Charge-ordered Phase Research Articles

Mean-Field Study of Charge, Spin, and Orbital Orderings in Triangular-Lattice CompoundsANiO2(A= Na, Li, Ag)

We present our theoretical results on the ground states in layered triangular-lattice compounds ANiO2 (A=Na, Li, Ag). To describe the interplay between charge, spin, orbital, and lattice degrees of freedom in these materials, we study a doubly-degenerate Hubbard model with electron-phonon couplings by the Hartree-Fock approximation combined with the adiabatic approximation. In a weakly-correlated region, we find a metallic state accompanied by \sqroot3x\sqroot3 charge ordering. On the other hand, we obtain an insulating phase with spin-ferro and orbital-ferro ordering in a wide range from intermediate to strong correlation. These phases share many characteristics with the low-temperature states of AgNiO2 and NaNiO2, respectively. The charge-ordered metallic phase is stabilized by a compromise between Coulomb repulsions and effective attractive interactions originating from the breathing-type electronphonon coupling as well as the Hund's-rule coupling. The spin-orbital-ordered insulating phase is stabilized by the cooperative effect of electron correlations and the Jahn-Teller coupling, while the Hund's-rule coupling also plays a role in the competition with other orbital-ordered phases. The results suggest a unified way of understanding a variety of low-temperature phases in ANiO2. We also discuss a keen competition among different spin-orbital-ordered phases in relation to a puzzling behavior observed in LiNiO2.

The origin of different magnetic properties in nanosized Ca0.82La0.18MnO3: Wires versus particles

A comparative investigation has been conducted on the nanowires and nanoparticles (both in the size range of 20–35 nm) of Ca0.82La0.18MnO3, finding that the nanowires have similar magnetic properties to the bulk, whereas the nanoparticles behave evidently different. In the nanoparticles, charge ordering and antiferromagnetic phase disappear; instead, a ferromagnetic transition is observed. Analysis of the crystal structure indicates that, for nanosized manganite systems, whether charge ordering is suppressed and ferromagnetism is developed as the size scales down depends on the level of intrinsic structural distortions; nanodimensional effect or surface effect is not determinant.

Interrelationship between Li+diffusion, charge, and magnetism inLi7Mn2O4andLi71.1Mn1.9O4spinels: Elastic, inelastic, and quasielastic neutron scattering

Using quasielastic neutron scattering (QENS), we have investigated a self-diffusive behavior of Li${}^{+}$ ions for both $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$ and $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ spinels. In addition, we have carried out elastic and inelastic neutron scattering measurements using the same samples, to study the interrelationship between Li${}^{+}$ self-diffusion, magnetism, and charge distribution in the lattice. From the QENS results, the self-diffusion of Li${}^{+}$ was observable above 280 K, and a self-diffusion coefficient (${D}_{s}^{\mathrm{Li}}$) for $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$ was estimated as $~{10}^{\ensuremath{-}8}$ cm${}^{2}$/s at 400 K. ${D}_{s}^{\mathrm{Li}}$ for $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ was comparable to that for $^{7}\mathrm{Li}$Mn${}_{2}$O${}_{4}$. Furthermore, combining with the results of elastic and inelastic measurements, it was found that $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$ undergoes a transition from a low-temperature ($T$) short-range charge-ordered (SRCO) phase to a high-$T$ charge-disordered (CDO) phase at 280 K. The structure of the SRCO was determined as a hexagon, because the formation of hexagon spin clusters was deduced from a magnetic diffuse scattering at low $T$. Assuming the presence of the SRCO-CDO transition at 280 K, both the anomaly of the diffusive behavior at 280 K and the local lattice distortion below 280 K are reasonably explained, despite the absence of long-range CO for $^{7}\mathrm{Li}$${}_{1.1}$Mn${}_{1.9}$O${}_{4}$.

Effective Hamiltonians for Unconventional Charge Ordering in Pr-Based Filled Skutterudite Compounds

The origin of unconventional charge ordering in Pr-based filled skutterudite compounds are studied theoretically. We derive an effective interaction by analyzing the hybridization between the main p band composed of pnictogen molecular orbitals and the Pr 4 f 2 states, and show that it depends strongly on the occupancy of the f orbitals. Together with the nesting instability of the conduction band, this interaction naturally leads to a predominant charge order with modulated f -electron crystal field levels. It is also shown that the magnetic interaction among residual triplets in the charge order phase is very small. These characteristics account well for the available experimental results of PrRu 4 P 12 . In addition, we discuss briefly a charge order in PrFe 4 P 12 , focusing on the differences from that of PrRu 4 P 12 . The multiband properties in PrFe 4 P 12 and a possible interpretation on recent pressure experiments are discussed.

Suppressed charge ordering and cluster glass behavior of Pr0.75Na0.25Mn0.95Fe0.05O3

The cusps of charge ordering and the long range AFM order of the "pseudo"-CE type are suppressed with increasing Fe concentration in Pr0.75Na0.25Mn1−xFexO3(0≤x≤0.30). Ac susceptibility at low temperatures of Pr0.75Na0.25Mn0.95Fe0.05O3 was investigated. The real component (χ') shows that freezing temperature Tf is linear in the logarithm of the frequency. The normalized slope P = ΔTf /TfΔlogl0ω, which is much lower than canonical insulating spin glass systems in which 0.06≤P≤0.08. The χ' peak at Tf are suppressed with the increasing frequency. The intensity of imaginary component (χ") at Tf and spin blocking temperature TB∼10 K shows a complicated variation trend with increasing frequency. The ground state of the sample is cluster glass with a coexistence of charge-ordered phase, correlated ferromagnetic clusters in spin glass matrix.

Quantum Phase Transition and Emergent Symmetry in a Quadruple Quantum Dot System

We propose a system of four quantum dots designed to study the competition between three types of interactions: Heisenberg, Kondo, and Ising. We find a rich phase diagram containing two sharp features: a quantum phase transition (QPT) between charge-ordered and charge-liquid phases and a dramatic resonance in the charge liquid visible in the conductance. The QPT is of the Kosterlitz-Thouless type with a discontinuous jump in the conductance at the transition. We connect the resonance phenomenon with the degeneracy of three levels in the isolated quadruple dot and argue that this leads to a Kondo-like emergent symmetry from left-right Z2 to U(1).

Competing two-phase coexistence in doped manganites: Direct observations byin situLorentz electron microscopy

We examined thin epitaxial films ${\text{La}}_{5/8\ensuremath{-}y}{\text{Pr}}_{y}{\text{Ca}}_{3/8}{\text{MnO}}_{3}$ $(\text{LPCMO}:y=0.275\ensuremath{-}0.3)$ in situ by Lorentz transmission electron microscopy (TEM) and other microscopy methods. Clear evidence was obtained for the competing two-phase coexistence of antiferromagnetic charge-ordered (CO) and ferromagnetic (FM) phases that exhibit mesoscale phase separation below the metal-to-insulator transition (MIT) at $\ensuremath{\sim}164\text{ }\text{K}$. In addition, we observed some regions of mixed CO- and FM-domain contrast attributed earlier to formation of the new CO-FM phase. Using in situ heating/cooling TEM experiments, we interpret this effect as the interfacial wetting phenomenon inherent to the first-order CO-FM phase transition, rather than to the formation of new CO-FM phase. It is evidenced by the partial magnetic melting of CO phase at interfaces with the FM phase, thereby creating charge-disordered spin-glass metastates. For coexisting CO- and FM-domain configurations, we directly refined the relationship between charge-orbital and spin-ordering vectors, consistent with FM moments pinned by (101)-crystal twins in LPCMO films. We also discuss the striking linear dependence observed below MIT for the log-resistance behavior and the CO fraction in LPCMO directly measured by TEM. Such linear dependence does not follow typical percolation equations, suggesting that percolation model needs further revisions for transport description of manganites.

Ferromagnetic and Charge-Ordered Phases in (Nd, Na)–Mn–O Compounds

Polycrystalline Nd1−xNaxMnO3 (\(x=0.05\mbox{--} 0.20\)) compounds were prepared in single-phase form with a Pbnm space group. Paramagnetic to ferromagnetic transitions were observed up to a doping concentration of 15% with a maximum TC of 113 K. The x=0.20 sample exhibits a charge order transition at 180 K, with a distinct behavior in magnetization versus field curve. The magnetization versus temperature plot of all these materials exhibits an anomaly at around 40 K, with a signature of spin-glass like behavior. The x=0.10 sample is found to exhibit a maximum spontaneous magnetization value of 3.1μB at 20 K. The magnetization data could be analyzed based on the Brillouin function model, by accounting for the ferromagnetic interaction. The effective spin contribution of 3d electrons for the double-exchange ferromagnetic interaction and the value of magnetic spin-canting angles were estimated. The measured magnetization is explained on the basis of competing magnetic interaction due to spin-canting. The charge order quenching and the first-order transition from the charge-ordered to ferromagnetic phase are observed for a threshold field of 3 T at 12 K. The electrical resistivity of the above samples exhibits insulating behavior.

A-site disorder effects in electron-doped manganite La0.4Ca0.6MnO3

The A-site disorder effects on the stability of charge-ordered state in electron-doped manganite R0.4A0.6- MnO3 (where R = La, Pr, Nd, Sm, and A= Ca, Sr, Ba) are investigated by detailed measurements of transport, mag- netism, and specific heat. The robust charge-ordered phase within the prototype sample can be successively suppressed by enhancing the A-site disorder, and finally an atomic-scale spin-glass state is evidenced in the highest disordered sam- ple. However, no ferromagnetic phase can be observed; even the long-range charge-ordered antiferromagnetic phase is fully suppressed, which can be associated with the insuffi- cient eg electron density in such an electron-doped mangan- ite. This revealing is helpful for understanding the origin of ferromagnetic phase in nano-sized or B-site doped charge- ordered manganites with small eg electron density.

Ultrafast near-infrared spectroscopic study of coherent phonons in the phase-separated manganiteLa1/4Pr3/8Ca3/8MnO3

We report the generation of coherent optical and acoustic phonons in mixed valent manganite ${\text{La}}_{1/4}{\text{Pr}}_{3/8}{\text{Ca}}_{3/8}{\text{MnO}}_{3}$ using femtosecond infrared pump-probe spectroscopy. Temperature-dependent measurements of the time-resolved optical reflectance, obtained over a range of 5--300 K, revealed that the energy of the photoexcited electrons dissipated during relaxation to acoustic phonons, in the high-temperature paramagnetic phase, and to optical phonons, in the low-temperature charge-ordering phase. Analysis of the temperature-dependent behavior reveals that the modal amplitudes of the coherent phonons appear strongly correlated with the charge-ordering phase.

Abnormal magnetization and field-induced transition in (La 0.73Bi 0.27) 0.67Ca 0.33MnO 3

The magnetic field dependence of magnetization of Bi doped manganites (La 1− x Bi x ) 0.67Ca 0.33MnO 3 ( x=0.27) was investigated at different temperatures with a pulsed high magnetic field. A metamagnetic transition was observed in the magnetization measurement, which revealed the coexistence of charge ordering (CO) and ferromagnetic (FM) phases. With decreasing magnetic field, the field-induced FM phases remained stable even when the magnetic field decreased to zero. This result suggests that ferromagnetic interactions are enhanced due to the effect of the pulsed high magnetic field, which makes the doped manganites a good system for magnetoresistance materials.

Experimental evidence of coupling interaction between charge ordering and spin ordering

In order to experimentally probe into the complicated interaction between charge ordering and spin ordering in manganites, two sets of samples Nd0.5Sr0.5Mn1−xGaxO3 (NSMGO) and Nd0.5Sr0.5Mn1−yCryO3 (NSMCO) (0.0 ⩽ x, y ⩽ 0.075), have been studied by means of electrical transport, magnetization, electron spin resonance and transmission electron microscopy analysis. By comparing the influence of Cr-doping and Ga-doping in the Nd0.5Sr0.5MnO3 (NSMO) system, large difference between the evolution of charge ordering temperature Tco in the Cr-doping and the Ga-doping cases is found. In the NSMCO system, the CE-type antiferromagnetic (AFM)/charge ordering (CO) phase disappears with only 2.5 percent Cr doping; but in the NSMGO system, the CE-type AFM/CO phase always exists. This phenomenon indicates that the charge ordering formation is dominated by the spin ordering. As a result, it is experimentally proved that there is strong coupling interaction between charge ordering and spin ordering in NSMO system.

Collective Excitations in the Charge-Ordered Phase ofα−(BEDT−TTF)2I3

The charge response of a charge-ordered state in the organic conductor α-(BEDT-TTF)2I3 is characterized by dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the two-dimensional conduction layer. Two dielectric modes are detected. The large mode is related to the phasonlike excitation of the 2k(F) bond-charge density wave which forms in the ab plane. The small dielectric mode is associated with the motion of domain-wall pairs along the a and b axes between two types of domains which are created due to inversion symmetry breaking.

Nature of the low-energy excitations of a charge-ordered phase of La0.25Ca0.75MnO3 manganites

The nature of the low-energy excitations of polycrystalline and nanostructured La0.25Ca0.75MnO3 samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that the electrodynamic response spectra of La0.25Ca0.75MnO3 in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies of 20–40 and 80–100 cm–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem.

Electronic ferroelectricity, charge ordering and structural phase transitions in LuFe2O4(LuFeO3)n (n = 0 and 1)

AbstractThe nature of the charge order, multiferroic features, and structural phase transitions in LuFe2O4(LuFeO3)n (n = 0 and 1) materials have been extensively investigated. In situ transmission electron microscopy (TEM) investigations on LuFe2O4 (n = 0) show the presence of well‐defined charge‐ordered states at low temperatures that could give rise to a notable ferroelectric polarization. Measurements of the structural and physical properties on LuFe2O4(LuFeO3)n (n = 1) demonstrate that the intercalation of the LuFeO3‐block could lead to visible changes in the dielectric properties and the charge ordering (CO) features. Measurements of ferroelectric properties suggest that Mn substitution for Fe in Lu2Fe2Fe1–xMnxO7 (0 < x ≤ 0.86) can notably suppress the conducting leakage current and results in clear hysteresis loops in the polarization–electric field switching curves.

Current-induced nonlinear conduction of two-electron doped manganites Ca1−xCexMnO3

We have investigated the current-induced nonlinear conduction in a charge-ordered phase of two-electron doped manganites Ca1−xCexMnO3 (x = 0.1 and 0.167). The substitution of Ce4+ ion for Ca2+site of the parent matrix causes two-electron doping with the chemical formula . Seebeck coefficient data of Ca0.9Ce0.1MnO3 are, in both its temperature dependence and its magnitude, very similar to those of Ca0.8Sm0.2MnO3, leading to further evidence for two-electron doping. The VI characteristics measured using a long pulsed current show a negative differential resistance and its associated giant electroresistance effect. It is demonstrated that a temperature rise across the samples due to Joule heating is not responsible for a huge decrease in resistance observed here.

Correlation function for the one-dimensional extended Hubbard model at quarter filling

We examine the density-density correlation function in the Tomonaga-Luttinger liquid state for the one-dimensional extended Hubbard model with the on-site Coulomb repulsion $U$ and the intersite repulsion $V$ at quarter filling. By taking into account the effect of the marginally irrelevant umklapp scattering operator by utilizing the renormalization-group technique based on the bosonization method, we obtain the generalized analytical form of the correlation function. We show that, in the proximity to the gapped charge-ordered phase, the correlation function exhibits anomalous crossover between the pure power-law behavior and the power-law behavior with logarithmic corrections, depending on the length scale. Such a crossover is also confirmed by the highly-accurate numerical density-matrix renormalization group method.

Order–disorder type of charge-ordering phase transition in narrow-bandwidth compound, α′-(BEDT-TTF) 2IBr 2

The kinetic energy of α′-(BEDT-TTF) 2IBr 2, α-(BEDT-TTF) 2I 3, and α-(BEDT-TTF) 2NH 4Hg(SCN) 4 was estimated from the room-temperature optical conductivity measured from 50 to 30 000 cm −1. The kinetic energy of α′-(BEDT-TTF) 2IBr 2 was significantly smaller than that of α-(BEDT-TTF) 2I 3 that shows metal–insulator phase transition. A clear hysteresis was found in the magnetic susceptibility near the phase transition at around 30 K.

Effective Connection of Phase-Separated Metallic Pathways under Low Magnetic Fields in Charge-Ordered Insulators of Micropatterned Perovskite Manganite Thin Films

We investigated the probe size dependence of the electrical transport properties of perovskite manganite La 0.67 Ca 0.33 MnO 3 (LCMO) films with electronic phase separation (EPS) between ferromagnetic metal and charge-ordered insulator phases. A micropatterned wire was fabricated by an excimer-laser-assisted metal organic deposition process. A patterned wire of an LCMO film with a width of 10 µm had a higher insulator–metal transition temperature associated with the ferromagnetic transition than that of an LCMO film with a large probe size. Moreover, a low-magnetic-field magnetoresistance (LFMR) effect was observed for the micropatterned wire film; the resistivity decreased only from H ⊥ c > 0.02 T. The origin of this LFMR effect is thought to be the effective connections of the ferromagnetic metal domains between the probe contacts with small dimensions similar to the size of the phase-separated metal and insulator domains. These properties were qualitatively explained by numerical simulations of resista...

Study of Dynamics in Inhomogeneous States by Temperature-Modulated Optical Reflectivity Measurement: Application to Perovskite Manganites

We developed an experimental system for measuring temperature-modulated optical reflectivity with a frequency up to 20 kHz to study the dynamics of inhomogeneity in a crystal. By this system, we studied doped perovskite manganites, in which a ferromagnetic metallic phase and a (short-ranged) charge-ordered phase coexist near the Curie temperature ( T C ). We found an appreciable frequency dependence of the temperature-modulated reflectivity around T C . This result can be explained by the dynamics of the domains in the two coexisting phases, i.e., by the relaxation time to reach a new equilibrium state of the two-phase coexistence when the temperature is modulated.