Ž . Applied Surface Science 144–145 1999 575–579 Tunneling spectroscopy of bound and resonant states in superconducting proximity structures Y. Levi a , O. Millo a, ) , N.D. Rizzo b , D.E. Prober b , L.R. Motowidlo c a Racah Institute of Physics, the Hebrew UniÕersity, Jerusalem 91904, Israel b Applied Physics, Yale UniÕersity, New HaÕen, CT 06520-8284, USA c IGC-AS, Waterbury, CT 06704, USA Abstract Scanning tunneling microscopy and spectroscopy are employed in order to map with nanometer resolution the local quasi-particle density of states in superconducting proximity structures. Our experimental configuration is unique, in that the tunneling current flows in parallel to the interfaces between different materials. We focus here on the measurement of bound and resonant states, arising from multiple Andreev reflections of quasi-particles propagating in a plane perpendicular to the tunneling current. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Superconductivity; Superconductor—metal interfaces 1. Introduction Ž. The mutual influence of a superconductor S in Ž . electrical contact with a normal metal N , a phe- Ž . nomenon known as the proximity effect PE , has been studied extensively in the past three decades w x 1,2 . Central issues of the PE are the spatial varia- tions of the local quasi-particle density of states Ž . wx DOS in the vicinity of the N–S boundary 3 , and the presence of quasi-particle bound and resonant states. Recently, research of the PE gained further momentum thanks to technological advances that enable the fabrication of complex mesoscopic de- w x vices. Nevertheless, many of these works 4–6 fol- wx lowed earlier studies 2 and addressed macroscopic properties of the samples, thus obtaining average ) Corresponding author. Tel.: q972-2-658-5670; Fax: q972-2- 658-4437; E-mail: milode@vms.huji.ac.il wx rather than local information. In other cases 7 spatially resolved data were sought, but only in a limited number of locations. Scanning tunneling mi- Ž . croscopy STM can be very effective in the research of PE, since the DOS can be measured locally. At T s 0, the tunneling d IrdV vs. V curve is directly proportional to the sample’s DOS, while at finite temperatures it is thermally smeared. By taking topo- graphic images simultaneously with tunneling cur- Ž . rent–voltage I–V or d IrdV vs. V characteristics, one obtains a spatially resolved map of the DOS. Some STM experiments treated systems composed of a normal metal island film deposited onto a S w x substrate 8–10 so that tunneling took place perpen- dicular to the S–N interface. Hess et al. measured a type-II superconductor in the Abrikosov vortex lat- tice state, mapping the DOS around and within a w x single flux-vortex 11,12 . However, this study did not address a structure involving different metals. 0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-4332 98 00868-X