Showing papers on "Charge ordering published in 2017"
TL;DR: The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low-energy electronic excitations as discussed by the authors.
Abstract: The phase diagram of cuprate high-temperature superconductors features an enigmatic pseudogap region that is characterized by a partial suppression of low-energy electronic excitations. Polarized neutron diffraction Nernst effect, terahertz polarimetry and ultrasound measurements on YBa_2Cu_3O_y suggest that the pseudogap onset below a temperature T^∗ coincides with a bona fide thermodynamic phase transition that breaks time-reversal, four-fold rotation and mirror symmetries respectively. However, the full point group above and below T^∗ has not been resolved and the fate of this transition as T^∗ approaches the superconducting critical temperature T_c is poorly understood. Here we reveal the point group of YBa_2Cu_3O_y inside its pseudogap and neighbouring regions using high-sensitivity linear and second-harmonic optical anisotropy measurements. We show that spatial inversion and two-fold rotational symmetries are broken below T^∗ while mirror symmetries perpendicular to the Cu–O plane are absent at all temperatures. This transition occurs over a wide doping range and persists inside the superconducting dome, with no detectable coupling to either charge ordering or superconductivity. These results suggest that the pseudogap region coincides with an odd-parity order that does not arise from a competing Fermi surface instability and exhibits a quantum phase transition inside the superconducting dome.
166 citations
TL;DR: This is the first time that the highly reversible Co2+/Co3+ redox couple is observed in P2‐layered cathodes for sodium‐ion batteries, and may open new approaches to design advanced intercalation‐type cathode materials.
Abstract: Developing sodium-ion batteries for large-scale energy storage applications is facing big challenges of the lack of high-performance cathode materials. Here, a series of new cathode materials Na 0.66Co xMn 0.66–xTi 0.34O 2 for sodium-ion batteries are designed and synthesized aiming to reduce transition metal-ion ordering, charge ordering, as well as Na+ and vacancy ordering. An interesting structure change of Na 0.66Co xMn 0.66–xTi 0.34O 2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na 0.66Co 0.22Mn 0.44Ti 0.34O 2 with a P2-type layered structure delivers a reversible capacity of 120 mAh g -1 at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g -1 can still be obtained, indicating a promising rate capability. The low valence Co 2+ substitution results in the formation of average Mn 3.7+ valence state in Na 0.66Co 0.22Mn 0.44Ti 0.34O 2, effectively suppressing the Mn3+-induced Jahn–Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na 0.66Co 0.22Mn 0.44Ti 0.34O 2 during charge/discharge is contributed by Co 2.2+/Co 3+ and Mn 3.3+/Mn 4+ redox couples. This is themore » first time that the highly reversible Co 2+/Co 3+ redox couple is observed in P2-layered cathodes for sodium-ion batteries. This finding may open new approaches to design advanced intercalation-type cathode materials.« less
81 citations
09 Jul 2017
TL;DR: The origin of the charge density wave (CDW) is still under debate as discussed by the authors, partly because the origin and origin and evolution of CDW are still open issues in condensed matter physics.
Abstract: Charge density wave (CDW) is an important concept in condensed matter physics, germane to a number of physical phenomena. But the origin of CDW is still under debate, partly because the origin and ...
79 citations
TL;DR: In this paper, the authors show that the computational cost of the hybrid-space DMRG is independent of the width of the lattice, in contrast to the real-space formulation, for which it is proportional to the width squared.
Abstract: The performance of the density matrix renormalization group (DMRG) is strongly influenced by the choice of the local basis of the underlying physical lattice. We demonstrate that, for the two-dimensional Hubbard model, the hybrid--real-momentum-space formulation of the DMRG is computationally more efficient than the standard real-space formulation. In particular, we show that the computational cost for fixed bond dimension of the hybrid-space DMRG is approximately independent of the width of the lattice, in contrast to the real-space DMRG, for which it is proportional to the width squared. We apply the hybrid-space algorithm to calculate the ground state of the doped two-dimensional Hubbard model on cylinders of width four and six sites; at $n=0.875$ filling, the ground state exhibits a striped charge-density distribution with a wavelength of eight sites for both $U/t=4.0$ and $8.0$. We find that the strength of the charge ordering depends on $U/t$ and on the boundary conditions. Furthermore, we investigate the magnetic ordering as well as the decay of the static spin, charge, and pair-field correlation functions.
67 citations
TL;DR: The results show that, upon releasing the symmetry constraint on the density but not on the geometry, charge disproportionation is observed, resulting in a band gap of around 0.2 eV at the Fermi level, which implies that the Verwey transition is probably a semiconductor-to-semiconductor transition and that the conductivity mechanism above TV is small polaron hopping.
Abstract: Magnetite exhibits a famous phase transition, called Verwey transition, at the critical temperature TV of about 120 K. Although numerous efforts have been devoted to the understanding of this interesting transition, up to now, it is still under debate whether a charge ordering and a band gap exist in magnetite above TV. Here, we systematically investigate the charge ordering and the electronic properties of magnetite in its cubic phase using different methods based on density functional theory: DFT+U and hybrid functionals. Our results show that, upon releasing the symmetry constraint on the density but not on the geometry, charge disproportionation (Fe2+/Fe3+) is observed, resulting in a band gap of around 0.2 eV at the Fermi level. This implies that the Verwey transition is probably a semiconductor-to-semiconductor transition and that the conductivity mechanism above TV is small polaron hopping.
59 citations
TL;DR: Using resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states, which are distinct from optical excitations and are best explained as magnetic polarons.
Abstract: The first known magnetic mineral, magnetite, has unusual properties, which have fascinated mankind for centuries; it undergoes the Verwey transition around 120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition, however, remains contentious. Here we use resonant inelastic X-ray scattering over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe2+ and Fe3+ states. Comparison of the experimental results with crystal-field multiplet calculations shows that the spin-orbital dd excitons of the Fe2+ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe2+O6 octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and are best explained as magnetic polarons.
47 citations
TL;DR: In this paper, the role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior, and the authors directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature chargeordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy.
Abstract: In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal-insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.
47 citations
TL;DR: The present findings suggest that rapid cooling is useful for exploring and controlling the metastable electronic/magnetic state, which is potentially hidden behind the ground state.
Abstract: Electrons in condensed matter have internal degrees of freedom, such as charge, spin, and orbital, leading to various forms of ordered states through phase transitions. However, in individual materials, a charge/spin/orbital ordered state of the lowest temperature is normally uniquely determined in terms of the lowest-energy state, i.e., the ground state. Here, recent results are summarized showing that under rapid cooling, this principle does not necessarily hold, and thus, the cooling rate is a control parameter of the lowest-temperature state beyond the framework of the thermoequilibrium phase diagram. Although the cooling rate utilized in low-temperature experiments is typically 2 × 10−3 to 4 × 10−1 K s−1, the use of optical/electronic pulses facilitates rapid cooling, such as 102–103 K s−1. Such an unconventionally high cooling rate allows some systems to kinetically avoid a first-order phase transition, resulting in a quenched charge/spin state that differs from the ground state. It is also demonstrated that quenched states can be exploited as a non-volatile state variable when designing phase-change memory functions. The present findings suggest that rapid cooling is useful for exploring and controlling the metastable electronic/magnetic state, which is potentially hidden behind the ground state.
46 citations
TL;DR: In this paper, the half-filled extended Hubbard model on a two-dimensional square lattice using cluster dynamical mean field theory on clusters of size 8-20 was studied, and it was shown that the model exhibits metallic, Mott-insulating, and charge-ordered phases.
Abstract: We study the half-filled extended Hubbard model on a two-dimensional square lattice using cluster dynamical mean-field theory on clusters of size 8--20. We show that the model exhibits metallic, Mott-insulating, and charge-ordered phases, and determine the location of the charge-ordering phase-transition line and the properties of the phases as a function of temperature, local interaction, and nearest-neighbor interaction. We find strong nonlocal correlations outside the charge-ordered phase and a pronounced screening effect in the vicinity of the phase transition, where nonlocal interactions push the system towards metallic behavior. In contrast, correlations in the charge-ordered phase are mostly local to the unit cell. Finally, we demonstrate how strong nonlocal electron-electron interactions can increase electron mobility by turning a charge-ordered insulator into a metal. We analyze finite-size effects and the convergence of our data to the thermodynamic limit. Control of all sources of errors allows us to assess the regime of applicability of simpler approximation schemes for systems with nonlocal interactions.
44 citations
TL;DR: It is shown that the average valence distribution of Pb3.5+Co2.5-3Co2+2Co3+2O12 quadruple perovskite structure can be stabilized by tuning the energy levels of P b 6s and transition metal 3d orbitals.
Abstract: Perovskite PbCoO3 synthesized at 12 GPa was found to have an unusual charge distribution of Pb2+Pb4+3Co2+2Co3+2O12 with charge orderings in both the A and B sites of perovskite ABO3. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb2+Pb4+3Co2+2Co3+2O12 quadruple perovskite structure. It is shown that the average valence distribution of Pb3.5+Co2.5+O3 between Pb3+Cr3+O3 and Pb4+Ni2+O3 can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.
44 citations
TL;DR: A hidden stripe-type charge ordering in multilayer iron selenide films on strontium titanate, resembling that in high-temperature cuprate superconductors, could help to explain the complex behaviour of this unusual iron-based superconductor as mentioned in this paper.
Abstract: A hidden stripe-type charge ordering in multilayer iron selenide films on strontium titanate, resembling that in high-temperature cuprate superconductors, could help to explain the complex behaviour of this unusual iron-based superconductor.
TL;DR: In this article, the quantum many-body instabilities for electrons on the honeycomb lattice at half-filling with extended interactions were investigated, motivated by a description of graphene and related materials.
Abstract: We investigate the quantum many-body instabilities for electrons on the honeycomb lattice at half-filling with extended interactions, motivated by a description of graphene and related materials. We employ a recently developed fermionic functional Renormalization Group scheme which allows for highly resolved calculations of wavevector dependences in the low-energy effective interactions. We encounter the expected anti-ferromagnetic spin density wave for a dominant on-site repulsion between electrons, and charge order with different modulations for dominant pure $n$-th nearest neighbor repulsive interactions. Novel instabilities towards incommensurate charge density waves take place when non-local density interactions among several bond distances are included simultaneously. Moreover, for more realistic Coulomb potentials in graphene including enough non-local terms there is a suppression of charge order due to competition effects between the different charge ordering tendencies, and if the on-site term fails to dominate, the semi-metallic state is rendered stable. The possibility of a topological Mott insulator being the favored tendency for dominating second nearest neighbor interactions is not realized in our results with high momentum resolution.
TL;DR: It is shown that the presence, or absence, of a radiation scattering pre-peak is principally related to the symmetry breaking, or not, of the global charge order, induced by the peculiarities of the molecular shapes.
Abstract: The structural properties of ionic liquids and alcohols are viewed under the charge ordering process as a common basis to explain the peculiarity of their radiation scattering properties, namely the presence, or absence, of a scattering pre-peak. Through the analysis of models, it is shown that the presence, or absence, of a radiation scattering pre-peak is principally related to the symmetry breaking, or not, of the global charge order, induced by the peculiarities of the molecular shapes. This symmetry breaking is achieved, in practice, by the emergence of specific types of clusters, which manifests how the global charge order has changed into a local form. Various atom–atom correlations witness the symmetry breaking induced by this re organization, and this is manifested into a pre-peak in the structure factor. This approach explains why associated liquids such as water do not show a scattering pre-peak. It also explains under which conditions core-soft models can mimic associating liquids.
TL;DR: The largest reported orbital molecules are V717+ heptamers that emerge below a 700 K charge ordering transition in the spinel AlV2O4 as mentioned in this paper, and these molecules persist to at least 1100 K although they become 'hidden' by disorder in the average cubic structure above the charge order transition.
Abstract: Electronic instabilities in transition metal compounds can lead to ground states containing orbital molecules when direct metal-metal orbital interactions occur. The largest reported orbital molecules are V717+ heptamers that emerge below a 700 K charge ordering transition in the spinel AlV2O4. However, x-ray total scattering analysis shows that the apparent heptamers are actually pairs of spin-singlet V39+ trimers and V48+ tetramers, and that these orbital molecules persist to at least 1100 K although they become `hidden' by disorder in the average cubic structure above the charge ordering transition.
TL;DR: A charge-ordered organic material with an isosceles triangular lattice shows charge dynamics associated with crystallization and vitrification of electrons, which can be understood in the context of an energy landscape arising from the degeneracy of various CO patterns.
Abstract: Charge ordering (CO) is a phenomenon in which electrons in solids crystallize into a periodic pattern of charge-rich and charge-poor sites owing to strong electron correlations. This usually results in long-range order. In geometrically frustrated systems, however, a glassy electronic state without long-range CO has been observed. We found that a charge-ordered organic material with an isosceles triangular lattice shows charge dynamics associated with crystallization and vitrification of electrons, which can be understood in the context of an energy landscape arising from the degeneracy of various CO patterns. The dynamics suggest that the same nucleation and growth processes that characterize conventional glass-forming liquids guide the crystallization of electrons. These similarities may provide insight into our understanding of the liquid-glass transition.
TL;DR: It is suggested that the normal-polarization state can be induced by an electric field applied normal to the superlattice layers, yielding an antiferroelectric double-hysteresis loop.
Abstract: The structure and properties of the $1\ensuremath{\mathbin:}1$ superlattice of ${\mathrm{LaVO}}_{3}$ and ${\mathrm{SrVO}}_{3}$ are investigated with a first-principles density-functional-theory-plus-$U$ ($\mathrm{DFT}+U$) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered La/Sr cation ordering to produce a polar structure with a large nonzero spontaneous polarization normal to the interfaces. This polarization, comparable to that of conventional ferroelectrics, is produced by electron transfer between the ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{4+}$ layers. The energy of this normal-polarization state relative to the ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the normal-polarization state can be induced by an electric field applied normal to the superlattice layers, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the ground state on removal of the field, a ferroelectric-type hysteresis loop could be observed.
TL;DR: In this article, a design strategy to realize small band gap polar oxides with high carrier mobilities by combining small radii A cations with Bi3+/Bi5+ charge disproportion was conceived.
Abstract: The applications of transition metal oxides as photovoltaic and photocatalytic materials are mainly impeded by their poor visible light absorption, low photogenerated carrier mobility, and low valence band position, which originate from the generally large band gap (≥3 eV), narrow transition metal d states, and deep oxygen 2p states Here, we conceive a design strategy to realize small band gap polar oxides with high carrier mobilities by combining small radii A cations with Bi3+/Bi5+ charge disproportion We show that these cation sizes and chemical features shift the valence band edge to higher energies and therefore reduce the band gap, promoting the formation of highly dispersive Bi 6s states near the Fermi level as a byproduct By means of advanced many-electron-based first-principles calculations, we predict a new family of thermodynamically stable/metastable polar oxides ABiO3 (A = Ca, Cd, Zn, and Mg), which adopt the Ni3TeO6-type (space group R3) structure and exhibit optical band gaps of ∼20 eV,
TL;DR: Unexpected 5-fold cation/charge ordering in high-pressure-synthesized RMn3O6 perovskites with R = Gd-Tm and Y is reported on.
Abstract: Cation and anion ordering plays an important role in the properties of materials, in particular, in the properties of perovskite materials. Here we report on unusual 5-fold cation/charge ordering in high-pressure-synthesized (at 6 GPa and ∼1670 K) RMn3O6 perovskites with R = Gd–Tm and Y. R3+, Mn2+, and Mn3+ cations are ordered at the A site in two separate chains consisting of R3+ and alternating Mn2+ (in tetrahedral coordination) and Mn3+ (in square-planar coordination), while Mn3+ and mixed-valent Mn3+/Mn4+ are ordered at the B site in layers. The ordering can be represented as [R3+Mn2+0.5Mn3+0.5]A[Mn3+Mn3.5+]BO6. The triple cation ordering observed at the A site is very rare, and the layered double-B-site ordering is also scarce. RMn3O6 compounds crystallize in space group Pmmn with a = 7.2479(2) A, b = 7.4525(3) A, and c = 7.8022(2) A for DyMn3O6 at 213 K, and they are structurally related to CaFeTi2O6. They are prone to nonstoichiometry, R1−δMn3O6–1.5δ, where δ = −0.071 to −0.059 for R = Gd, δ = 0 fo...
TL;DR: In this article, the authors studied the physical properties of trirutile-type materials, including magnetism, electronic structure, phase transition, and charge ordering, and showed that the magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate compressive strain.
Abstract: Trirutile-type ${\mathrm{LiFe}}_{2}{\mathrm{F}}_{6}$ is a charge-ordered material with an ${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$ configuration. Here, its physical properties, including magnetism, electronic structure, phase transition, and charge ordering, are studied theoretically. On one hand, the charge ordering leads to improper ferroelectricity with a large polarization. On the other hand, its magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate compressive strain. Thus, ${\mathrm{LiFe}}_{2}{\mathrm{F}}_{6}$ can be a rare multiferroic with both large magnetization and polarization. Most importantly, since the charge ordering is the common ingredient for both ferroelectricity and magnetization, the net magnetization may be fully switched by flipping the polarization, rendering intrinsically strong magnetoelectric effects and desirable functions.
TL;DR: The studies show that the first-order phase transition near room temperature in LiMn2O4 is associated with charge ordering, which ultimately is a consequence of the Jahn-Teller effect.
Abstract: The phase transition near room temperature in LiMn2O4 was studied using thermal expansion measurements, and directly compared with the electrochemical performance of the material. Studies based on thermal expansion indicate the onset of a first-order phase transition at Tc ∼ 220 K for the nearly half-doped material, with [Mn3+]/[Mn4+] ≈ 1. The Tc shifts to a higher temperature, ∼290 K, and signatures for Verwey-type charge ordering at 290 K can be observed when the fraction of Jahn–Teller Mn3+ in LiMn2O4 is increased, i.e., [Mn3+]/[Mn4+] > 1. These studies show that the first-order phase transition near room temperature in LiMn2O4 is associated with charge ordering, which ultimately is a consequence of the Jahn–Teller effect. In addition, the Jahn–Teller effect is proven to be an important cause of magnetoresistance and electrochemical capacity fading in LiMn2O4. Electrochemical measurements show that both materials, either with a Tc ∼ 220 K or Tc ∼ 290 K, exhibit capacity fading to almost the same extent. Electrochemical capacity retention is observed only in nanosized LiMn2O4, for which the phase transition anomalies are completely absent.
TL;DR: In this article, the Sr doping has been found to stabilize the crystal structure of hole-doped La2CoO4 and the high-spin state has been concomitant with the delocalization of the e g electrons.
TL;DR: In this article, a generalized gradient approximation (GGA), an extension to include van der Waals interactions and a recently proposed meta-GGA are considered, and local Coulomb interactions for the Mn $3d$ electrons are more explicitly considered with the DFT+$U$ approach.
Abstract: The phases of A$_2$Mn$_8$O$_{16}$ hollandite group oxides emerge from the competition between ionic interactions, Jahn-Teller effects, charge ordering, and magnetic interactions. Their balanced treatment with feasible computational approaches can be challenging for commonly used approximations in Density Functional Theory. Three examples (A = Ag, Li and K) are studied with a sequence of different approximate exchange-correlation functionals. Starting from a generalized gradient approximation (GGA), an extension to include van der Waals interactions and a recently proposed meta-GGA are considered. Then local Coulomb interactions for the Mn $3d$ electrons are more explicitly considered with the DFT+$U$ approach. Finally selected results from a hybrid functional approach provide a reference. Results for the binding energy of the A species in the parent oxide highlight the role of van der Waals interactions. Relatively accurate results for insertion energies can be achieved with a low $U$ and a high $U$ approach. In the low $U$ case, the materials are described as band metals with a high symmetry, tetragonal crystal structure. In the high $U$ case, the electrons donated by A result in formation of local Mn$^{3+}$ centers and corresponding Jahn-Teller distortions characterized by a local order parameter. The resulting degree of monoclinic distortion depends on charge ordering and magnetic interactions in the phase formed. The reference hybrid functional results show charge localization and ordering. Comparison to low temperature experiments of related compounds suggests that charge localization is the physically correct result for the hollandite group oxides studied here. . . .
TL;DR: In this paper, temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with the addition of ferrite in the composites, which is confirmed by measuring magnetoelectric voltage coefficient, α ME and the maximum value of α ME is 5.389 mV/cm-Oe.
TL;DR: In this paper, the stability of antiferromagnetic (AF), charge density wave (CDW), and superconducting (SC) states within the t-J-U-V model of strongly correlated electrons by using the statistically consistent Gutzwiller approximation (SGA).
Abstract: In the first part of the paper, we study the stability of antiferromagnetic (AF), charge density wave (CDW), and superconducting (SC) states within the t-J-U-V model of strongly correlated electrons by using the statistically consistent Gutzwiller approximation (SGA). We concentrate on the role of the intersite Coulomb interaction term V in stabilizing the CDW phase. In particular, we show that the charge ordering appears only above a critical value of V in a limited hole-doping range δ. The effect of the V term on SC and AF phases is that a strong interaction suppresses SC, whereas the AF order is not significantly influenced by its presence. In the second part, separate calculations for the case of a pure SC phase have been carried out within an extended approach (the diagrammatic expansion for the Gutzwiller wave function, DE-GWF) in order to analyze the influence of the intersite Coulomb repulsion on the SC phase with the higher-order corrections included beyond the SGA method. The upper concentration for the SC disappearance decreases with increasing V, bringing the results closer to experiment. In appendices A and B we discuss the ambiguity connected with the choice of the Gutzwiller renormalization factors within the renormalized mean filed theory when either AF or CDW orders are considered. At the end, we overview briefly the possible extensions of the current models to put descriptions of the SC, AF, and CDW states on equal footing.
TL;DR: In this paper, the authors demonstrate that light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition.
Abstract: Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the ordering field are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions.
TL;DR: LiFe$_2$F$_6$ can be a rare multiferroic with both large magnetization and polarization, and since the charge ordering is the common ingredient for both ferroelectricity and magnetization, the net magnetization may be fully switched by flipping the polarization, rendering intrinsically strong magnetoelectric effect as mentioned in this paper.
Abstract: Trirutile-type LiFe$_2$F$_6$ is a charge-ordered material with Fe$^{2+}$/Fe$^{3+}$ configuration. Here its physical properties, including magnetism, electronic structure, phase transition, and charge ordering, are studied theoretically. On one hand, the charge ordering leads to improper ferroelectricity with a large polarization. On the other hand, its magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate compressive strain. Thus, LiFe$_2$F$_6$ can be a rare multiferroic with both large magnetization and polarization. Most importantly, since the charge ordering is the common ingredient for both ferroelectricity and magnetization, the net magnetization may be fully switched by flipping the polarization, rendering intrinsically strong magnetoelectric effect and desirable function.
TL;DR: This work studies the zero-bandwidth limit of the extended Hubbard model, which can be considered as a simple effective model of charge ordered insulators and derives the exact ground state diagrams for different lattice dimensionalities.
Abstract: The phenomenon associated with inhomogeneous distribution of electron density is known as a charge ordering. In this work, we study the zero-bandwidth limit of the extended Hubbard model, which can be considered as a simple effective model of charge ordered insulators. It consists of the on-site interaction $U$ and the intersite density-density interactions ${W}_{1}$ and ${W}_{2}$ between nearest neighbors and next-nearest neighbors, respectively. We derived the exact ground state diagrams for different lattice dimensionalities and discuss effects of small finite temperatures in the limit of high dimensions. In particular, we estimated the critical interactions for which new ordered phases emerge (laminar or stripe and four-sublattice-type). Our analysis show that the ground state of the model is highly degenerated. One of the most intriguing finding is that the nonzero temperature removes these degenerations.
TL;DR: In this article, a giant TeraHertz third harmonic signal is observed in La 1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature TCO = 55 K. The results provide compelling experimental support for the presence of hidden superfluid order in the normal state of cuprates.
Abstract: Unconventional superconductivity in the cuprates emerges from, or coexists with, other types of electronic order. However, these orders are sometimes invisible because of their symmetry. For example, the possible existence of superfluid charge stripes in the normal state of single layer cuprates cannot be validated with infrared optics, because interlayer tunneling fluctuations vanish on average. Similarly, it is not easy to establish if charge orders are responsible for dynamical decoupling of the superconducting layers over broad ranges of doping and temperatures. Here, we show that TeraHertz pulses can excite nonlinear tunneling currents between linearly de-coupled charge-ordered planes. A giant TeraHertz third harmonic signal is observed in La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature TCO = 55 K. We model these results by considering large order-parameter-phase oscillations in a pair density wave condensate, and show how nonlinear mixing of optically silent tunneling modes can drive large dipole-carrying super-current oscillations. Our results provide compelling experimental support for the presence of hidden superfluid order in the normal state of cuprates. These experiments also underscore the power of nonlinear TeraHertz optics as a sensitive probe of frustrated excitations in quantum solids.
TL;DR: This work synthesised crystalline α-MnO2 nanorods are synthesised using a facile co-precipitation method to exhibit ferroelectric behaviour for the first time and the novel properties observed are the result of structural rearrangements sparked by electrons in mixed valence cations.
Abstract: Nanostructuring followed by incorporation of defect induced non-stoichiometry is an emerging field of prominence due to its capacity to introduce unprecedented properties in materials with potential applications. In this work, crystalline α-MnO2 nanorods are synthesised using a facile co-precipitation method to exhibit ferroelectric behaviour for the first time. The evolution mechanism of the nanorods is investigated using XRD, HRTEM and FTIR spectra, while their thermal stability is probed using TGA/DTA. The novel properties observed are the result of structural rearrangements sparked by electrons in mixed valence cations (Mn3+/Mn4+). The high density of Jahn-Teller active Mn3+ cations breaks the inversion symmetry in α-MnO2, thereby altering the atomic environment inducing distortion in the basic MnO6 octahedra. Since variable temperature XRD analysis confirms the phase stability of the crystal structure up to very high temperatures, the ferroelectric phase exhibited by the material below Tc is an outcome of the combined effects of orbital ordering (OO) of the eg electron in Mn3+ and charge ordering (CO) of Mn3+ and Mn4+ cations. This is confirmed by DSC analysis. The breakdown of the ferroelectric nature is identified to originate as a result of octahedral tilting as suggested by temperature-dependent Raman studies. Magnetic and electrical transport studies provide additional evidence of a CO ferroelectric phase as they predict the existence of double-exchange hopping conduction and surface ferromagnetism in the sample.
TL;DR: In this paper, the electric spontaneous polarization in YbFe2O4, one candidate "electronic ferroelectric,” where Ferroelectricity originates from the polar charge order, was observed.
Abstract: We report the observation of the electric spontaneous polarization in YbFe2O4, one candidate “electronic ferroelectric,” where ferroelectricity originates from the polar charge order. Though we have proposed that in LuFe2O4 polar charge ordering of iron ions having different valence states give rise to ferroelectricity, some reports questioned not only the electronic origin but also the existence of the ferroelectricity itself in this system. In response to this, we show a direct macroscopic evidence of the existence of the ferroelectricity by a clear P-E hysteresis loop of YbFe2O4 belonging to the same system as that of LuFe2O4. Although further investigations are required about the electronic origin, we report the direct observation of the macroscopic electric polarization.