Magnetization and susceptibility of grain-aligned HoBa2Cu3O7−δ superconductors

Physica C 160 (1989) 55-64 North-Holland. Amsterdam

MAGNETIZATION AND SUSCEPTIBILITY SUPERCONDUCTORS M. WACENOVSKY

OF GRAIN-ALIGNED

HoBa2Cu307-6

and H.W. WEBER

Atominstitut der iisterreichischen Universitiiten, A-1020 Wien, Austria

O.B. HYUN and D.K. FINNEMORE Ames Laboratory and Department ofphysics, Iowa State University, Ames, IO 50011, USA

K. MEREITER Institut ftir Mineralogie, Kristallographie und Strukturchemie, Technische Universitiit, A-1060 Wien, Austria Received 8 June 1989 Revised manuscript received

30 June 1989

Magnetization and AC susceptibility measurements were carried out on grain-aligned HoBa2Cu~07_-d in epoxy. Because of the high degree of alignment, the anisotropic superconducting properties could be assessed in a satisfactory way. Based on careful considerations of demagnetizing effects, results on the anisotropy and temperature dependence of the first flux penetration field are presented and compared with data on YBa$usO,_+ In the magnetization curves significant effects of paramagnetism due to the Has+-ions are observed. An evaluation of critical current densities in terms of the Bean model shows very high j,-values at low temperatures and fields ( > 10” Ame2), but strong reductions with increasing temperature (factor of 10 at 40 K) and field (factor of 2 below 2 T). The anisotropy ofj, amounts to a factor of 20 at 4.2 K.

1. Introduction One of the most important properties needed for applications of high-temperature superconductors is their current carrying capacity in strong magnetic fields. Whereas in the usual sintered ceramics, the current transport is limited by Josephson-coupling between adjacent grains (leading to generally rather poor “intergrain” critical current densities), magnetization measurements on single crystals [ 1,2] revealed at an early stage that the critical current densities achievable inside the grains (“intragrain” currents) were much higher and comparable to those observed in optimized classical superconductors such as NbTi or Nb$n. However, in contrast to these materials, all the physical properties of (123)-superconductors are extremely anisotropic. Therefore, in order to achieve an improved understanding of their superconducting properties, an investigation of single crystals seems to be necessary. Although single 0921-4534/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division )

crystals of excellent quality have been grown successfully recently, their main drawback is their extremely small size, which makes the application of SQUID-magnetometers mandatory. Because of the characteristic twinning within the basal (&)-plane of ( 123)-compounds, an alternative to single crystals, namely grain-aligned materials [ 3 1, which have a preferential orientation along the c-axis, have become very attractive and seem to be equally well suited for the study of the anisotropic properties in the superconducting state [ 41. In the present work, we report on studies of grainaligned HoBa2Cu@_6 cured in epoxy, which were carried out by measuring the DC magnetization and the AC susceptibility for the two major crystal orientations. After a brief description of sample preparation and characterization (section 2) as well as of the experimental techniques applied in the present study (section 3 ), we will present our results on the demagnetizing factors and the critical fields H,, and B.V.

56

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBa2Cu307_8

Hc2 in section 4. Section 5 is devoted to the results of magnetization measurements and the evaluation of critical current densities obtained in two ways from the magnetization experiments. Finally, in section 6 a summary of the results will be presented.

2. Materials

The samples used for the present experiments have been prepared according to a method first described by Farrell et al. [ 31. Polycrystalline powder of HoBa2Cu307_-6 single grains, as obtained from standard ceramic methods, were dispersed in commercial epoxy resin and cured for 12 h in a magnetic field of 2.4 T at room temperature. For rare-earth substituted (123)-materials, the normal-state paramagnetic susceptibility is strongly dominated by the RE3+-ions. Because of small torques due to the different energies for the two possible orientations in an external field (parallel or perpendicular to c) [5], AE=V(~,,H)2Ax/2, the grains align themselves with their c-axis parallel to the applied external field. In this way, samples of roughly cubic shape with a volume of about 1 cm3 were made. The volume fraction of the superconducting grains was about 33%. From these cubes, cylinders were cut using a low-speed diamond saw so that their long axis (I= 10 mm) was either parallel or perpendicular to the c-axis (the diameter of the cylinders was 3 mm). In order to characterize the grains and their alignment, optical micrographs and X-ray diffraction patterns were taken. The micrographs shown in fig. 1 demonstrate nicely the alignment of the grains. The average grain dimensions obtained from measuring a large number of grains under the microscope were determined to be 40% 30 x 30 urn3 for the a-, b- and c-directions, respectively. X-ray diffraction patterns for the beam perpendicular and parallel to the c-axis are shown in figs. 2a and b. It will be noted that in the first case only (001) reflections are observed, whereas for the beam parallel to the c-axis, only (MO) reflections occur within experimental resolution. In order to measure the degree of alignment in a more quantitative way, rocking curves of several reflection peaks were measured. As an example, the rocking

Fig. 1. Optical micrographs of grain-aligned HoBa2Cu307_6 in the a-b-plane (upper part) and in the c-a(b)-plane (lower part).

curve of the (00 11 )-reflection is shown in fig. 2c, together with a fit to a Gaussian distribution (a= 3.29” ), which represents a convenient measure of the average grain-misalignment.

3. Experimental techniques Two experimental techniques were employed to determine the susceptibility and the magnetization of cylindrical samples of identical shape, but with different “internal? grain orientation. Firstly, the AC susceptibility was measured with a two-channel lock-in amplifier operating at 73 Hz and providing peak-to-peak magnetic field amplitudes of

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBalCu@_d

Fig. 2. X-ray diffraction patterns taken after alignment and curing in epoxy for the beam perpendicular (a) and parallel (b ) to the c-direction (Cu Ku radiation, A= 0.1547 18 nm; refined unit dimensions: b=0.3889(2) nm, cell a=0.3827( 1) nm, c= 1.1667(4) nm). In the lowest part (c) the rocking curve of a (00 11 )-reflection is shown.

10 PT. The samples were placed into one of a pair of compensated coils, wound on quartz-tubes in order to avoid changes of the compensation with temperature. Both the real and the imaginary parts of the response were recorded either in zero magnetic field or in various DC fields up to 4T with the temperature changing from room temperature down to 4.2 K at a rate of roughly 100 K per hour. In this way, the transition temperature T,, the diamagnetic saturation signals as a function of grain orientation and the depression of “T,” in DC-fields were obtained. Secondly, the magnetization of the cylinders was measured by applying a differential low-tempera-

57

ture-chopper technique, which has been used extensively by our group for magnetization measurements on classical low-~ superconductors previously [ 6,7]. This method is based on sweeping the magnetic field at an exactly constant rate, which can be kept very small (0. l-l mTs- ’ ) in order to achieve “quasistatical”conditions, and recording the induced voltages in a carefully compensated pair of pick-up coils. In order to avoid thermoelectric and other noise voltages generated between the sample holder and the measuring device, these small DC voltages are transformed into rectangular AC signals by a lowtemperature chopper system operating at 73 Hz. The subsequent phase-sensitive detection of this transformed signal yields directly the derivative of the magnetization, dM/dp&, which can be integrated numerically with an evaluation program. Because of the very small slopes of the magnetization, which are characteristic of extreme type-II superconductors such as the ( 123)-materials, superimposed voltages due to slightly imperfect compensations of the pickup coils turned out to be the limiting factor for the resolution of the present system. In fact, a suitable compensation could only be achieved over comparatively narrow field ranges, with the best results being achieved at very low magnetic fields. In this way, the first deviation of the magnetization from the Meissner signal could be detected very sensitively for all temperatures and grain-orientations, but the magnetization of the material with H parallel to the (a, b)-plane could be measured only at low temperatures. The resolution of the present device is estimated to be z 2 x low3 Am*T- ‘, but alterations of the coil-system are expected to improve this limit by an order of magnitude.

4. Demagnetization

and critical fields

4.1. Demagnetizing factors A crucial aspect for the correct evaluation of any magnetic measurement on superconductors with finite geometrical dimensions is the appropriate treatment of demagnetizing effects [8-l 11. This has been ignored in most of the papers on high T, superconductors published recently. The influence of demagnetization for elliptically

58

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBazCu30,_a

shaped samples is commonly described by a demagnetizing factor, which defines a relation between the effective field at the sample surface H,, and the applied field H, as follows: poHeff=pLoH,-Da,borcM, where factors c-axis, tizing

0 -m

(1)

Da,b Or’ denotes the effective demagnetizing for the field applied parallel to the a-, b- or respectively. The sum of the three demagnefactors is equal to 1:

D”+Db+Dc=

1.

(2)

Using eq. 2.20 of ref. (m=a,b/c> l),

[8]

for oblate

spheroids

m2 DC= m2- 1 x

I

l-

1 (m2-1)‘12

arcsin[

(“lil

““I}

(3) 1

0

and assuming, in view of the brick-like shape of the grains, that D”=Db (a=b=35 urn, c=20 urn), we obtain D”,‘=0.255

(4a)

and DC= 0.49 ,

(4b)

i.e., demagnetization corrections, which differ by a factor of 2 depending on which crystal direction is oriented parallel to the external field. This theoretical result, which certainly represents only a rough approximation for the geometry of the grains, can be compared with experiment in two ways, if we assume that a full Meissner state is established within the grains at very low magnetic fields. Firstly, the slope of the magnetization in the Meissner state is given by dM -=-dhH

1 1_D”.b

or

_-

1 1 -DC

depending on the grain-orientation within the sample cylinders. (Note, that the actual geometrical dimensions of both samples containing the differently oriented grains are exactly identical.) The corresponding results are shown in fig. 3a. The ratio of these slopes is found to be 1.67:

I

I

I

I

I

I

I

20

40

80

80

100

120

140

1

T(K)

Fig. 3. (a) Low-field differential magnetization curves for the two cylindrical samples having their c-axis orientation parallel and perpendicular to the cylinder axis (field direction), respectively. The y-axis shows the output voltage of the lock-in amplifier recorded for a field-sweeping time constant of 1 mTs-‘. (b) AC response of the two samples in zero field, measured with an AC ripple field of 10 uT (peak-to-peak) at 73 Hz. T, (onset) is found to be = 92 K in both cases.

1_D”,b ~ = 1.67 1 -DC

(Meissner)

Secondly, the AC susceptibility netization by M=x’,u,,H,

,

x‘=

-?t-.1 +xD’

. is related to the mag-

(7)

where x and x’ denote the true and effective (measured) susceptibilities, respectively. In the full Meissner state, i.e., for x= - 1, we obtain 1 x’=-l_D. Accordingly, AC susceptibility measurements on the same samples (fig. 3b ) provide us again with a ratio of appropriate demagnetizing factors:

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBa2CuJ07_a

X

F-

1 _D”.b

,C

- ~1-D’

= 1.5

.

(AC susceptibility)

(9)

From these ratios and using eq. (2) with Da=Db, we obtain D”,b=0.23 D’=O.54

(M) ,

0.25 (ACs.)

,

(loa)

0.50 (AC s.) .

(M),

(lob)

These results compare favorably with the calculated demagnetizing factors [eq. (4) 1. A closer examination of eq. (3) shows that the calculated demagnetizing factors depend on the actual dimensions of the grain in a most sensitive way. The corresponding calculations are shown in fig. 4 together with experimental results deduced from the magnetization and AC susceptibility experiments. In view of the actual spread of grain dimensions observed in the micrographs, we consider the results to be completely consistent, and a proof that individual decoupled grains are causing the observed magnetic response. For all further evaluations, the “experimental” values D”lb=0.5 and D’~0.25 were used. 4.2. Lower criticaljelds A variety of attempts to determine the lower critical fields H,, of high-temperature superconductors has been reported in the literature [ 12- 19 1. The experimental situation, however, is not satisfactory at all, because the measurements are always done on

59

samples with strong flux pinning effects, which tend to mask the thermodynamic equilibrium flux penetration field H,, . (For a discussion of these problems occurring even in nearly reversible low-~ superconductors, cf. ref. [ 201. ) No reference will be made to small penetration effects, which are found in ceramic materials at minute magnetic fields and related to intergrain properties of these materials. With the present equipment, where we actually measure the derivative of the magnetization, a comparatively sensitive method for detecting the first deviation of the signal from the constant Meissner slope has become available. This is demonstrated in fig. 5, where we compare the experimental differential curve for BIIa,b at 4.2 and 86 K with the integrated magnetization curve obtained by most other techniques. It should be noted that the “deviation” field can be determined quite easily as the intersection point of two tangents fitted to the experimental data, even at very high temperatures and low fields (fig. 5b). The corresponding evaluation of data, which relies heavily on the application of correct demagnetizing factors, is shown in fig. 6. A particularly strange feature is the upturn of H,, for T-+0, which has also been

15 -

0.6

0.7 0

‘O ~oH(mT)

0.6

2o

?a 0.5

0.4

10

15

20

35

30

35

40

43

50

0

Fig. 4. Demagnetizing factors DC calculated from eq. (3) for the field parallel to the c-axis as a function of “transverse” grain dimensions (a, b). Parameter of the curves is the grain thickness (c). Also shown are the experimental results deduced from Meissner effect (M) and AC susceptibility (AC) measurements.

’

2

3

4

kP(mT)

Fig. 5. Differential magnetization for Blla, bat 4.2 (a) and 86 K (b). Also shown is the result of the numerical integration (INT) for the 4.2 K-experiment.

60

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBa2Cu307_d

T(K) Fig. 6. Temperature dependence of the first flux penetration field for the field parallel and perpendicular to the c-axis, respectively.

observed for YBCO [ 141, but not reported in other work, presumably because of the lack of a sufficiently dense grid of temperatures. In view of the fact that the critical current densities also increase significantly at low temperatures (cf. section 5 ), Beanmodel calculations of the reversible magnetization, M,,, = l/2 (M+M- ) , were made and extrapolated to the low-field Meissner line, in order to determine the penetration field. This procedure always resulted in much larger values of “Hcl”, a result which is not unexpected, if the experimental differential magnetization data are compared to the integrated magnetization curves (fig. 5a). Concerning the absolute values of H,, and their anisotropy, comparison can be made only for Y- and EuBa2Cu307_-6 single crystal data [ 13,14,18,19 1. At T=O, the magnitude of Hc,, 54 and 15 mT for the cand a&directions, respectively, as well as the anisotropy factor of x 4, are similar to those reported for the other materials. The slopes at T, are found to be 0.4 and 0.2 mTK-‘. In summary, the temperature dependence of H,, is not understood and may still be affected by hysteresis effects. The results also disagree with recent theoretical work based on strong-coupling calculations [21]. 4.3. Upper critical fields Because of the non-connective structure of the present samples, resistive measurements of the transition fields into the normal conducting state were

ruled out. Furthermore, the magnetization measurements near T, turned out to provide only insufficiently accurate information on the change of slopes at HC2.We, therefore, made an attempt to deduce HC2 from AC susceptibility measurements in various external DC fields using AC ripple fields with amplitudes ranging from 10 to 100 PT. In contrast to results on sintered ceramics (e.g. ref. [ 22]), no amplitude dependence of the response signal was detected, which confirms again that we are measuring the response of individual grains. Concerning the temperature dependence of the apparent transition field for B]]c and Blla,b, respectively, we obtain results which are typical for AC measurements: small broadening of the transition curve for Bllc, significant broadening for B]]a, b and a characteristic curvature of the transition fields (defined arbitrarily by a loo/6 criterion) with temperature. In the light of recent work on time-dependent effects (e.g. refs. [ 23-25 ] ), an interpretation of our results in terms of an irreversibility line rather than an upper critical field is straightforward. This is also supported by the extremely small slopes of the transition field (-0.44TK-’ and -0.87TK-’ for Bllc and Blla, b, respectively), which deviate by factors of 5-10 from recent DC magnetization results obtained on YBCO single crystals ( - 1.9 TK- ’ and - 10.5 TK-‘, [26]). In view of this situation and because of the lack of DC magnetization data for HC2 in HoBa2Cu307-fi, no further attempt is made to evaluate anisotropic Ginzburg-Landau parameters and characteristic lengths [27,28], which would depend critically on HC2 as an input parameter.

5. Magnetization

and critical currents

5.1. Magnetization curves Complete magnetization cycles were measured with the method described in section 2, at temperatures between 2 and 50 K and in magnetic fields up to 4 T. Typical examples are shown in fig. 7 for fields parallel to the c- and a, b-directions, respectively. The dramatic difference in flux pinning between these two orientations is most obvious. Furthermore, the paramagnetic contribution of the Ho3+-ions to the total

61

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBa2CuJ07_-6

ceptibility data [ 33 ] showed a nearly free ion moment of 10.4~~.

Hllc T-4.2K

5.2. Critical current densities As a first step, an analysis of the magnetization curves presented in section 5.1. was made in terms of the Bean model [ 341, in order to derive the critical current densities within the grains. This can be done with some confidence, because flux profile measurements based on AC waveform analysis [ 221 or AC amplitude variation [ 35 ] have proved that the flux density gradients indeed penetrate in an essentially linear form, with the exception of small surface contributions. Accordingly, depending on the shape of the grains, the critical current densities can be calculated from the magnetization in increasing (M+ ) and decreasing (M- ) fields as follows [ 34,36 ]

H Ila,b T.4.2K 0

-0.15

-

I

-0.25

-

-0.30

-

-0.35

r

,

,

,

,

,

,

,

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

.

PO ” (T)

Fig. 7. Integrated magnetization loops (0-4-O T) for the field parallel and perpendicular to the c-axis, respectively.

M+-M-=$hj,R

(cylinder)

M+-M-=p,,j,a/2

(slab),

M+-M-=,u,,jc (rectangular

magnetization

is clearly observed,

in particular

for

RIM, b. In this case, where we nearly reach saturation at 4T and 4.2 K, an evaluation in terms of magnetic moments of the Ho3+-ions was made and led to pc5.5~~. This value is in agreement with data obtained on a single crystal of HoBazCuJO,_g [ 29 1, but smaller than the result of Furrer et al. [ 301 for the easy direction of magnetization M, (7.9~~), which was obtained from a calculation of crystalline electric field interactions based on inelastic neutron scattering experiments. Of course, both the results on grain-aligned and single crystal HoBa2CuJ0,_-6 represent an unknown mixture of a- and b-axis contributions, but the low saturation field and the small moments are in remarkable contrast to the calculations presented in ref. [ 301. As a concluding remark we wish to mention results on sintered ceramics reported in the literature. Whereas magnetization measurements up to very high fields (25 and 40T, respectively) have led to moments of 7.6 [ 3 1 ] and 8.6,~~ [32], respectively, an analysis of initial sus-

,

(11) (12)

a2/2 [ 1 -aJ3a,] parallelepiped)

,

(13)

where 2R, a, and a,, a2 (a, > a2 ) denote the sample (grain) dimensions in the plane perpendicular to the external field. It is important to note that the actual magnetization values, i.e. those corrected by the appropriate demagnetizing factors, have to be inserted into eqs. ( 1 1 )-( 13). (We assume that the demagnetizing effects are unchanged at these comparatively low fields.) In view of the brick-like shape of the grains in our samples, eq. ( 13) was used for the evaluation with average grain dimensions of 30 and 40 urn in the basal plane and 20 pm in the c-direction. The results obtained in this way are shown in fig. 8 for the field parallel to the c-direction. The critical current densities at low fields and low temperatures are very high and comparable to the results obtained on single crystals. With increasing fields, j, initially drops by a factor of about 2, but then levels off for fields above x 1.5 T. The decrease of j, with temperature is even more drastic: at 40 K, e.g., the critical current densities have dropped by a factor of 10 compared to 4.2 K, again in agreement with results on single crystals. Concerning the anisotropy of j,, we have data for BIIa,b only at 4.2 K due to sensi-

M. Wacenovsky et al. /Magnetization and susceptibility ofgrain-aligned HoBa2CuJ07--6

(16)

Fig. 8. Critical current densities as a function of magnetic for Bllc and temperatures between 2 and 50 K.

field

limitations of our equipment (cf. section 2 and lig.7).At2T,j,isfoundtobe8.4X10sAm-*,afactor of z 20 smaller than for B/c. From DC magnetization measurements, information on the critical current densities can be deduced in a second way, namely by sweeping the field through “minor hysteresis loops” [ 37,381 between the two branches, M+ and M-, of the irreversible magnetization curve. If the field is reduced from a certain starting point with magnetization M+ by a certain amount H’, until the magnetization M- on the decreasing branch of the major hysteresis loop is reached, the total amount of flux moving out of the sample is given by [ 391

tivity

d@=pO dH’ [2x(a,

+aZ)-4x*]

,

which is identical to the results for cylindrical samples, if a, = a2 = 2R. A measuring cycle of this type is shown in fig. 9. Bearing in mind that the direct measuring signal is the derivative of the DC magnetization, dM/d&Y, the desired information on j, is given by the slope of the signal immediately after the spikes, which indicate the points of field reversal. An evaluation of this type and a comparison with results obtained from eq. ( 13) is shown in fig. 10. It will be noted immediately that the agreement between the two evaluation procedures is very satisfactory at low temperatures, but deteriorates with decreasing j,. Although the reasons for these discrepancies are not completely clear, we assume that the resolution of the present equipment is the limiting factor for a sufficiently accurate determination of the second derivative, d2M/d(m)*. Indeed, with decreasing flux pinning, the field variation H’ needed to cycle the magnetization from M+ to M- becomes so small that an accurate evaluation of the slopes of the differential curves (fig. 9) is affected by experimental settings, such as the time-constant of the lockin amplifier or the sampling rate (data processing) of the measuring process.

(14)

if a sample of rectangular cross-section (a, > a2) is considered. The expression within the brackets describes the area of the sample, which is affected by the change of fields, and x is the “penetration” depth, over which the reversal of flux density gradients occurs,

x=H'/2j,,

(15)

if a linear flux profile is assumed. After some transformations, j, can be related to the second derivative of the magnetization as follows:

Fig. 9. Differential recordings of minor hysteresis loops at Tc4.2 K. The slopes of the curves immediately after the spikes are the second derivatives of the magnetization at the field values, where the field is reversed.

M. Wacenovsky et al. /Magnetization and susceptibility of grain-aligned HoBa2CuJ07_d

63

made in much detail, because no DC magnetization measurements of Hc2 have been reported for HoBazCus0,_6 so far. 3) The critical current densities, determined from the magnetization in two ways, have been found to be very high ( > 10” Ame2) at low temperatures and fields, very anisotropic (factor of 20) and strongly temperature-dependent, basically in agreement with expectations from measurements on single crystals. Further work, which is aimed at an extension of the present experimental techniques to higher fields and to an inclusion of DC magnetization measurements in a SQUID-magnetometer as well as AC flux profile measurements, is under way and will be reported on later. Fig. 10. Comparison of critical current densities evaluated the integrated curves [ eq. ( 13) ] and the minor hysteresis [ eq. ( 16) ] at a constant field of 2 T.

from loops

6. Conclusions In the present work, we have shown that magnetic measurements on grain aligned high-T, superconductors provide valuable information on the anisotropic properties of these materials. Due to the decoupling of individual grains by the epoxy matrix, intragrain properties become directly accessible to experiment. The main results may be summarized as follows: 1) Based on a combination of experimental techniques and on some theoretical considerations, demagnetizing factors could be established for the anisotropic grain shapes within the samples, which were used throughout the evaluation of critical fields and critical currents. Because of their significant effect on measurements with the field parallel and perpendicular to the basal plane (Pb=0.25, P=O.SO), this procedure is considered essential, in order to arrive at intrinsic, shape-independent data. 2) Detailed studies of the first flux penetration field as a function of temperature and grain-orientation have confirmed the highly anisotropic nature of critical fields in these materials. If we interpret the data as being representative of the lower critical field H,,, the sign of the anisotropy as well as its magnitude are in qualitative agreement with expectations based on the Ginzburg-Landau theory ( HE2 H:ib ). Unfortunately, these comparisons cannot be

Note added in proof Recent careful measurements on YBCO single crystals with a SQUID-magnetometer (A. Umezawa and G.W. Crabtree, ANL, private communication) show qualitatively identical results on the temperature dependence of H,,. The “upturn” temperature in YBCO is somewhat higher ( x 40 K), the anisotropy at T= 0 larger than in HoBa2Cu307_-6 (Fig. 6).

Acknowledgements We wish to thank Mr. H. Niedermaier for invaluable help with the experiments and Dr. E. Seidl for numerous discussions. This work was supported in part by Fonds zur Fiirderung der Wissenschaftlichen Forschung, Wien, under grant # 7098.

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