Comparative investigation of intrinsic Josephson contacts in HTC superconductors by modulated microwave absorption measurements

Physica C 209 (1993) North-Holland

362-368

Comparative investigation of intrinsic Josephson contacts in HTC superconductors by modulated microwave absorption measurements B. Nebendahl, C. Kessler, D.-N. Peligrad ’ and M. Mehring ’ 2. Physikalisches Received

Institut, Universitiit Stuttgart, D- 7000 Stuttgart 80, Germany

15 January

1993

A quantitative analysis of the temperature dependence of modulated microwave absorption measurements in a sputtered film, a single crystal and a ceramic sample reveals two different types of intrinsic Josephson contacts. By modelling the complete temperature dependence of the microwave absorption signal, we were able to distinguish SNS- and SIS-type junctions. As a result we found SIS-type junctions for the granular (sputtered) layer, whereas the single crystal reveals SNS-type junctions.

1. Introduction Microwave absorption measurements on superconductors have proven to be an efficient method for the investigation of their superconducting properties and the quality qf the samples. In particular, the magnetic field dependence of microwave absorption in low and high magnetic fields can provide detailed information on the flux line dynamics and their pinning behaviour. In this contribution we focus on modulated microwave absorption in the low field regime ( -20 mT < Ho< 20 mT). This technique is applied by placing the sample inside a typical EPR type cavity in the region of the homogeneous microwave magnetic field B,. A field modulation is applied parallel to the external magnetic field B0 and lock-in detection of the signal is recorded. The setup is that of a typical EPR spectrometer. We have analysed the observed signal by applying a model developed recently by DulS et al. [ 11. Within the framework of this model the microwave absorption is caused by the field and temperature dependent currents in the Josephson links. It turns out that different charac’ On leave from: Institute of Physics and Technology

of Mate-

rials, PO Box MG-6, Bucharest, Romania. 2 Reprint requests should be addressed to M. Mehring. 0921-4534/93/$06.00

0 1993 Elsevier Science Publishers

teristic features of the observed signals, like ( 1) the phenomenon of modulation hysteresis, (2 ) an absorption minimum at vanishing external field and ( 3 ) a strong dependence on the magnetic field at low fields can qualitatively be explained by this model. The simulated signal shapes are in very good agreement with those observed experimentally. In section 2 we discuss this model briefly and extract some quantitative predictions. In section 3 we present some experimental details and address some experimental problems. In section 4 the data are presented and the results obtained are discussed.

2. Model It is generally accepted that HTC superconductors consist of weakly coupled grains - even in single crystals - with differing size and coupling strength depending on the preparation process and the sample morphology. It is this granularity which is probed very efficiently by the microwave absorption technique. Inspired by this view which is supported by many observations [ 2-5,1], DulEiC et al. [ I] used a RSJ-type equation to model this intrinsic Josephson network and replaced the applied magnetic fields, i.e. the external, the modulation and the microwave

B.V. All rights reserved

363

B. Nebendahl et al. /Intrinsic Josephson contacts

field by their corresponding shielding currents. After sorting out the relevant contributions to the phase variation of the superconducting wavefunction across a junction, they were able to calculate the expected modulated microwave absorption signal in the form s

$RI& mod=_

(fi~,,.,/(2eR))~

I,

(l+r~)~‘~

(l)

with I: cos2 q&J ‘=

(Sto,,,,/2eR)2

(2)

’

Already at this point we remark that the negative sign guarantees an absorption minimum at zero field. We come back to this point later. I,, and I”,,,* represent the microwave and modulation field shielding currents, o,, the microwave frequency. q. is the net phase difference of the superconducting wavefunction across the junction network introduced by the shielding currents of the external field Ho, and R is an averaged junction resistance. Note that the signal is comprised of a sum of two terms, where the term containing sin p. changes sign upon field sweep reversal (p. changes sign due to reversal of the boundary currents). Correspondingly, the signal can be transformed into two alternative forms by taking the sum and the difference of the recorded signal for both field sweep directions. The sum and the difference signal can be expressed as 4RILw ~,,,,/(2eR))~ iRI& sdiffz2

(fi

o,,/(2eR))2

II

(l+~)~‘~

II (T, H) =II (T) F(H) to be valid, separating field and temperature dependence. F(H) becomes the envelope function mentioned before. The temperature dependence of II ( T) now allows one to distinguish between two types of Josephson contacts [ 6 ] ( t = T/ T, ) : I,(T)cc(l--t)

SIS-type ,

I,(T)cc(1-t)2

SNS-type ,

(5)

where SIS stands for “Superconductor-insulator-superconductor” and SNS for “Superconductor-normal metal-superconductor”. If we assume F(H) not to vary at all with temperature we can express the peak to peak intensity of the sum signal as 1

I, (1 +q)3'2

In our case the effective maximum critical current a Josephson junction can carry is a random variable and has to be averaged over the Josephson network. Its averaged value is included in II ( T, H). The I, (H)curve of a single junction is the well-known “diffraction pattern”. In the simplest form of approximation this is washed out and broadened due to the distribution of junction sizes, orientations and internal fields leading to a simple envelope function F(H) for the whole network of junctions. However, F(H) might deviate more or less severely from this simplified picture as discussed later. Furthermore, I, (T, H) is modified if the junction parameters or their number is changing significantly with temperature (or field). We follow DulEic et al. and assume the ansatz

K$$%md~ a

2a

(3)

I,P(T)=aO(~+~o(~_t)2")3/2

(4)

with the parameters r,+,= q ( T= 0)) ao, T, and (Yto be determined from a fitting procedure. We note that this allows one to determine the critical temperature T, quite accurately.

Imod

H,,dHmod.

Both signal parts can be determined from the experimentally observed microwave absorption signal and can be analysed separately. The sum signal is dominated by the averaged maximum critical current of the Josephson junctions, whereas the difference signal contains the modulation hysteresis which is connected with the dynamics and strength of the boundary currents. In the following we focus on the sum signal S,,,.

(1-t)

'

(6)

3. Experimental All our measurements were performed using a Bruker EPR-spectrometer ESP 300 equipped with additional coils to achieve a true zero crossing of the external magnetic field. The microwave absorption signal shape proved to be very reproducible. No

364

B. Nebendahl et al. /Intrinsic Josephson contacts

shielding of the earth magnetic field was applied and we cannot exclude a small influence of this field on the signal. Since low field excursions showed perfect reproducibility of the signal in subsequent runs we suppose its effects to be negligible or at most additive. The cooling was performed in zero external field, the field was then swept to the starting field (typ. - 10 mT) and the data recording was started. A field sweep was performed for a whole cycle beginning at the starting field up to the opposite field value and back. We could not find any difference in the spectra for different temperature cycling histories. Cycling above T,between measurements had no corresponding effects on the signal intensity or shape. Therefore,we refrained from heating above T,between different measurements. We used a modified Oxford continous flow cryostat, ER 900, for temperature variation of the sample. In order to improve the accuracy of the temperature reading we inserted a new heater, which was wound directly on the inner part of the helium transfertube within the isolation vacuum. With this modification we were able to establish a far better temperature stability and reproducibility which was almost completely independent of the gas flow. The samples were mounted on a teflon holder fixed to an evacuated glas tube and were placed directly in the gas stream. This solved the problem encountered in standard EPR-equipment where there is no direct contact between the sample and the thermocouple used for the temperature control and measurement. Therefore, one can never be sure how close the agreement is between the temperature reading of the temperature control unit and the actual sample temperature. When using higher heater power, additional heating of the thermocouple can cause even larger deviations. Usually this problem is not so severe in conventional EPR. We believe, however, that many investigations on superconductors in the temperature regime around T,have suffered from this inaccuracy. The samples used for the current investigation are ( 1) a sintered bulk ceramics, YBa2Cu@_s prepared in a solid state reaction, (2) a granular layer, YBazCu@_6, thickness 4000 A, sputtered on Y-stabilized ZrO, (3) a single crystal, YBaZCu307_-6 grown from a melt of the solid starting materials, (4) sintered ceramic of La,CuO,+, prepared in a

solid state reaction and a subsequent annealing cycle. Electrochemically oxygen was loaded to a value of x= 0.03 (estimated from its actual Neeltemperature).

4. Results

Let us begin with the presentation of the YBCO bulk ceramic sample. Figure 1 displays the plot of the peak to peak intensity of the sum signal versus temperature. The solid line is a fit according to eq. (6). The lit parameters are given in the inset. It is most remarkable that we find an exponent (YX 1 for the temperature dependence which is indicative for SIS-type junctions. For comparison we have included a best lit for (Y= 2 as a broken line in fig. 1. Moreover, the figure demonstrates that the Josephson links remain active down to very low temperatures as given evidence for by the remarkably good agreement of the data with the theoretical expression. Next we present the results of the YBCO granular layer (fig. 2) where we extract an exponent of cx= 2 for the temperature dependence. As mentioned before, this value is characteristic for SNS-type junc-

a

6

5 54 3

2

0

0

20

40

60

60

T [Kl Fig. 1. Intensity of the modulated microwave absorption signal vs. temperature for the YBCO bulk ceramic sample. The solid line is a fit to eq. (6) with the set of parameters shown in the inset. The dashed line indicates the deviations when using an exponent 01 = 2.

365

B. Nebendahl et al. /Intrinsic Josephson contacts

0.5 T. = 86.30

? 0.07

--l

K

a-2

0.4

6

0.3

q.

= 2.5

+ 0.6*107

a,

-

? 0.7*10”

2.7

3

3

2

T!

-2

-24

0.2

0.1

a I40 0

20

4.0

60

80

50

70

60

80

90

T [Kl

T [Kl Fig. 2. I,,,(T) for the granular YBCO layer. Solid and dashed line have the same meaning as in fig. 1. Here an exponent of 01= 2 is obtained.

tions. In comparison to the previous sample the value q. is much larger, indicating a stronger coupling between the grains which leads to a larger effective critical current and therefore results in the larger value of 90. The YBCO single crystal is expected to exhibit a much smaller zero field signal due to the reduced number of weak links. For a perfect crystal (with no weak links) the signal should even vanish completely. This expectation is confirmed by the corresponding data shown in fig. 3. The signal intensity is smaller by more than one order of magnitude compared with the ceramic sample and the granular layer, although the crystal volume is about the same or even larger as compared to the other two samples. The signal increases very rapidly to its maximum value with decreasing temperature below T,and falls off almost as fast before it disappears in noise-like but reproducible fluctuations only u 10 K below T,.Performing the same fitting procedure as before results in an exponent (Y= 2, which is again typical for an internal SNS-type coupling among the superconducting areas of the single crystal. Comparing q. to the former two samples, it has grown enormously by a factor of nearly 104, indicating a strong coupling situation. We note that an increasing discrepancy is found between

Fig. 3. I,,( 7) for the single crystal microwave absorption vs. temperature. Note the sharp rise and fall over a very narrow temperature range of < 10 K. Data fitting gives an exponent of (Y= 2. Deviations on the falling edge of the signal are explained in the text. no has the highest value for this sample, indicating strong coupling as is expected for defect planes in single crystals.

the theoretical expression and the experimental data for decreasing temperature. This can be accounted for by a significant reduction of the number of active junctions at lower temperatures. On lowering the temperature one expects some of the junctions to become stronger in the sense that they have larger critical currents, eventually reaching a critical current similar to the bulk material. In particular, this might hold for those junctions which are linked to defect planes or related microscopic planar defects. It is therefore quite conceivable that these junctions contribute no longer to the microwave absorption and consequently the signal should decrease more rapidly than predicted by theory, where it is assumed that the number of active junctions is constant. As a last example we turn to a ceramic lanthanum cuprate sample. The sample was doped by an electrochemical technique as mentioned in the experimental section. In contrast to the YBCO ceramic sample the temperature dependence of the microwave absorption signal deviates strongly from the former and shows a rather moderate monotonic increase with decreasing temperature below T,.As far as the characteristic temperature dependence is con-

B. Nebendahl et al. /Intrinsic Josephson contacts

366

cerned, we observe again a value of a! close to one, indicating SIS-type behaviour as for the YBCO ceramic sample (fig. 4). The critical current parameter qo, however, is found to be extremely small, indicating weak coupling over the whole temperature range from the extrapolated T, down to 4 K.

5. Discussion It is most remarkable that both sintered ceramics reveal the same SIS-type coupling of the grains, whereas the other two samples show SNS-type behaviour. We explain this difference by the different sample morphologies caused by the different preparation procedures. The ceramic samples were baked, ground, pressed and sintered, and were exposed to air which might result in a strong distortion of the intergranular layers. It is expected that this could give rise to an insulating surface layer. Pressing and sintering of the sample introduces point contacts which obviously act as SIS junctions as concluded from our measurements. Gradual growth procedures (as in single crystal preparation or sputtering) might highly reduce surface degradation. As a consequence, only junctions

I

I

1.25

a = 0.99t 0.07 1

\

I

I

0

1

6

10

20

25

30

Fig. 4. I,,,(T) and fit as found for the granular La2Cu04+X sample. c~= 1 is seen to follow the behaviour very accurately except at very low temperatures. The very small q0 indicates only weak coupling between adjacent grains.

due to intrinsic defect planes or boundaries dominate. It is conceivable that these junctions behave as metals and weak superconductors. Their modified electronic structure might result in entirely metallic and non-superconducting character or in a weak superconductor with reduced T,.Therefore, part of these junctions could become superconducting at lower temperatures with critical currents reaching values similar to the bulk. This is obviously the situation in the single crystal, where some of the junctions seemed to have disappeared at lower temperatures, i.e. they reach critical currents similar as the superconducting material, leading to a more rapid fall off of the microwave absorption signal as expected from eq. (6 ). It is well known that low field microwave absorption in single crystals often shows a series of very narrow peaks at low temperatures [ 21. These are due to a small number of still active weak links forming SQUID-like structures. It is worth mentioning that the single crystal used here indeed showed these typical narrow line spectra in the low temperature regime ( < 50 K). The corresponding results are published elsewhere [ 7 1. It is, however, surprising that we observe a similar line structure even in the sintered (ceramic) sample of LaZCu04+X (fig. 5). There, the line structure is superimposed on the broad feature typical for granular superconductors. In order to characterize the area of the underlying SQUIDlike structure we derive from the distance of two neighbouring lines an effective area of 7.5 um2. This could be caused by superconducting clusters embedded in an insulating surrounding; this was partially discussed already in the context of a percolative phase separation [ 81. This demonstrates clearly that different microscopic morphologies in the superconducting samples give rise to distinctly different microwave absorption signals. SQUID-like structures give rise to narrow microwave absorption lines, whereas a network of Josephson junctions results in the broad line feature as explained by the above model. In some of our samples, namely in the YBCO-layer and the ceramic lanthanum cuprate we found a splitting of the microwave absorption line around zero field. This effect has been observed also by others and corresponds to a local maximum in the microwave absorption at zero field. This feature was re-

367

B. Nebendahl et al /Intrinsic Josephsoncontacts

3.5

3.0

2 8

2.5

B 0 2

2.0

E 1.5

1.0 -10

5

-5 0

10

PmTl

Fig. 5. Superposition of a sequence of narrow lines and the broad absorption minimum in the La,Cu04+, sample, demonstrating the coexistence of two different mechanisms as mentioned in the text.

cently discussed in the context of an unusual “paramagnetic MeiBner-effect”as proposed by Braunisch et al. [ 9 1. These authors found a “paramagnetic” susceptibility when cooling a HTC-sample in very low but non-vanishing fields and linked its appearance to the “paramagnetic” maximum around Ho = 0 seen in microwave absorption as mentioned above. We examined the behaviour of this splitting with respect to its dependence on the magnetic field used when cooling the sample. We observed a far better reproducibility than is expected from their proposal. A significant change of this splitting was only observed for fields large enough to influence the broad feature as well. We, therefore, believe a more natural explanation can be found in the distribution of junction orientations. Miller et al. [ 10,111 analysed a junction cooled in low fields with the field perpendicular to the junction plane and were able to demonstrate a drastic change of the critical current versus field curves depending on the orientation of the measuring field which was either parallel or perpendicular to the junction plane. As a consequence a splitting of the characteristic “diffraction”-like pattern can occur at zero field. An example for such a splitting is shown in fig. 6. For comparison we have included a microwave absorption signal of the YBCO

-3

-2

-1

0

L 1

2

3

BWI

Fig. 6. Alteration of the “diffraction pattern” of a single junction due to vortex pinning for a field perpendicular to the junction plane as discussed in ref. [ 1 I 1.The inset demonstrates the corresponding splitting of the central absorption minimum in the microwave absorption signal of the granular sample.

layer showing this feature in the inset of fig. 6. To our opinion, most of the reproducible structures seen in low field microwave absorption are a consequence of a modified diffraction pattern or incomplete averaging of side maxima.

6. Conclusion We have demonstrated that low field microwave absorption can be analysed in terms of a sum and difference signal which yield different types of information on the superconducting properties of the samples under investigation. In particular the sum signal proved to be extremely useful since it allows one to discriminate between SIS- and SNS-type Josephson junctions. We were able to identify clearly the SNS-type character in single crystals and their large critical currents in contrast to ceramic samples where SIS-type behaviour was observed. The model allows one to incorporate small modifications due to variations of the Josephson network and the strengthening of weak links at lower temperatures. Moreover, we could offer a natural explanation for the so-called “paramagnetic MeiBner-effect” as

368

B. Nebendahl et al. /Intrinsic Josephson contacts

caused by a splitting of the “diffraction”-like pattern of critical current versus field around zero field.

Acknowledgements We would like to Htibener, Ttibingen paring the samples. Bundesministerium (BMFT), Germany, the Ministerium fur Wtirttemberg.

thank T. Wolf, Karlsruhe, R.P. and P. Gergen, Stuttgart for preThis work was supported by the fur Forschung und Technologie project number 13N5845 and Wissenschaft und Kunst, Baden-

References [ 11A. DulEiC, B. Rakvin and M. Poiek, Europhys.

Lett. 10 (1989) 593; M. Poiek, A. DulEiC and B. Rakvin, Physica C 169 ( 1990) 95. [ 2 ] K.W. Blazey, A.M. Portis, K.A. Miiller, J.G. Bednorz and F. Holtzberg, Physica C 153-155 (1988) 56; K.W. Blazey, A.M. Portis, K.A. Mtlller and F. Holtzberg, Europhys. Lett. 6 (1988) 457.

[ 31 K.W. Blazey, K.A. Milller, J.G. Bednorz, W. Berlinger, G. Amoretti, E. Buluggiu, A. Vera and F.C. Matacotta, Phys. Rev. B 36 (1987) 7241; K.W. Blazey, A.M. Portis and J.G. Bednorz, Solid State Commun. 65 (1988) 1153. [4] E.J. Pakulis and T. Osada, Phys. Rev. B 37 (1988) 5640. [ 51 S.H. Glarum, J.H. Marshall and L.F. Schneemeyer, Phys. Rev. B 37 (1988) 7491. [ 61 A. Barone and G. Paternb, Physics and Application of the Josephson effect (Wiley, New York, 1982). [7] C. Kessler, B. Nebendahl, A. DulEiC, Th. Wolf and M. Mehring, Physica C 192 (1992) 79. [ 8 ] M. Mehring, M. Baehr, P. Gergen, J. GroB, C. Kessler and N. Winzek, Phase Separation in Cuprate Superconductors from NMR and Microwave Absorption Measurements, in: Proc. Erice-Workshop on High-temperature Superconductivity, May 1992, Sicily, Italy, eds. G. Benedek and K.A. Mtlller (World Scientific, Singapore, 1993), to be published. [ 91 W. Braunisch, N. Knauf, S. Neuhausen, A. Grtitz, A. Koch, B. Rode, D. Khomskii and D. Wohlleben, Phys. Rev. Lett. 68 (1992) 1908. ilO ] S.I. Miller, K.R. Biagi, J.R. Clem and D.K. Finnemore, Phys. Rev. B 31 (1985) 2684. [11 ] V.N. Gubankov, M.P. Lisitskii, I.L. Serpuchenko and M.V. Fistul’, Sov. Phys. JETP 73 ( 1991) 734.