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ON WEAK SUPERCONDUCTING SPOTS IN Nb3Sn
J. Halbritter
To cite this version:
JOURNAL D E PHYSIQUE Colloque C6, supplPment au no 8, Tome 39, aotit 1978, page C6-396
ON
WEAK
SUPERCONDUCTING SPOTSIN
NbjSnJ. Halbritter
Kernforschungszentmon KmZsruhe,IK,Postfach 3640 D 7500 KarZsruhe, Federal Republic G e m m y
RLsum6.- Des mesures rLcentes sur des cavitLs h.f. supraconductrices en Nb3Sn prouvent l'existence d'Ltats d'excitation dans le gap d'ct~ergie de Nb~sn. Ceux-ci correspondent 5 deux classes de domaines
1 supraconductivit6 affaiblie avec T < I K at Tc % 2.5 K.
Abstract.- Recent rf results in superconducting NbeSn cavities prove the egistence of quasiparticles in the energy gap of Nb3Sn corresponding to 2 classes of weak spots with T < I K and :T % 2.5 K.
INTRODUCTION.- In recent years the preparation of Nb3Sn has been improved very much. For example, fairly good tunnel data111 and rf resultsl2-41 have been obtained with Nb3Sn layers. But there exist se- veral indications, that these layers are already deteriorated, e.g. : the tunnel data worsen with oxidation time, especially in humid air111 ; the surface barrier hindering flux entry is weakened by chemical treatments151 ; and the rf properties of Nb3Sn cavities, which are heavily oxidizedl2-41, are
still systematically inferior compared to Nb cavi- ties. These degradations can be explained by weakly superconducting spots existing or growing on oxidi- zed Nb3Sn surfaces. The chemistry and transition temperature T~(B) of these weak spots are not yet
C
known. Possibilities for the nature of such weak spots are : Sn rich phases (T: = 2.1 K and 2.6 K) 141, "amorphous Nb3SnU (T: = 3.1 K) 161 and precipi- tates caused by H take-up/5/. Till now the nature of amorphous Nb3Sn/6/ or of H induced precipitates 151 is not known, so it is not clear if they exist as well defined phases or only as mixtures of micro-
n
cristallites of different T-. To analyse these pro-
L.
blems further, rf measurements ar,e well suited be- cause they are sensitive to small densities of low lying excitations and average in a well defined way over a layer of about 200 nm thickness. Here, we present surface resistance measurements, the T- and Brf-dependence of which prove the existence of spots with T* < 3 K in Nb3Sn surface layers. The spots,.appearing in a volume ratio below at Nb3Sn surfaces handled in air, H20 or acids, can be split into 2 classes ; one with T* 2.5 K and the other with T~ < 1 K. Such weak spots in proximity with NbsSn offer explanations for the experimental observations mentioned abovell-61 and for pinning
SURFACE RESISTANCE OF INHOMOGENEOUS Nb3Sn.- The sur- face resistance of well prepared superconductors can be split into R = RBCS(T) + Rres, where RBCS(T) des- cribes the rf losses in the superconductor assumed homogeneous/7/ and where Rres describes the rf los- ses by inelastic scattering of conduction electrons into localized states of the oxidel81. Because Nb3Sn has a large magnetic field penetration depth
X
,
the electric fieldI E
I
-
wpoAHI 1
-
I I
islarge and hence the partial surface resistance of small (<<A) weak spots R~ = u*dn(wpo~) can become
*
large, too. Here a is a surface ratio and d a layer equivalent to the weak spots. So, the surface resis- tance reads now : R(T) : RBCS(T) + Rres + a~~ with RBCS(2.8 GHz) = 1.3 x I O - ~ S ~ ~ X ~ ( - A / ~ T ) ~ T (figure 1) and
X
= 161 nm for T>Tc/2 /7/.These results for ho- mogeneous Nb3Sn describe the measured impedancesl3, 41 h(T) and R(T) for %CS(T)>> ,,:R = a ~ * + Rres. Rres(2.8 GHz)<
1.2 x 10-~51 is the upper limit of residual losses of NbsSn due to surface scattering 181.3 x ~ O - ~ Q ~ O * ~ " T > T ~ ( ~ ~ £ 1 a~~(2.8 GHz) = {
A exp (-A*/ICT)/T - T < T ~ ( B ~ ~ ) %CS(2.8 GHz,T) was calculated with the follo- wing material parameters : 2A/kTc
"
4.5,AL(T=06R=m) = 60 nm,
t o =
6 nm andR
= 1 nm171. Comparing these results with figure 1, one rea- lize that R* (<2 K) > 2 x 1 0 ~ ~ ~ 2 cannot be explaiires ,..
ned by interface lossesl81. As verified by the ob- servation that the smallest R~ % 5 x 10-'52
re2
and Brf Q, 0.7 mT grew to 2 x 10 751 by applying
, B ~ ~ Q, 7 mT, these losses are caused by normal con-
ducting spots, which were superconducting by proxi- mity effect only/9/. Using a~~ Q, 2 x 10-~8 one ob-
Fig. 1 : Surface resistance of NbsSn reacted at 1 0 5 0 ~ ~ . The data are taken at 2.8 GHz in a TMolo
-
cavity after subsequent oxipolishing treatments of the NbsSn surface. For T%
2 K the fit R(T)-
Ca exp(-A /kT)/T yields A~(T % 2 K) % 0.9 meV.tains ~*d~~5xl0-~S2, i.e.
u
= 1/40 pQcm of damaged /6/ Nb3Sn results in dx = 2.3 nm. This mean thick- ness seems plausible, but because of the unknown nature of these weak spots and the deviations/lO/ of the conductivity bX(w) = aX(o) / + (u'rin)7 of the localized excitations from thermal equili-brium by a long inelastic scattering time T ~ ~ ~ T - ~ this dX is a crude estimate only.
The slow rise in figure 1 of R(T) above 2 K indicates transitions of domains to the normal
x
conducting state with T > 2 K. Using proximity ef- fect theories/9/, the plateau of R(T) in figure 1
x
indicates Tc % 2.5 K. The gap value
A*
% I O - ~ ~ V ob- tained from the rise of R(T), is larger than the gap A(T~ % 2.5 K) % 4 x IO-'~V, which is due to the proximity/9/ with Nbs~n@=3.5 meg and due to rf induced superconductivity by overheating because of 'rin > l/f./10/ Because of these effects, an es- timate of the amount of such spots is difficult anda < seems plausible/5/. Various surface treat- ments like : reaction at 1050°C and 1500°C with and without SnC12, homogenization at 1050°C and oxi-
polishing have been tried : The tendency was, that homogenization improved R* (<2 K), but 2 x IO-'Q
res
obtained by oxipolishing alone could not be overco- me. The relative success of homogenization hints to the strained grain-boundaries as weak spots. But the 1 5 0 0 ~ ~ cavity with NbsSn cristallites being a factor 9 larger than the 1050~ NbsSn shows with ~:,,(<2 K, 7 mT) 'L 2 x IO-'Q no improvement. So, these weak spots cannot simply be grain boundaries. More likely reasons are 0 or H take-up during hand- ling of the cavities. Experiments are in progress to clear these questions.
ACKNOWLEDGEMENT.- The author would like to thank P. Kneisel and 0. Stoltz for carrying out the mea- surements discussed in this paper and A. Citron for helpful comments.
References
/ I / Moore,D.F., Rowel1,J.M. and Beasley,M.R., Solid
State Commun.
20
(1976) 305/2/ Hillenbrand,B., Martens,H., Pfister,H., Schnit- zke,K. and Uzel,Y., IEEE Trans. Mag-13 (1977) 491
/3/ Arnold,G., Proch. D. ; ibid. p. 500
/4/ Kneisel,P., Kiipfer,H., Schwarz,W., Stoltz,O., and Halbritter,J., ibid p. 496 and
Kneisel,P., ~Gpfer,~., Stoltz,O. and Halbritter, J., Advances in Cryogenic Engineering vol. 24
(Eds. Timmerhaus et al. Plenum Publ. N.Y. 1978)
/5/ BussiSre,J.F. and Kovachev,V.T., J. Appl. Phys. 49 (1978)
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/6/ Poate,J.M., Dynes,R.C., Testardi,L.R. and Ham-
mond,R.H., Phys. Refr. Lett.
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(1976) 1308 171 Hal.britter,J., 2. Phys.266
(1974) 209 /8/ Halbritter,J., IEEE Trans Mag-11 (1975) 427 /9/ Fischer,G. and Klein,R., Phys. kondens. Materie7 (1968) 7
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/lo/ Schwarz,W. and Halbritter,J., J. Appl. Phys. 48 (1977) 4618
