Charge-ordered State Research Articles

Terahertz-field-induced polar charge order in electronic-type dielectrics

Ultrafast electronic-phase change in solids by light, called photoinduced phase transition, is a central issue in the field of non-equilibrium quantum physics, which has been developed very recently. In most of those phenomena, charge or spin orders in an original phase are melted by photocarrier generations, while an ordered state is usually difficult to be created from a non-ordered state by a photoexcitation. Here, we demonstrate that a strong terahertz electric-field pulse changes a Mott insulator of an organic molecular compound in κ-(ET)2Cu[N(CN)2]Cl (ET = bis(ethylenedithio)tetrathiafulvalene), to a macroscopically polarized charge-order state; herein, electronic ferroelectricity is induced by the collective intermolecular charge transfers in each dimer. In contrast, in an isostructural compound, κ-(ET)2Cu2(CN)3, which shows the spin-liquid state at low temperatures, a similar polar charge order is not stabilized by the same terahertz pulse. From the comparative studies of terahertz-field-induced second-harmonic-generation and reflectivity changes in the two compounds, we suggest the possibility that a coupling of charge and spin degrees of freedom would play important roles in the stabilization of polar charge order.

Experimental observation of weak antiferromagnetic correlation between blocked spins in the nanocrystalline La0.45Ca0.55MnO3 compound at low temperature

In the present study, an attempt has been taken to probe the meta-stable nature of the magnetic ground state in the nanocrystalline La0.45Ca0.55MnO3 compound at the low temperature through magnetic and magneto-transport properties and compared the results from the observation with the bulk polycrystalline form of the La0.45Ca0.55MnO3 compound. The charge-ordered antiferromagnetic ground state in the bulk compound transformed into a ferromagnetic state in the nanocrystalline sample. Additionally, in the nanocrystalline form of the compound, a short-range antiferromagnetic correlation between the blocked spin is noticed at the low temperature region (T ⩽ 50 K). Such weak magnetic interaction between magnetic spins or sublattices is probed through the magneto-transport and magnetocaloric measurements study in the nanocrystalline La0.45Ca0.55MnO3 compound.

Dynamic electron correlations with charge order wavelength along all directions in the copper oxide plane

In strongly correlated systems the strength of Coulomb interactions between electrons, relative to their kinetic energy, plays a central role in determining their emergent quantum mechanical phases. We perform resonant x-ray scattering on Bi2Sr2CaCu2O8+δ, a prototypical cuprate superconductor, to probe electronic correlations within the CuO2 plane. We discover a dynamic quasi-circular pattern in the x-y scattering plane with a radius that matches the wave vector magnitude of the well-known static charge order. Along with doping- and temperature-dependent measurements, our experiments reveal a picture of charge order competing with superconductivity where short-range domains along x and y can dynamically rotate into any other in-plane direction. This quasi-circular spectrum, a hallmark of Brazovskii-type fluctuations, has immediate consequences to our understanding of rotational and translational symmetry breaking in the cuprates. We discuss how the combination of short- and long-range Coulomb interactions results in an effective non-monotonic potential that may determine the quasi-circular pattern.

Structural, magnetic and magneto-transport properties of Pr0.5Ca0.5Mn0.9V0.1O3: Indication of large field coefficient of resistance (FCR)

Structural, magnetic and magneto-transport properties of polycrystalline Pr0.5Ca0.5Mn0.9V0.1O3 have been studied. The incorporation of vanadium attributes to enhancement in the Mn3+/Mn4+ ratio as well as density of conducting eg carriers, which affects the magnetic and electrical behavior of the system. On cooling, a multi-phase separated “paramagnetic (PM) – charge-ordered antiferromagnetic (COAFM) – ferromagnetic (FM) – reentrant spin glass (RSG)” phases are observed with the characteristic charge ordering temperature TCO = 233 K and Neel temperature TN = 143 K. Field-induced first-order magnetic phase transition from a charge-ordered antiferromagnetic state to a ferromagnetic cluster glass state takes place at TC = 44 K. A 7 T magnetic field induces metal–insulator transition at TMI = 163 K. At low temperatures, strong memory-effect and colossal magnetoresistance (CMR) ~ 99.99% are observed in resistivity data. The sample shows a high field coefficient of resistance (FCR) value of about 855% in the low-temperature regime.

Isotropic parallel antiferromagnetism in the magnetic field induced charge-ordered state of SmRu4P12 caused by p−f hybridization

Nature of the field-induced charge ordered phase (phase II) of SmRu$_4$P$_{12}$ has been investigated by resonant x-ray diffraction (RXD) and polarized neutron diffraction (PND), focusing on the relationship between the atomic displacements and the antiferromagnetic (AFM) moments of Sm. From the analysis of the interference between the non-resonant Thomson scattering and the resonant magnetic scattering, combined with the spectral function obtained from x-ray magnetic circular dichroism, it is shown that the AFM moment of Sm prefers to be parallel to the field ($m_{\text{AF}} \parallel H$), giving rise to large and small moment sites around which the P$_{12}$ and Ru cage contract and expand, respectively. This is associated with the formation of the staggered ordering of the $\Gamma_7$-like and $\Gamma_8$-like crystal-field states, providing a strong piece of evidence for the charge order. PND was also performed to obtain complementary and unambiguous conclusion. In addition, isotropic and continuous nature of the phase II is demonstrated by the field-direction invariance of the interference spectrum in RXD. Crucial role of the $p$-$f$ hybridization is shown by resonant soft x-ray diffraction at the P $K$-edge ($1s\leftrightarrow 3p$), where we detected a resonance due to the spin polarized $3p$ orbitals reflecting the AFM order of Sm.

Intercalated phases of transition metal dichalcogenides

AbstractTwo‐dimensional transition metal dichalcogenides (TMDs) play host to a wide range of novel topological states, such as quantum spin Hall insulators, superconductors, and Weyl semimetals. The rich polymorphism in TMDs suggests that phase engineering can be used to switch between different charge order states. Intercalation of atoms or molecules into the van der Waals gap of TMDs has emerged as a powerful approach to modify the properties of the material, leading to phase transition or the formation of substoichiometric phases via compositional tuning, thus broadening the electronic and optical landscape of these materials for a wide range of applications. Here, we review the current efforts in the preparation of intercalated TMD. The challenges and opportunities for intercalated TMDs to create a new device paradigm for material science are discussed.

Zero-temperature phases of the two-dimensional Hubbard-Holstein model: A non-Gaussian exact diagonalization study

We propose a numerical method which embeds the variational non-Gaussian wavefunction approach within exact diagonalization, allowing for efficient treatment of correlated systems with both electron-electron and electron-phonon interactions. Using a generalized polaron transformation, we construct a variational wavefunction that absorbs entanglement between electrons and phonons into a variational non-Gaussian transformation; exact diagonalization is then used to treat the electronic part of the wavefunction exactly, thus taking into account high-order correlation effects beyond the Gaussian level. Keeping the full electronic Hilbert space, the complexity is increased only by a polynomial scaling factor relative to the exact diagonalization calculation for pure electrons. As an example, we use this method to study ground-state properties of the two-dimensional Hubbard-Holstein model, providing evidence for the existence of intervening phases between the spin and charge-ordered states. In particular, we find one of the intervening phases has strong charge susceptibility and binding energy, but is distinct from a charge-density-wave ordered state, while the other intervening phase displays superconductivity at weak couplings. This method, as a general framework, can be extended to treat excited states and dynamics, as well as a wide range of systems with both electron-electron and electron-boson interactions.

Correlated insulating states at fractional fillings of moiré superlattices.

Quantum particles on a lattice with competing long-range interactions are ubiquitous in physics; transition metal oxides1,2, layered molecular crystals3 and trapped-ion arrays4 are a few examples. In the strongly interacting regime, these systems often show a rich variety of quantum many-body ground states that challenge theory2. The emergence of transition metal dichalcogenide moiré superlattices provides a highly controllable platform in which to study long-range electronic correlations5-12. Here we report an observation of nearly two dozen correlated insulating states at fractional fillings of tungsten diselenide/tungsten disulfide moiré superlattices. This finding is enabled by a new optical sensing technique that is based on the sensitivity to the dielectric environment of the exciton excited states in a single-layer semiconductor of tungsten diselenide. The cascade of insulating states shows an energy ordering that is nearly symmetric about a filling factor of half a particle per superlattice site. We propose a series of charge-ordered states at commensurate filling fractions that range from generalized Wigner crystals7 to charge density waves. Our study lays the groundwork for using moiré superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional extended Hubbard model3,13,14 or spin models with long-range charge-charge and exchange interactions15,16.

Thermally superactive artificial kagome spin ice structures obtained with the interfacial Dzyaloshinskii-Moriya interaction

Artificial kagome spin ice exhibits exotic magnetic correlations driven by a combination of geometric frustration and dipolar interactions that, at low-enough temperature, can result in ordered phases. This order, whether it is the ground state of several kagome rings, or the theoretically predicted long-range order of an extended array, has yet to be experimentally observed. By introducing an interfacial Dzyaloshinskii-Moriya interaction, we are able to reduce the blocking temperature of the individual nanomagnets, allowing a system of 30 kagome nanomagnets to explore its vast manifold of microstates and find its ground state. Furthermore, the extracted magnetic correlations in an extended artificial kagome spin ice are found to exhibit quantitative signatures of long-range charge order, providing evidence of the theoretically predicted continuous phase transition to the charge-ordered state. The significant lowering of the blocking temperature in nanomagnets is important for the exploitation of superparamagnetism in artificial spin systems and devices.

Neutron powder diffraction study of NaMn2O4 and Li0.92Mn2O4 : Insights on spin-charge-orbital ordering

The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed.

Crystal Structures and Physical Properties of Charge-Ordered Organic Conductor β-(MTDT-TTP)2BF4

Abstract We have prepared a new donor MTDT-TTP (bis(methylthio)dithio-tetrathia-pentalene) and obtained β-(MTDT-TTP)2BF4, which undergoes structural transition by ordering of the bis(methylthio) groups at 190 K. A crystal structure analysis indicated that the BF4 salt is in a charge-ordered (CO) state at room temperature (RT), although it shows a metallic behavior above 260 K. The CO pattern at RT has a four-molecular cycle. The BF4 salt has also the CO state below the transition temperature, but the CO pattern is changed by the structure change.

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

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

Anomalous 2D-Confined Electronic Transport in Layered Organic Charge-Glass Systems.

To get insight into the nature of the electronic fluid in the frustration-driven charge glasses, we investigate in-plane and out-of-plane charge transport for several quasitriangular-lattice organic systems: θ-(BEDT-TTF)_{2}X [X=RbZn(SCN)_{4}, CsZn(SCN)_{4}, and I_{3}]. These compounds host a charge order, charge glass, and Fermi liquid, depending on the strength of the charge frustration. We find that the resistivity exhibits extreme 2D anisotropy and contrasting temperature dependence between the in-plane and out-of-plane directions in the charge-glass phase, unlike in the charge order and metallic states. The experimental features indicate that the frustration-induced charge glass carries an anomalous 2D-confined electronic fluid with possible charge excitations other than conventional quasiparticles.

Thermal stability of the crystallographic structure of nanocrystalline Nd0.7Sr0.3MnO3 manganite with enhanced magnetic properties

We report the effect of temperature on the crystallographic structure and magnetic properties of ultrasonically prepared nanostructured Nd0.7Sr0.3MnO3 perovskite manganite. The crystal structure of as-synthesized nanoparticles remains unaltered over a wide scanning temperature range. Temperature dependent magnetization measurements demonstrate that the Curie temperature (Tc) of Nd0.7Sr0.3MnO3 nanoparticles is in the range of 211 K–220 K under largely varying applied magnetic fields. Below Tc, the soft ferromagnetic nature of these nanoparticles is confirmed by the field-dependent magnetization measurements. The absence of the charge-ordered state is also revealed in this nanomanganite down to 20 K, which is strikingly different from analogous Nd–Sr based nanocrystals. The experimentally observed effective paramagnetic moment and saturation magnetic moment have matched quite well with the values calculated theoretically. The Tc values up to a temperature of 220 K, nearly perfect ferromagnetically ordered Mn ions below Tc, high saturation magnetization, and magnetic softness of synthesized nanostructured Nd0.7Sr0.3MnO3 manganite can be associated with their good crystallinity as well as the nominal internal and surface disorder effect owing to intermediate particle size (∼75 nm to 150 nm). Our investigation elucidates the promising potential of nanocrystalline Nd0.7Sr0.3MnO3 particles for numerous technological applications.

Resurgence of ferromagnetic behavior coupled with Griffiths singularity in the electron doped La0.3Ca0.7MnO3 compound following the creation of oxygen deficiency

We report the effect of the oxygen deficiency on the structural and magnetic properties of La0.3Ca0.7MnO3-δ (0 ≤ δ ≤ 0.2) perovskites. The parent compound (δ = 0) was prepared by the solid- state reaction. This sample was used to prepare the oxygen deficiency compounds using iron as oxygen absorber. The X-ray diffraction (XRD) data have been analyzed by the Rietveld method using the FullProf program. We have noticed that all samples crystallized in the orthorhombic structure with Pbnm space group. Magnetic measurements show that the parent compound La0.3Ca0.7MnO3 exhibits a paramagnetic behavior at high temperature and an antiferromagnetic state (AFM) coupled with charge ordering (CO) at low temperature. With oxygen deficiency a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) state is fixed at decreasing temperature. The compound with δ = 0.15 exhibit a PM-FM transition when decreasing the temperature at TC = 191 K with persistence of the charge ordering state at low temperatures below 81 K. The compound with δ = 0.2 exhibit a PM-FM transition when the temperature decreases at TC = 134 K with vanishing of the CO state at low temperature. Further investigations of the magnetization measurements reveal features consistent with the appearance of Griffiths phase (GP) in the sample with δ = 0.15 and δ = 0.2 which proves the existence of ferromagnetic clusters in the paramagnetic domain below TG = 250 K and 234 K for δ = 0.15 and δ = 0.2 respectively.

How inactive d0 transition metal controls anionic redox in disordered Li-rich oxyfluoride cathodes

The disordered Li-rich oxyfluoride cathodes not only offer potentials for high energy density through cooperative cationic and anionic redox but also for facile lithium diffusion by formatting activated percolating channels. Generally, the d0 transition metals (TM) are considered as an essential facilitator for stabilization of the cation-disordered phase, but no redox activity. Nevertheless, their influence on reversible cationic and anionic redox is not fully understood, and consequently preventing further material design. Here, our studies for typical material Li2Mn1/2Ti1/2O2F unequivocally demonstrate that the local configurations containing redox-inactive d0 TM make more contribution to reversible capacity than those containing electrochemically active Mn2+. The peculiar effect is attributed to forming low-energy charge-ordering state that d0 TM prefers staying with Li–O–Li and pushes down O 2p unbonding state into a low-energy level. The present study reveals that a disordered oxyfluoride cathode design achieving a high reversible capacity may necessarily consider a suitable ratio of active low-charge TM to high-charge d0 TM.

Chemical Vapor Deposition of IrTe2 Thin Films

Two-dimensional (2D) IrTe2 has a profound charge ordering and superconducting state, which is related to its thickness and doping. Here, we report the chemical vapor deposition (CVD) of IrTe2 films using different Ir precursors on different substrates. The Ir(acac)3 precursor and hexagonal boron nitride (h-BN) substrate is found to yield a higher quality of polycrystalline IrTe2 films. Temperature-dependent Raman spectroscopic characterization has shown the q1/8 phase to HT phase at ~250 K in the as-grown IrTe2 films on h-BN. Electrical measurement has shown the HT phase to q1/5 phase at around 220 K.

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Charge ordering in superconducting copper oxides

Charge order has recently been identified as a leading competitor of high-temperature superconductivity in moderately doped cuprates. We provide a survey of universal and materials-specific aspects of this phenomenon, with emphasis on results obtained by scattering methods. In particular, we discuss the structure, periodicity, and stability range of the charge-ordered state, its response to various external perturbations, the influence of disorder, the coexistence and competition with superconductivity, as well as collective charge dynamics. In the context of this journal issue which honors Roger Cowley’s legacy, we also discuss the connection of charge ordering with lattice vibrations and the central-peak phenomenon. We end the review with an outlook on research opportunities offered by new synthesis methods and experimental platforms, including cuprate thin films and superlattices.

Magnetotransport studies of Fe vacancy-ordered Fe4+δSe5 nanowires

We studied the electrical transport of Fe4+δSe5 single-crystal nanowires exhibiting √5 × √5 Fe-vacancy order and mixed valence of Fe. Fe4+δSe5 compound has been identified as the parent phase of FeSe superconductor. A first-order metal-insulator (MI) transition of transition temperature T MI ∼ 28 K is observed at zero magnetic fields (B). Colossal positive magnetoresistance emerges, resulting from the magnetic field-dependent MI transition. T MI demonstrates anisotropic magnetic field dependence with the preferred orientation along the c axis. At temperature T < ∼17 K, the state of near-magnetic field-independent resistance, which is due to spin polarized even at zero fields, preserves under magnetic fields up to B = 9 T. The Arrhenius law shift of the transition on the source-drain frequency dependence reveals that it is a nonoxide compound with the Verwey-like electronic correlation. The observation of the magnetic field-independent magnetoresistance at low temperature suggests it is in a charge-ordered state below T ∼ 17 K. The results of the field orientation measurements indicate that the spin-orbital coupling is crucial in √5 × √5 Fe vacancy-ordered Fe4+δSe5 at low temperatures. Our findings provide valuable information to better understand the orbital nature and the interplay between the MI transition and superconductivity in FeSe-based materials.