J Supercond Nov Magn (2013) 26:1473–1477 DOI 10.1007/s10948-012-2051-x ORIGINAL PAPER Tunneling Density of States of Indium Oxide Films Through the Superconductor to Insulator Transition D. Sherman · G. Kopnov · E. Farber · D. Shahar · A. Frydman Received: 5 November 2012 / Accepted: 3 December 2012 / Published online: 5 January 2013 © Springer Science+Business Media New York 2013 Abstract We present planar tunneling junction spectro- scopy measurements on disordered amorphous indium ox- ide films on both sides of the superconductor–insulator tran- sition. Our measurements directly reveal a superconducting gap in the insulating phase. The measured energy gap has the same energy scale on both sides of the transition. Unlike the case of granular films, the tunneling curves cannot be fitted to the BCS density of state expression, even when in- troducing a broadening parameter to account for nonthermal broadening sources. The results are consistent with the pres- ence of superconducting islands of which superconducting properties depend on film disorder and on the carrier den- sity of the superconducting material. Keywords Superconductor to insulator transition · Disorder · Tunneling · Electron concentration 1 Introduction The supercontuctor–insulator transition (SIT) in disordered thin films has gained increasing interest lately, mainly due to the realization that it is a prototype of a quantum phase transition, which is easily accessible experimentally [1]. De- D. Sherman ( ) · A. Frydman The Department of Physics, Bar Ilan University, Ramat Gan 52900, Israel e-mail: shredan@gmail.com G. Kopnov · D. Shahar Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel D. Sherman · E. Farber Laboratory for Superconductivity and Optical Spectroscopy, Ariel University Center of Samaria, Ariel 40700, Israel spite recent intensive study, the nature of superconductivity in these systems, and in particular, the nature of the insu- lating phase, is still not understood. A number of experi- ments on amorphous thin films of indium oxide, titanium nitride, and niobium nitride have shown evidence for su- perconductivity at temperatures above T C [2–5], as well as on the insulating side of the transition [3, 6–8]. Two main models have been suggested for these results: disorder in- duced granularity and the presence of preformed pairs. The first invokes the emergence of “electronic granularity,” i.e., regions of superconducting islands embedded in an insu- lating matrix [9–15]. Unlike the situation in granular films which exhibit an SIT as a function of intergrain coupling, in the amorphous films, the superconducting islands result from disorder and not from metallurgic agglomeration. In this picture, global superconductivity occurs when super- conducting percolation is achieved throughout the film via intergrain Josephson coupling. Hence, superconductivity in single grains may exist above T C or in the insulating state despite the fact that long range superconductivity cannot be observed by transport measurements. Unlike “real” grains, the size of these emergent islands can decrease with disor- der, temperature, or magnetic field [12, 13, 16–18]. The sec- ond model suggests the existence of preformed pairs above T C or in the insulating side of the transition [19, 20]. These pairs are uncorrelated, and hence do not form a condensate in any region of space. Recently, we presented [8] tunnel- ing spectroscopy results that showed that a superconducting gap persists into the insulating side of the SIT, and that its energy scale is similar to that in the superconductor. This was taken as an indication for the presence of superconduct- ing islands in the insulating phase. In this paper, we further discuss these findings. We compare results on a number of amorphous indium oxide films having different disorder, W , and carrier concentration, n. We study the effect of W and