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{\small Abstract, 225th WE-Heraeus Seminar,
        Bad Honnef, 11-15 October 1999}\\
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{\large\bf Rare regions, local moments, and annealed disorder: A novel
mechanism
       for metal-insulator transitions} \\

\bigskip
{\underline{Thomas Vojta}}$^1$, Dietrich Belitz$^2$ and Theodore R.
Kirkpatrick$^3$\\

\bigskip
{\small\em
$^1$Institut f\"ur Physik, Technische Universit\"at Chemnitz,
                                D-09107 Chemnitz, Germany \\
$^2$Department of Physics and Materials Science Institute,
                               University of Oregon, Eugene, OR 97403\\
$^3$Institute for Physical Science and Technology, and Department
             of Physics, University of Maryland, College Park, MD 20742\\
}
{\tt vojta@physik.tu-chemnitz.de}
\end{center}

\smallskip
\noindent
Metal-insulator transitions (MITs) remain a fascinating and only incompletely
understood phenomenon [1]. Conceptually, one distinguishes between
Anderson transitions in models of noninteracting electrons, and Mott-Hubbard
transitions of clean, interacting electrons. At the former, the
electronic charge diffusivity $D$ is driven
to zero by quenched, or frozen-in, disorder, while the thermodynamic
properties do not show critical behavior. At the latter,
the thermodynamic density susceptibility $\partial n/\partial\mu$
vanishes due to electron-electron interaction effects. In either case, the
conductivity $\sigma = (\partial n/\partial\mu)D$ vanishes at the MIT. In
many real systems both quenched disorder and interactions are present,
which makes a theoretical understanding of the resulting Anderson-Mott
transition very difficult.

One particular complication is provided by
the presence of magnetic local moments in such systems. There is much
experimental evidence for LMs [2], and their formation has been studied
theoretically [3], but no existing theory can describe their
interplay with the transport properties near the MIT.

Here we show that local magnetic moments in disordered systems can be
described in terms of {\it annealed} magnetic disorder, i.e. magnetic
disorder that is in thermal equilibrium with the rest of the system.
This annealed magnetic disorder is generically self-generated in
quenched disordered electronic systems.
We then develop a method for incorporating these `annealed local moments'
into a transport theory.

As an example we consider a model system of non-interacting electrons
with local magnetic moments. We show that the annealed magnetic disorder
leads to a new mechanism, and a new universality
class, for a metal-insulator transition. The transition is driven by
a vanishing of the thermodynamic density susceptibility rather than by
localization effects. The critical behavior in $d=2+\epsilon$ dimensions
is determined, and the underlying physics is discussed.

\bigskip
\noindent
[1] \parbox[t]{15.4cm}{For a review, see, e.g., D. Belitz and T.R.
Kirkpatrick, Rev. Mod.
 Phys. {\bf 66}, 261 (1994)}

\noindent
[2] \parbox[t]{15.4cm}{M.A. Paalanen, S. Sachdev, R.N. Bhatt, and A.E.
Ruckenstein, Phys.
 Rev. Lett. {\bf 57}, 2061 (1986); M.A. Paalanen, J.E. Graebner, R.N. Bhatt,
 and S. Sachdev, Phys. Rev. Lett. {\bf 61}, 597 (1988); Y. Ootuka and
 N. Matsunaga, J. Phys. Soc. Japan {\bf 59}, 1801 (1990)}

\noindent
[3] \parbox[t]{15.4cm}{M. Milovanovic, S. Sachdev, and R.N. Bhatt, Phys.
Rev. Lett.
 {\bf 63}, 82 (1989)}
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