  A very remarkable feature of the layered transition metal oxides (TMOs),
whose most famous members are the high-temperature superconductors (HTSs), is
that even though they are prepared as bulk three-dimensional single crystals,
they display hugely anisotropic electrical and optical properties, seeming to
be insulating perpendicular to the layers and metallic within them. This is the
phenomenon of confinement, a concept at odds with the conventional theory of
solids and recognized as due to magnetic and electron-lattice interactions in
the layers which must be overcome at a substantial energy cost if electrons are
to be transferred between layers. The associated energy gap or 'pseudogap' is
particularly obvious in experiments where charge is moved perpendicular to the
planes, most notably scanning tunneling microscopy (STM) and polarized infrared
spectroscopy. Here, using the same experimental tools, we show that there is a
second family of TMOs - the layered manganites La2-2xSr1+2xMn2O7 (LSMO) - with
even more extreme confinement and pseudogap effects. The data, which are the
first to resolve atoms in any metallic manganite, demonstrate quantitatively
that because they are attached to polarons - lattice and spin textures within
the planes -, it is equally difficult to remove carriers from the planes via
vacuum tunneling into a conventional metallic tip, as it is for them to move
between Mn-rich layers within the material itself.
