Charge-ordering Transition Research Articles

Positron lifetime study of Pr 1−xCa xMnO 3 ( x=0.5, 0.3) during magnetic transition

We measured the positron lifetime in perovskite manganites Pr 1−xCa xMnO 3 ( x=0.3, 0.5). Two lifetime components were observed for each compound; they were attributed to the annihilation of free positrons and positrons trapped at the A-site vacancies. The positron lifetime at the A-site vacancies changed significantly during the antiferromagnetic transition in both the compounds, whereas it was constant around the charge-ordering transition. This change indicates that the electron distribution at the vacancies changed possibly due to the change in the electron distribution of neighboring oxygen atoms. This result indicates that positron lifetime measurements can provide unique information on electronic states during a spin-related phase transition in various oxide materials.

Oxygen Isotope Effect in Cr- and Ru-Doped Pr<sub>0.5</sub>Ca<sub>0.5</sub>MnO<sub>3</sub> Manganites

The effect of 16О → 18О isotope substitution on the properties of Pr0.5Ca0.5MnO3 manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic phase. The 16О → 18О isotope substitution leads to the growth of the charge-ordering transition temperature (TCO), and to the lowering of ferromagnetic transition temperature (TFM) accompanied by a decrease in the content of ferromagnetic phase. The difference in the behavior of the Cr- and Ru-substituted samples is analyzed.

Magnetic ordering in the spinel compound Li[Mn2−xLix]O4(x=,0.04)

The two B-site ions Mn3+ and Mn4+ in the stoichiometric spinel structure LiMn2O4 form a complex, columnar ordered pattern below the charge-ordering transition at room temperature. On further cooling to below 66 K, the system develops long-range antiferromagnetic order. In contrast, whereas lithium-substituted Li[Mn2−xLix]O4 also undergoes a charge-ordering transition around room temperature, it only displays frozen in short-range magnetic order below ∼25–30 K. We investigate to what extent the columnar charge-order pattern observed in LiMn2O4 can account for the measured magnetic ordering patterns in both the pure and Li-substituted (x=0.04) compounds. We conclude that eightfold rings of Mn4+ ions form the main magnetic unit in both compounds (x=0,0.04), and that clusters formed out of these rings act as superspins in the doped compound.

Structure and charge ordering behavior of the colossal magnetoresistive manganite Nd0.5Sr0.5MnO3

The structure and charge ordering (CO) behavior of the three-dimensional colossal magnetoresistive manganite Nd0.5Sr0.5MnO3 have been studied by transmission electron microscopy. The electron diffraction analysis suggested its room temperature structure as orthorhombic, with a Pnma space group. By controlling the experimental temperature setting either above or below the CO transition temperature, it was observed that the CO grew through the propagation of the CO front. The twin boundaries were the barriers of the CO front movement. In heavily ion irradiated samples, it was observed that the CO front was pinned by the intracrystalline defects, forming a zigzag shape interface. Moreover, the electron beam was observed to influence the CO.

Thermodynamic studies at the charge-ordering and spin-Peierls transitions in

Abstract We report results of high-resolution thermal expansion measurements on the charge-ordering ( T CO ) and spin-Peierls ( T SP ) transition in ( TMTTF ) 2 X for X = AsF 6 and PF 6 . Based on distinct lattice effects observed at T CO in the c * -axis expansivity, a scheme is proposed which involves a charge modulation along the TMTTF stacks and its coupling to displacements of the anions. It is also the c * -axis expansivity which is most strongly affected by the transition at T SP . The combination of expansivity and specific heat results for the X = AsF 6 salt enables us to separate for both quantities the contribution associated with the spin-Peierls transition from the ordinary lattice effects and to provide a thermodynamic determination of the uniaxial-pressure dependence of T SP .

DC and high-frequency conductivity of the organic metals [formula omitted]-(BEDT-TTF) 2SF 5RSO 3 (R = CH 2CF 2 and CHF)

The temperature dependences of the electric-transport properties of the two-dimensional organic conductors β ″ -(BEDT-TTF) 2SF 5CH 2CF 2SO 3, β ″ -(d 8-BEDT-TTF) 2SF 5CH 2CF 2SO 3, and β ″ -(d 8-BEDT-TTF) 2SF 5CH 2CF 2SO 3 are measured by dc methods in and perpendicular to the highly conducting plane. Microwave measurements are performed at 24 and 33.5 GHz to probe the high-frequency behavior from room temperature down to 2 K. Superconductivity is observed in β ″ -(BEDT-TTF) 2SF 5CH 2CF 2SO 3 and its deuterated analogue. Although all the compounds remain metallic down to low-temperatures, they are close to a charge-order transition. This leads to deviations from a simple Drude behavior of the optical conductivity which become obvious already in the microwave range. In β ″ -(BEDT-TTF) 2SF 5CH 2CF 2SO 3, for instance, charge fluctuations cause an increase in microwave resistivity for T < 20 K which is not detected in dc measurements. β ″ -(BEDT-TTF) 2SF 5CHFSO 3 exhibits a simple metallic behavior at all frequencies. In the dc transport, however, we observe indications of localization in the perpendicular direction.

Effect of oxygen isotope substitution on charge ordering and magnetic and transport properties inPr0.5Ca0.5MnO3doped by chromium and ruthenium

The effect of the $^{16}\text{O}\ensuremath{\rightarrow}^{18}\text{O}$ isotope substitution on the properties of the ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_{3}$ manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic clusters. The $^{16}\text{O}\ensuremath{\rightarrow}^{18}\text{O}$ isotope substitution leads to an increase in the charge-ordering transition temperature $({T}_{\text{CO}})$ and to the lowering of ferromagnetic transition temperature $({T}_{\text{FM}})$ accompanied by a decrease in the content of ferromagnetic phase. The difference in the properties of the Cr- and Ru-doped compounds is analyzed.

Charge ordering correlated ultrasonic anomalies in La 1/3Sr 2/3Fe 1− xCr xO 3 perovskites

The charge ordering (CO) transition in polycrystalline La 1/3Sr 2/3Fe 1− x Cr x O 3 ( x = 0, 0.015, 0.025, 0.05) has been studied by measuring the low temperature magnetization, resistivity and longitudinal ultrasonic velocity ( V l). It is found that with increasing Cr content, the resistivity increases and the charge ordering transition temperature T CO shifts to lower temperature. For all samples, upon cooling down from 300 K, a substantial softening in V l above T CO and a dramatic stiffening below T CO are observed. This abnormal elastic softening above T CO can be described well by the mean-field theory, which indicates that the Jahn–Teller effect plays an important role for the charge ordering transition in La 1/3Sr 2/3Fe 1− x Cr x O 3. Below T CO, another softening in V l is observed for x = 0 sample, which is attributed to the breathing-type distortion of Fe–O octahedron. With Cr doping, this ultrasonic anomaly weakens. This behavior suggests that the charge disproportionation (CD) transition is suppressed by the Cr substitution.

Suppression of charge ordering transition by the reduction in particle size in the manganite Bi0.2Ca0.8MnO3

We have prepared a series of Bi0.2Ca0.8MnO3 samples with different particle sizes ranging from 60 to 1500 nm by the sol-gel method and systematically investigated their static and dynamic magnetic properties through dc magnetization, ac susceptibility, and electron spin resonance measurements. It was found that the reduction in particle size suppresses the charge ordering (CO) transition in this composition, resulting in the occurrence of the spin glass state in low temperatures. The observations have been interpreted in terms of martensitic strain and surface effect. Moreover a scenario has been posed for the waning process of the CO transition. With the reduction in the particle size, the CO phase transition survives only in the core regions until its complete disappearance, and connected with this, the magnetic cluster phenomenon becomes more pronounced.

Electron spin resonance across the charge-ordering transition inYBaMn2O6

We investigated the metal-ordered manganite system ${\text{YBaMn}}_{2}{\text{O}}_{6}$ using electron spin resonance (ESR) in the paramagnetic regime across the charge ordering and structural phase transition at ${T}_{\text{CO}}=480\text{ }\text{K}$ and ${T}_{t}=520\text{ }\text{K}$, respectively. All ESR parameters exhibit jumplike changes at ${T}_{t}$ while the charge ordering at ${T}_{\text{CO}}$ manifests itself only as a weak and broad anomaly. Above ${T}_{t}$ the ESR spin susceptibility is reduced with respect to the dc susceptibility, indicating that only the ${t}_{2g}$-core spins of Mn ions contribute to the resonance absorption. The contribution of the ${e}_{g}$ spins is suppressed by the time scale of the polaronic hopping process of the ${e}_{g}$ electrons. The linewidth in this regime is reminiscent of a Korringa-type relaxation behavior. In this picture the ESR properties below ${T}_{t}$ are dominated by the slowing down of the polaronic hopping process. The charge fluctuations persist down to the temperature ${T}^{\ensuremath{\ast}}\ensuremath{\approx}410\text{ }\text{K}$, below which the system can be described as a charge-ordered assembly of ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ spins.

Charge Order, Dynamics, and Magnetostructural Transition in MultiferroicLuFe2O4

We investigated the series of temperature and field-driven transitions in LuFe2O4 by optical and Mössbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low-temperature monoclinic distortion that can be driven by both temperature and magnetic field.

Specific heat in quasi-one-dimensional conductors K0.3MoO3 and (TMTTF)2AsF6

Abstract We measured the specific heat c p ( T ) of single crystals of quasi-one-dimensional conductors K 0.3 MoO 3 and (TMTTF) 2 AsF 6 by differential thermal analysis (DTA) method. In K 0.3 MoO 3 , a clear discontinuous change Δ c p 0 ∼1.2 J/mol/K was detected at the charge density wave (CDW) transition temperature T CDW =180 K. On the other hand, in (TMTTF) 2 AsF 6 , c p ( T ) showed a weak anomaly at the charge order (CO) transition temperature T CO =100 K. The entropy lost at around T CO was Δ S ∼0.24 J/mol/K at most, that is much smaller than R ln 2∼5.8 J/mol/K, which is the expected magnitude in the case of order-disorder transition. The CO of (TMTTF) 2 AsF 6 might be related to the displacive type ferroelectric.

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)2SbF6 under Constant Pressure

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)₂SbF₆ under Constant Pressure

13C NMR study of the chemical pressure effect in (TMTTF)2[(AsF6)x (SbF6)1-x] (x ∼ 0.5)

13C NMR measurements of (TMTTF)2[(AsF6)x (SbF6)1-x] (x ∼ 0.5) alloy were performed in order to understand the chemical pressure effect and the electronic state at low temperatures. The charge-ordering transition temperature is the intermediate value between those of SbF6 and AsF6 salts. The broadening of NMR spectra and the weak temperature dependence of the spin-lattice relaxation rate at low temperatures indicate that this salt is situated in the vicinity of the phase boundary between the spin-Peierls and antiferromagnetic phases.

Electron spin resonance study of spin correlations in charge-ordered La2−2xSr1+2xMn2O7(x=0.6)

Electron spin resonance (ESR) studies have been performed on a bilayered manganite La2−2xSr1+2xMn2O7(x=0.6) single crystal. The charge ordering (CO) and antiferromagnetic (AFM) transitions can be well identified from the ESR spectra in both the ab plane and c axis directions. A coexistence of paramagnetic resonance signal and low-field AFM resonance signal was observed below the CO temperature TCO. The analysis of the ESR parameters as a function of temperature suggests the existence of weak ferromagnetic (FM) correlations in the paramagnetic state and AFM correlations below TCO. The competition of FM and AFM correlations results in a broad peak in ESR intensity. Our results also show that the intensity of AFM correlations increases with decreasing temperature as the system enters from the CO state into the A-type AFM ground state. Therefore, the evolution and anisotropy of spin correlations can be fully mapped by ESR study.

Roughening Surface of Layered Manganite La0.5Sr1.5MnO4 by Scanning Tunneling Microscopy

We have investigated the layered manganite La0.5Sr1.5MnO4 surface at 100 K by scanning tunneling microscopy (STM). The surface roughness at 100 K below the charge and orbital order transition temperature (TCO = 220 K) is less than that at room temperature. By electron injection from the tip, STM measurements generate protrusions of about 2.5 nm diameter and 0.3 nm height in the scanned area on the surface. The induced protrusions at 100 K are aligned along the specific crystallographic directions, [100] or [010], whereas those at 300 K are randomly distributed.

The effect of strain and strain symmetry on the charge-order transition in Bi0.4Ca0.6MnO3 films

The transition to a charge and orbital ordered (CO/OO) state in epitaxial manganite films is strongly influenced by lattice strain. Bi1− x Ca x MnO3 is a particularly interesting material due to its high transition temperature and its relation to other Bi-based materials. Here, we review its properties and show the effects of strain and strain symmetry on Bi0.4Ca0.6MnO3 films on SrTiO3 (STO) and LaAlO3 substrates with (pseudocubic) (001) and (011) orientations. Transport and magnetization data are compared to four-circle X-ray diffraction and high-resolution Z-contrast scanning transmission microscopy data. We observe the spontaneous formation of single-unit-cell thick, Bi-rich layers only on (001) STO substrates and different defect structures depending on the substrate type and orientation. This shows that the details of epitaxial strain play a role not only at phase transitions, but also during the growth of these materials. Results are compared to those published for other CO/OO manganite films.

Superconductivity, charge order and magnetic transitions under high pressure in vanadium bronzes

The electric properties of β-vanadium bronze β-Na0.33V2O5 have been investigated under high pressure up to 13 GPa. The metal-insulator transition accompanied by charge order is suppressed by pressure, and the superconductivity is observed around 7 GPa and 8 K (Tc). The superconducting phase adjacent to the charge ordered phase suggests an important role of charge fluctuation for the superconductivity. With further increase of pressure beyond 12 GPa, the superconductivity disappears in relevance to the appearance of a nonsuperconducting new phase.

Influence of charge ordering on magnetocaloric properties of nanocrystalline Pr0.65(Ca0.7Sr0.3)0.35MnO3

Nanocrystalline Pr0.65(Ca0.7Sr0.3)MnO3 show large magnetocaloric effect at their charge order transition temperature (TCO) as well as at the temperature at which the spontaneous destabilization of charge ordered state occurs (TM). In comparison to their polycrystalline bulk form, TM’s are substantially enhanced in the cases of nanocrystalline samples, whereas their TCO’s remain almost unchanged. Although there is no clear signature of charge order transition in the temperature dependence of magnetic susceptibility and resistivity for the sample with the lower particle size, a clear maxima due to charge order transition is visible in its temperature dependence of change in magnetic entropy.

Direct evidence for the suppression of charge stripes in epitaxialLa1.67Sr0.33NiO4films

We have successfully grown epitaxial ${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ films with a small crystalline mosaic using pulsed laser deposition. With synchrotron radiation, the x-ray-diffraction peaks associated with charge stripes have been successfully observed for relatively thick films. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurements. X-ray scattering provides direct evidence for suppression of the stripe phase in thinner samples; the phase disappears for film thicknesses $\ensuremath{\leqslant}2600\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The suppression appears to be a result of shrinking the stripe phase domains. This may reflect the stripe phase progressing from nematic to isotropic.