t i f i e d CEF levels at 12.3, 14.8 and 24.8 meV f o r Tm-07 and at 14.5 and 20.7 meV for the Tm-O6 compound. From the observed i n t e n s i t i e s we believe that the level at 14.5 meV in tetragonal Tm-06 is doubly degenerated and is s p l i t into two levels at 12.3 and 14.8 meV in orthorhombic Tm-07. We expect a d d i t i o nal levels at higher energies up to 80 meV corresponding to the neighbouring systems Er-O7-x and Ho-O7-x ( I ) . Similar measurements on the i s o s t r u c t u r a l Nd-O7-compounds revealed levels at 12.1 meV, 20.2 meV and 34 meV. Since J=9/2 a l l states are doubly degenerate. There is one level missing which we conclude from our measurements must l i e at higher energies than 80 meV. In agreement with a doubly degenerate ground-state large quasielastic scattering is observed which, as Fig. 2 shows, broadens l i n e a r l y with temperature, independent of the p a r t i c u l a r oxygen content.

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FIGURE I spectra f o r TmBa2Cu307 and CEF e x c i t a t i o n s at 12.3, 14.5, 20.7 meV. The small due to a contamination.

0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

o'

I 0 TEMPERATURE

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2 0' 0 (K)

FIGURE 2 Linewidth (HWHM) of quasielastic magnetic scattering in NdBa2Cu307-x as determined from f i t s with a Lorentzian.

E. Gering et al. / Crystal electric field excitations

Quite a d i f f e r e n t behaviour is found f o r samples of (Pr,Tb)6Y1_~Ba2Cu307-x. For Pr compounds with x=O, 6=.I, .5, I and x=1, ~=I were investigated. In a l l cases we f i n d large magnetic scattering f o r h~C6 meV centered around zero energy (Fig. 3). From f i t s which we have performed, we conclude that the scattering is e s s e n t i a l l y i n e l a s t i c with a broad maximum around h ~ 3 meV. For the Tb samples with 6=.I, .2 and x=O s i m i l a r results were obtained at 50 K and 100 K. In measurements performed at 10 K and 20 K we got evidence f o r magnetic ordering. This is concluded from the occurrence of an additional e l a s t i c component and pronounced coherent effects in the magnetic scattering. Our INS investigations are complemented by specific heat C(T) measurements between 1.4 K and 45 K on samples with RE-Gd,Pr,Nd and Tm (Fig. 4). We found that a f t e r correcting f o r differences in mass the heat capacities f o r the Gd- and the Y-compounds are p r a c t i c a l l y the same above ~12 K. Thus the Gd r e s u l t can be taken as a reference in order to extract the contribution AC(T) of CEF e x c i t a t i o n s to the t o t a l C(T). For the Nd- and Tm samples AC(T) corresponds well to the specific heat which is calculated using the level energies found in our INS experiments. The difference in C(T) between RE-07 and RE-O6 corresponds well to the s o f t e r phonon spectra of RE-O6 compounds (3). A CEF doublet around 4 K f o r Nd-07 as suggested by Ramirez et a l . (2) would not be compatible with our measurements. The steep upturn in C(T)/T at T<5 K is probably an onset of magnet i c ordering. Magnetic ordering was also found f o r our samples Pr-07 and Pr-06 at 15 K and 10 K, respectively. From the occurrence of kinks in the magnetic s u s c e p t i b i l i t y at the I 615 0

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185

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FIGURE 4 Specific heat Cp(T)/T for RE Ba2Cu307_ x compounds. Broken l i n e s r e f e r to oxygen d e f i c i e n t compounds with x=1. same temperature we conclude that the ordering is antiferromagnetic. This ordering was not observed by Ramirez et a l . and is subject of f u r t h e r i n v e s t i g a t i o n s . Unfortunately, the magnetic ordering anomaly masks the Schottky anomaly due to the CEF e x c i t a t i o n s around 3 meV observed by INS. In agreement with the neutron data no evidence is obtained f or f u r t h e r CEF-transitions at higher energies. In conclusion we have found s i g n i f i c a n t d i f ferences in the spectrum of magnetic e x c i t a tions f o r the superconducting Nd- and Tm-samples and the isostructural Pr- and Tb-compounds. The l a t t e r two ions have a tendency to be 4+ which might explain that the corresponding RE Ba2Cu307 compounds are not superconducting. Furthermore, f l u c t u a t i o n s between the valence states 3+ and 4+ could be responsible f o r the d i f f e r e n t magnetic behaviour. Structural changes between Tm-07 and Tm-O6 are c l e a r l y r e f le c t ed in the CEF e x c i t a t i o n spectrum. F i n a l l y from our data no evidence is obtained f o r an anomalous behaviour near Tc which shows that an i n t e r a c t i o n between the 4f and the conduction electrons must be small. REFERENCES ( I ) A. Furrer et a l . , this volume (2) A.P. Ramirez, L.F. Scheemeyer, and J.V. Waszcak, Phys. Rev. B 36 (1987) 7145 (3) B. Renker et a l . , this v o l u ~