Charge-ordered Phase Research Articles

Effects of Bi3+ doping on the charge ordering and high-field phase diagram of La0.5-xBixCa0.5MnO3

La0.5-xBixCa0.5MnO3 (x = 0, 0.05, 0.1 and 0.2) compounds have been prepared to study the effects of Bi3+ substitution on the structure, magnetic and magnetotransport properties. Experimental results show that Bi3+ dopants have induced a distinct lattice distortion and phase separated state in which the long range ferromagnetic order turns into short range ferromagnetic clusters, and the charge ordering/antiferromagnetism and ferromagnetic clusters coexist in the system. Bi3+ doping induced disorders lead to the presence of a Griffith-like phase. The hybridization between Bi3+-6s orbitals and O2–-2p orbitals produces a localization of O2–-2p electrons and lattice distortion, which plays an important role in favoring charge ordering state and multiple magnetic ordering. The charge ordering antiferromagnetic collapses into ferromagnetic phase through a metamagnetic transition with applying a sufficiently high magnetic field. A T-HC phase diagram was determined based on the magnetization and electrical transport measurements, in which the phase boundary is figured out.

Effects of Vanadium Doping on the Charge Ordering and Low-Temperature Spin-Glass Phase in Pr0.45ca0.55mno3

Effects of Vanadium Doping on the Charge Ordering and Low-Temperature Spin-Glass Phase in Pr0.45ca0.55mno3

Required nearest-neighbor Coulomb interactions for a charge-ordered phase transition in (TMTTF)2MF6 with inversion symmetry breaking in crystal

We examined the charge-ordered phase transition in (TMTTF)2MF6 systems by exact diagonalization of an extended Hubbard model Hamiltonian whose parameters were referred from quantum chemical calculations with periodic boundary conditions along with inversion symmetry breaking in the crystal. We showed that a nearest-neighbor Coulomb interactions for the charge-ordered phase transition should be twice as much as those previously obtained by fitting for conductivities with a minute inversion symmetry breaking of the crystal (∼0.01 Å) which was confirmed by a recent experimental study.

Break of symmetry at the surface of IrTe2 upon phase transition measured by x-ray photoelectron diffraction

IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical x-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T-IrTe2 in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2 trigonal to a monoclinic unit cell and confirm the occurrence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

Ultrafast control of electronic states by a terahertz electric field pulse in the quasi-one-dimensional organic ferroelectric (TMTTF)2PF6

A strong terahertz pulse is effective for controlling the macroscopic polarization in ferroelectrics. In the present study, we investigated the response of an organic molecular compound, namely ${(\mathrm{TMTTF})}_{2}\mathrm{P}{\mathrm{F}}_{6}$ (TMTTF: tetramethyltetrathiafulvalene), to a strong electric field using terahertz pulse-pump optical-reflectivity probe spectroscopy. This compound undergoes a transition from Mott insulator to charge-order insulator with lowering temperature, and exhibits electronic ferroelectricity in the charge-order phase. When the terahertz pulse is applied in the Mott-insulator phase, an ultrafast reflectivity change proportional to the square of the electric field waveform of the terahertz pulse emerges, which is attributed to the generation of charge disproportionation in each dimer and the resultant creation of macroscopic polarization. When the terahertz pulse is applied in the charge-order phase, an ultrafast reflectivity change proportional to the electric field waveform of the terahertz pulse is observed, which originates from the modulation of the original charge disproportionation and polarization. These ultrafast reflectivity changes can be ascribed to purely electronic responses. In the midinfrared region, where totally symmetric $({a}_{\mathrm{g}})$ modes of intramolecular vibrations coupled with intermolecular charge transfers exist, a large reflectivity change is commonly observed in the Mott-insulator and charge-order phases. To interpret this feature, we constructed a model that incorporates a charge-transfer transition and ${a}_{g}$-mode intramolecular vibrations. The analyses of the results with this model revealed that the change in the reflectivity spectrum by the terahertz electric field can be explained by the energy shift of the charge-transfer transition caused by the electric field-induced change of charge disproportionation in each dimer, and the transfer of the spectral weight from the intradimer charge-transfer (CT) transition to the interdimer CT transition resulting from the weakening of the dimerization. Our model can be used to analyze the optical responses to electric fields in various organic molecular compounds with electron-intramolecular vibration couplings.

The effect of La3+ substitution on the remanent magnetization, ferromagnetism, and antiferromagnetic charge-ordered phase in Sm0.5Ca0.5MnO3

The effect of La3+ substitution on the remanent magnetization, ferromagnetism, and antiferromagnetic charge-ordered phase in Sm0.5Ca0.5MnO3

Interacting spinless fermions on the square lattice: Charge order, phase separation, and superconductivity

We investigate the phase diagram of spinless fermions on a square lattice with nearest-neighbor interaction, using the recently developed projective truncation approximation in Green's function equation of motion. For attractive interaction, the ground state is in an homogeneous p + ip superconducting (SC) phase at high or low electron densities. Near half filling is a phase separation (PS) between the SC phases. Allowing inhomogeneous solution, we obtain p-wave SC domains with positive interface energy. As temperature increases, the SC phases transit into normal phases above Tsc, generating an homogeneous normal phase (far away from n = 1/2), or a PS between normal phases with different densities (close to n = 1/2). Further increasing temperature to Tps, the PS disappears and the particle-hole symmetry of the Hamiltonian is recovered. For repulsive interaction, depending on electron filling, the ground state is in charge-ordered phase (half filling), charge-disordered phase (large hole/electron doping), or PS between them (weak doping). At finite temperature, the regime of charge order phase moves to finite V and extends to finite doping regime.

First-Principles Study on the Stability and Electronic Structure of the Charge-Ordered Phase in α-(BEDT-TTF)2I3

We theoretically study the structural and electronic properties of a molecular conductor, α-(BEDT-TTF)2I3, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with a conventional exchange-correlation functional within the generalized gradient approximation. By performing structural optimization, we found a stable charge-ordered solution for the former, in contrast to the latter approach where the magnitude of the charge imbalance becomes considerably small compared to that when the experimental structure is adopted. The electronic band structure near the Fermi level, with and without structural optimization, as well as the molecule-dependent local density of states of the charge-ordered state are discussed.

Schottky-like anomaly in the heat capacity and magnetocaloric effect of charge-ordered single-crystalline (Sm, Ca, Sr)MnO[formula omitted] compound

We present a comprehensive experimental study on the magnetic and magnetocaloric properties of a charge-ordered single-crystalline Sm0.5Ca0.25Sr0.25MnO3 compound. The studies on x-ray photoelectron spectroscopy (XPS) reveals the presence of an equal distribution of Mn3+ and Mn4+ ions in the studied system. The Oxygen, O1s-core level spectra have been simulated with three binding energies curves, which correspond to the O2− ions, O1− ions, and chemically adsorbed oxygens, Ochem. The XPS analysis of the O1s-core-level spectra and magnetic characterizations indicate the proper stoichiometry of the present sample. Considering the change of volume phase fraction in the isofield magnetization measurements during the first-order magnetic phase transition from paramagnetic state to ferromagnetic state, the isothermal magnetic entropy change (ΔS) has been estimated based on the modified Clausius–Clapeyron equation. An inverse magnetocaloric effect has also been noticed in the -ΔS vs. T plot calculated by Maxwell’s thermodynamic relation, suggesting the dominant antiferromagnetic ground state supported by a charge-ordered phase of the studied system. The high-temperature zero-field heat capacity (CP) data can be well-interpreted quantitatively using the Debye model of heat capacity. With the extracted magnetic heat capacity (Cmag) data, the temperature variation of the magnetic entropy (S(0)), as well as the adiabatic temperature change (ΔTad), have been estimated. In addition to that, the low-temperature CP data displays a Schottky-like anomaly in the temperature region between 2 K and 20 K. The experimental data points are successfully fitted by considering the various contributing factors of the low-temperature heat capacity such as the lattice-phonon vibration (Clat), antiferromagnetic spin-wave (Cmag), and the two-level Schottky function (Csch) due to the energy splitting of the Sm3+ cations.

Stability of charge ordering in La0.5−xHoxCa0.5MnO3 polycrystalline manganites

Magnetotransport properties of La0.5−xHoxCa0.5MnO3 (x = 0.05 and 0.1) polycrystalline samples were investigated in order to study the effect of magnetic Ho3+ ions on the stability of the charge ordering (CO) and magnetic phase coexistence in pristine La0.5Ca0.5MnO3. Our samples were synthesized by using solgel method. Temperature dependence of resistivity shows an insulating behavior with high resistivity values, confirming the presence of long-range CO. The application of 7 T magnetic field slightly reduces resistivity values and demonstrates relatively small magnetoresistance (MR) values (−MR(T) did not exceed 14% under 7 T applied field). This observation indicates that the CO in the Ho-based specimens is strong, which indicates that Ho substitution for La in La0.5Ca0.5MnO3 gives more strength to the CO.

Coexistence of Charge Order and Antiferromagnetic Order in an Extended Periodic Anderson ModelSupported by the National Natural Science Foundation of China (Grant Nos. 11974048 and 11974049), and the Beijing Science Foundation (Grant No. 1192011).

The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition, which occurs ubiquitously in heavy fermion materials. However, there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce- and Y-based compounds. We study an extended periodic Anderson model (EPAM) which includes the onsite Coulomb repulsion Ucf between the localized electrons and conduction electrons. By employing the density matrix embedding theory, we investigate the EPAM in the symmetric case at half filling. By fixing the onsite Coulomb repulsion U of the localized electrons to an intermediate value, the interplay between the RKKY interaction, the Kondo effect and the Coulomb repulsion Ucf brings rich physics. We find three different phases, the antiferromagnetic phase, the charge order phase and paramagnetic phase. When the hybridization strength V between the localized orbital and the conduction orbital is small, the Kondo effect is weak so that the AF phase and the CO phase are present. The phase transition between the two long-range ordered phase is of first order. We also find a coexistence region between the two phases. As V increases, the Kondo effect becomes stronger, and the paramagnetic phase appears between the other two phases.

Tracking motion of topological defects in a stripe charge-ordered phase with continuously variable temperature cryo-STEM

Tracking motion of topological defects in a stripe charge-ordered phase with continuously variable temperature cryo-STEM

Electrostatic carrier doping of charge-ordered YbFe2O4 thin films using ionic liquids

We report the modulation of electrical properties of charge-ordered YbFe2O4 films using electrostatic carrier doping. By utilizing the current switching effect under a high DC electric field as a probe, we detected the charge-ordered phase in the c-axis-oriented YbFe2O4 films grown by pulsed-laser deposition. A three-terminal electric double-layer device with a thin channel of YbFe2O4 and an ionic liquid (IL) gate dielectric showed reversible modulation of channel conductance under a positive gate voltage. Furthermore, we demonstrated that conductance modulation could be enhanced by customizing the molecular structure of the IL.

Insensitivity of the striped charge orders in IrTe2 to alkali surface doping implies their structural origin

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide IrTe$_2$ upon potassium (K) deposition on its surface. Pristine IrTe$_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Supported by density functional theory calculations, we first show that the K atoms dope the topmost IrTe$_2$ layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of $0.13$ and $0.21$~monolayer (ML). Using LEED, we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the charge-ordered phase transitions of IrTe$_2$ are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in IrTe$_2$.

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.

Charge-order dynamics in underdoped La1.6−xNd0.4SrxCuO4 revealed by electric pulses

The dynamics of the charge-order domains has been investigated in La1.48Nd0.4Sr0.12CuO4, a prototypical stripe-ordered cuprate, using pulsed current injection. We first identify the regime in which nonthermal effects dominate over simple Joule heating and then demonstrate that, for small enough perturbation, pulsed current injection allows access to nonthermally induced resistive metastable states. The results are consistent with the pinning of the fluctuating charge order, with fluctuations being most pronounced at the charge-order onset temperature. The nonequilibrium effects are revealed only when the transition is approached from the charge-ordered phase. Our experiment establishes pulsed current injection as a viable and effective method for probing the charge-order dynamics in various other materials.

Charge and anion ordering in the quasi-one-dimensional organic conductor (TMTTF)2NO3

The quasi-one-dimensional organic conductors (TMTTF)$_2X$ with non-centrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X$ = BF$_4^-$, ReO$_4^-$, and ClO$_4^-$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (TMTTF)$_2$NO$_3$. Here we explore the electronic and structural transitions by transport experiments, optical and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance 2$\delta$ and an activated behavior of $\rho(T)$ with $\Delta_{\rm CO}\approx 530$~K below $T_{\rm CO} = 250$~K. Since (TMTTF)$_2$NO$_3$ follows the universal relation between charge imbalance 2$\delta$ and size of the gap $\Delta_{\rm CO}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at $T_{\rm AO}=50$~K. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.

Effects of charge ordering in electronic subsystem of quasi-2D BEDT-TTF conductors

The interplay of the inter-site Coulomb repulsion which induces charge ordering, and the correlated hopping of electrons, which causes peculiar dependence of energy characteristics on a doping, is studied in a model of BEDT-TTF organic conductor electronic subsystem. By applying the projection method in the Green function equation of motion approach the single-particle energy spectrum, sublattice magnetization in charge-ordered phase, and the transition temperature have been calculated. Dependence of Verwey temperature on the band filling shows that inter-site Coulomb correlation significantly influences the width of the concentration interval in which a charge order stabilizes. The correlated hopping causes the decrease of the critical value of electron concentration and increase of Verwey temperature in whole concentration interval of charge ordering stability.

Comparative studies on the structural, magnetic, and optical properties of perovskite Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) manganite nanoparticles synthesized by sol–gel method

Comparative studies of the structural, magnetic, and optical properties of the sol–gel synthesized Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) nanoparticles were carried out focusing on the effect of the A-site average cation size ⟨rA⟩. Rietveld refinements of x-ray diffraction data demonstrate all nanoparticles crystallize in an orthorhombic crystal structure (Pnma space group). Their unit cell volumes and the Mn–O–Mn bond angle decreased with reducing ⟨rA⟩, whereas the Mn–O bond length increased. The morphologies of nanoparticles evolved from spherical to irregular shapes, and their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching mode of the Mn–O bond near 600 cm−1, and the softening of this phonon mode as reducing ⟨rA⟩ is ascribed to the elongation of the Mn–O bond length. X-ray photoelectron spectroscopy reveals the mixed Mn3+ and Mn4+ cations with a content ratio of Mn3+/Mn4+ = 2:1, divalent Ca cations, and trivalent rare earth Ln cations in all nanoparticles and oxygen element existing as lattice oxygen and chemically absorbed oxygen. The La0.67Ca0.33MnO3 nanoparticles exhibited ferromagnetic behavior, whereas Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles displayed antiferromagnetic behavior and strong exchange bias effect. Temperature dependence of dc magnetizations suggests the spin-glass behavior established in the La0.67Ca0.33MnO3 nanoparticles, while magnetic cluster-glass behavior formed in the Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles, in which the charge-ordered and antiferromagnetic phases were completely suppressed. Electronic bandgaps of the nanoparticles were about 1.55 eV–1.66 eV, which was ascribed to the electronic charge transfer between two eg bands of the Mn cation with up-spins and down-spins separated by Hund’s coupling energy.

Substitutional tuning of electronic phase separation in CaFe3O5

Electronic phase separation into charge-ordered (CO) and charge-averaged (CA) phases in $\mathrm{Ca}{\mathrm{Fe}}_{3\ensuremath{-}x}{M}_{x}{\mathrm{O}}_{5}$ samples for dopants $M=\mathrm{Mn}$ and Co has been investigated using powder neutron and x-ray diffraction, magnetization, and M\"ossbauer spectroscopy measurements. Electronic phase separation is observed in lightly doped $\mathrm{Ca}{\mathrm{Fe}}_{3}{\mathrm{O}}_{5}$ samples, where $4--10%$ Ca is replaced by Mn, Fe, or Co, and the CA ground state is stabilized at higher doping levels. The CO and CA phases are found to emerge below a common magnetic ordering temperature at $300--320\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ providing strong evidence for a lattice strain-driven mechanism that couples their magnetic transitions.