Charge-ordered Insulator Research Articles

Phase Competition and Superconductivity in κ-(BEDT-TTF)2X: Importance of Intermolecular Coulomb Interactions

We theoretically study the competition among different electronic phases in molecular conductors $\kappa$-(BEDT-TTF)$_2$X. The ground-state properties of a 3/4-filled extended Hubbard model with the $\kappa$-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.

Doping-driven metal-insulator transitions and charge orderings in the extended Hubbard model

We perform a thorough study of an extended Hubbard model featuring local and nearest-neighbor Coulomb repulsion. Using dynamical mean-field theory we investigated the zero temperature phase-diagram of this model as a function of the chemical doping. The interplay between local and non-local interaction drives a variety of phase-transitions connecting two distinct charge-ordered insulators, i.e., half-filled and quarter-filled, a charge-ordered metal and a Mott insulating phase. We characterize these transitions and the relative stability of the solutions and we show that the two interactions conspire to stabilize the quarter-filled charge ordered phase.

Visualizing the evolution from the Mott insulator to a charge-ordered insulator in lightly doped cuprates

A central question in the high temperature cuprate superconductors is the fate of the parent Mott insulator upon charge doping. Here we use scanning tunneling microscopy to investigate the local electronic structure of lightly doped cuprate in the antiferromagnetic insulating regime. We show that the doped charge induces a spectral weight transfer from the high energy Hubbard bands to the low energy in-gap states. With increasing doping, a V-shaped density of state suppression occurs at the Fermi level, which is accompanied by the emergence of checkerboard charge order. The new STM perspective revealed here is the cuprates first become a charge ordered insulator upon doping. Subsequently, with further doping, Fermi surface and high temperature superconductivity grow out of it.

Avalanche-like metamagnetic transition in (LaNd)CaMnO manganites

We investigate the nature of ultrasharp metamagnetic transitions in La5/8-yNdyCa3/8MnO3 manganites. These compounds change from a low temperature ferromagnetic metallic state at low Nd doping to a charge-ordered antiferromagnetic insulator for high Nd content. At an intermediate doping a phase-separated state is established. At low temperatures (∼2 K), an avalanche-like field-induced metamagnetic transition is observed, where the entire compound changes abruptly from one phase to the other. The behavior of this ultrasharp transition is investigated using magnetization and specific heat measurements. We found a first order transition in the specific heat associated with discontinuous jumps in the magnetization. A strong increase of the sample temperature is simultaneously observed. The results are interpreted in terms of latent heat released from the field-induced enhancement of the ferromagnetic fraction, triggering the avalanche process.

Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy.

In electronic-type ferroelectrics, where dipole moments produced by the variations of electron configurations are aligned, the polarization is expected to be rapidly controlled by electric fields. Such a feature can be used for high-speed electric-switching and memory devices. Electronic-type ferroelectrics include charge degrees of freedom, so that they are sometimes conductive, complicating dielectric measurements. This makes difficult the exploration of electronic-type ferroelectrics and the understanding of their ferroelectric nature. Here, we show unambiguous evidence for electronic ferroelectricity in the charge-order (CO) phase of a prototypical ET-based molecular compound, α-(ET)2I3 (ET:bis(ethylenedithio)tetrathiafulvalene), using a terahertz pulse as an external electric field. Terahertz-pump second-harmonic-generation(SHG)-probe and optical-reflectivity-probe spectroscopy reveal that the ferroelectric polarization originates from intermolecular charge transfers and is inclined 27° from the horizontal CO stripe. These features are qualitatively reproduced by the density-functional-theory calculation. After sub-picosecond polarization modulation by terahertz fields, prominent oscillations appear in the reflectivity but not in the SHG-probe results, suggesting that the CO is coupled with molecular displacements, while the ferroelectricity is electronic in nature. The results presented here demonstrate that terahertz-pump optical-probe spectroscopy is a powerful tool not only for rapidly controlling polarizations, but also for clarifying the mechanisms of ferroelectricity.

Visualization of Melting of Antiferromagnetic Insulator Phase in Phase-Separated Manganite Film using Magnetic Force Microscopy

The phase separation and magnetic-field-induced transition of the antiferromagnetic charge-ordered insulator (AFI) phase into the ferromagnetic metal (FM) phase in an anisotropically-strained manganite thin film is directly imaged using a home-built magnetic force microscope (MFM). The MFM images at 10 K show that the two competing phases already coexist in zero magnetic field. Remarkably anisotropic distribution of the stripe-like phase domains are observed, which can qualitatively account for the anisotropic transport. Above 2.2 T, the AFI phase starts to transform into FM phase gradually. The melting of AFI phase is completed at 3.2 T. The FM phase can be retained after the magnetic field is removed, suggesting the metastable nature of the AFI phase at this temperature.

Evolution and control of the phase competition morphology in a manganite film.

The competition among different phases in perovskite manganites is pronounced since their energies are very close under the interplay of charge, spin, orbital and lattice degrees of freedom. To reveal the roles of underlying interactions, many efforts have been devoted towards directly imaging phase transitions at microscopic scales. Here we show images of the charge-ordered insulator (COI) phase transition from a pure ferromagnetic metal with reducing field or increasing temperature in a strained phase-separated manganite film, using a home-built magnetic force microscope. Compared with the COI melting transition, this reverse transition is sharp, cooperative and martensitic-like with astonishingly unique yet diverse morphologies. The COI domains show variable-dimensional growth at different temperatures and their distribution can illustrate the delicate balance of the underlying interactions in manganites. Our findings also display how phase domain engineering is possible and how the phase competition can be tuned in a controllable manner.

Exchange-bias field induced by surface inhomogeneities in ferromagnetic/charge-ordered bilayer structure

Epitaxial bilayer structure consisting of ferromagnetic metallic Pr0.7Sr0.3MnO3 (PSMO) and charge-ordered insulator La0.5Ca0.5MnO3 (LCMO) was fabricated on (001) SrTiO3 substrate by pulsed laser deposition. High-resolution synchrotron X-ray diffraction showed high quality of epitaxial layer. However, besides diffraction peaks from PSMO layer, LCMO layer and SrTiO3 substrate, we observed an additional shoulder peak, which might stem from the inhomogeneities of composition in PSMO/LCMO. Further the atomic force microscopy measurement showed the presence of non-stoichiometric large particulates at surface, imparting an overall inhomogeneous composition to the film. This implied that the variation of crystalline structure of PSMO/LCMO occurred due to inhomogeneous composition. Moreover, studies on magnetic properties showed that surface inhomogeneities and ferromagnetic clusters at the PSMO/LCMO interface probably influenced the ferromagnetism of the bilayer film together, tuning exchange bias effect.

Correlation between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4

We examine the interplay between quantum charge fluctuations and magnetic ordering in multiferroic LuFe2O4 and show that this can couple spin and charge degrees of freedom in a LuFe2O4 bilayer below the Neel temperature TN. Our analysis supports the idea that the double exchange mechanism normally used in metallic systems can be applied to charge-ordered insulators. This causes ferrimagnetic spin order to reduce the transfer integrals between Fe2+ and Fe3+ in LuFe2O4, decreasing charge fluctuations and increasing the polarization in this system below TN. This work thus provides a more detailed understanding of the mechanism for spin-charge coupling in LuFe2O4.

Characterization of the quasi-one-dimensional compounds δ-(EDT-TTF-CONMe2)2X, X=AsF6 and Br by vibrational spectroscopy and density functional theory calculations

We have investigated the infrared spectra of the quarter-filled charge-ordered insulators δ-(EDT-TTF-CONMe2)2X (X= AsF6, Br) along all three crystallographic directions in the temperature range from 300 to 10 K. DFT-assisted normal mode analysis of the neutral and ionic EDT-TTF-CONMe2 molecule allows us to assign the experimentally observed intramolecular modes and to obtain relevant information on the charge ordering and intramolecular interactions. From frequencies of charge-sensitive vibrations we deduce that the charge-ordered state is already present at room temperature and does not change on cooling, in agreement with previous NMR measurements. The spectra taken along the stacking direction clearly show features of vibrational overtones excited due to the anharmonic electronic molecule potential caused by the large charge disproportionation between the molecular sites. The shift of certain vibrational modes indicates the onset of the structural transition below 200 K.

Current-Induced Gap Suppression in the Mott Insulator Ca2RuO4

We present nonlinear conduction phenomena in the Mott insulator Ca2RuO4 investigated with a proper evaluation of self-heating effects. By utilizing a non-contact infrared thermometer, the sample temperature was accurately determined even in the presence of large Joule heating. We find that the resistivity continuously decreases with currents under an isothermal environment. The nonlinearity and the resulting negative differential resistance occurs at relatively low current range, incompatible with conventional mechanisms such as hot electron or impact ionization. We propose a possible current-induced gap suppression scenario, which is also discussed in non-equilibrium superconducting state or charge-ordered insulator.

Theory of strain-controlled magnetotransport and stabilization of the ferromagnetic insulating phase in manganite thin films.

We show that applying strain on half-doped manganites makes it possible to tune the system to the proximity of a metal-insulator transition and thereby generate a colossal magnetoresistance (CMR) response. This phase competition not only allows control of CMR in ferromagnetic metallic manganites but can be used to generate CMR response in otherwise robust insulators at half-doping. Further, from our realistic microscopic model of strain and magnetotransport calculations within the Kubo formalism, we demonstrate a striking result of strain engineering that, under tensile strain, a ferromagnetic charge-ordered insulator, previously inaccessible to experiments, becomes stable.

Resistivity anisotropy of layered cobaltite Na x CoO2 thin films

Layered cobaltite NaxCoO2 thin films with tilted c-axis have been grown on vicinal cut SrTiO3 (100) substrates by pulse laser deposition. The single phase and the epitaxial growth aligned to the c-axis of vicinal cut substrates have been studied by X-ray diffraction analysis. The resistivity measurements along and perpendicular to the substrate tilted direction in the plane, and the physical model that current in the inclined direction is composed of the in-plane and out-of-plane parts are employed to attain the resistivity anisotropy ρc/ρab. In NaxCoO2, ρc is larger than ρab, which is consistent with its structure that insulating Na+ layers stack along c-axis. ρc/ρab decreases with increasing Na concentration due to the enhancement of inter-plane electron hopping for large x. The charge order insulator Na0.5+δCoO2 has a sharp rise of ρc/ρab when temperature below 50 K. These observations can improve the present understanding of the layered cobaltite thermoelectric oxides.

Coherent Electron Dynamics in 10 fs Time Scale in Organic Charge Ordered and Dimer-Mott Insulators

Coherent oscillations of correlated electrons were detected in the early dynamics of photoinduced melting and/or construction of the organic charge ordered/ferroelectric cluster and the dimer Mott insulator by using 3 optical-cycle infrared pulse.

Photoinduced Phase Transitions in α-, θ-, and κ-type ET Salts: Ultrafast Melting of the Electronic Ordering

Photoinduced phase transitions in organic compounds with strong electron correlation ET [bis(ethylenedithio)-tetrathiafulvalene)-based salts α-(ET)2I3, θ-(ET)2RbZn(SCN)4, κ-(d-ET)2Cu[N(CN)2Br] were discussed based, on time resolved optical pump-probe spectroscopy using ~150 fs mid-infrared pulse, 12 fs near infrared pulse, and sub-picosecond terahertz pulse. (i) In charge-ordered insulators α-(ET)2I3 and θ-(ET)2RbZn(SCN)4, we captured ultrafast snapshots of charge dynamics i.e., immediate (ca. 15 fs) generation of a microscopic metallic state (or equivalently the microscopic melting of the charge order) which is driven by the coherent oscillation (period; 18 fs) of correlated electrons. Subsequently, condensation of the microscopic metallic state to the macroscopic scale occurs in α-(ET)2I3. However, in θ-(ET)2RbZn(SCN)4, such condensation is prevented by the large potential barrier reflecting the structural difference between the insulator and metal; (ii) In a Dimer–Mott insulator κ-(d-ET)2Cu[N(CN)2Br], photogeneration of the metallic state rises during ca. 1 ps that is much slower than the melting of charge order, because the photoinduced insulator to metal transition is driven by the intradimer molecular displacement in the dimer Mott insulator. The ultrafast dynamics of photoinduced insulator–metal transitions depend strongly on the molecular arrangement, reflecting various competing phases in the ET sheets.

Interplay between Correlated Electrons and Quantum Phonons in Charge-Ordered and Mott-Insulating Organic Compounds

At an early stage of the photoinduced transition from an insulator to a metal in quasi-two-dimensional organic conductors, a coherent motion of electrons is observed in a charge-ordered insulator, but not so far in a Mott insulator. The mechanisms of these different photoinduced charge dynamics are theoretically studied by numerical solutions to the time-dependent Schrodinger equation for exact many-electron–phonon wave functions on small clusters of model systems. We use two-dimensional three-quarter-filled extended Holstein–Hubbard models on anisotropic triangular lattices. For a charge-ordered insulator on a lattice simplified from the structure of α-(BEDT-TTF)2I3, we indeed find a low-energy collective electronic motion coupled with quantum phonons even if the energy of photoexcitation is away from this energy. For a Mott insulator on a lattice simplified from the structure of κ-(BEDT-TTF)2X, however, such a collective motion does not appear, and quantum phonons are hardly excited.

Theory of Photoinduced Phase Transitions in Molecular Conductors: Interplay Between Correlated Electrons, Lattice Phonons and Molecular Vibrations

Dynamics of photoinduced phase transitions in molecular conductors are reviewed from the perspective of interplay between correlated electrons and phonons. (1) The charge-transfer complex TTF-CA shows a transition from a neutral paraelectric phase to an ionic ferroelectric phase. Lattice phonons promote this photoinduced transition by preparing short-range lattice dimerization as a precursor. Molecular vibrations stabilize the neutral phase so that the ionic phase, when realized, possesses a large ionicity and the Mott character; (2) The organic salts θ-(BEDT-TTF)2RbZn(SCN)4 and α-(BEDT-TTF)2I3 show transitions from a charge-ordered insulator to a metal. Lattice phonons make this photoinduced transition hard for the former salt only. Molecular vibrations interfere with intermolecular transfers of correlated electrons at an early stage; (3) The organic salt κ-(d-BEDT-TTF)2Cu[N(CN)2]Br shows a transition from a Mott insulator to a metal. Lattice phonons modulating intradimer transfer integrals enable photoexcitation-energy-dependent transition pathways through weakening of effective interaction and through introduction of carriers.

Magnetotransport properties of Pr0.5Ca0.5MnO3 thin films grown by a solution route

Thin films of Pr0.5Ca0.5MnO3 were fabricated on (001) oriented SrLaAlO4, NdGaO3, and SrTiO3 substrates using a hybrid solution route and spin coating techniques. Good crystalline and epitaxial quality of the films was confirmed with X-ray diffraction and transmission electron microscopy studies. Strain in the film grown on NdGaO3 substrate did not relax during annealing process and the film exhibited charge-ordered insulator phase at low temperatures even with magnetic fields up to 9 T. However, the films on SrLaAlO4 and SrTiO3 substrates (with partially relaxed compressive and tensile strain, respectively) displayed melting of the charge-ordered phase with applied magnetic fields of less than 5 T. The results suggest that strain-relaxation rather than only the type of strain plays an important role in lowering critical melting magnetic fields in these films.

Photogenerated Metallic States in Charge-Ordered Insulators in (BEDT-TTF)2X

We theoretically investigate the physical properties of the photoexcited states in the charge-ordered states in α-(BEDT-TTF) 2 I 3 and θ-(BEDT-TTF) 2 RbZn(SCN) 4 . We adopt the 1/4-filled extended Hubbard Hamiltonian for holes on the two-dimensional anisotropic triangular lattice. The photoexcited states are numerically calculated by solving the time-dependent Schrodinger equation subject to pump light. In the lower-energy region of pump light photons, an extremely small number of energy eigenstates dominate the transition moment from the ground state. These energy eigenstates are collective excited states that have the charge order, and they are insulating as a result of the charge order. In the higher-energy region, the density of energy eigenstates that contribute to the light absorption spectrum, is much larger than that in the lower-energy region, and these energy eigenstates are metallic and have no charge order. The present results show that the metallic states are directly generated in the early s...

Early-Stage Dynamics of Light-Matter Interaction Leading to the Insulator-to-Metal Transition in a Charge Ordered Organic Crystal

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator α-(BEDT-TTF)2I3 were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.