Effect of inhomogeneous coupling on BCS superconductors

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Yue Zou,¹ Israel Klich,² and Gil Refael¹
¹Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
²Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, California 93106, USA

Received 18 December 2007; published 25 April 2008

Weinvestigate the influence of inhomogeneity in the pairing coupling constantU( r-vector ) on dirty BCS superconductors, focusing on T_c, the orderparameter Delta (), and the energy gap E_g(). Within mean-field theory,we find that when the length scale of the inhomogeneityis comparable to or larger than the coherence length, theratio 2E_g/T_c is significantly reduced from that of a homogeneoussuperconductor, while in the opposite limit, this ratio stays unmodified.In two dimensions, when strong phase fluctuations are included, theKosterlitz-Thouless temperature T_KT is also studied. We find that whenthe inhomogeneity length scale is much larger than the coherencelength, 2E_g/T_KT can be larger than the usual BCS value.We use our results to qualitatively explain recent experimental observationof a surprisingly low value of 2E_g/T_c in thin films.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.77.144523
DOI:	10.1103/PhysRevB.77.144523
PACS:	74.78.-w; 74.20.-z; 74.81.-g 74.78.-w Superconducting films and low-dimensional structures 74.20.-z Theories and models of superconducting state 74.81.-g Inhomogeneous superconductors and superconducting systems YEAR: 2008
KEYWORDS:	BCS theory, coherence length, dirty superconductors, Kosterlitz-Thouless transition, superconducting energy gap, superconducting thin films, superconducting transition temperature

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