Polarons and confinement of electronic motion to two dimensions in a layered transition metal oxide

TL;DR

This paper investigates the electronic confinement in layered transition metal oxides, revealing that in manganites, polarons cause extreme confinement and pseudogap effects, hindering electron movement both between layers and into vacuum.

Contribution

The study provides the first atomic-resolution evidence that polarons in layered manganites cause significant electronic confinement and pseudogap phenomena, extending understanding beyond high-temperature superconductors.

Findings

Layered manganites exhibit even more extreme confinement than other TMOs.

Polarons are responsible for the pseudogap and confinement effects.

Electron removal from planes is as difficult as interlayer transfer due to polaron attachment.

Abstract

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.