Charge Ordering Phase Transition Research Articles

Columnar defect induced phase transformation in epitaxial La0.7Ca0.3MnO3 films

Epitaxial La0.7Ca0.3MnO3 thin films on the SrTiO3(100) surface have been irradiated with 250 MeV Ag17+ ions at different nominal fluence values in the range of 5×1010–4×1011 ions/cm2, resulting in columnar defects. At low fluences these defects cause changes in material properties that are small and scale linearly with dosage. Above a threshold fluence value ∼3×1011 ions/cm2 dramatic changes are observed, including an order of magnitude increase in the resistivity and 50 K drop in the Curie temperature. Transmission electron microscopy measurements show that the changes are associated with a phase transformation of the undamaged region between the columnar defects. The transformed phase has a diffraction pattern very similar to that seen in charge-ordered La0.5Ca0.5MnO3. We propose that above a critical level of ion damage, strains caused by the presence of the columnar defects induce a charge-ordering phase transition that causes the observed dramatic changes in physical properties. We speculate that a conceptually similar surface-induced charge ordering may be responsible for the “dead layer” observed in very thin strained films, and the dramatic changes in optical properties induced by polishing, and that an impurity-induced charge ordering causes the extreme sensitivity of properties to lattice substitution.

Optical Spectroscopy of Perovskite-Type Manganites

Review is given on late optical studies on colossal magnetoresistance (CMR) manganites with perovskite or related structures. In La1−xSr x MnO3 which undergoes a ferromagnetic transition, optical conductivity spectrum σ(ω) shows a change from a gap-like feature to metallic but highly diffussive one with increasing spin-polarization. Such a remarkable spectral weight transfer is also observed in a layered manganite, La2−2xSr1+2xMn2O7 (x = 0.4), with a mid-infrared peak structure due to inter-orbital \((d_{x^2 - y^2 } \to d_{3z^2 - r^2 } )\) transition. In Pr1−xCa x MnO3 (x = 0.4) which undergoes the charge ordering phase transition, σ(ω) shows an anisotropic feature reflecting charge/orbital ordering pattern at 10 K. The charge-ordered state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T at 30 K, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV–3 eV).

First evidence for charge ordering in NaV 2O 5 from Raman spectroscopy

We argue on the basis of symmetry selection rules and Raman scattering spectra that NaV 2O 5 undergoes a charge ordering phase transition at T c=34 K. Such a transition is characterized by the redistribution of the charges at the phase transition and corresponds to the change of the vanadium ions, from uniform V 4.5+ to two different V 4+ and V 5+ states. In the low temperature phase the V 4+ ions are forming a “zig-zag” ladder structure along the b-axis, consistent with the symmetry of the P2/b space group.

Infrared spectroscopy investigation of the charge ordering transition in LiMn 2O 4

Using infrared spectroscopy we have investigated LiMn 2O 4 through the charge ordering phase transition taking place around 290 K. We show that the phonon spectrum becomes much more structured below the transition temperature, confirming that the crystal structure is more distorted. The transition is reversible and both the cubic and the orthorhombic phases obtained at room temperature, depending on the thermal history of the sample, are stable. The evolution of the electronic gap confirms that charge carriers localise as temperature decreases down to 100 K, where the electronic properties freeze. This freezing process is not accompanied by structural changes.

Local lattice distortions and thermal transport in perovskite manganites

Measurements of thermal conductivity versus temperature and magnetic field are reported for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and/or structural phase transitions. The data reveal a dominant lattice contribution to the heat conductivity with $\ensuremath{\kappa}\ensuremath{\sim}1\ensuremath{-}2$ W/mK near room temperature. The rather low values, implying a phonon mean free path on the order of a lattice spacing, are shown to correlate with static local distortions of the ${\mathrm{MnO}}_{6}$ octahedra. Modifications of the local structure are responsible for abrupt anomalies in the zero-field \ensuremath{\kappa} at the FM, CO, and structural transitions, and for colossal magnetothermal resistance near the FM transition.