MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Electronic and Magnetic Measurements 33
varies exponentially with temperature and can be used to determine the sign
of the carrier and the carrier concentration.
3.1.3.2.2 Polarons
A localized electron will always distort its surroundings relative to an
unoccupied site simply because of the coulombic interaction of the electron
and the surrounding atoms. The potential well produced by this distortion
acts as a trapping center for the self-trapped carrier. The quasiparticle
composed of a self-trapped electronic carrier taken together with the pattern
of atomic displacements that produces the self-trapping became known as a
polaron because self-trapping was first considered in ionic (polar) materials.
The quasiparticle can move as a whole, the electron and the distortion
moving together.
The spatial extent of the self trapped states depends on the range of the
interaction. Long-range electron-lattice interactions [57] produces large
polarons with a finite radius. In such an instance, the self-trapped electronic
carrier extends over multiple sites. The radius of the large polarons decreases
continuously as the strength of the electron-lattice interaction is increased.
The multi-site extension of the large polaron results in its itinerant motion.
In contrast, small polarons are confined to a single lattice position due to
strong short-range electron-lattice interaction. If the electron-lattice
interaction is too weak, the carriers remain unbound. The extreme
confinement of the small polaron typically leads to its moving by thermally
assisted hopping.
One of these two different types of polarons will be stable when both long-
range and short-range electron-lattice interactions coexists. The presence of
long-range interactions eases the requirements for forming a small-polaron,
but only once the interactions are sufficiently strong will the polaron change
from large to small.