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Decay mechanisms of quantum-well states and Superconductivity in ultrathin Pb islands grown on Si(111) : scanning tunneling spectroscopy study

Christophe BRUN - Institut de Physique de la Matière Condensée, École Polytechnique Fédérale de Lausanne


Christophe Brun, I-Po Hong, François Patthey, Wolf-Dieter Schneider

 

A clear picture of the decay mechanisms of low-energy electronic excitations has been obtained in several types of bulk metals as well as at various metal surfaces [1]. However, due to various technical limitations, few studies have reported so far detailed lifetime investigations of metallic quantum well states [2]. Only one scanning tunneling spectroscopy (STS) study reported a quantitative linewidth analysis of a QWS metal system, Yb(111)/W(110) [3], but the results were subsequently questioned by a combined time-resolved two-photon photoemission and DFT study on bulk Yb [4]. Using low temperature STS in ultrahigh vacuum (UHV) environment, we studied the linewidth of unoccupied quantum well states (QWS) in Pb nanocrystals of thicknesses between 7 and 22 monolayers, deposited on Si(111) and grown in UHV on two different Pb/Si interfaces. The Pb/Si(111) system is very suited to study thickness dependent properties because the thickness of the Pb film or islands can be controlled at the atomic monolayer, due to very prononced quantum size effects occuring in this system [5]. A quantitative analysis allowed us to determine the various decay contributions of the QWS excitations in terms of electron-electron, electron-phonon and interface-defects scattering. These quantities are in good agreement with the corresponding ab initio calculated quantities [6]. In addition, the superconducting energy gap was measured as a function of island thickness between 5ML and 60 ML. It is found to decrease with decreasing island thickness, from about the bulk value for 60 ML to about 60% of the bulk value for 5ML islands. The observed reduction with decreasing island thickness was rationalized employing DFT calculations for free-standing Pb films [7] and is in good agreement with ex-situ magnetic susceptibility measurements on Ge-capped Pb ultrathin filmsgrown on Si(111) [8].

 

Support by the Swiss National Science Foundation is gratefully acknowledged.

 

References :

 

[1] E.V. Chulkov /et al./ Chem. Rev. 106, 4160 (2006)

[2] J.J. Paggel, T. Miller, and T.-C. Chiang, Science 283, 1709 (1999)

[3] D. Wegner, A. Bauer, and G. Kaindl, Phys. Rev. Lett. 94, 126804 (2005)

[4] V. P. Zhukov /et al./ Phys. Rev. B 76, 193107 (2007)

[5] Z. Zhang /et al./ Phys. Rev. Lett. 80, 5381 (1998)

[6] I.-P. Hong /et al./ Phys. Rev. B 80, 081409 (R) (2009)

[7] C. Brun /et al./ Phys. Rev. Lett. 102, 207002 (2009)

[8] M. M. Ozer /et al./ Nature Physics 2, 173 (2006)

 

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