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{\large\bf Ultrathin epitaxial Pb films on Si(111) 7 $\times$ 7~:
Electronic transport and magnetoconductance at low temperatures} \\

\bigskip
{\underline{O. Pfennigstorf}}, V. Borchers, K. Lang, H.-L.
G\"unter and M. Henzler\\

\bigskip
{\small{\em
Institut f\"ur
Festk\"orperphysik, Universit\"at Hannover, D-30167 Hannover, Germany}}
\\
http://www.fkp.uni-hannover.de/FKP/surface.html\\

{\tt pfennigstorf@fkp.uni-hannover.de}
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\noindent

Epitaxial metallic monolayers are model systems for 2D conduction. They
provide a bulk like structure
in monatomic, well ordered films. For clarification of the conductance
behavior of ultrathin 
Pb films deposited in ultrahigh vacuum onto clean Si(111) 7$\times$7
substrates
at 15~K the magnetoconductance and the conductance 
have been measured {\it in situ} with four-point probes in van der Pauw
arrangement.

For
thicknesses from 1.5~ML to 3.5~ML and temperature close to 6~K the
magnetoconductance shows a
non-classical behavior. The negative change of the conductance in the
magnetic field 
of $\pm$4~T can not be explained by the classical magnetic field dependence
of the 
conductance $\Delta L\sim -\mu ^2B^2$ with the measured  Hall-mobility for
$\mu$. Also 
the quantum effects like weak localization and weak antilocalization as
measured in ultrathin 
Ag films [1,2] can not explain this behavior. Since the superconductivity
is destroyed by magnetic 
fields below 0.1~T there is not yet a theory, which describes the results.
The results may be discussed
with models of amorphous or granular films, because former experiments
[3,4] and the optical LEED system 
showed that the Pb film is badly ordered at coverages below 4~ML.


\bigskip
\noindent
[1] M. Henzler, T. L\"uer and A. Burdach, Phys.Rev.B 58
(1998) 10046.

\noindent
[2] M. Henzler, T. L\"uer and J. Heitmann, Phys.Rev.B 59 (1999)
2383.

\noindent
[3] M. Jalochowski and E. Bauer, Phys.Rev.B 38, (1988) 5272.

\noindent
[4] M. Jalochowski, H. Knoppe and G. Lilienkamp and E. Bauer, Phys.Rev.B
46, (1992) 4693.

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Mit freundlichen Grüssen,



Olaf Pfennigstorf




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Dipl. Phys. Olaf Pfennigstorf
Institut für Festkörperphysik
Universität Hannover
Appelstrasse 2
30167 Hannover
Tel.: 0511/7624828
FAX:  0511/7624877
email: pfennigstorf@fkp.uni-hannover.de
Internet: http://www.fkp.uni-hannover.de/~olaf/main.html
          http://www.fkp.uni-hannover.de/FKP/RESEARCH/LEIT/leit.html	
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