•F. Pérez-Willard¹, C. Sürgers¹, H. v. Löhneysen^1,2, E. Scheer³, P. Pfundstein⁴ und D. Gerthsen^4
1Physikalisches Institut and Center for Functional Nanostructures, Universität Karlsruhe, D-76128 Karlsruhe
²Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76344 Karlsruhe
³Fachbereich Physik, Universität Konstanz, D-78457 Konstanz
⁴Laboratorium für Elektronenmikroskopie, Universität Karlsruhe, D-76128 Karlsruhe

We report on low-temperature (20 mK - 2 K) measurements of the electrical differential resistance R = dU/dI of bismuth nanocontacts. The samples consist of 200 nm bismuth deposited under ultra-high vacuum conditions on both sides of an insulating 50-nm thick silicon-nitride membrane in which a single nanohole of diameter d  £  50 nm has been patterned previously by means of e-beam lithography and reactive ion etching. The resistance of the samples shows a negative temperature coefficient which is ascribed to finite size effects [1]. The resistance ratio R (2 K) / R (293 K) is between 2.5 and 3. For magnetic fields up to B = 8.5 T applied perpendicular to the contact region, i. e. parallel to the membrane, we observe a large magnetoresistance effect of approximately 40%. Superimposed on this background, magnetoconductance fluctuations are resolved. In addition, we observe time-dependent fluctuations due to two-level systems in the contact region.
 

[1] K. Lui et al., Phys. Rev. B 58, 14681 (1998); F. Y. Yang et al., Science 284, 1335 (1999)