Evidence for ferroelectric phase transitions and excitonic high-Tc superconductivity in YBa2Cu3O7−δ from ultrasonic measurements

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Solld State Communications, Vol. 72, No. I0, pp. 997-1001, 1989. Printed in Great Britain.

EVIDENCE EXCITONIC

FOR FERROELECTRIC HIGH-To FROM

PHASE

TRANSITIONS

SUPERCONDUCTIVITY

ULTRASONIC

0038-I098/8953.00+.00 Pergamon Press plc

AND

IN YBa2Cu3Or-6

MEASUREMENTS

V. Miiller, C. Hucho, K. de Groot, D. Winau, D. Maurer, and K.H. Rieder Freie Universit~t Berlin, Fachbereich Physik, Arnimallee 14, D-1000 Berlin 33, FRG

(Received October 3, 1989 by B. Miihlschlegel)

We report on pronounced anomalies of the absorption and dispersion of the sound-induced rf magnetic field in superconducting YBa2Cu307_~ (6 < 0.15). The experimental findings are highly indicative for a ferroelectric phase transition in the temperature range of 150 K< T < 180 K and for a near coincidence of the superconducting transition and a ferroelectric phase transition at about 90 K.

from other charges than the conduction-electrons) is large (i.e. 800 - 3600) in (1-2-3) compounds15,16. Furthermore, there is a number of indirect pieces of evidence17-21 suggesting that at low temperature a ferroelectric state exists in (1-2-3) compounds. Although ultrasound is a highly sensitive probe of phase transitions and therefore should be an axiequate tool in giving a more rigorous proof of the formation of a ferroelectric (antiferroelectric) phase, practice teaches us that in actually available single crystals and ceramic (1-2-3) samples the sound-field quantities are completely dominated by the background (lattice) terms 21 which prevent a reliable determination of the electronic contribution to the sound velocity and ultrasonic attenuation. On the other hand, taking into account that an ultrasonic wave will modulate the lattice as well as the electronic charge distribution, and therefore should give rise to sound-induced electromagnetic fields e,b, it is intuitively evident that measurements of these fields should be a promising method to overcome the afore mentioned difficulties and to separate the electronic response from the background terms. In the MHz range, however, where most of the ultrasonic experiments are performed, the respective fields usually turn out to he negligible small in conducting materials, because in response to the acoustically distorted charge distribution of the ions, the highly polarizable electron gas distributes itself so as to shield or screen the fields produced by the ions. Note, that in normal metals and for a 10 MHz ultrasonic wave the dielectric permittivity is of the order of 10l° in the Thomas-Fermi limit14. Nevertheless, the sound-induced electromagnetic fields may become fairly large 22 if a strong dc magnetic field B0 is applied, since in consequence of the different mass, the Lorentz force will influence by far more the electron dynamics than that of the ions, so that in the presence of a d c magnetic field the conduction-electron screening is less perfect. Hence, it is the aim of this paper to present measurements of the ultrasound-induced electromagnetic fields and to demonstrate that, upon heating from temperatures well below To, the experimental findings are highly indicative that near the superconducting transition temperature YBa2Cu3Or_6 (6 < 0.15) undergoes a phase transformation at 90 K into a ferroelectric state and at about 170 K from a ferroelectric into another pyroelectric phase 23. The measurements were performed with a computer-

Since the early discussions1 of the possibilities of raising the superconducting transition temperature well above the "electron-phonon limit" of Te ~ 30 - 40 K 1, "sandwich"type compounds with alternating dielectric (polarizable) and metallic layers were regarded 1-3 as the most promising candidates for high-To superconductivity, because excitons (huexc ~ 0.1 - 0.3 eV) in the dielectric layers may provide1-3 a more effective pairing-mechanism and a substantially higher Te than the interaction of conduction-electrons with phonons (huph ,,~ 10-2 eV) in normal metals, i.e. with the comparatively weakly polarizable lattice. The discovery4 of high-Tc superconductivity in defect perovskite oxides, on the other hand, i.e. in a species known for its tendency to undergo ferroelectric phase transitions, and the finding that the defect perovskite cnprates with the highest Tc valuess - r belong to the class of layered compounds, therefore raises the question, whether in these materials metallic and dielectric layers coexist and whether the pairing mechanism is of excitonic nature. Following the arguments of Ginzburgs, a rigorous verification of ferroelectricity would be highly indicative for an excitonic pairing mechanism, since excitonic superconductivity and ferroelectricity are of the same origin, namely the presence of soft dipoles (with electric polarizability a). In YBa2Cu3Or-~ it is most likely that the soft dipoles are formed by the Cu ions and the highly polarizable 0 2- ions9 placed on the Cu(1)-0 zigzag chains1°. If n is the number density of soft dipoles, P the electric polarization and fP/Eo the Lorentz contribution to the acting (effective) electric field (Eel,, = E + f P/~o), the dielectric permittivity is given by s ~ - 1 = an/(1 - fan). Therefore, a spontaneous polarization of the soft dipoles, i.e. ferroelectricity, will appear if f a n = 1 for k = 0, whereas superconductivity with a high Tc requiress a large negative value of ,(0,k) (i.e. f a n > 1) at k ~ k F . Accordingly, ferroelectricity and excitonic high-To superconductivity should intimately be related to each other n-13. From an experimental point of view, however, the difficulties of establishing the formation of a spontaneous electric polarization in these highly conducting systems are very high, because a gas of conduction-electrons stands out for its extremly high electric polarizability, so that the total permittivity 14 is dominated by the conduction-electrons. Nevertheless, there is much evidence that the dielectric permittivity (originating 997

998

FERROELECTRIC PHASE TRAI~SITIONS

controlled sampled-continuous wave spectrometer 24 on cylindrical coarse grained (grain size> 20pm) YBa2Cu30~-6 (/~ < 0.15) ceramic samples (ire -~ 91 K, onset) in a dc magnetic field of 6 T. After polishing, the samples were carefully annealed in oxygen (Po2 = l b ) at about 450°C. In order to avoid pollution by the bonding agent (Thiokol LP32) when attaching the 6 MHz X-cut quartz transducer, and for reasons which will be discussed below, both end faces of the samples were gold-plated. The thickness of the gold layer (d ,~ lltm) was choosen to be smaller than the skin depth (6A~ ~ 10pm) at 6 MHz. The sound-induced ff magnetic field b oriented perpendicular to the acoustic wave vector q was detected by means of a specially designed rf coil. The latter was mounted opposite to that end-face of the specimen at which the transducer was fixed. Regarding that the sound-induced total electric polarization p may be written as the sum P = Pee + Pb

(1)

of the sound-induced polarization poe of the conductionelectrons and the contribution Pb of the bound charges (soft dipoles), and taking into account that (in a coordinate system moving with the acoustic displacement velocity d u / d t of the ions) the total current density may be written as j(r, t) = Op'(r, t)/cgt ,

(2)

in the presence of a strong dc magnetic field B0 Maxwell's equations yield for the space and time Fourier transforms of the sound-induced rf magnetic field --(C.f/Co)2(~-- 1)

b(q,~) = 1 - - ~ - ~ )

i

q

X (u X B0)

(3)

where cs is the sound velocity and co the speed of light in vacuum. In deriving this equation we have put p'(q,w) = E0(E -- 1)e', where e' = e + / ~ u x B0 is the sound-induced electric field in the moving coordinate system, w the angular frequency of the sound wave, ~0 the dielectric vacuum permittivity and ~(q,w) is the total dielectric function (i.e. a c-number) which, according to E(q,w) = (~ee + Eb - -

1)

(4)

,

may he expressed by the dielectric functions ¢c~ and eb of the conduction-electrons and soft dipoles, respectively. We mention that (¢b - 1) = ( a n ) / ( 1 - fc~n) is not the sum of (~bi - 1) of the different kinds of soft dipoles, since only their polarizabilities al = ((Ebi- 1)/nl)/(l+f(e~-- 1)) are additive quantities, where n i is the number density of the individual ions or molecules of one species. Following the lane of Ref.22 and regarding that for wr << 1 and Pb = 0 (i.e. Cb = 1) the current density of the conduction-electrons may be written as jc, = iwp'~ = aoe', with p ' , = ~0(~e~- 1)e' it is easy to show 14 that the relation (,~, - 1) (co/e,)~/(i~) must hold, i.e. w i t h , = d - ig' we have

(dce-1)=O

(coV!~ '

d~= \c,]

and

(5)

where 1 / r is the scattering rate of the conduction-electrons, a0 the dc conductivity,/~ = (q*)~/2 and df the skin depth. Hence, combining Eqs.(3) and (5) we obtain for b, with e~ = 1, the same expression as derived in Ref.22. In the superconducting (mized) state, i.e. for Bc~ < Bo < Be2, where Be~ is the lower and Be2 the upper critical field, and for ~b = 1 the respective equation for the superconducting current density j , = - i ( e + iwu x (B0))/(p0wA 2) also yields (~, - 1) = (Co/Co)2/(i1~,), but

(<-

1):

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penetration depth, IZo the vacuum permeability and (Bo) the spatially averaged magnetic field in the sample. W e note that the (temporal) phase shift~ of the sound-induced rf magnetic field with respect to the sound wave is related to the real part of/3 in virtue of the identity 22 ~ = arctg{Re/3}. In YBa2Cu3Or_~ Re/3 is about 20 in the normal-conducting state and zero in the superconducting state which gives rise to the observed phase shift of A ~ _,2 x / 2 at Tc, and therefore may be used as an efficient tool in determining the superconducting transition temperature of the bulk material. As for the highest known dynamic dielectric permittivities Eb, the magnitude of I~b -- 1[ is small as compared to that of [ece - 11 in metals, and as (c,/co) 2 ~_ 10 -1°, it follows from Eqs.(3)-(6) that the contribution of (~b -- 1) to the sound-induced rf magnetic field is negligible small. However, as we will see below, the situation ch:.nges markedly if the measurements of b are extended to the determination of the absorption and dispersion of the sound-induced ff magnetic field, because the product (ee, - 1)(~b -- 1) of the dielectric permittivities of the conduction-electrons and bound charges (soft dipoles) enters into the respective expressions. This is the reason why dielectric contributions which axe hidden from the view in conventional ultrasonic experiments may now become visible. Furthermore, by combining Eqs.(3) and (6), and as demonstrated in Fig.l, it is easy to realize that measurements of the magnitude of b should be a powerful tool in determining the temperature dependence of the magnetic penetration depth A(T) for T _< Te. Although the magnitude of b (see Fig.l) increases considerabely for T < Te (note that in the superconducting state I/~,1 varies between 10 -4 _< I/~sl < ~ for 0 < T < Te ), it deviates from the temperature dependence (see Eqs.(3) and (6)) expected if A would follow the empirical formula A(T)/A(0) = (I - (Y/Tc)4) -1D. However, as this finding will be subject of another paper, we just mention that in ceramic samples the spatial distribution of the crystal axes of the single grains and the anisotropic nature of A and Tc(Bo) must be taken into account when evaluating the experimental data quantitatively. Furthermore, one should bear in mind that, due to the anisotropic nature of T~(Bo), the superconducting transition-width (i.e. the distribution of the

t0

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i

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50

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, II

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1

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"--0 ,

(6)

since, contrary to the normal-conducting state, fls = i(qA) 2 is an imaginary number. Here, A(T) is the magnetic

Fig.1. Temperature dependence of the magnitude of the 6 MHz sound-induced rf magnetic field in coarse grained (grain size > 20pm) YBa2Cu3Or-8 (6 < 0.15) at Be = 6 T upon heating at an average heating rate of 20 K/h.

Vol. 72, No. I0

transition temperatures) becomes very large in ceramic samples if a d c magnetic field of 6 T is applied. In this context it is worth to mention that for w/2x ,~ 6 MHz the parameter/~ is of the order of 10 -1 in a thin ( ~ l p m ) gold layer, whereas in YBa2Cu3Or-s ~ is about 20 for T > T¢. Hence, the magnitude of b becomes very small for T > T~. However, the detected sound-induced rf magnetic field bd (and consequently the signal-to-noise ratio) may considerably be enhanced if the surface in front of the detection coil is coated with a thin gold layer, for in that case (d ~:/fa~) the detected rf magnetic field bd may approximately be written as bd "" - b r ( 1 + i[3r)/(i~a~). Here, b r , bA~ are the respective fields given by Eq.(3), and the indices A u , Y refer to the gold layer and to the YBa2Cu3Or-$ compound, respectively. For low cooling and heating rates of about 20 K/h, the temperature dependence of the acoustic standing wave resonance frequency w , / 2 x (which is proportional to the acoustic phase vdocity e°) is presented in Fig.2 and shows a pronounced dastic hysteresis. We mention, in passing, that pronounced elastic hystereses have recently also been found 2s in large single crystals YBazCu3Or-6 and therefore cannot be considered any longer as a specific property of coarse grained ceramics 1~,21 only. As a function of temperature and for the same cooling and heating rates as in Fig.2, both the decay rate 1/r,, of the sound-induced rf magnetic fidd and the difference in frequency (win - w,)/w, are presented in Fig.3 upon cooling, and in Fig.4 upon heating the sample. The most striking features of Figs.2-4 are the findings that, in contrast to the sound velocity (and ultrasonic absorption), the sound-induced rf magnetic field shows a marked change in frequency and a huge absorption peak at 90 K. However, both phenomena could be observed only in those samples which show a pronounced elastic hysteresis and only in those heating runs where, before heating, the specimen was first cooled well below T¢. Furthermore, the anomalies were not present (or at least much smaller) if t h e sample was cooled from room temperature to T < Tc. On the other hand, if (upon raising the temperature from T <60 K) the heating run was stopped at about 10-20 K above ire and the sample was cooled again below To, then 5 . 9 5

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FERROELECTRIC PHASE TRANSITIONS

I

. . . .

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5 . 7 5

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150

200

250

Temperature in K Fig.2. Temperature dependence of a longitudinal acoustic standing wave resonance frequency ua = w J 2 r of about 6 MHz upon cooling and heating at an average cooling/heating rate of 20 K/h. The sample is the same as in Fig.1. Note, that the frequency is proportional to the ultrasonic phase velocity.

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Temperature in K Fig.3. Temperature dependence of the absorption (upper part) and dispersion (lower part) of the sound-induced rf magnetic field upon cooling. The sample and the experimental conditions are the same as in Fig.2.

both features could be observed also upon cooling. Accordingly, the appearance of the anomalies at 90 K are less indicative for the superconducting transition than for the transformation of a hitherto non specified low-temperature phase (which upon cooling seems to be fully established21 below 60 K) into a new state. In this context it is worth to emphasize that only for very few temperature cycles the absorption peak turned out to be as sharp as the one shown in Fig.4. This clearly indicates that the homogeneity of the low-temperature phase depends sensitively on the kind of thermal cycling. We further note that our findings agree well with those of previous zero-field specific heat measurements 26,2r, where upon heating a pronounced peak in the specific heat was found 2e at 90 K and identified to reflect a first-order phase transition of so far unidentified physical origin, whereas in samples, where the superconducting and the 90 K transition coincide, Wohlleben et al.2r specified the phase transition to be of higher order than n = 2. As can be seen in Figs.3 and 4, upon both cooling and heating a further (but less pronounced) peak in the electromagnetic decay rate 1/rm as well as a marked change in frequency appears between 150 K and 180 K, indicating a high-temperature phase transition. This phase transition has also been observed in specific heat measurements 2s but at a higher temperature and only upon heating. It should be mentioned, however, that the respective transition temperature depends strongly on sample preparation, grain size and the thermal history of the specimen under investigation and may vary between 170 and 220 K. The question now arises, what is the physical origin of the unusual behaviour of the absorption and dispersion of the sound-induced rf magnetic field near T~? Magnetic screening effects due to the presence of a strong dc magnetic field (6T) are unlikely, since otherwise it would be hard to understand, why these phenomena are observed upon heating the sample from low temperature but not upon cooling from room temperature, and analogous effects do not appear in magnetic susceptibility measurements. Furthermore, fiuxpinning effects become negligible small at To, and there is no reason why upon cooling the flux-lattice should give rise

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to a pronounced absorption peak and a marked change in frequency near Tc only in that particular case, where the sample was first cooled well below Tc and then heated to about 10-20 K above Tc before cooling it again. Thus we are led to conclude that the anomalies near Tc are most likely of electric but not of magnetic origin. This conclusion may also be founded more rigorously by means of theoretical arguments. Regarding that the external (unscreened) electric field acting within a dielectric, homogeneously polarized domain on the conduction-electrons is the electric field - f p b / e o of the soft dipoles (in a coordinate system at rest) plus iwu x Bo (where in isotropic or cubic crystals f = 1/3 for a sphere and f = 1 for a layer), it can be shown29 that for fir :~ 1, q .l_ B0 and with reference to the sound field quantities wa, 1/v,, the leading term in the expressions for the change in the dispersion and absorption of the sound-induced rf magnetic field may be written as / - - ~ C

wa

-- 21~opC~ ]

4

___

-B2osin2$ f ('c. ]4 (ec, - 1)2w.e~'

(8)

Here, p is the mass density, 1/r., the decay rate of the soundinduced rf magnetic field and ¢ the angle between u and B0. As (%~ - 1) "~ 0 for Tc and for temperatures slightly below the superconducting transition temperature, it follows that both ( l / r , , - 1Iv,) and (win - w , ) / w , should vanish near T~. In YBa2Cu30~_6, however, Tc(Bo) is highly anisotropic, and for B0 = 6 T we have3° (Tc).L ~ 86 K for B0 J- c and (T~)]I -~ 77 K for B0 [[ e. In ceramic samples with randomly

PHASE TRANSITIONS

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distributed orientations of the single grains, the dielectric permittivity ece - 1 is therefore a weighted sum of ((ece)± - 1) and ((ece)[I - 1), so that for (Tc)lI < T < (Tc)± and with (c,/co)2(eee - 1) = 1/it~ the t e r m f ( c s / c o ) 4 ( e c e - 1)2(eb -- 1) has to be substituted by --f(eb -- 1)/(fill) 2, where(fl l) is the averaged fl value of those grains which still are in the normalconducting state for T < (Tc)j.. For T > 77 K the frequency shift in Figs.3 and 4 therefore should reflect, above all, the temperature dependence of - ( d b - 1), and the absorption the temperature dependence of WadbI (plus additive background terms). With (/31[) ~_ ~ "" 20,(B~/p0)/(pc~) ~ 3 - 1 0 -4 it thus follows from .Eq.(7) and Figs.3 and 4 that in the vicinity of To ~ 90 K and T1 - 170 K (where (win - w,)/w~ is of the order of 10-3 ) the dispersive part of the dynamic permittivity (e~ - 1) must be of the order of 103 - 104. The magnitude of (db - 1) may be considered as evidence that close to these temperatures YBa~Cu3Ov_$ transforms into a ferroelectric or antiferroelectric state. The very specific structure of (w,n - wa)/wa at To and T1 further indicates that the phase transformations should be achieved by a resonant process with a temperature dependent resonance frequency. Hence, nonresonant orientational ordering of preexisting permanent electric dipoles may be excluded, because this should result in a relaxator type behaviour. Accordingly, the reasons for the phase transitions have to be searched for in the atomic and displacement polarizabilities of the material where, in the frequency range investigated here, the former are considered not to be relevant, since the atomic eigenfrequencies are by far too high and should not depend on temperature. Thus we are led to conclude that the phase transitions near To and T1 are achieved by atomic displacements, where the formation of a spontaneous electric polarization is closely tied to a softening of the lattice31,32,33, i.e. of a polar optical phonon mode. This means that on approaching the transition temperatures, the respective eigenfrequencies wto should tend to zero. As our experiments were performed at very low frequencies (6 MHz) and at q "~ 0, it is most likely that the temperature dependent anomalies in the dispersion and absorption of the sound-induced electromagnetic fields reflect ferroelectric phase transitions, because at a ferroelectric transition a polar optical phonon softens at q = 0, whereas a transition into an antiferroelectric state is driven by a softening at q = =I=G, i.e. at the edge of the Brillouin zone. In this context it is worth to mention that, irrespective of the particular value of To, we have recently found 34 a small, but detectable change in the elastic behaviour of single crystals YBa2Cu307-8 at 90 K which is compatible with a phonon softening. Furthermore, our results are in agreement with considerations of Kurtz at al. 19 who have predicted a near coincidence of a ferroelectric transition and the high-temperature superconducting phase transition. Our interpretation is further corroborated by previous NQR measurements3s, where at the Cu(1) sites and in the vicinity of Tc a marked change in the NQR frequency has been observed which most easily may be explained by a ferroelectric ordering of the oxygens on the Cu(1)-O zigzag chains. Nevertheless, it seems to be surprising that in our experiments the eigenmodes are not overdamped. Our findings, however, turn out to be quite natural if the ferroelectric phase transitions are of second or higher order, because in that case the correlation time rto (and consequently wtorto which is a measure of the coherency of the eigenmodes), should be large at To and T1 • Summarizing our results, there is much evidence that within the limits of the simple isotropic model used here, both ferroelectricity and superconductivity coexist in YBa2Cu30~-~, so that in this material superconductivity is most likely of excitonic nature. This work was supported by the Bundesministerium fib Forschung und Technologie (F+E-Vorhaben 13N 5479/9).

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1001

References 1. "High-Temperature Superconductivity" edited by V.L. Ginzburg and D.A. Kirzhnits, Consultants Bureau, New York (1977) and references therein. 2. V.L. Ginzburg, Phys. Lett. 13, 101 (1964) 3. M.L. Cohen and P.W. Anderson, Proc. AIP-Conference on d- and f-band superconductivity, Rochester 1971, Am. Inst. Phys. New York (1972), p. 17 4. J.G. Bednorz and K.A. MiUler, Z. Physik B64, 189 (1986) 5. M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L.L. Gao, Z.J. Huang, V.Q. Wang and C. Chu, Phys. Rev. Left. 58, 908 (1987) 6. H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jap. J. Appl. Phys. 27, L209 (1988) 7. Z.Z. Sheng and A.M. Hermann, Nature 332,55 (1988); Nature 332,138 (1988) 8. V.L. Ginzhurg, Ferroelectrics 76, 3 (1987) and references therein. Note, that a is defined by P -eonc~Eell, where eo is the vacuum permittivity and n the number density of soft dipoles. 9. A. Bussmann-Holder, H. Bilz and P. Vogel in Springer Tracts in Modern Physics, Springer Verlag, Berlin, Heidelberg, New York, Tokyo 1983, p.74 and references therein 10. M. Francois, A. Junod. K. Yvon, A.W. Hewat, J.J. Capponi, P. Str6bel, M. Marezio and P. Fischer, Solid State Comm. 66,1117 (1988) 11. Based on purely symmetry arguments it has been shown by J.L. Birman1~ that Matthias' conjecture13: "Superconductivity and Ferroelectricity are mutually incompatible", cannot be justified. 12. J.L. Birman, Ferroelectrics 16,171 (1977) 13. B.T. Matthias, Mat. Res. Bull. 5, 665 (1974) 14. N.W. Ashcroft, N.D. Mermin, Solid State Physics, Holt, Rinehart and Winston, New York (1976) 15. F.M. Mueller, S.P. Chen, M.L. Pruitt, J.F. Smith, J.L. Smith, and D. Woldleben, Phys. Rev. B37, 5837

(1988) 16. L.R. Testardi, W.G. Muolton, H. Matthias, H.K. Ng and C.M. Rey, Phys. Rev. B37, 2324 (1988) 1(7~ V. Miiller, D. Maurer, Ch. Roth, C. Hucho, D. Winan, K. de Groot, H. Eickenbusch, and R. Schbllhorn, Physica C153-155, 280 (1988). Parts of the considerations reported there, were already presented at the Yamada Conference XVIII, Sendal, September 1987. Also see: V. Miiller, K. de Groot, D. Maurer, Ch. Roth, K.H. Rieder, H. Eickenbusch and R. Sch~llhorn, Jpn. J. Appl. Phys. 26, Suppl. 26-3, 2139 (1987)

18. S.K. Kurtz, L.E. Cross, N. Setter, D. Knight, A. Bhalla, W.W. Cao and W.N. Lawless, Mater. Lett 6, 317 (1988) 19. S.K. Kurtz, J.R. Hardy and J.F. Flocken, Ferroelectrics 87, 29 (1988) ~0. A.S. Shcherbakov, M.I. Katsnelson, A.V. Trefflov, N.L. Sorokin, V.E. Startzev, E.G. Valiulin, V.P. Dyakina, V.L. Kozhernikov and S.M. Cheshnitsky, Scientific reports "Problems of high-temperature superconductivity" Komi Scientific Centre and Sverdiovsk Science Centre of the Ural Branch, USSR Academ. Sci. Issue 7, 24 (1988). Also see: A.S. Shcherbakov, M.I. Katsnelson, A.V. Trefilov, N.L. Sorokin, V.E. Startzev, E.G. Valiulin, V.P. Dyakina and V.L. Kozhernikov, JETP Lett. 49,121 (1989) 2~1~ V. M~iller and D. Maurer, Special Issue of "Phase Transitions" (1989), in press. 22. V. MiUler, G. Schanz, E. Fischer, and E.J. Unterhorst, phys. stat. sol (b) 80, 629 (1977) 23. V.M. Ishchuk,L.A. Kvichko, V.P. Seminozhenko, V.L. Sobolev, N.A. Spridonov, JETP Lett. 49, 389 (1989) 24. J.G. Miller and D.I. Bolef, Rev. Sci. Instr. 40, 915 (1969) 125. T.J. Kim, J. Kowalewski, W. Assmus and W. Grill, Z. Physik B, 1989, in press 26. R. Butera, Phys. Rev. B37, 5909 (1989) 27. F. Seidler, P. BShm, H. Geus, W. Braunisch, E. Braun, W. Schnelle, Z. Drzazga, N. Wild, B. Roden, H. Schmidt, D. Woblleben, I. Felner and Y. Wolfus, Physiea C157, 375 (1989) 28. T. Laegreid and K. Fossheim, Europhys. Lett. 6, 81 (1988) 29. V. Miiller, to be published 30. T.K. Worthington, W.J. Gallagher and T.R. Dinger, Phys. Rev. Lett. 59, 1160 (1987) 31. V.L. Ginzburg JETP 10, 36 (1949). Also see references in Ref.8 32. P.W. Anderson in Fizika Dielectricov, Ed. by G.I. Skanavi, Moscow, Acad. Nank USSR (1960) 33. W. Cochran, Adv. in Phys. O, 387 (1960) 34. D. Winan, diploma work, Freie Universit~t Berlin (1989) 35. H. Riesemeier, Ch. Grabow, E.W. Scheidt, V. Miiller, K. Liiders and D. Riegel, Solid State Comm. 64, 309 (1987). Note, that in this early assignment of the NQR frequencies to the Cu(1) and Cu(2) sites the former have to be interchanged.