IOURNAL DE PHYSIQUE Colloque C6, supplhent au no 8, Tome 39, aolir 1978, page C6-606 RESISTIVE TRANSITION OF LEAD SUPERCONDUCTING STRIPS 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 ~ient<ficas, 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 dTdpaisseurvariable (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--probetechnique 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 vFR~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 sheath A , which for typical K and H values is of order 2 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/, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786273