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Charge Ordering in the Manganites

Front Cover of Nature Materials: 7(1) 2008

Charge density waves in the manganites

Certain manganite compounds such as La1-xCaxMnO3 exhibit superstructures which can be observed using TEM (transmission electron microscopy).

Superlattice observed with TEM

The superlattice is an unusual example of a charge density wave in a material with a high resistivity.

Indications of Weak Coupling from Simulations

Simulations of diffraction patterns for charge ordered system with very strong electron-phonon coupling give rise to peak shapes which are not observed. The simulated lattice reflections were either much too broad or contain extra reflections which are not observed.

The Frenkel-Kontorova model was then used to model various levels of coupling by modelling electron-electron interaction as springs and electron-lattice interaction as interaction with a sinusoidal potential:

The Frenkel-Kontorova Model starting state Start
The Frenkel-Kontorova Model end state End

Video of the model relaxing into a low temperature state.

The random starting state and frustrated end state causes variance in the wavenumber calculated from the simulation, and the variance increases as the coupling increases. Variance in the measured wave vector of real samples is caused by a number of factors, so the simulation puts an upper limit on the coupling in real systems. This upper limit indicates that the electron-phonon coupling is very weak.

Confirming the weak coupling by controlling the strain

The periodicity changes when strain in a sample is changed. This strongly indicates that the periodicity is not strongly coupled to the lattice because changing the strain changes the periodicity of the superlattice much more than the periodicity of the parent lattice.

This was tested by growing a thin film of manganite on an NGO substrate, creating a strained film due to the lattice mismatch. Then, some of the NGO was milled away to relieve the strain in a certain region.

The superlattice periodicity was then found at 18 locations, by accurately measuring the positions of most of the superlattice reflections in the TEM images, using some specially developed computer vision techniques.

Measuring the position of all of the superlattice reflections

All the superlattice measurements are shown in a false colour map overlaid on a TEM picture of the sample. Note that the black regions have been cut away to relieve the strain in the central region.

Superlattice periodicity varying with strain

Heat capacity consistent with CDW

Presence of CDW demonstrated by electrical measurements

History dependent resistivity

Hysteretic resistivity features are typical of CDWs in pinned and sliding states. As the sample is cooled, the CDW settles into a minimum-free-energy pinned configuration, corresponding to maximum electrical resistivity. On the application of a strong electric field, and the CDW starts to slide. As the field is reduced again, the CDW freezes into a distorted state, characterized by a lower resistivity; the initial, minimum energy state cannot be regained without thermally cycling the sample, explaining the hysteresis in our data.

History dependent resistivity

The hysteresis loop only occurs on the first cycling of the field. On subsequent cyclings without thermally resetting the sample, only the lower curve is followed.

Broadband noise

Broadband noise

Despite the small difference in resistivity (left), huge amounts of broadband 1/f noise are exhibited along the superlattice direction (centre), compared to across the superlattice (right). Note that the coloured surfaces represent noise equipotentials. The noise is very high with an effective temperature of 1011 K), and is strongly aligned with the direction of the superlattice. These characteristics are typical of a CDW system. Additionally, the onset of noise is correlated with a sharp drop in resistivity:

Noise and resistivity at 97K as the field varies
Red: Resistivity. Blue: Noise.

This shows that the most likely cause of the noise is due to the CDW sliding.

Random telegraph noise

Random Telegraph Noise
Red: parallel to q Blue: perpendicular to q

The sample also exhibits strong random telegraph noise in the direction of the superlattice. This is characteristic of a CDW.

Updated February 18th 2011, 08:06