Conduction channels of superconducting quantum point contacts
E. Scheer, A. Mayer-Gindner*
Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsruhe

Modern nanofabrication techniques including modified scanning tunneling microsopes (STM) and mechanically controllable breakjunctions (MCB) provide adjustable atomic-size contacts between two metallic banks. Since the Fermi wavelength in metals is of the order of the inter-atomic distance, these contacts accommodate only a small number of conduction channels. The conductance of the smallest contacts is usually of the order of the conductance quantum 2e2/h.

The number N and the individual transmission coefficients 0 < Tn < 1 of these channels can be revealed when using superconducting metals. The current-voltage characteristics of superconducting contacts display the strong nonlinearities - called subgap structure - associated with multiple Andreev reflection (MAR).

Using high-stability lithographically fabricated MCBs [1], the quantitative agreement between our experimental results and the full quantum theory of MAR [2-4] allows to determine the number of conduction channels and the individual transmission coefficents of contacts of elements covering several series of the periodic table. We find that in the smallest stable contacts, usually three channels contribute to the transport in the case of Al and Pb, five in Nb, while only one does in the case of Au. The values Tn, however, strongly depend on the exact geometrical and chemical environment of the central atom. (For the determination of the channel content of gold contacts we use the proximity effect with superconducting leads made of Al.)

These findings are explained from the quantum chemistry point of view that the channels are determined by the valence atomic orbitals of the central atom of the contact.

[1] J.M. van Ruitenbeek et al., Rev. Sci. Inst. 67, 108 (1996)
[2] D. Averin and A. Bardas, Phys. Rev. Lett. 75, 1831 (1995)
[3] J.C. Cuevas, A. Martín-Rodero, and A. Levy Yeyati, Phys. Rev. B. 54,7366 (1996)
[4] E.N. Bratus et al., Phys. Rev. B 55, 12666 (1997)

* In collaboration with N. Agraït, W. Belzig, J.C. Cuevas, Y. Naveh, J.M. van Ruitenbeek, and C. Urbina.