Very weak electron-phonon coupling and strong strain coupling in manganites

We describe transmission electron microscopy experiments that demonstrate the validity of the charge density wave (CDW) Landau theory in describing the so-called stripe phase of the manganites and that permit quantitative estimates of some of the theoretical parameters that describe this state. In polycrystalline Pr0.48Ca0.52MnO3 a lock-in to q/a(*)=0.5 in a sample with x>0.5 has been observed. Such a lock-in has been predicted as a key part of the Landau CDW theory of the stripe phase. Thus it is possible to constrain the size of the electron-phonon coupling in the CDW Landau theory to between 0.08% and 0.50% of the electron-electron coupling term. In the thin film samples, films of the same thickness grown on two different substrates exhibited different wave vectors. The different strains present in the films on the two substrates can be related to the wave vector observed via Landau theory. It is demonstrated that the elastic term, which favors an incommensurate modulation, has a similar size to the coupling between the strain and the wave vector, meaning that the coupling of strain to the superstructure is unexpectedly strong.