  Of the perovskites, ABX_<3>, a subset of special interest is the family in
which the A site is occupied by a lanthanide ion, the B site by a rare earth
and X is oxygen, as such materials often exhibit a large change in electrical
resistance in a magnetic field, a phenomenon known as "colossal"
magnetoresistance (MR). Two additional phenomena in this family have also drawn
attention: the metal-insulator transition (MIT) and electroresistance (ER). The
MIT is revealed by measuring resistance as a function of temperature, and
observing a change in the sign of the gradient. ER - the dependence of the
resistance on applied current - is revealed by measuring resistance as a
function of applied current. Up until now, the phenomena of MIT and ER have
been treated separately. Here we report simultaneous observation of the MIT and
ER in the lanthanum/calcium manganites. We accomplish this by measuring
voltage-current curves over a wide temperature range (10-300 K) allowing us to
build up an experimental voltage surface over current-temperature axes. These
data directly lead to resistance surfaces. This approach provides additional
insight into the phenomena of electrical transport in the lanthanum/calcium
manganites, in particular the close connection of the maximum ER to the
occurrence of the MIT in those cases of a paramagnetic insulator (PMI) to
ferromagnetic metal (FMM) transition.
