Weak Charge Ordering Research Articles

Experimental Confirmation of Massive Dirac Fermions in Weak Charge-Ordering State in α-(BEDT-TTF)2I3

The electronic structure of weak charge-ordering (CO) state just below the critical pressure in an organic conductor α-(BEDT-TTF)2I3 was experimentally investigated using peak structure in the temperature dependence of interlayer magnetoresistance (MR). Based on a minimal model considering multiple Landau levels (LLs), we discuss herein the MR peak as characteristic to multilayer massless/massive Dirac fermion (DF) systems. MR measured in the weak CO state exhibited a clear MR peak, and its magnetic-field dependence was consistent with the LL behavior of a massive DF with a small gap. Results indicate that the weak CO state in α-(BEDT-TTF)2I3 is a massive DF state.

Possible Current-Induced Phenomena and Domain Control in an Organic Dirac Fermion System with Weak Charge Ordering

We show that when the electron and hole densities are unbalanced, observable nonlinear anomalous Hall effect and current-induced orbital magnetization appear at zero magnetic field in the weak charge ordering (CO) state of an organic two-dimensional Dirac fermion system, α-(BEDT-TTF)2I3. These current-induced phenomena are caused by a finite Berry curvature dipole resulting from inversion symmetry breaking and Dirac cone tilting. In the actual system, however, these effects are canceled out between different types of inversion asymmetric CO domains. To avoid the cancellation, we propose a new experimental method to realize the selective formation of a single type of domain using the current-induced magnetization.

X-ray magnetic diffraction under high pressure.

Advances in both non-resonant and resonant X-ray magnetic diffraction since the 1980s have provided researchers with a powerful tool for exploring the spin, orbital and ion degrees of freedom in magnetic solids, as well as parsing their interplay. Here, we discuss key issues for performing X-ray magnetic diffraction on single-crystal samples under high pressure (above 40 GPa) and at cryogenic temperatures (4 K). We present case studies of both non-resonant and resonant X-ray magnetic diffraction under pressure for a spin-flip transition in an incommensurate spin-density-wave material and a continuous quantum phase transition of a commensurate all-in-all-out antiferromagnet. Both cases use diamond-anvil-cell technologies at third-generation synchrotron radiation sources. In addition to the exploration of the athermal emergence and evolution of antiferromagnetism discussed here, these techniques can be applied to the study of the pressure evolution of weak charge order such as charge-density waves, antiferro-type orbital order, the charge anisotropic tensor susceptibility and charge superlattices associated with either primary spin order or softened phonons.

Bond patterns and charge-order amplitude in quarter-filled charge-transfer solids

Metal-insulator transition accompanied by charge-ordering has been widely investigated in quasi-one-dimensional conductors, including in particular organic charge-transfer solids. Among such materials the 1/4-filled band charge-transfer solids are of strong interest, because of the commensurate nature of the charge-ordering in these systems. The period-four charge-order pattern ...1100... here is accompanied by two distinct bond distortion patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2. Using quantum Monte Carlo methods, we determine the phase diagram within the extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the thermodynamic limit. We further investigate the effect of electron-electron and electron-phonon interactions on the amount of charge disproportionation. Our results show that between these two bond patterns, one (BCDW2) in general coexists with a large magnitude charge order, which is highly sensitive to electron-phonon interactions, while the other (BCDW1) is characterized by weak charge order. We discuss the relevance of our work to experiments on several 1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and (DMEDO-TTF)_2X with large amplitude charge-order.

Magnetic reversal of an artificial square ice: dipolar correlation and charge ordering

Magnetic reversal of an artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal axis is investigated via magnetic force microscopy of the remanent states that result. Sublattice independent reversal is observed via correlated incrementally pinned flip cascades along parallel dipolar chains, as evident from analysis of vertex populations and dipolar correlation functions. Weak dipolar interactions between adjacent chains favour antialignment and give rise to weak charge ordering of ‘monopole’ vertices during the reversal process.

Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries ${X}_{3}$ and ${\ensuremath{\Delta}}_{5}$ induce the structural transformation from the high-temperature cubic to the low-temperature monoclinic phase. The coupling between the order parameters is described by the Landau free energy functional. The electronic and crystal structure for the cubic and monoclinic phases were optimized using the ab initio density functional method. The electronic structure calculations were performed within the generalized gradient approximation including the on-site interactions between $3d$ electrons at iron ions---the Coulomb element $U$ and Hund's exchange $J$. Only when these local interactions are taken into account, the phonon dispersion curves, obtained by the direct method for the cubic phase, reproduce the experimental data. It is shown that the interplay of local electron interactions and the coupling to the lattice drives the phonon order parameters and is responsible for the opening of the gap at the Fermi energy. Thus, it is found that the metal-insulator transition in magnetite is promoted by local electron interactions, which significantly amplify the electron-phonon interaction and stabilize weak charge order coexisting with orbital order of the occupied ${t}_{2g}$ states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.

Weak charge-ordering behavior in phase separated (Ln 1/3Sm 2/3) 2/3Ca 1/3MnO 3 (Ln=Pr and La) manganites

We have studied a weak charge-ordering behavior in phase separated (Ln 1/3Sm 2/3) 2/3Ca 1/3MnO 3 (Ln=Pr and La) manganites with same hole doping but slightly different tolerance factors. The M ( T ) and ρ ( T ) curves of the Pr-sample showed an anomaly caused by a weak charge-order (CO) transition at ∼210 K, while that of La-sample did not. However, the La-sample displayed a weak anomaly related to the CO instability in the derivative curve of the temperature dependent magnetoresistance. This result is understood on the basis that the CO phase in La-sample is partly suppressed by a large magnetic field of ∼80 kOe, while in Pr-sample it is not collapsed in that field scale. Our data support the existence of charge-ordering, electronic phase separation and a field-induced reduction of weak CO instability for La-sample, which can contribute towards high MR especially at lower temperatures due to partial melting of CO phase and/or percolation behavior.

Verwey transition inFe3O4investigated usingLDA+DMFT

Motivated by recent structural data questioning the adequacy of the charge order (CO) or disorder picture for the Verwey transition (at $T={T}_{V}$) in magnetite, we reinvestigate this issue within a new theoretical picture. Using the local density $\text{approximation}+\text{dynamical}$ mean-field theory ($\mathrm{LDA}+\mathrm{DMFT}$) method, we show that the nontrivial interplay between octahedral distortions and strong, multiorbital electronic correlations in the half-metallic state is a necessary ingredient for a proper quantitative understanding of the physical responses across ${T}_{V}$. While weak CO is found to have very small effects on the low-$T$ spectral function, the low-$T$ charge gap and the resistivity jump across ${T}_{V}$ are quantitatively reproduced only upon inclusion of CO in the local spin density $\text{approximation}+\mathrm{DMFT}$ scheme. Our results strongly suggest that the Verwey transition is dominantly driven by multiorbital electronic correlations with associated Jahn-Teller distortions on the $B$ sublattice, and constitutes a nontrivial advance in attempts to understand the physics of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$.

Low-current-induced electrical hysteresis in Nd0.7Ca0.3MnO3

The electrical characteristics of Nd0.7Ca0.3MnO3 were investigated by measuring voltage versus current (V-I) at various temperatures. The V-I curves of the Nd0.7Ca0.3MnO3 sample change from linear to nonlinear behavior at temperatures below the charge ordering (CO) temperature (TCO∼200K). An unexpected and reproducible electrical hysteresis loop is observed in the linear region below the Curie temperature (TC∼110K); it is associated with the “tearing” of a two-domain structure of weak and strong CO state. The required current for this hysteresis is of the order of microamperes, which is appropriate for the low-current memory devices.

High energy x-ray scattering studies of strongly correlated oxides

Many transition metal oxides display strongly correlated charge, spin, or orbital ordering resulting in varied phenomena such as colossal magnetoresistance, high temperature superconductivity, metal–insulator transitions etc. X-ray scattering is one of the principle techniques for probing the structural response to such effects. In this paper, we discuss and review the use of synchrotron radiation high energy x-rays (50–200 keV) for the study of transition metal oxides such as nickelates (La2−xSrxNiO4) and manganites (La2−2xSr1+2xMn2O7). High energy x-rays have sufficient penetration to allow us to study large flux-grown single crystals. The huge increase in sample scattering volume means that extremely weak peaks can be observed. This allows us to study very weak charge ordering. Measurements of the intensity, width and position of the charge ordering satellites as a function of temperature provide us with quantitative measures of the charge amplitude, inverse correlation length and wavevector of the charge ordering.

Novel diffractometer optimized for the study of weak superlattice reflections using crossed parabolic mirrors

We present the development of a novel laboratory-based diffractometer optimized for high intensity and resolution matched to that of flux grown single crystal transition metal–oxide samples studied in solid state physics. This has been implemented using crossed graded d-spacing parabolic multilayer mirrors, a severely off-cut asymmetric analyzer crystal, and a microminiature cryostat. We demonstrate that the wider bandpass of the multilayer mirrors provides a significant increase in intensity compared to the use of silicon and germanium optics, but still provides the necessary resolution to obtain accurate measurements for inverse correlation lengths. The increase in flux allows the observation of features that were previously only visible with third generation synchrotron x-ray sources, shown by the observation of the very weak charge order peak without the use of a synchrotron source. Results on samples previously studied show an increase of a factor of 10 in intensity, coupled together with a factor of 7.5 increase in resolution over the previous system employed using a rotating anode source and flat pyrolytic graphite (0001) crystals. With such a diffractometer it is now possible to carry out detailed studies of charge ordering in transition metal oxides in the laboratory and the benefits of this are threefold. First, this will improve the quality of any preparation work, for subsequent experiments at synchrotron sources. This has the advantage of maximizing the efficiency of synchrotron measurements. Second, the enhanced intensity and resolution will allow experiments to be conducted in the laboratory which previously required access to synchrotron sources. Third, it will allow for synchrotron time to be used for its unique properties, i.e., wavelength tunability and polarization, which are necessary for the observation of magnetic reflections and orbital ordering by means of resonant enhancements and polarization analysis. This means that laboratory based measurements can be easily incorporated with synchrotron data, maximizing the potential of both techniques and receiving a net gain in data output.