On the microstructure of the charge density wave observed in La_1−xCa_xMnO₃

Abstract

We have used low temperature (90 K) transmission electron microscopy to investigate the 'charge ordering' modulation in the mixed valent manganite, La 1Àx Ca x MnO 3 . It has been stated that Mn 3þ and Mn 4þ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5 mm in La 0.48 Ca 0.52 MnO 3 had a wavevector which varied on a scale of tens of nanometres with an average of hqi ¼ 0.450a* and a standard deviation Áq ¼ 0.004 a* in its magnitude and Á ¼ 0.56 in its direction at 90 K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90 K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30 K above the 'charge ordering temperature'.

Comment on “Pinning Frequencies of the Collective Modes inα-Uranium”

“…Owing to the large periodicity of the CDWs locking into the lattice at transition 3 (6b and 5:5c), it should be the transition most affected by disorder, explaining its disappearance in the polycrystal. This is consistent with observations of similar ordering in which the strain in a polycrystalline sample can prevent a lock-in in some grains [7]. Therefore the CDW transitions in both single crystal and polycrystalline -U can be modeled as DPTs, without erroneously invoking pinning modes of the CDWs.…”

Limited local electron-lattice coupling in manganites: An electron diffraction study

“…3͑c͔͒ measured using a Quantum Design superconducting quantum interference device. Below 150 K, ͑1͒ all but one value of is nonzero within error and the material is primarily antiferromagnetic, 9,17 and ͑2͒ q / a * Ϸ 0.46 varies little with temperature, but falls short of the expected 13 ͑1−x͒ = 0.48 perhaps due to pinning 21 ͓which would also produce the local variations in superlattice orientation seen in Figs. 2͑c͒ and 2͑e͔͒.…”

Investigation of the electronic structure of the charge-ordered phase in epitaxial and polycrystallineLa1−xCaxMnO3Schmidt1 2008Phys. Rev. B370901View full textAdd to dashboardCite

“…Electronically "soft" phases with weak electron-lattice coupling have been suggested 17 and a phenomenological Ginzburg-Landau theory was postulated 16 to describe these phases in a delocalised charge-density wave (CDW) picture. 22,23 At this point, it has to be stated that any CO pattern could be regarded as some kind of CDW. However, the term CDW is conventionally used only for systems which undergo a Peierls transition by cooling below the Peierls transition temperature T PE .…”