Charge-orbital Ordering Research Articles

Spin Wave Spectra in Pseudoperovskite Manganites with Superexchange Interaction Competition

In compounds La1/3Ca2/3MnO3 and BiMnO3, a calculation of spin-wave dispersion dependences along pseudoperovskite directions of reciprocal space is made. The model includes orbitally dependent superexchange interaction and single-ion anisotropy. Considered compounds present competing exchange interactions within magnetic unit cell because of orbital or charge-orbital ordering. The nearest neighbor superexchange interaction is taken into account. The magnetic structure and dispersion dependences of spin waves frequencies are calculated within the framework of regular multi-sublattice model. The peculiarities of frustrated magnetic spin-wave spectra are found. It is shown, that crossing and splitting of spin-waves branches in non-symmetric points of magnetic Brillouin zone is a common feature of the spectra due to exchange competition. The band structure of Г-point spectra for both compounds are predicted.

Room-Temperature Ferroelectricity of Paraelectric Oxides Tailored by Nano-Engineering

Inducing clear ferroelectricity in the quantum paraelectric SrTiO3 is important for triggering methods to discover hidden phases in condensed matter physics. Several methods such as isotope substitution and freestanding membranes could introduce ferroelectricity in SrTiO3 toward nonvolatile memory applications. However, the stable transformation from quantum paraelectric SrTiO3 to ferroelectricity SrTiO3 at room temperature still remains challenging. Here, we used multiple nano-engineering in (SrTiO3)0.65/(CeO2)0.35 films to achieve an emergent room-temperature ferroelectricity. It is shown that the CeO2 nanocolumns impose large out-of-plane strains and induce Sr/O deficiency in the SrTiO3 matrix to form a clear tetragonal structure, which leads to an apparent room-temperature ferroelectric polarization up to 2.5 μC/cm2. In collaboration with density functional theory calculations, it is proposed that the compressive strains combined with elemental deficiency give rise to local redistribution of charge density and orbital order, which induce emergent tetragonality of the strained SrTiO3. Our work thus paves a pathway for architecting functional systems in perovskite oxides using a multiple nano-design.

Charge-orbital Ordering, Magnetic State, and Exchange Couplings in Quasi-One-Dimensional Vanadate V6O13

Charge and orbital ordering, magnetic state, and exchange couplings in quasi-one-dimensional vanadate V6O13, a potential cathode material for Li-ion batteries, are investigated using the density functional theory with Coulomb interaction correction method (DFT + U). While the difference between {{t}_{{2g}}} orbital occupancies of V4+ (with a nominal 3{{d}^{1}} electronic configuration) and V5+ ions is large and gives direct evidence for charge and orbital ordering, the screening is so effective that the total 3d charge disproportionation is rather small. Our results show that the occupied {{t}_{{2g}}} states of V4+ ions in the single V–V layer form a spin-singlet molecular orbital, while the rest half of V4+ ions in the structurally distinct double V–V layers order antiferromagnetically in the low-temperature insulating phase of V6O13. We conclude that the metal-insulator transition and low-temperature magnetic properties of V6O13 involve the spin-Peierls transition assisted by orbital ordering and concomitant distortions of the crystal structure.

Frenkel pair formation energy for cubic Fe3O4 in DFT + U calculations

The cubic phase of magnetite is stabilized above the Verwey transition temperature of about 120 K via a complex electron–phonon interaction that is still not very well understood. In this work using the DFT + U method we describe our attempt to calculate point defect formation energies for this cubic phase in the static approximation. The electronic structure calculations and atomic relaxation peculiarities are discussed in this context. Only the cubic phase model with a small band gap and charge disproportionation (Fe2+/Fe3+) gives an adequate point defect formation energies, not the semi-metallic model. The relaxation of the local defect atomic structure and the relaxation of the surrounding crystal matrix are analyzed. Point defects cause only local perturbations of atomic positions and charge-orbital order. After analysis of the supercell size effects for up to 448 atoms, we justify the use of small supercells with 56 atoms to make calculations for the cubic phase. The extensive experimental results of Dieckmann et al on defects in magnetite at high temperature are deployed for comparison of our DFT + U results on Frenkel pair formation energies.

Observation of orbital order in the van der Waals material 1T−TiSe2

Besides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant x-ray-scattering experiments on the layered van der Waals compound $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$, we establish that the known charge density wave in this weakly correlated, quasi-two-dimensional material corresponds to an orbital ordered phase. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in ${\mathrm{TiSe}}_{2}$, and their importance throughout the family of correlated van der Waals materials.

Enhanced Magnetoresistance Effect in Graphene Coupled to a Ferromagnetic Oxide with Charge Orbital Ordering

In this paper, we fabricated graphene/Fe3O4 heterostructure devices by stacking monolayer graphene on magnetite (Fe3O4) substrate and investigated their magneto-transport properties. Interestingly, graphene/Fe3O4 heterostructure devices exhibit a giant magnetoresistance (MR) of 70% at a low magnetic field of 0.65[Formula: see text]T and at 11[Formula: see text]K, which is three times greater than that of graphene on SiO2. Based on standard two-fluid model and LDA[Formula: see text]U simulation, we showed that the observed enhanced MR effect is due to the increased disorder in graphene induced through the charge polarization via the alignment of C atoms of graphene over the charge ordered B-site cations of Fe3O4. Our results demonstrate a potential way to enhance graphene MR effect through coupling graphene with a suitable substrate with charge orbital ordering.

Symmetry mode analysis of distorted polar/nonpolar structures in A -site ordered SmBaMn2O6 perovskite

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the $A$-site ordered $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at ${T}_{\mathrm{CO}1}\ensuremath{\approx}380\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{CO}2}\ensuremath{\approx}190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{N}}\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The main difference between the two charge-ordered phases concerns the stacking sequence along the $c$ axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about $0.35{e}^{\ensuremath{-}}$ in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above $\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the $a$ axis.

Trimeron-phonon coupling in magnetite

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation.

Study of modification of the magnetic and magnetocaloric properties with Gd doping in Sm-Sr based manganite compounds: Influence of short range charge-orbital ordering

Detail investigation on the influence of Gd-doping both in the magnetic and magnetocaloric properties of (Sm1-yGdy)0.55Sr0.45MnO3 (y = 0.5 and 0.7) compounds has been carried out. With the Gd-content, the studied compounds exhibit quite distinct physical properties in the presence of an external magnetic field. In addition to the field-induced meta-magnetic transition and quantitatively large magnetocaloric effect, both the compounds reveal significant relative cooling power and refrigerant capacity. However, in the low temperature region, different nature in magnetocaloric effect was addressed by considering the field induced modification of short-range ordering.

Theoretical study of frustrated magnetic ordering in La1/3Ca2/3MnO3: The role of charge-orbital ordering

This investigation is devoted to a theoretical study of the role of orbital ordering in the magnetic frustration of the pseudoperovskite manganite La1/3Ca2/3MnO3. In the current study, the effect of orbital ordering on magnetic competition is described within the framework of orbitally-dependent superexchange interaction. The ranges of the mixing angles in the orbital structure of frustrated magnetic structures are determined. An orbitally-dependent single-ion anisotropy that takes octahedra tilting into account is proposed as the cause for strongly non-collinear magnetic ordering. The magnetic field dependences of complicated magnetic structures are discussed.

Reversible control of magnetic and transport properties of NdNiO3–δ epitaxial films

Rare-earth nickelates possess intrinsic charge order, orbital order, and electron-lattice coupling, which make them very interesting for applications in oxide-based electronic devices. In this study, we grew NdNiO3–δ (NNO) films with oxygen pressures changing from 27 to 10−5 Pa. With decreasing oxygen pressure, the antiferromagnetic state of the NNO film becomes a ferromagnetic state, and the resistance increases significantly. According to combined X-ray absorption spectroscopy and X-ray linear dichroism measurements, the ratio of Ni2+-ions increases with decreasing oxygen-pressure, and the preferred orbital occupation changes from x2-y2 to 3z2-r2. In addition, using the ionic-liquid gating method to control the migration of oxygen vacancies, both the magnetic properties and resistance of NNO films can be modulated reversibly. The oxygen vacancy induces a valence in the Ni ions and the orbital occupation changes, which alters the magnetic properties and the electronic transport in these NNO films. This study describes a novel tunable method for electronic devices that use NdNiO3–δ films, and opens new doors for future improvements and functionalities.

Impact of weak ferromagnetism on the magnetocaloric properties of A-site-doped PrMnO3 compound

Investigation of the magnetic ground state and magnetocaloric properties of the polycrystalline Pr0.65(Ca0.80Sr0.20)0.35 MnO3 compound has been carried out in detail. In addition to the charge-orbital ordering and antiferromagnetic ordering transition, a weak ferromagnetic (FM) phase has been recognized from its temperature dependence of magnetization as well as from the change of magnetic entropies. The influence of such ferromagnetic phase on magnetocaloric effect have been examined properly in the presence of different magnetic fields. Moreover, a magnetic phase diagram, derived from the magnetic isotherms and magnetocaloric effect has been constructed to understand evolution of the weak ferromagnetic phase with external magnetic field at low temperature region.

Discovery of the soft electronic modes of the trimeron order in magnetite

The Verwey transition in magnetite (Fe$_3$O$_4$) is the first metal-insulator transition ever observed and involves a concomitant structural rearrangement and charge-orbital ordering. Due to the complex interplay of these intertwined degrees of freedom, a complete characterization of the low-temperature phase of magnetite and the mechanism driving the transition have long remained elusive. It was demonstrated in recent years that the fundamental building blocks of the charge-ordered structure are three-site small polarons called trimerons. However, electronic collective modes of this trimeron order have not been detected to date, and thus an understanding of the dynamics of the Verwey transition from an electronic point of view is still lacking. Here, we discover spectroscopic signatures of the low-energy electronic excitations of the trimeron network using terahertz light. By driving these modes coherently with an ultrashort laser pulse, we reveal their critical softening and hence demonstrate their direct involvement in the Verwey transition. These findings represent the first observation of soft modes in magnetite and shed new light on the cooperative mechanism at the origin of its exotic ground state.

Effect of the magnetic impurity on the charge diffusion in highly dilute Ce doped LaMnO3

LaMnO3+δ is a complex oxide, which, depending on the oxygen excess concentration, presents different crystalline structure and interesting magnetic and electric properties such as colossal magnetoresistance, polaron dynamics, multiferroic behavior, and charge-orbital ordering. This complexity requires different characterization techniques to draw a picture as complete as possible allowing a good understanding of these phenomena. Here, we have used the perturbed angular correlation (PAC) technique to measure hyperfine interactions at La and Mn sites of LaMnO3+δ(δ ∼0.15) using 140Ce and 111Cd at La sites as probe nuclei in order to investigate within an atomic scale the magnetic and electric interactions in this compound. The results show that 111Cd nuclei occupy highly symmetric local sites in agreement with a rhombohedral structure. The magnetic hyperfine field (Bhf) measured with 111Cd at La sites is very small (Bhf = 0.40 T) due to the supertransferred magnetic field from Mn neighbors through oxygen orbitals. On the other hand, 140Ce nuclei at La sites present a saturation field of around 3.7 T much higher than that expected for La sites (due to the weak transfer field by superexchange mechanism). In addition, for temperature range above the magnetic ordering (200-300 K) a dynamic hyperfine interaction was observed characterized by the attenuation parameter λ(T) whose temperature dependence allowed to determine the activation energy (Ea) associated to charge transfer. The polarization of the 4f-electron of Ce impurities affects the local magnetic field at impurity sites as well as the Ea.

Orbital Domain Dynamics in Magnetite below the Verwey Transition.

The metal-insulator phase transition in magnetite, known as the Verwey transition, is characterized by a charge-orbital ordering and a lattice transformation from a cubic to monoclinic structure. We use x-ray photon correlation spectroscopy to investigate the dynamics of this charge-orbitally ordered insulating phase undergoing the insulator-to-metal transition. By tuning to the Fe L_{3} edge at the (001/2) superlattice peak, we probe the evolution of the Fe t_{2g} orbitally ordered domains present in the low temperature insulating phase and forbidden in the high temperature metallic phase. We observe two distinct regimes below the Verwey transition. In the first regime, magnetite follows an Arrhenius behavior and the characteristic timescale for orbital fluctuations decreases as the temperature increases. In the second regime, magnetite phase separates into metallic and insulating domains, and the kinetics of the phase transition is dictated by metallic-insulating interfacial boundary conditions.

Biaxial strain engineering of charge ordering and orbital ordering in HoNiO3

Strain in epitaxial ${\mathrm{HoNiO}}_{3}$ is found to be capable of influencing the charge and the spin of Ni ions and the orbital hybridization of Ni-O and further manipulating charge ordering and orbital ordering by our first-principles calculation. The tensile strain can encourage Jahn-Teller distortion instead of breathing distortion, which leads to the phase transition from the charge-ordering state to the orbital-ordering state. Furthermore, we calculated the potential energy of the two modes, which reveals that the biaxial strain tunes the competition between the breathing and Jahn-Teller distortions.

Interplay between spin-orbit interaction and stripe-type charge-orbital order of IrTe2

We have studied the electronic structure of the stripe-type charge-orbital ordered phase of IrTe2 by means of angle-resolved photoemission spectroscopy. An electron pocket at the Brillouin zone boundary and hole Fermi surfaces around its center are clearly observed in agreement with the band structure calculation for the stripe-type charge-orbital ordered phase. The present study indicates that the hole and electron Fermi surfaces can survive due to the strong Te 5p spin-orbit coupling even under the charge-orbital order. The participant of the Te 5p orbitals to the conduction band provides an interesting interplay between the charge-orbital order and the spin-orbit interaction in IrTe2.

Biaxial strain effect induced electronic structure alternation and trimeron recombination in Fe3O4

The Verwey transition in Fe3O4 is the first metal-insulator transition caused by charge ordering. However, the physical mechanism and influence factors of Verwey transition are still debated. Herewith, the strain effects on the electronic structure of low-temperature phase (LTP) Fe3O4 with P2/c and Cc symmetries are investigated by first-principles calculations. LTP Fe3O4 with each space group has a critical strain. With P2/c, Fe3O4 is sensitive to the compressive strain, but it is sensitive to tensile strain for Cc. In the critical region, the band gap of LTP Fe3O4 with both two symmetries linearly increases with strain. When strain exceeds the critical value, DOS of spin-down t2g electron at Fe(B4) with P2/c and Fe(B42) with Cc changes between dx2-y2 and dxz + dyz. The trimerons appear in Cc can be affected by strain. With a compressive strain, the correlation of trimeron along x and y axes is strengthened, but broken along the face diagonal of FeB4O4, which is opposite at the tensile strains. The results suggest that the electronic structure of Fe3O4 is tunable by strain. The narrower or wider band gap implies a lower or higher transition temperature than its bulk without strains, which also gives a glimpse of the origin of charge-orbital ordering in Fe3O4.

Competition Between Anti-Phase Boundaries and Charge-Orbital Ordering in Epitaxial Stepped Fe3O4(100) Thin Films

Magnetite is a highly utilized transition metal oxide with many interesting magnetic and transport properties. The presence of anti-phase boundaries (APBs) and charge-orbital ordering (COO) are two of the most exciting properties of epitaxial magnetite thin films. Here, epitaxial stepped Fe3O4 films were prepared to investigate the competition between APBs and COO via measurements of in-plane anisotropy. The anisotropy was probed for two orthogonal configurations, with magnetic field applied or electrical-contacts aligned either along or perpendicular to the steps. We reveal that the APBs dominate the magnetic and transport properties of the films above the Verwey transition temperature ([Formula: see text]. However, below [Formula: see text] film thickness becomes a decisive factor in determining the magnetic nature of stepped magnetite films, due to its correlation with domain size. When the film is thinner than a critical thickness, the anisotropy is dominated by the APBs, and a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented along the steps. Conversely, for sufficiently thick films, below [Formula: see text], the magnetic and electrical transport properties are dominated by COO. Thus a higher anisotropy constant and MR ratio are observed when the magnetic field or contacts are oriented perpendicular to the steps.

Granular superconductivity and magnetic-field-driven recovery of macroscopic coherence in a cuprate/manganite multilayer

We show that in Pr$ _{0.5} $La$ _{0.2} $Ca$ _{0.3} $MnO$ _{3} $/YBa$ _{2} $Cu$ _{3} $O$ _{7} $ (PLCMO/YBCO) multilayers the low temperature state of YBCO is very resistive and resembles the one of a granular superconductor or a frustrated Josephson-junction network. Notably, a coherent superconducting response can be restored with a large magnetic field which also suppresses the charge-orbital order in PLCMO. This coincidence suggests that the granular superconducting state of YBCO is induced by the charge-orbital order of PLCMO. The coupling mechanism and the nature of the induced inhomogeneous state in YBCO remain to be understood.