Mixed-stack Organic Charge-transfer Research Articles

Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides

We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity ${\mathrm{Cu}}^{2+}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{Cu}}^{1+}$ in both families upon doping, at or near optimal doping in the conventionally prepared electron-doped compounds and at the pseudogap phase transition in the hole-doped materials. In thin films of the ${\mathrm{T}}^{\ensuremath{'}}$ compounds, the valence transition has occurred already in the undoped state. The phenomenology of the valence transition is closely related to that of the neutral-to-ionic transition in mixed-stack organic charge-transfer solids. Doped cuprates have negative charge-transfer gaps, just as rare-earth nickelates and ${\mathrm{BaBiO}}_{3}$. The unusually high ionization energy of the closed shell ${\mathrm{Cu}}^{1+}$ ion, taken together with the doping-driven reduction in three-dimensional Madelung energy and gain in two-dimensional delocalization energy in the negative charge transfer gap state drives the transition in the cuprates. The combined effects of strong correlations and small $d\ensuremath{-}p$ electron hoppings ensure that the systems behave as effective 1/2-filled Cu band with the closed shell electronically inactive ${\mathrm{O}}^{2\ensuremath{-}}$ ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole band, now with electronically inactive closed-shell ${\mathrm{Cu}}^{1+}$ ions, in the doped state. The model thus gives microscopic justification for the two-fluid models suggested by many authors. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states. The robust commensurate antiferromagnetism in the conventional ${T}^{\ensuremath{'}}$ crystals, the strong role of oxygen deficiency in driving superconductivity and charge carrier sign corresponding to holes at optimal doping are all manifestations of the same quantum state. In the hole-doped pseudogapped state, there occurs a biaxial commensurate period 4 charge density wave state consisting of ${\mathrm{O}}^{1\ensuremath{-}}\ensuremath{-}{\mathrm{Cu}}^{1+}\ensuremath{-}{\mathrm{O}}^{1\ensuremath{-}}$ spin singlets that coexists with broken rotational ${\mathrm{C}}_{4}$ symmetry due to intraunit cell oxygen inequivalence. Finite domains of this broken symmetry state will exhibit two-dimensional chirality and the polar Kerr effect. Superconductivity within the model results from a destabilization of the 1/4-filled band paired Wigner crystal [Phys. Rev. B 93, 165110 (2016) and Phys. Rev. B 93, 205111 (2016)]. We posit that a similar valence transition, ${\mathrm{Ir}}^{4+}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{Ir}}^{3+}$, occurs upon electron doping ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. We make testable experimental predictions in cuprates including superoxygenated ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ and iridates. Finally, as indirect evidence for the valence bond theory of superconductivity proposed here, we note that there exist an unusually large number of unconventional superconductors that exhibit superconductivity proximate to exotic charge ordered states, whose band fillings are universally 1/4 or 3/4, exactly where the paired Wigner crystal is most stable.

Temperature-induced valence and structural instability in DMTTF-CA: Single-crystal Raman and infrared measurements

We report a detailed spectroscopic investigation of temperature-induced valence and structural instability of the mixed-stack organic charge-transfer (CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal exhibiting the neutral-ionic transition by lowering temperature. We confirm that DMTTF-CA undergoes a continuous variation of the ionicity on going from room temperature down to $\sim$ 20 K, but remains on the neutral side throughout. The stack dimerization and cell doubling, occurring at 65 K, appear to be the driving forces of the transition and of the valence instability. In a small temperature interval just below the phase transition we detect the coexistence of molecular species with slightly different ionicities. The Peierls mode(s) precursors of the stack dimerization are identified.

Anomalous Dispersion of Optical Phonons at the Neutral-Ionic Transition: Evidence from Diffuse X-Ray Scattering

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.

Electronic and structural instabilities of mixed-stack organic charge-transfer salts

Electronic (valence) and structural (Peierls) instabilities occur either separately or together in organic charge-transfer (CT) crystals with mixed stacks of donors (D) and acceptors (A). A Peierls–Hubbard model, H CT, with site D, A energies and linear electron–phonon coupling in a harmonic lattice is shown to provide a unified microscopic description and to distinguish between thermal and quantum Peierls transitions. Vibrational, magnetic and structural data are interpreted as thermal Peierls transitions in two ionic CT salts and as dimerized ground states with spin solitons in other salts. Quantum Peierls transitions are identified in largely neutral salts, and a neutral–ionic crossover is discussed in a salt with dipolar disorder that suppresses the Peierls instability. The coincident Peierls and neutral–ionic transition of tetrathiafulvalene-chloranil (TTF-CA) is another special case of H CT.

Crystallographic investigation of temperature-induced phase transition of the tetrathiafulvalene-p-bromanil, TTF-BA charge transfer complex

The phase transition of 1:1 mixed-stack organic charge-transfer crystal of tetrathiafulvalene-$p$-bromanil (TTF-BA) has been investigated by high resolution x-ray diffraction. The temperature-induced phase transition to a nonmagnetic state at 53 K was characterized by a dimeric distortion of the donor-acceptor stack similar to that observed in charge transfer complexes displaying a neutral-to-ionic transition. But in this case the dimerization process does not change the charge transfer between donor and acceptor molecules. Molecular packing in mixed stacks along two orthogonal crystallographic axes and the role of inter- and intrastack interactions are discussed.

Theory of photoinduced phase dynamics in organic charge-transfer complexes

In the quasi-one-dimensional mixed-stack organic charge-transfer complex, TTF-CA, photoirradiation is known to trigger transitions between the neutral and ionic phases. Here we use a one-dimensional extended Peierls–Hubbard model with alternating potentials and calculate the mean-field dynamics of charge density coupled with that of lattice displacements. We show qualitative differences between the photoinduced ionic-to-neutral and neutral-to-ionic transitions, which are consistent with recent experimental findings. That is, the ionic-to-neutral transition proceeds cooperatively, characterized by threshold behavior, while the neutral-to-ionic transition proceeds uncooperatively, characterized by linear behavior. The threshold absorption in the former depends on the strength or the duration of the pulse. The coherence during the transition dynamics is also different between the two transitions. In the ionic-to-neutral transition, a clear interference effect is observed as a function of the interval when the pulse is spilt into two.

Phase transition on (BEDO-TTF) (Cl2TCNQ): Spin dynamics observed by NMR measurements

13C NMR measurements were carried out to investigate the electronic state of Mixedstack organic charge-transfer complex (BEDO-TTF)(Cl 2 TCNQ). From the remarkable temperature dependence of the central shift and relaxation rate, we have confirmed that the transition at 110 K is from paramagnetic to nonmagnetic spin-singlet phase.

Finite-temperature phase diagram of mixed-stack charge-transfer complexes

The mixed-stack organic charge-transfer complex, (BEDO–TTF)(Cl 2TCNQ), has shown a phase transition accompanied with a sharp drop in the magnetic susceptibility and with an increase in the intermolecular overlap, as the temperature is lowered. Here, we theoretically consider the possibility for a phase transition from the ionic phase to a neutral phase, which is driven by charge-transfer fluctuations. Using finite-temperature density-matrix renormalization-group calculations for the one-dimensional extended Hubbard model with alternating potentials, we show that, with increasing transfer integral t, a transition from the neutral phase to the ionic phase is induced by spin fluctuations for small t, while another transition from the ionic phase to the neutral phase is induced by charge-transfer fluctuations for large t. Near the phase boundary, the free energy is easily lowered by dimerization of transfer integrals or by staggered magnetic field. Thus, a further transition is also possible to a dimerized nonmagnetic phase or to an antiferromagnetic phase at a lower temperature. In two dimensions, we expect that the effect of charge-transfer fluctuations is larger and that the transition to the dimerized nonmagnetic phase is suppressed.

A new type of neutral–ionic interface in mixed stack organic charge transfer crystals: Temperature induced ionicity change in ClMePD–DMeDCNQI

Raman and polarized infrared spectra of the mixed stack charge transfer crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed spectral interpretation allows us to gain new insight into the temperature induced neutral-ionic transition in this compound. In particular, the crossing of the neutral-ionic borderline appears to be quite different from that of the few known temperature induced neutral-ionic phase transitions. First of all, the ionicity change is continuous. Furthermore, the onset of stack dimerization precedes, rather than accompanies, the neutral-ionic crossing. The (second order) phase transition is then driven by the dimerization, but the extent of dimerization is in turn affected by the ionicity change.

Mixed-stack organic charge-transfer complexes with intercolumnar networks

We report synthesis and examination of the electronic properties of an isomorphous series of mixed-stack organic charge-transfer complexes, composed of [bis(ethylenedithio)tetrathiafulvalene] (BEDT-TTF)-based electron donors and electron acceptors of halogen-substituted tetracyanoquinodimethanes (TCNQ's). Single crystals of ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{M}\mathrm{e}}_{2}\mathrm{TCNQ}),$ (BEDT-TTF)(ClMeTCNQ), and ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{O}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{C}\mathrm{l}}_{2}\mathrm{TCNQ})$ were investigated as to their crystal structures, and optical and magnetic properties. The intermolecular overlaps between the stacked columns were comparable to the face-to-face overlap between the donor and the acceptor molecules inside the stacks. As seen from the optical and magnetic measurements, the ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{M}\mathrm{e}}_{2}\mathrm{TCNQ})$ and (BEDT-TTF)(ClMeTCNQ) complexes were neutral, while the ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{O}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{C}\mathrm{l}}_{2}\mathrm{TCNQ})$ complex was found to be ionic. Among these complexes, anomalous magnetic properties were observed in the ionic complex of ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{O}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{C}\mathrm{l}}_{2}\mathrm{TCNQ}).$ ${(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{O}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})(\mathrm{C}\mathrm{l}}_{2}\mathrm{TCNQ})$ is a magnetic insulator with Curie-Weiss behavior in magnetic susceptibility followed by a sharp drop at around 120 K, which is in sharp contrast to conventional ``spin-Peierls-type'' nonmagnetic mixed-stack ionic compounds.

Temperature-induced neutral-ionic transition in 2-chloro-5-methyl-p-phenylenediamine-2,5-dimethyl-dicyanoquinonediimine

A mixed-stack organic charge-transfer crystal of 2-chloro-5-methyl- $p$-phenylenediamine-2,5-dimethyl dicyanoquinonediimine (ClMePD-DMeDCNQI), which undergoes a temperature-induced neutral-ionic transition (TINIT), is reported. From infrared spectroscopy, the molecular ionicity (\ensuremath{\rho}) of ClMePD-DMeDCNQI was found to change continuously from \ensuremath{\rho}=0.3 at 300 K to \ensuremath{\rho}=0.6 at 100 K. The neutral-ionic interface was crossed without a discontinuity in \ensuremath{\rho} at around 200 K, accompanied by stack dimerization. These features were quite different from the discontinuous change in \ensuremath{\rho} observed for the TINIT of tetrathiafulvalene-$p$-chloranil (TTF-CA) and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The transition characteristics in ClMePD-DMeDCNQI are discussed by drawing a comparison with those seen in TTF-CA and TMB-TCNQ.

Effects of electrostatic field on the phase properties of mixed-stack organic charge-transfer compounds.

We have studied the effects of an external electric field on the phase properties of mixed-stack organic charge-transfer complexes through illustrative calculations based on the ionic crystal model, paying special attention to the roles of electrostatic energy. The ground-state charge configurations of tetrathiafulvalene-p-chloranil (TTF-CA) under electric fields have been explored mainly by the Monte Carlo simulated annealing method in the cases of one-dimensional chain, three-dimensional bulk crystal, and thin film. It has been found for TTF-CA bulk crystals at room temperature and ambient pressure that the applied electric field beyond approximately ${10}^{6}$ V/cm would induce a transition from the neutral phase to the charge-separated phase in which positively and negatively charged donor-acceptor pairs, ${\mathit{D}}^{+}$${\mathit{A}}^{0}$ and ${\mathit{D}}^{0}$${\mathit{A}}^{\mathrm{\ensuremath{-}}}$, separately appear near both ends of DA stack chains. Investigating the phase structure of charge-transfer complexes through the variations in ionization energy, Madelung energy, and external field, we have also found that the applied field can cause a neutral-ionic transition of mixed-stack organic charge-transfer compounds in the close vicinity of neutral-ionic phase boundary. We then discuss the feasibility of this electric-field-induced neutral-ionic transition in actual organic-complex systems in terms of employing compounds other than TTF-CA or of varying temperature, pressure, and film thickness to approach the systems to the neutral-ionic phase boundary.

Neutral-ionic transition in dimethyl-TTF-chloranil

Abstract We report on a new mixed-stack organic charge-transfer crystal of 2,6-dimethyltetrathiafulvalene- p -chloranil (DMTTF-CA), which undergoes a temperature-induced neutral-ionic transition (TINIT). The discontinuous change in molecular ionicity was found to occur at 65K. A most striking feature of the TINIT in DMTTF-CA was that a neutral-ionic coexisting phase appeared at lower temperatures, unlike the homogeneous ionic phase observed for the TINIT of TTF-CA. The pressure-induced transition in DMTTF-CA was found to occur at 1.2 GPa. The transition characteristics of DMTTF-CA are discussed in comparison with those of TTF-CA.

Temperature-induced neutral-ionic transition in dimethyltetrathiafulvalene-p-chloranil.

A new 1:1 mixed-stack organic charge-transfer crystal of 2,6-dimethyltetrathiafulvalene-p-chloranil (DMTTF-CA), which undergoes a temperature-induced neutral-ionic transition (TINIT), was reported. At a critical temperature of 65±5 K the molecular ionicity and the stack dimerization were found to change discontinuously. A most striking feature in the phase transition of DMTTF-CA was that a neutral-ionic coexisting phase did appear at lower temperatures, unlike the homogeneous ionic phase observed for the TINIT of tetrathiafulvalene-p-chloranil (TTF-CA). These transition characteristics were discussed by taking into account the structural differences between these two complexes composed of similar constituents

A Role of Molecular Compressibility in Transformation Mechanism of Mixed-Stack Organic Charge-Transfer Crystals

The neutral-ionic transformation in mixed-stack organic charge-transfer (CT) crystals has been analysed within a phenomenological theory. The theory uses two parameters: lattice distortion (the stack dimerization) which breaks inversion symmetry and molecular deformation (related to molecular ionicity) which preserves site symmetry. A coupling of totally symmetric molecular vibrations with a charge-transfer between donor and acceptor molecules allows to introduce the concept of molecular compressibility which includes both elastic and electronic contributions. The concept helps to understand discontinuity of the neutral-ionic transformation and can be usefull in the theory of solid state chemistry.

Negative-resistance effect in rubidium tetracyanoquinodimethane (Rb-TCNQ).

Nonlinear-conductivity phenomena, including negative-resistance and current-switching effects, have been observed in the radical-anion salt of rubidium tetracyanoquinodimethane (Rb-TCNQ) known as a spin-Peierls system. From the current and temperature dependences of infrared ${\mathit{a}}_{\mathit{g}}$ CN-stretching-vibration spectra, it is concluded that the observed nonlinear properties are not due to the Joule-heating effect but are attributable to the dynamics of soliton-type defects in the bond-ordered (dimerized) stacks of TCNQ molecules. The possible soliton-mediated transport mechanism in Rb-TCNQ is discussed, in parallel with previous discussions on similar nonlinear transport properties in mixed-stack organic charge-transfer crystals.

Anomalous dielectric response in tetrathiafulvalene-p-chloranil as observed in temperature- and pressure-induced neutral-to-ionic phase transition.

The temperature and pressure dependences of the dielectric constants have been measured on the mixed-stack organic charge-transfer crystal, tetrathiafulvalene-p-chloranil, in a frequency range from 100 Hz to 10 MHz. The dielectric response increases remarkably with decreasing temperature down to ${\mathit{T}}_{\mathit{c}}$=81 K, where the neutral-to-ionic (NI) phase transition occurs. On the other hand, with applied pressure at room temperature, the change in the dielectric response is less remarkable, whereas the dc conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$ increases considerably. Such dynamical responses are explained by a Debye-type relaxation model. The strength of the dielectric response is closely correlated with both the relaxation time \ensuremath{\tau} and ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$. These anomalous dielectric responses are attributed to the dynamics of the NI domain-wall pairs and ionic domains in the neutral lattice.

Temperature-induced neutral-ionic transition in tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ).

Experimental evidence is presented that indicates the occurrence of a temperature-induced neutral-ionic transition occurring at ${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}205 K in the mixed-stack organic charge-transfer crystal, 3,3',5,5'-tetramethylbenzidine-7,7,8,8-tetracyanoquinodimethane (TMB-TCNQ). The transition is characterized by a large first-order-like jump in the magnetic susceptibility and a dimeric distortion of the stacks similar to those observed in tetrathiafulvalene-chloranil (TTF-CA). However, contrary to the case in TTF-CA the molecular ionicity jump at ${\mathit{T}}_{\mathit{c}}$ is comparatively small in TMB-TCNQ. The nature of this transition observed in TMB-TCNQ is discussed drawing comparisons with that seen in TTF-CA.

Intrinsic negative-resistance effect in mixed-stack charge-transfer crystals.

A stable and reproducible negative-resistance effect has been observed in various mixed-stack organic charge-transfer crystals having ionicities close to the neutral-ionic phase boundary. Systematic studies on several kinds of single crystals indicate that this effect is an intrinsic property, arising from the dynamics of the charge-carrying defects with fluctuating molecular valence in the one-dimensional (1D) molecular stacks. A microscopic mechanism of the negative resistance is proposed taking account of the three-dimensional electrostatic interaction between 1D stacks.

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