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Submitted on 1 Jan 1978
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RESISTIVE TRANSITION OF LEAD
SUPERCONDUCTING STRIPS IN THE SURFACE
SHEATH STATE
J. Aponte, M. Octavio, R. Callarotti
To cite this version:
IOURNAL DE PHYSIQUE Colloque C 6 , supplhent au no 8, Tome 39, aolir 1978, page C6-606
R E S I S T I V E T R A N S I T I O N OF LEAD SUPERCONDUCTING S T R I P S I N THE SURFACE SHEATH STATE J.M. Aponte, M. Octavio and R.C. Callarotti
Laboratorio de Ingenieria EZgctrica, Institute VenezoZano de I n ~ e s t i g ~ i o n e s ~ i e n t < f i c a s , Apartado 1827, Caracas, Venezue Za.
Rdsum6.- On a ddtermind des transitions resistives en presence d'un champ magndtique pour des cou- ches minces de plomb sur une microbande dTdpaisseur variable (0,l +2pm). On observe des gradins dans les courbes caractsristiques courant-tension et nous les expliquons par des centres de phase glissante. Les autres pertes observdes sont expliqudes par un modsle de mouvement des lignes de flux.
Abstract.-Resistive transitionsina magnetic field have been measured in lead microstrips of vary- ing thicknesses (0.1 +2pm). Steps in the I-V characteristics are observed and they are explained in terms of phase-slip centers. Additional observed losses are assumed to be due to flux flow dis- sipation.
The appearance of steps in the I-V characte- ristics of a variety of superconducting geometries have been reported. They have been observed in whiskers, microbridge and microstrips /I/, and they have been explained as due to the formation of pha- se-slip centers. A similar phenomenon has been re- ported in lead microstrips in the presence of a magnetic field /2,3,4/, for fields near above HcZ, and it has been suggested / 2 , 3 / , in this case that the step structure is due to conduction through quantized quasiparticle states in the normal region between the sheathsat each surface of the film. In this paper, we report on experimental measurements of this phenomenon. We find that all the phenomena observed can be explained in terms of the formation of localized phase-slip centers due to the current- induced breakdown of the superconducting surface sheath.
Our samples are rectangular strips of lead (length'L7 mm, width Q 1 mu, thickness Q 0.1-2 ym) evaporatedin a vacuum of 3x10-6 torr. Critical fields determined from the R vs.H transition are of order 600 to 1000 gauss for Hc2 and 2000 to 4000 gauss for Hll, thus, since Hc2= K Hc, K is of order 1 and our samples are type I1 supercon- ductors. I-V characteristics and R vs.H curves are measured by the standard four--probe technique with voltage tabs placed along the center of the long strip (14 mm).
Typical I-V characteristics as a function of field are shown in figure 1.At low fields some steps are visible, but only a few of them can be traced out, probably due to the heating effects
due to the large currents flowing through the samples. As the field is increased numerous steps are visible (up to n= 14)
,
and in order to trace the complete step structure,the curves have to be traced repeatedly, as observed by others/2,3/. As the field is increased the variations of the indi- vidual step critical currents is not large, and it becomes harder to trace the complete step structu- re. Except for the first step, all higher order steps (n >])have a definite critical current I.
C The insert of figure 1 shows how the dif- ferential resistance dV/dI of each step increases by the same constant amount with n, each unit cor- responding to the same resistance of order 9 d in this sample: While the geometry of the last re- sistance unit caused by the destruction of the sheath is unknown, this resistance level corres- ponds to what is expected from a unit of lenght
1
and widht A
=(7
v F R ~ 2 the quasiparticle dif- fusion lenght, where r2 is the electron-phonon scattering time for quasiparticles, and the thick- ness of the region is the thickness of the sheathA
,
which for typical K and H values is of order2
5
(T)/5/. The value of r2 is approximately 5x10-12s. in lead / 6 / . The variation of resistance with magnetic field is consitent with what is excep- ted from the phase-slip model. As the field is in- creased there is only a small variation of the differential resistance, until one approaches H(T), where the differential resistance diverges. This has been shown to be the case for tin micro- bridges 171, where the appropriate relaxation time was found to be the transverse relaxation time /8/,
me angular dependence of the observed processes clearly indicates the role of the surface sheath.
the magnetic field. Other aspects of the data also agree with the phase-slip model. The time averaged supercurrent Is, extrapolated to zero voltage, is of order 0.7 Ic, for all steps except the first one, with I being the critical current of each individual step. While alternative explanations for this phenomenon have been proposed in terms of quasiparticle-bound states, we find that the step structure is observed even for fields in which the thickness of the sheathis as large as the sample151 and thus the normal region required for this expla- nation would not exist for these field values. We are currently performing experiments in order to control the geometry of the final resistive region better, in order to quantitatively compare the ob- served resistances to the expected changes in
A (T,H 1.
The general behavior of the R vs.H curves is given in figure 2, and it clearly shows the different regions of existence of the step regime. The insert of this figure shows the data as
[%/R (H)
-
ll
-'vs .I at fixed field values. We can compare this with the behavior of bulk niobium in the surface sheathregime as discussed previous- ly 191. Not too close to Hcj [R~IR(H)-~-'~~ pro- portional to I. In the case of niobiunthe mechanism responsible for the observed losses in the sheath was thought to be the viscous flow of flux quanta in the sheaththrough the effect of the measuring current. In the present samples of lead we inter- pret our measured losses both as phase slip cen- ters dissipation and losses associated with the movement of flux lines. In either case the extre- Fig.1 : I-V characteristics showing the step struc- ture (thickness : 3800 8 ) . (T = 4.02 K)modified to include the effects of the effects of
Fig.2 : Typical R vs.H curves thickness:1700 8
)
.
(T = 4.02 K) ~nsert: [%/R(B)-~‘' vs.1 dc at fixed H values.References
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30
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(1965) A 1937171 Kadin,A., Skocpol,W.J.and Tinkham,M. to be published.
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