Charge-ordering Transition Research Articles

Hydrothermal synthesis, magnetism and resistivity of orthorhombic perovskite manganates Y1−xCaxMnO3 (x = 0, 0.07, 0.55, 0.65)

Orthorhombic Y1−xCaxMnO3 (x=0, 0.07, 0.55, 0.65) crystals were successfully prepared in one step via the mild hydrothermal synthesis, and the magnetic and electric properties were investigated in detail. The formation and composition of Y1−xCaxMnO3 crystals mainly depend on the molar ratios of starting materials, reaction temperature and alkalinity of the reaction system. The magnetic properties of Y1−xCaxMnO3 strongly depend on the doping content of Ca. Y1−xCaxMnO3 (x=0, 0.07 and 0.55) demonstrate the spin-glass-like behaviors. Y1−xCaxMnO3 (x=0.55 and 0.65) show charge ordering and antiferromagnetic transition, and the antiferromagnetic state is stabilized by charge ordering. The signal of charge ordering transition of Y1−xCaxMnO3 (x=0.65) is very strong and its magnetization increases significantly below 46K due to the Dzyaloshinsky–Moriya (DM) anisotropic super-exchange interaction. The charge transport of samples that have charge ordering takes on the behavior of semiconductors. They show a small magnetoresistance effect. Their electric conduction mechanism accords with the variable-range hopping mode of highly correlated electrons.

Anomalous Magnetic Properties of the Bilayered LaSr2Mn2−z Co z O7 (z=0–0.15) Manganite

The Co-doped bilayered LaSr2Mn2O7 manganite at low Co concentrations (0–0.15) was synthesized by the sol–gel process. The X-ray diffraction (XRD) technique confirms phase formation for all the samples under investigation. The results of ac magnetic susceptibility measurements indicate the effect of Co doping on the magnetic ordering phases. The indications of charge ordering (CO) transitions were observed in all the prepared compounds. The CO magnetic phase transition temperature was observed systematically shift to lower temperatures as the Co concentration increases. There was an anomalous oscillating magnetic behavior in all samples with a few peaks before the CO temperature in the paramagnetic (PM) region so that with an increasing Co doping, the number of peaks and amplitude were decreased. Also, the ac susceptibility measurements were performed in the presence of an applied dc magnetic field to further study of this oscillating behavior.

Phase precipitation in the Bi1−xCaxMnO3 compounds (x = 0.4–0.6)

Charge ordering (CO) in Bi1−xCaxMnO3 (x = 0.4–0.6) has been investigated by temperature-dependent high-resolution neutron powder diffraction (NPD), X-ray powder diffraction (XRD) and physical property measurements. Anomalies in magnetization, resistivity and lattice parameters have been observed around the CO transition temperature (TCO) reported in the literature. However, a scrutiny of the NPD and XRD data at different temperatures and the electron diffraction experiments at 300 K give no evidence of a long-range CO below the reported TCO. Instead, NPD and XRD reveal a coexistence of two orthorhombic phases in the samples below the reported TCO. All the experiments suggest that one of the coexisting phases is originated from a precipitation or an incomplete structural transition of the other phase and the reported TCO corresponds to the precipitation reaction or structural transition temperature, rather than a CO transition temperature.

Electron-spin-resonance of Gd3+ ions in Y1−xGdxBaCo2O5 (x = 0.01, 0.25, and 1)

While charge-ordering transition is well established in the oxygen-deficient layered cobaltite YBaCo2O5, there has been no evidence for the transition in the analogous compound GdBaCo2O5 either from neutron diffraction or from magnetic susceptibility experiments. In this paper, we present comparative studies on the electron spin resonance (ESR) of Gd3+ ions in Y1−xGdxBaCo2O5 (x = 0.01, 0.25, and 1) polycrystalline samples. Magnetic susceptibility was also performed for comparison with the ESR results. It was found that charge-ordering transition for Y0.75Gd0.25BaCo2O5 revealed by the susceptibility is also manifested in the characters of the ESR data. The similar temperature-evolution of the ESR parameters for GdBaCo2O5 provides a support for the existence of charge-ordering transition, although no characteristic of the transition is evidenced from susceptibility experiments, which can be understood as being hidden by dominant paramagnetism of Gd3+ ions in this compound. We have interpreted the temperature variation of ESR parameters in terms of electron localization associated with Co2+/Co3+ charge ordering.

Effect of substrate orientation on charge ordering behavior in Sm0.5Ca0.5MnO3 epitaxial films

The effect of anisotropic strain induced by SrTiO3 (STO) substrate with different orientations on charge ordering (CO) behavior in epitaxial Sm0.5Ca0.5MnO3 (SCMO) films was investigated using cross-sectional transmission electron microscopy at 103K. Incommensurate modulated structure caused by CO transition was observed in the SCMO film grown on a [101]-oriented STO substrate while none were found for SCMO film grown on a [100]-oriented STO substrate. This distinctive orientation-dependent CO transition is correlated with anisotropic strain in the films induced by lattice mismatch, confirming the previous results for growth on different substrates.

Collapse of Charge Ordering Due to Disorder in Quasi One-Dimensional Electron Systems

Effects of disorder on charge ordering (CO) is investigated for one-dimensional (1D) and quasi 1D extended Hubbard model with infinite on-site repulsion at quarter filling. Only the forward scattering due to disorder is taken into account. In the 1D model, the resistivity due to Umklapp scattering that is an origin of CO is shown to be suppressed by the forward scattering. In addition, we obtain that the CO transition temperature TCO for quasi 1D model becomes smaller with increasing strength of the forward scattering.

Spectral Properties of Correlated Materials: Local Vertex and Nonlocal Two-Particle Correlations from CombinedGWand Dynamical Mean Field Theory

We present a fully self-consistent combined GW and dynamical mean field (DMFT) study of the extended two-dimensional Hubbard model. The inclusion of the local dynamical vertex stemming from the DMFT self-energy and polarization is shown to cure the known problems of self-consistent GW. We calculate momentum-resolved spectral functions, two-particle polarizations, and electron-loss spectra, as well as the effective dynamical interaction induced by nonlocal screening. The momentum-dependence introduced by GW into the extended DMFT description leads to a narrowing of the quasiparticle width and more pronounced Hubbard bands in the metallic regime as one approaches the charge-ordering transition. It further affects the shape of collective modes, giving rise to dispersive plasmon-like long-wavelength and stripe modes.

Magnetocaloric Effect and Refrigerant Capacity of Non-Stoichiometric ${\rm Nd}_{0.5}{\rm Sr}_{0.5}{\rm MnO}_{3}$ Single Crystalline

Non-stoichiometric Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (NSMO) single crystalline has been grown by the floating zone method. Their magnetic properties, magnetocaloric effect (MCE) and refrigerant capacity ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ) at near both first- and second-order phase transitions were investigated. The NSMO system goes through an antiferomagnetic charge-ordered transition at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> ~ 152 K followed by a ferromagnetic to paramagnetic transition at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ~ 272 K. The transition region spread over a broader temperature range at the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> but it had a narrower temperature range at the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> . The maximum magnetic entropy change Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> gives about 1.65 J · kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the first order magnetic transition and -1.13 J · kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the second order magnetic transition, respectively. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> is obtained and it shows much higher value about 34.85 J/kg at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> than the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> 19.06 J/kg at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> . The slight non-stoichiometric NSMO single crystalline system raises the Curie temperature around 30 K towards room temperature without degradation of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> and temperature span corresponding to the full width at half maximum of Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> (Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FWHM</sub> ) compared to pure single crystalline NSMO system.

Ultrasonic study on charge ordering in Bi1−xCaxMnO3 (x=0.8, 0.85)

The charge ordering (CO) transition in polycrystalline Bi1−xCaxMnO3 (x=0, 0.8, 0.85) has been studied by measuring the resistivity and longitudinal ultrasonic velocity (Vl). It is found that with increasing the Ca content, the resistivity decreases and the charge ordering transition temperature TCO shifts to lower temperature. For both samples, upon cooling down from 300K, an obvious softening in Vl above TCO and dramatic stiffening below TCO are observed. This abnormal elastic softening above TCO 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 Bi1−xCaxMnO3. And this effect is weakened with increasing the Ca content, which may lead to the shift of TCO.

Effects of Cr substitution and oxygen reduction on elastic anomaly and ultrasonic velocity in charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics

Charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics have been investigated by electrical resistivity, AC susceptibility and ultrasonic velocity measurements to elucidate the effects of Cr substitution and oxygen reduction on charge ordering (CO). Resistivity and susceptibility measurements showed that the x=0 sample exhibits insulating behavior and an anti-ferromagnetic transition at 230K as well as a CO transition at 280K. The substitution of Cr induces a ferromagnetic–paramagnetic and metal–insulator (MI) transition as well as gradually suppressing the CO state due to weakening of the Jahn–Teller (JT) effect. Quenching to reduce the oxygen content of the x=0.05 sample caused the MI transition temperature to shift to lower temperatures most likely due to oxygen reduction. On the other hand, both longitudinal and shear velocities at 100K increased significantly with Cr substitution indicating improvement in elastic properties. However, quenching the x=0.05 sample slightly deceased both velocities and related elastic moduli. A step-like longitudinal velocity anomaly characterized by a slope change suggests the existence of CO state for x=0, 0.02 and 0.05 samples. The step-like anomaly shifts to lower temperatures from 266K (x=0) to 222K (x=0.05) with Cr substitution indicating a weakening of the CO state. Absence of the step-like anomaly for the quenched x=0.05 sample suggests suppression of the CO state due to oxygen reduction. Analysis of the step-like anomaly using the mean-field theory suggests involvement of the JT effect which transforms from dynamic to static type with decreasing temperature. Cr substitution weakened the CO state as a result of weakening the JT effect while reducing the oxygen content suppresses the CO state as a result of oxygen reduction.

Stability of Charge and Magnetic Orderings Due to Competing Interactions in PrFe4P12

The origin of a mysterious phase transition at 6.5K in the skutterudite PrFe4P12 has long been a subject of intensive research. Recent microscopic experiments such as NMR, X-ray, and neutron scattering commonly showed that the local symmetry around each Pr site is unbroken below the transition temperature, although the two neighboring Pr sites become inequivalent. It means that the symmetry of the order parameter belongs to a class of nonmagnetic charge. These experimental facts remind oneself of a similarity with unconventional charge ordering (CO) in PrRu4P12. It should be noted, however, that there are marked differences in magnetic properties of the two materials. It was recently shown that the CO phase of PrFe4P12 is transformed to an antiferromagnetic order (AFM) phase under high pressure and to a ferromagnetic order (FM) phase by the La substitution. This is apparently contrasted with the absence of proximity to any magnetic ordering in PrRu4P12. In the present paper, we shall clarify the origin of the competing phases in PrFe4P12 within a simple theory, which is based upon the similarity with the CO phase of PrRu4P12 and a specific nature of the df interaction between Fe and Pr ions. It is now established for PrRu4P12 that the conduction band is simply composed of molecular p orbitals in the pnictogen cages. A similar p band with a nested Fermi surface was identified also in PrFe4P12, 9) which seems responsible for an increased resistivity just below the CO transition temperature. A remarkable disparity in the band structures of the two materials is a d-band contribution to the Fermi surface in PrFe4P12, which has been found in many other skutterudites as well. Recently, it was shown that the df interaction brings sizable magnetic moments of both Pr and Fe ions in PrFe4X12 for X 1⁄4 As, Sb, which form complex magnetic structures. Therefore, it is reasonable to assume that the d electrons are relevant also for X 1⁄4 P. Let us start the analysis with reproducing an effective pf Hamiltonian that describes the interaction between the au conduction band and the singlet–triplet f 2 crystal field (CF) states, Hp 1⁄4 X

Pyroelectric detection of spontaneous polarization in magnetite thin films

We have investigated the spontaneous polarization in Fe${}_{3}$O${}_{4}$ thin films by using dynamic and static pyroelectric measurements. The magnetic and dielectric behavior of Fe${}_{3}$O${}_{4}$ thin films grown on Nb:SrTiO${}_{3}$(001) substrates was consistent with bulk crystals. The well-known metal-insulator (Verwey) transition was observed at 120 K. The appearance of a pyroelectric response in the Fe${}_{3}$O${}_{4}$ thin films just below the Verwey temperature shows that spontaneous polarization appeared in Fe${}_{3}$O${}_{4}$ at the charge-ordering transition temperature. The polar state characteristics are consistent with bond- and site-centered charge ordering of Fe${}^{2+}$ and Fe${}^{3+}$ ions sharing the octahedral $B$ sites. The pyroelectric response in Fe${}_{3}$O${}_{4}$ thin films was dependent on the dielectric constant. Quasistatic pyroelectric measurement of Pd/Fe${}_{3}$O${}_{4}$/Nb:SrTiO${}_{3}$ junctions showed that magnetite has a very large pyroelectric coefficient of 735 nC cm${}^{\ensuremath{-}2}$ K${}^{\ensuremath{-}1}$ at 60 K.

Revealing the insulating gap in α′-NaV 2O5 with resonant inelastic x-ray scattering

We measured the low energy excitation spectrum of α′-NaV 2O5 across its charge ordering and crystallographic phase transition with resonant inelastic x-ray scattering (RIXS) at the V L3 edge. Exploiting the polarization dependence of the RIXS signal and the high resolution of the data, we reveal the excitation across the insulating gap at 1 eV and identify the excitations from occupied 3dxy bonding orbitals to unoccupied bonding 3dxy and 3dyz/3dxz orbitals. Furthermore we observe a progressive change of the electronic structure of α′-NaV 2O5 induced by soft x-ray irradiation, with the appearance of features characteristic of sodium deficient NaxV 2O5 (x < 1).

Ultrasonic study on the Jahn–Teller effect near different phase transitions in La1/3Sr2/3MO3 (M=Mn, Fe, Co)

The longitudinal ultrasonic velocity (Vl), magnetization and resistivity of polycrystalline La1/3Sr2/3MO3 (M=Mn, Fe, Co) have been measured between 20 and 300K. Dramatic anomalies in Vl were observed near the temperature of the charge-ordering transition (CO), charge disproportionation transition (CD), and ferromagnetic transition (FM), which are explained by the Jahn–Teller effect originating from the M ions (M=Mn, Fe, Co). However, the detailed form of these anomalies is different, which is strongly depended on the M ion's unique electronic structure. For La1/3Sr2/3MO3 (M=Mn, Fe), the Vl exhibits a valley around the CO transition temperature because of the localization of the Jahn–Teller active ions (Mn3+, Fe4+). And due to the instability of Fe4+, the CD transition occurs in La1/3Sr2/3FeO3, which results in another softening in Vl, while only normal increase is observed in La1/3Sr2/3MnO3. For La1/3Sr2/3CoO3, the local lattice distortion via the Jahn–Teller effect of intermediate spin Co3+ leads to the velocity anomaly in the ferromagnetic metallic state.

Dielectric Properties of Charge-OrderedLuFe2O4Revisited: The Apparent Influence of Contacts

We show results of broadband dielectric measurements on the charge ordered, proposed to be multiferroic material LuFe(2)O(4). The temperature and frequency dependence of the complex permittivity as investigated for temperatures above and below the charge-order transition near T(CO)≈320 K and for frequencies up to 1 GHz can be well described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and hopping induced ac conductivity. No pronounced contribution of intrinsic dipolar polarization could be found, and thus the ferroelectric character of the charge order in LuFe(2)O(4) has to be questioned.

Dielectric signature of charge order in lanthanum nickelates

Three charge-ordering lanthanum nickelates La2−x A x NiO4, substituted with specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order, are investigated using broadband dielectric spectroscopy up to GHz frequencies. The transition temperatures of the samples are characterized by additional specific heat and magnetic susceptibility measurements. We find colossal magnitudes of the dielectric constant for all three compounds and strong relaxation features, which partly are of Maxwell-Wagner type arising from electrode polarization. Quite unexpectedly, the temperature-dependent colossal dielectric constants of these materials exhibit distinct anomalies at the charge-order transitions. This phenomenon is ascribed to a variation of intrinsic material properties affecting the formation of depletion layers at the electrode-sample interfaces.

Interorbital correlation effects on heavy-electron systems

We study an extended periodic Anderson model, which includes correlations between conduction and $f$ electrons, by employing dynamical mean field theory and continuous-time quantum Monte Carlo simulations. It is clarified how the antiferromagnetic phase, which is stabilized in the ordinary periodic Anderson model, changes into a charge ordered (CO) phase in the strong $c$-$f$ interaction region. A systematic analysis of the effects due to the hybridization and the $c$-$f$ interaction evidences the existence of a critical end point for the CO transition; the quantum phase transition between the CO phase and the paramagnetic phase is of first (second) order in the weak (moderate) hybridization region. We also address the effects of the Hubbard-type correlations among conduction electrons on the phase diagram.

Dielectric tunability and magnetoelectric coupling in LuFe2O4 epitaxial thin film deposited by pulsed-laser deposition

C-axis orientated LuFe2O4 thin films on (001) sapphire substrates are epitaxially deposited by pulsed-laser deposition. Temperature-dependent resistance characterization reveals the ferrimagnetic transition at 237K and charge-ordering transition at 340K in the film. Importantly, the dielectric constant of the film can be significantly changed by both electric and magnetic fields. The dielectric tunability reaches 35% when an electric field of 5V is applied, while this value reduces to 20% and 15%, respectively, when a magnetic field of 0.83T is applied perpendicular and parallel to the film normal direction. This suggests a magnetically controlled dielectric tunability and strong magnetoelectric coupling, and is therefore promising for tunable device applications in film form.

Electronic and magnetic properties in strongly correlated heterostructures

We present a theoretical study of a model heterostructure for a Mott-insulator sandwiched between two band insulators, such as SrTiO3/LaTiO3. Particular emphasis is given on the interplay between magnetism and inhomogeneous charge distributions. Our mean-field analysis of the generalized Hubbard model displays numerous ordered phases in the ground-state phase diagram. In particular, we find a canted antiferromagnetic state near the interface when antiferromagnetic-ordering exists inside the Mott insulator. A checkerboard charge-ordering proposed previously is also stabilized for large long-range Coulomb interactions. Regarding its origin we also point out the importance of interlayer spin-mediated interactions. It is further shown that such a strong spin-charge coupling gives rise to pronounced magnetic/charge order phase transitions in external magnetic fields: a firstorder metamagnetic transition and a reentrant charge-order transition with checkerboard pattern. The mechanisms stabilizing these intriguing phases are explored through a detailed analysis of the physical quantities with special focus on the spin-charge interplay.

Oxygen Isotope Effect in Ordered PrBaMn&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;

The oxygen isotope effect in ordered half-doped manganite PrBaMn216-18O6 has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 35 kOe. Substantial increase of charge ordering (CO) transition temperature TCO under oxygen isotope substitution 16O 18O and small decrease of ferromagnetic (FM) transition temperature TFM were found. Small systematic shift of ferromagnetic transition temperature in oxygen reduced manganite PrBaMn216 18O5 is also considered. The obtained results are compared with those observed in other manganites demonstrating the large isotope effect.