Observation of the Meissner effect in polysulphur nitride, (SN)x

Solid State Communications, Vol. 24, pp. 469472.

Pergamon Press.

Printed in Great Britain.

OBSERVATION OF THE MEISSNER EFFECT IN POLYSULPHUR NITRIDE, (SN) * x R.H. Dee, D.H. Dollard and B.G. Turrell Department of Physics, University of British Columbia Vancouver, B.C. Canada V6T 1W5 and J.F. Carolan Physics Laboratory, University of Nijmegen, The Netherlands and Department of Physics, University of British Columbia Vancouver, B.C. Canada V6T lW5t (Received 28 August 1977 by R. Barrie)

The superconductivity of (SN)~is definitively established by the observation of a Meissner effect with the magnetic field applied perpendicular and parallel to the (SN)~fibre axis. Considerable flux trapping is observed and the magnetization curves are similar to those expected for coupled filamentary type II superconductors.

In this communication we report the obser— vation of the Meissner effect in polysulphur nitride, (SN), providing conclusive evidence for the superconductivity of (SN)~. Previous work on the electrical resistance ~ particu— 2, larly thesuggests study of that the critical strongly (SN)x is field a type II superconductor, the properties of which are determ— med to a great extent by the morphology of the fibres forming the crystalline material. How— ever, recent tunneling measurements5 showed no evidence for superconductivity in (SN)~, and earlier measurements6 in our laboratory of the a.c. magnetic properties showed changes at ~O.25 K, but no d.c. Meissner effect was observed, A batch of (SN)~samples was s~nthesized by us using established techniques. Their residual resistance ratio was found to be ~3 and the critical temperature as defined by the mid—point of the resistance transition was 255 mK. A well—formed specimen (sample A), almost hexahedral in shape being 0.6 mm along the fibre axis and 0.45 mm wide, was placed together with a specimen of tantalum wire in a coil of diameter 2.2 mm which was one half of a compen— sated pair coupled to a SQUID magnetometer. This geometry is similar to that described by Giffard et al.8. The specimen coil was attached to a cold finger in a dilution refrigerator as described previously.6 Magnetic fields could be applied at the sample by means of a coaxial superconducting solenoid wound on the 1 K heat shield of the refrigerator. The SQUID was also the sensor for an s.c. bridge so that the a.c.

susceptibility could also be determined. For this measurement the compensated pair of coils described above formed the secondary of a mutual inductance. Assuming (SN)x is a type II superconductor, its high critical field H and low transition temperature (T~~ 0.3 K) ~iply H is very small. In order to obtain small~ields the earth’s magnetic field was approximately can— celled using an external Helmholtz coil, mu— metal shielding and a superconducting lead pot. At low temperature the field on the sample was reduced to ±0.0005 Oe by adjusting the current in the inner superconducting solenoid until no magnetic flux changes were observed on passing through the tantalum superconducting transition (“~4.48K). At this point a known magnetic field was applied to the tantalum specimen in order to calibrate the magnetometer. Temperatures were measured using a calibrated Speer carbon resis— tor. The sample was cooled to a low temperature in zero field and then a magnetic field was applied. On warming, a change in magnetization occurred, and a transition temperature could be identified at T~ = 230 mK. The response com— pared to that of the tantalum specimen mdi— cated that below T~ (SN)x is diamagnetic. Fig. 1 shows the change in magnetization, N, on warming from a temperature T = 59 mK to 300 mA in an applied field of 0.025 Oe, oriented par— allel and perpendicular to the (SN)~fibre axis. The temperature Tc 230 mA determined magne— tically is somewhat less than that defined by the mid—point of the resistive transition

*

Work supported by grants from the National Research Council of Canada. •1•Prmt address.

470

THE MEISSNER EFFECT IN POLYSIJLPHUR NITRIDE, (SN)~

(255 inK), but is similar to that observed from 6 On cooling from above Tc a.c. measurements. we observe a partial Meissner effect for both orientations, i.e. a significant fraction of the flux in the sample is excluded. The hysteresis in the curves implies flux tr apping 1 e a percentage of the flux which penetrates the sample on warming is trapped on cooling. For the curves of Fig. 1 about 15% of the flux is trapped for H~_and about 48% for H 11 . Measure—

0.5

H

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~___~

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0.25

24, No. 7

served flux change, t~4, due to the sample’s changing magnetization is scaled as its volume, V(SN) x ,with respect to the volume of the tanta— lum,VTa, i.e. we use the relation ~

rel /H

=

(~(SN)

‘Ta~~Ta”~~(SN) )‘ X

X

In scaling our data in this way we are neglect— ing demagnetizing effects and the possible dif— 9 For sample ference in the flux coupling factor forthe differused in this experiment estimate that the ent orientations of the we sample. absolute magnitude of the actual magnetization Changes may differ from Nrel by up to 50%. (ii) although it is difficult to determine M absolutely we can draw conclusions about its anisotropy. We note from Fig. 2 that at T = 59 inK N reaches a peak value for H,, at H eakll —— 0.18 Oe and for H~ at Hpeakl = 0.70 ~e. Since

—4irMrei _______ H

Vol.

the longer axis of the crystal coincides with the H,~ field direction, we expect a smaller demagnetizing factor than for H~and for a normal type II superconductor we would expect Hpeak > H~1~j. To compare M(H1 ) with M(H,,) it is necessary to include coupling factors as well as demagnetizing factors. However since these factors to introduce corrections which the longest and shortest dimension of the crystal differ only by 50% we would not expect

-

(iii) we observe a temperature dependence of the susceptibility for applied fields

H11 0

Fig. 1.

would H
0.2

0.3

I (K) The magnetization, Mrel, of (SN) (sample A) versus temperature, T, with a magnetic field applied parallel, H,,, or perpendicular, H1, to the fibre axis. After initially cooling the sample to field then the samplethe is T = 59 is inK applied; in zero applied field, warned to 0.33 K and finally cooled again. Mrel is the magnetization rela— tive to that of the tantalum test specimen (see text)

ments on another specimen (sample B) produced similar magnetization curves but yielded 55% flux trapping for H~ = 0.025 Oe. It is not possible to measure N versus H directly with our system because the magneto— meter is very sensitive to changes in the applied magnetic field. However, it is possible to generate M(H) by recording N versus T (warm— ing) curves at various applied fields. A samp— ling of our results is shown in Fig.2 where we display three curves of ~ versus Hj~at diffe— rent temperatures and one curve of M 11 versus H,, at T = 59 inK. For the data in Figs. 1 and 2 there are several points to note: (i) the vertical scales in the figures are determined relative to the diamagnetism of the 8that the (SN)~ sampleWe is make smallthe superconducting tantalum specimen. compared to the pick—up coil and that the oh— approximation

that appropriate to a normal type II supercon— ductor with the peak value of N corresponding 1’cl• However for H < Hpeak the magnetization should be temperature to the critical field independent and reversible which it is not. This implies that for our data H~ 1is much smaller than Hpeak~ Superficially our data also resemble those and in obtained in early work on0’1’ colloids of more the type recent experiments on aluminum particles’2, I superconductor mercury’ suggesting the magnetic properties of (SN)~ result fromthat uncoupled fibres of small diameter, d.

In that case a temperature dependent suscep-

tibility for low fields would result if the penetration depth, A, was greater than d. However we reject this model on two grounds. The magnitude of the observed diamagnetism is at least an order of magnitude larger than expected for the accepted fibre size of (SN)x(d < 300 ~)6• Also the temperature depend— ence of the susceptibility predicted’’ from the penetration, 3(T), of the field into uncoupled fibres is not consistent with the data in Fig.2. Since the observed diamagnetism is too large for an uncoupled fibre model, we should assume some contact between the fibres.13 Results on granular tin exhibiting flux trapping and similar magnetization curves have been interpreted’~ in terms of a connected filament model with the coupling being limited by the temperature dependent critical current between the filaments. In that case, the trapped flux for T < Tc was proportional to [i — (T/T)2] [1 — (T/T)1+] ½

Vol. 24, No. 7

THE NEISSNER EFFECT IN POLYSTJLPHUR NITRIDE, (SN)~

471

0. I~

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—==

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5

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l.0

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H (Oe) Fig. 2.

Magnetization curves for (SN)x at various temperatures (sample A) . Mrel ~ the magnetization relative to that of the tantalum test specimen (see text).

We have compared our data to this relation and we obtain a poor fit at low temperatures for sample A with Hj= 0.025 Oe, although we ob— 0.025, Oe. Further studies in pro— tamed 0.050 an excellent fit for sample are B with H1 = gress to check the sample dependence of the flux trapping and the validity of a coupled fila— mentary model, In our earlier work6 we reported no Meissner effect for H 11 and it is worthwhile to compare that result to the present data. The previous experiment was performed with an applied field of 10 Oe and, since the earth’s field was not cancelled, warming curves after cooling in zero field could not be obtained. In view of the present results, it is not surpris— ing that the broad transition could not be identified because of flux trapping effects, stray fields and the relatively high field applied to the specimen,

Measurements of the a.c. susceptibility in the perpendicular geometry produced results which were very similar to those obtained in our 6. measurements Thus the real and the imaginary parts of earlier using parallel geom— the etrysusceptibility were positive and dependent on the frequency of excitation. A quantitative explanation remains to be found for this s.c. response although our earlier suggestions6 are consistent with the filamentary model13’~ discussed above. Of course, the presence of flux trapping and eddy current effects makes the analysis difficult. Acknowledgements — We thank A.J. Berlmnsky and R.L. Greene for helpful discussions and A.W. Cordes, N.L. Paddock and G.B. Street for their assistance in the preparation of the samples. One of us (J.F.C.) is grateful to the Stichting voor Fundamenteel Onderzoek der Materie for financial support.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

GREENE R.L., STREET G.B. and SUTER L.J., Phys. Rev. Lett. 34, 577 (1975). AZEVEDO L.J., CLARK W.G., DEUTSCHER G., GREENE R.L., STREET G,B. and SUTER L.J., Solid State Commun. 19, 197 (1976). CIVIAX R.L., ELBAUN C., JUNKER W., COUGH C., KAO 1-1.1., NICHOLS L.F. and LABES M.M., Solid State Commun. 18, 1205 (1976). CIVIAK R.L., ELBAIJM C., NICHOLS L.F., KAO H.I. and LABES M.M., Phys. Rev. 14, 5413 (1976). CHAIKIN P.M., HANSMA P.R., GREENE R.L. and ENGLER E.M,, Bull. Am. Phys. Soc. 22, 256 (1977). DEE R.H., BERLINSKY A.J., CAROLAN J.P., KLEIN E., STONE N.J. and TUIORELL B.G., Solid State Coimnun. 22, 303 (1977). STREET G.B., ARNAL H., GILL W.D., GRANT P.M. and GREENE R.L., Mat. Res. Bull. 10, 877 (1975)

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THE MEISSNER EFFECT IN POLYSULPHUR NITRIDE, (sN)~ 8. 9. 10. 11. 12. 13. 14.

Vol. 24, No. 7

GIFFARD R.P., WEBB R.A. and WHEATLEY J.C., J. Low Temp. Phys. 6, 533 (1972). PROBER D.E., Ph.D. Thesis, Harvard University (1975). SHOENBERG D., Proc. Roy. Soc. A175, 49 (1940). WHITEHEAD C.S., Proc Roy. Soc. A238, 175 (1956). BUHRNAN R.A. and GRANQVIST C.G., J. Appl. Phys. 47, 2220 (1976). BEAN C.P., Phys. Rev. Lett. 8, 250 (1962). MOROZOV Yu. C., NAUNENKO I.G. and PETINOV V.1., Soy. Phys. Solid State 16, 1974 (1975).