\documentstyle[preprint,tighten,prabib,prl,aps]{revtex}
\def\tmi{T$_{mi}$}
\def\tc{T$_{c}$}
\def\tcm{T$_{c}^m$}
\begin{document}
\draft

\title{
Direct Observation of Lattice Polaron Formation
in the Local Structure of
$\bf La_{1-x}Ca_xMnO_3$  
 }

\author{S. J. L. Billinge,$^1$ R. G. DiFrancesco,$^1$
	G. H. Kwei,$^2$ J. J. Neumeier$^3$ and J. D. Thompson$^3$}
\address{
$^1$Department of Physics and Astronomy and Center for
Fundamental Materials Research,\\
Michigan State University, East Lansing, MI 48824-1116.\\
$^2$Chemistry and Materials Sciences Department,
Lawrence Livermore National Laboratory, Livermore, CA 94550.\\
$^3$Los Alamos National Laboratory,
Los Alamos, NM 87545.}

\date{\today}
\maketitle

\begin{abstract} 
The local atomic structure of $\rm La_{1-x}Ca_{x}MnO_3$ ($x= 0.12,$
0.21 and 0.25) has been
studied using pair-distribution-function analysis of neutron 
powder-diffraction data.  A change is seen in the local structure which can
be correlated with the metal-insulator transition in the 
$x=0.21$ and 0.25 samples.  This local structural
change is modeled as an isotropic collapse of oxygen towards Mn of
magnitude $\delta = 0.12$~\AA\ 
occuring on one-in-four Mn sites.  We argue that this is the direct
observation of lattice polaron formation, associated with the metal-insulator
transition in these materials.
\end{abstract}
\pacs{71.30+h,71.38.+i,61.12-q,72.80.Ga}

\narrowtext

The discovery in the 1950's 
of ferromagnetism coupled with a metal-insulator (MI) transition
in the class of materials 
$\rm La_{1-x}A_xMnO_3$, where A is a divalent 
ion~\cite{jonke;p50}, led to a theoretical
explanation which involved a new type of exchange interaction called 
double exchange (DE)~\cite{zener;pr51}.

$\ldots$

The results are shown in Fig.~\ref{fig;str chg}.  
The upper set of curves, (i), in Fig.~\ref{fig;str chg}(a) show
the appearance in the {\it data} of the structure change at 
T$^m_c$. The PDFs are from the $x=0.21$ sample at T=140~K (solid line) and
176~K (dotted line).
$\ldots$

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\begin{thebibliography}{10}

\bibitem{jonke;p50}
G.~M. Jonker and J.~H. van Santen,
\newblock Physica (Utrecht) {\bf 16}, 337 (1950);
E.~O. Wollan and W.~F. Koehler,
\newblock Phys. Rev. {\bf 100}, 545 (1955);
C.~W. Searle and S.~T. Wang,
\newblock Can. J. Phys. {\bf 47}, 2703 (1969);
{\it ibid}, {\bf 48}, 2023 (1970).

\bibitem{helmo;prl93}
R.~von Helmolt et~al.,
\newblock Phys. Rev. Lett. {\bf 71}, 2331 (1993).



\bibitem{magnote}
The structure change correlates with the MI transition and not with the
  appearance of ferromagnetism. This is clear since the $x=0.12$ sample becomes
  ferromagnetic at a T$_c$ of $\approx 165$~K, with an ordered moment which is
  comparable to that observed in the other composition samples, although it
  remains insulating to the lowest temperatures. If the PDF were sensitive to
  the ferromagnetism rather than the structure change associated with the MI
  transition, it would be evident in the $x=0.12$ sample and it is not
  [Fig.~\protect\ref{fig;dat}].


\end{thebibliography}

%
\begin{figure}
\caption{PDFs obtained from the $\rm La_{0.88}Ca_{0.12}MnO_3$ sample.  
$\ldots$
\protect\label{fig;dat}} 
\end{figure}
%

\end{document}