Superconductivity and Tunneling Spectroscopy in Granular and Homogeneous Quench Condensed thin Films
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SUPERCONDUCTIVITY AND TUNNELING SPECTROSCOPY IN GRANULAR AND HOMOGENEOUS QUENCH CONDENSED THIN FILMS
J. M. VALLES, JR.', and R. C. DYNES AT&T Bell Laboratories, Murray Hill, N.J. 07074. 'Present address: University of Oregon, Eugene, Oregon, 97403
ABSTRACT Quench-condensed films of simple superconductors can be deposited with a morphology which is either granular (grain sizes on the scale of tens of angstroms) or homogeneous. Transport and tunneling studies of the normal and superconducting states of these films as a function of sheet resistance have revealed profound differences between them. In uniform films T. and the energy gap decrease continuously with decreasing film thickness. Superconductivity is destroyed by the reduction of the amplitude of the order parameter. In granular films, the grains are sufficiently large to support superconductivity in each grain. Long range phase coherence is destroyed by intergrain phase coherence breaking.
INTRODUCTION Transport studies in ultra-thin metal films have shown the general nature of the transition to insulating behavior[l], the phenomenon of weak localization[2,3], and the influence of strong Coulomb interactions in the extremely dirty regime.[4] We have demonstrated that films can be prepared with morphologies in two extreme limits. At one extreme, "uniform" films can be prepared which exhibit behavior suggesting statistical uniformity[5,6] at the atomic level. At the other extreme, granular films can be prepared which clearly show a clustered nature with grains tens of Angstroms in radius.[1] Despite these rather substantial morphological differences, much of their normal state transport characteristics as a function of sheet resistance, R 0 , are similar. In particular, both show a transition from logarithmically to exponentially localized behavior at Ro - 10kfl/O. The superconducting properties of these films as a function of R 0 , on the other hand do reflect their structural differences. While superconductivity is destroyed in both systems when R3 is increased to - 10kfQ/0, the manner in which it is destroyed depends strongly on the morphology of the film.[6,7,8,9] This is due to the fact that superconductivity can be destroyed in two ways and each way applies to one of the two extreme morphologies. One can write the superconducting order parameter in terms of an amplitude -o and a phase €
V) =fOoe-io The amplitude Vo reflects the strength of electron-phonon coupling and governs the energy gap A and the critical temperature T,. The phase 0 reflects the long range phase coherence of the phenomenon and is the parameter determining the transport properties. The supercurrent I1 is proportional to the gradient of the phase V07 and long range superconductivity persists only if there is long range phase coherence.
Mat. Res. Soc. Symp. Proc. Vol. 195. @1990 Materials Research Society
376
Thus, by either reducing the amplitude Vo (thus reducing A and TJ)the uniform film case, or breaking the long range phase coherence ', the granular film case, we can destroy
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