Showing papers on "Charge ordering published in 2004"
TL;DR: A theoretical study of an interface between a strongly correlated Mott insulator and a band insulator finds dramatic interface-induced electronic reconstructions: in wide parameter ranges, the near-interface region is metallic and ferromagnetic, whereas the bulk phase on either side is insulating and antiferromagnetic.
Abstract: Surface science is an important and well-established branch of materials science involving the study of changes in material properties near a surface or interface. A fundamental issue has been atomic reconstruction: how the surface lattice symmetry differs from the bulk. 'Correlated-electron compounds' are materials in which strong electron-electron and electron-lattice interactions produce new electronic phases, including interaction-induced (Mott) insulators, many forms of spin, charge and orbital ordering, and (presumably) high-transition-temperature superconductivity. Here we propose that the fundamental issue for the new field of correlated-electron surface/interface science is 'electronic reconstruction': how does the surface/interface electronic phase differ from that in the bulk? As a step towards a general understanding of such phenomena, we present a theoretical study of an interface between a strongly correlated Mott insulator and a band insulator. We find dramatic interface-induced electronic reconstructions: in wide parameter ranges, the near-interface region is metallic and ferromagnetic, whereas the bulk phase on either side is insulating and antiferromagnetic. Extending the analysis to a wider range of interfaces and surfaces is a fundamental scientific challenge and may lead to new applications for correlated electron materials.
571 citations
TL;DR: In this article, the authors re-analyse the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe2+Fe3+ charge ordering model.
Abstract: This review puts in doubt the classical description of the Verwey (metal–insulator) transition in magnetite on the basis of the wide set of experiments carried out over the last 60 years. We re-analyse here the most relevant experiments used to study the Verwey transition from the point of view of their degree of agreement with the proposed Fe2+–Fe3+ charge ordering model. We will consider three groups of experimental studies, according to their capability of detecting different ionic species and/or a charge periodicity: (1) Experiments which have been interpreted using the charge ordering model as the starting point though they are not able to demonstrate its validity. This is the case for macroscopic properties such as the electrical resistivity, the heat capacity and the magnetic properties. (2) Experiments which can distinguish different types of Fe ions, such as Mossbauer, nuclear magnetic resonance (NMR) and electronic spectroscopies. However, we show that they are not able to associate them with a specific valence (2+ or 3+ in our case) and, in some cases, they observe more than two different kinds of iron atoms. (3) Diffraction (x-ray, neutron and electron) experiments, which are the most conclusive ones for determining a periodic ordering of different entities. These experiments, instead, point to the lack of ionic charge ordering. We will focus, in particular, on the discussion of the results of some recent x-ray resonant scattering experiments carried out on magnetite that directly prove the lack of ionic charge ordering in such mixed valence oxide. Furthermore, we also reconsider some so-called Verwey-type transition metal oxides in terms of the applicability of the Verwey charge ordering model. We show that a complete charge disproportionation (δ) is not experimentally observed in any of these compounds, the maximum δ being less than 0.5 e−. Regarding the theoretical framework, we will outline some relevant implications for the description of the physics of 3d transition metal oxides of this critical re-examination of the experimental facts on magnetite. Electronic localization should then occur involving more than one transition metal atom, so the definition of ionic d states loses its meaning in mixed valence transition metal oxides.
262 citations
TL;DR: All results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.
Abstract: A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.
108 citations
TL;DR: In this paper, the magnetic and structural properties of stoichiometric and cation-deficient magnetite Fe3−δO4 and γ-Fe2O3 particles were studied using neutron diffraction, magnetic measurements, and Mossbauer spectroscopy.
Abstract: Stoichiometric and cation-deficient magnetite Fe3−δO4 and γ-Fe2O3 particles have been prepared by the chemical method followed by heat treatments The magnetic and structural properties were studied using neutron diffraction, magnetic measurements, and Mossbauer spectroscopy Charge ordering of Fe3+ and Fe2+ and lattice distortion are not observed below the Verwey transition temperature in the stoichiometric and cation-deficient magnetite It is found that the lattice parameter and the Verwey transition temperature decrease as the cation vacancy increases The Verwey transition almost disappears in the Fe3−δO4 sample with δ=0066 Mossbauer spectra show that the ratio of Fe3+/Fe25+ in stoichiometric magnetite can be modified by heat treatment The Fe vacancies on the B sites change the nature of the Verwey transition No cation vacancy ordering is observed for γ-Fe2O3, due to the small amount of cation vacancies in the compound
97 citations
TL;DR: In this article, the dielectric response of La1.5Sr0.5NiO4 was investigated and the role of the electrical contacts used, sample thickness, and grain size on the experimental data was determined.
Abstract: We report the dielectric response of La1.5Sr0.5NiO4, a system that experiences charge ordering above room temperature (TCO=480K) and a rearrangement of its charge-order pattern in the temperature region 160–200K. A careful analysis of the role of the electrical contacts used, sample thickness, and grain size on the experimental data allows us to determine that this material exhibits a high intrinsic dielectric constant. In addition, the temperature dependence of the dielectric constant, that shows a maximum in the region of the rearrangement of the charge-order pattern, points to a link between the two phenomena.
75 citations
TL;DR: In this paper, the authors measured the dielectric constant and ac conductivity for the layered organic conductor theta-(BEDT-TTF)2CsZn(SCN)_4 along the out-of-plane direction, which show a relaxation behavior similar to those in the charge density wave conductor.
Abstract: The dielectric constant and ac conductivity have been measured for the layered organic conductor theta-(BEDT-TTF)_2CsZn(SCN)_4 along the out-of-plane direction, which show a relaxation behavior similar to those in the charge-density-wave conductor. Most unexpectedly, they exhibit a large bias dependence with a hysteresis, and changes in magnitude by 100-1000 times at a threshold. These findings are very similar to the collective excitation of the charge density wave. theta-$(BEDT-TTF)_2CsZn(SCN)_4 has collective excitations associated with charge ordering, though it shows no clear indication of long range order.
71 citations
TL;DR: In this paper, a self-interaction corrected local spin density approximation was applied to study the electronic structure of half-metallic double perovskites and charge order in Fe 3 O 4.
70 citations
TL;DR: In this article, NaxCoO2 is studied by 23Na NMR and SQUID magnetometry and three Na sites are identified from their single valued quadrupole effects and magnetic shifts, which implies a definite order of the Na+ ions and of the Co charges in the CoO2 planes.
Abstract: Oriented powder samples of NaxCoO2 are studied by 23Na NMR and SQUID magnetometry. In nominal 0.50
60 citations
TL;DR: In this paper, the authors derived the thermal and volume parameters of the Verwey transition in RBaFe2O5+w (R=a rare-earth element) as a function of R and w.
53 citations
51 citations
TL;DR: In this paper, the A-site-ordered/disordered manganese perovskites, RBaMn 2 O 6 /R 0.5 Ba 0.3 (R = Y and rare earth elements), are reviewed, in terms of the structural characteristic that the MnO 2 square sublattice is sandwiched by two types of rock-salt layer, RO and BaO with different lattice sizes.
Abstract: The A-site-ordered/disordered manganese perovskites, RBaMn 2 O 6 /R 0.5 Ba 0.5 MnO 3 (R = Y and rare earth elements), are reviewed. RBaMn 2 O 6 displays remarkable features: (1) the charge/orbital order (CO) transition at relatively high temperatures far above 300 K, (2) a new stacking variation of the CE-type CO with a fourfold periodicity along the c-axis, (3) the presence of structural transition possibly accompanied by d x 2 -y 2 orbital order and (4) electronic phase segregation. These novel structural and electromagnetic properties are discussed in terms of the structural characteristic that the MnO 2 square sublattice is sandwiched by two types of rock-salt layer, RO and BaO, with different lattice sizes. Such structure introduces a strong frustration to the MnO 2 sublattice and gives a new perturbation to the competition of multiple degrees of freedom among charge, orbital, spin and lattice. In R 0.5 Ba 0.5 MnO 3 with a primitive cubic perovskite cell, on the other hand, the magnetic glassy states are dominant as the ground state. A peculiar behaviour, steplike ultrasharp magnetization and resistivity changes, has been observed in Pr 0.5 Ba 0.5 MnO 3 .
TL;DR: In this paper, the crystal structures of a metal-ordered manganite YBaMn2O6 which undergoes successive phase transitions were determined by means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM).
TL;DR: In this article, the structural transition temperature, Ts, decreases with decreasing V2+ content to a minimum value of 105 K. Magnetic susceptibility data show evidence for two phase transitions at 135 and 40 K.
TL;DR: In this article, the crystal and magnetic structures of the manganite Pr0.75Na0.25MnO3 with non-integer mean valence (Mn3+/Mn4+) have been studied by means of powder neutron diffraction at high pressures up to 8 GPa in the temperature range 1.5-295k.
Abstract: The crystal and magnetic structures of the manganite Pr0.75Na0.25MnO3 with non-integer mean valence (Mn3+/Mn4+) have been studied by means of powder neutron diffraction at high pressures up to 8 GPa in the temperature range 1.5–295 K. At ambient pressure, the system undergoes a charge ordering at TCO = 220 K, followed by an onset of the AFM arrangement of the pseudo-CE (charge-exchange) type at TN = 170 K. Under high external pressures, a suppression of the pseudo-CE-type AFM state and the concurrent appearance of the A-type AFM state are observed. The unconventional orientation of manganese magnetic moments in the A-type AFM phase inclined out of the [0 1 0] crystallographic plane indicates the presence of magnetic coupling between the A-type AFM regions, presumably of nanoscopic thickness, and the pseudo-CE-type AFM residual regions.
Journal Article•
TL;DR: In this article, the authors show that the tunneling transport of electrons causes a spin-dependent magnetoresistance of about −4.7% for Fe3O4 films and −1.8% for (Fe 3O4)0.6(SiO2) 0.4 composite films under a 46 kOe magnetic field at room temperature.
Abstract: (Fe3O4)1−x–(SiO2)x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by log ρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.
TL;DR: In this paper, the effect of Cr doping on the charge-ordered (CO) state of bilayered manganite LaSr2Mn2O is investigated by electronic transport and magnetic property measurements.
Abstract: The effect of Cr doping on the charge-ordered (CO) state of the bilayered manganite LaSr2Mn2O is investigated by electronic transport and magnetic property measurements. With Cr doping, the metal-insulator transition disappears and the temperature dependence of resistivity ρ(T) exhibits semiconducting behavior. Both the antiferromagnetic transition temperature TN and the CO temperature TCO shift to lower temperature and the cluster glass state appears with increasing Cr doping level. The increase of resistivity and the slight change in the magnetization with increasing Cr content may be related to the combined effect of the partial destruction of the Mn-O-Mn interaction network within the ferromagnetic (FM) layers of an A-type antiferromagnet and the appearance of Cr3+-O-Mn3+ FM interaction. Additionally, the introduction of the random Coulomb potential caused by the substitution of Cr3+ for Mn3+ is also a source of increased resistivity. Both, the different effects of Mn-site doping on the CO state betwe...
TL;DR: In this paper, the results of temperature-dependent electron paramagnetic resonance (EPR) studies on $Pr 1-x}Ca_xMnO_3$ were compared.
Abstract: We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on $Pr_{1-x}Ca_xMnO_3$ for x = 0.64, which is electron-doped, with results of studies on the hole-doped, x = 0.36 composition. The temperature dependence of the various parameters obtained from the powder and single crystal spectra showsignificant differences between the two manganites. At room temperature the ‘g’ parameter for the electrondoped system is less than the free electron ‘g’ value ‘ge’, whereas for the hole-doped system it is more than ‘ge’. Further, the linewidth obtained from the powder spectra as well as the single crystal spectra show different functional dependences on temperature in the two systems. Quite strikingly, the peak observed at $T_{co}$ in the temperature dependence of the asymmetry parameter, \alpha, of the single crystal spectra in the hole-doped system, is absent in the electron-doped system. We understand this contrasting behaviour of the EPR parameters in the two systems in terms of the very different nature of microscopic interactions in them.
TL;DR: In this article, single crystal neutron scattering measurements have been performed on the spinladder compound Sr 14 Cu 24 O 41 at 7 K. The incommensurate composite structure has been determined using the superspace group formalism.
Abstract: Single crystal neutron scattering measurements have been performed on the spin-ladder compound Sr 14 Cu 24 O 41 at 7 K. The incommensurate composite structure has been determined using the superspace group formalism. The low-temperature structure does not show a significant shift of the CuO 2 chains along the c direction. In addition to the main and satellite nuclear reflections compatible with the incommensurate composite structure, new weak super-structure peaks at Q=(h k L c ) , with L c = n ±0.20 (±0.01) have been observed. Their origin is probably due to the charge ordering associated to a three-dimensional configuration of the spin dimers in the CuO 2 chain.
TL;DR: In this paper, the magnetic and electronic properties of the insulating charge transfer salt (TMTTF) 2 AsF 6 by electron spin resonance (ESR), SQUID magnetometer and optical conductivity measurements were investigated in the vicinity of the charge order transition.
Abstract: We studied the magnetic and electronic properties of the insulating charge-transfer salt (TMTTF) 2 AsF 6 by electron spin resonance (ESR), SQUID magnetometer and optical conductivity measurements. For the first time, the lattice dynamics was investigated in the vicinity of the charge order transition. We found a splitting of the electron-molecular-vibration coupled totally symmetric intramolecular a g (v 3 ) mode below the charge-order transition at 100 K. From this result a charge disproportionation on the TMTTF molecular stacks of about 2:1 can be estimated. The spin degrees of freedom were investigated by ESR and SQUID. At high temperatures (TMTTF)2AsF6 behaves like a antiferromagnetic S=1/2 Heisenberg chain. Below 13 K, we observed an exponential decrease of the spin susceptibility indicating the phase transition into the spin-Peierls ground state.
TL;DR: In this article, it is shown that a first-order phase transition taking place at, TV at ambient pressure opens a small gap within the oxygen p-band, resulting in the observed insulating state at T. TV is shown to be unrelated to the actual mechanism of the metal - insulator transition attributed to the Verwey transition.
TL;DR: YBaMnCoO5 as discussed by the authors is a double-cell perovskite with Y and Ba ordered in layers so as to occupy 8-coordinate and 12 coordinate sites, respectively, and Mn and Co disordered over a square pyramidal site.
TL;DR: The Nd0.5Sr0.25Pr 0.25Sr 0.5MnO3 perovskites have been prepared by solid reaction technology.
TL;DR: In this article, the magnetic properties of electron-doped manganite La0.3Ca0.7MnO3 and La 0.8Cr0.2O3 polycrystalline samples were investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T.
TL;DR: In this paper, the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy was studied, and the experiments demonstrate that the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large.
Abstract: (TMTTF) 2 AsF 6 and (TMTTF) 2 SbF 6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (TMTTF) 2 SbF 6 . An extension of the well-known temperature/pressure phase diagram is proposed.
TL;DR: In this paper, specific heat and simultaneous AC magnetic susceptibility (χ′) and electric resistance of stoichiometric magnetite single crystal are presented, and it is concluded that the magnetic degrees of freedom do not actively participate in the transition and that the entropy released at TV may come from ordering electrons.
Abstract: Studies of the specific heat and simultaneous AC magnetic susceptibility (χ′) and electric resistance of stoichiometric magnetite single crystal are presented The temperature hysteresis of the Verwey transition is of 003 K found from the specific heat data confirming its first-order character The continuous temporal change of χ′ at TV can be switched off by an external magnetic field without affecting the transition The electrical resistance decreases continuously with increasing temperature with a rapid change of slope at the point when the phase transition is completed It was concluded that the magnetic degrees of freedom do not actively participate in the transition and that the entropy released at TV may come from ordering electrons
TL;DR: In this paper, a negative thermal expansion was found in the perovskite of a simple perovskiy material, where the spins are ordered in an A-type magnetic structure.
Abstract: We found an unusual negative thermal expansion in the simple perovskite ${\mathrm{MnF}}_{3}$ below the Neel point, where the spins are ordered in an A-type magnetic structure. The structure of ${\mathrm{MnF}}_{3}$ has been studied between 5 and 500 K using powder neutron and synchrotron diffraction. The structure remains monoclinic $C2/c$ over the entire temperature range. We have found that the interplay of spin and orbital ordering is sufficient to result in a large positive magnetovolume effect without charge ordering. This expansion is reminiscent of the Invar effect observed in certain alloys.
TL;DR: In this paper, a charge ordered state for the crystal structure reported by Radaelli et al. has been addressed with quantum chemical calculations on large embedded clusters, and important O toMn charge transfer effects are observed for the Zener polaron.
Abstract: has been addressed with quantum chemicalcalculations on large embedded clusters. We find a charge ordered state for the crystal structure reported byRadaelli et al. [Phys. Rev. B 55, 3015 (1997)] and Zener polaron formation in the crystal structure withequivalent Mn sites proposed by Daoud-Aladineet al. [Phys. Rev. Lett. 89, 097205 (2002)]. Important O toMn charge transfer effects are observed for the Zener polaron.DOI: 10.1103/PhysRevB.70.235104 PACS number(s): 75.47.Lx, 71.15.Ap, 71.27.1aI. INTRODUCTION