We report on the anisotropic response, the charge and lattice dynamics of normal and chargeordered phases with horizontal stripes in single crystals of the organic conductor α-(BEDT-TTF)2I3 determined by dc resistivity, dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations ν27(Bu) evidences the abrupt onset of static charge order below TCO = 136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens of approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength ∆εLD ≈ 5000 and ∆εSD ≈ 400, centered at 1 kHz < νLD < 100 MHz and νSD ≈ 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phason-like excitation is best described as a long-wavelength excitation of a 2kF bond-charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectric-like nature of the charge-ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.
We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 by dc conductivity, Hall effect and dielectric spectroscopy. At room temperature the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity ρ(T ). Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-ofplane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also the relaxation levels off. The observed behavior bears features of relaxor-like ferroelectricity. Because heterogeneities impede its long-range development, only a weak tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.
The charge response of the ladders in Sr 14−x Ca x Cu 24 O 41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x = 0, 3, and 6, in addition to the mode along the legs. For x = 8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.The physics of doped Mott-Hubbard insulators challenges conventional theories of metals and insulators. 1 The effect of strong Coulomb interactions produces a rich variety of exotic ordering phenomena, which have been the focus of intense scientific activity in recent years. The spin-chain and ladder self-doped compound Sr 14−x Ca x Cu 24 O 41 has attracted much attention since it is the first superconducting copper oxide with a nonsquare lattice. 2 Theoretically, in doped two-leg Cuu O ladders, superconductivity ͑SC͒ is tightly associated with the spin gap and in competition with charge-density wave ͑CDW͒. 3 While both the spin gap and CDW were established in the ladders of Sr 14−x Ca x Cu 24 O 41 , 4-6 the relevance of these objects to electronic properties and superconductivity is still subject of intensive discussion. Recently, it was shown, on the basis of dielectric response data, that substitution of Sr 2+ by Ca 2+ gradually suppresses the insulating CDW phase, which eventually vanishes for x Ͼ 9, Ref. 7. In contrast to these results, dynamical Raman response observed above RT for x = 0 was assigned to CDW fluctuations and found to persist in the metallic phase of x = 12, a system which becomes SC under pressure. 8 It is of particular interest to learn more about the nature of CDW order in the spin ladders, which presents a nice experimental system of strongly interacting electrons. Although the ground state for 0 ഛ x ഛ 9 reveals a number of well-known fingerprints of the conventional CDW, such as the pinned phason 9 and the broad dispersion at radio frequencies due to screening of the CDW by free carriers, 5-7 its origin is certainly more complicated, since the system does not undergo a metal-to-insulator but an insulator-to-insulator transition. The role of Ca substitution is another open issue. Suppression of the CDW phase was ascribed 7 to worsened nesting conditions 10 implying that the system becomes more 2D already at ambient pressure for large x. On the other hand, it was suggested that at ambient pressure ͑for all x͒ the charge dynamics is essentially one-dimensional 2,11 ͑1D͒ and that only the application of pressure induces the dimensional crossover from 1 to 2. 2 In this Report, we address these important questions concerning the charge-ordered ground state in the ladders of Sr 14−x Ca x Cu 24 O 41 . Our results give evidence that the CDW is two dimensional with an anisotropic dispersion: the longrange charge order develops only in ladder planes, leading to a screen...
The Mott insulator κ-(BEDT-TTF)2Ag2(CN)3 forms a highly-frustrated triangular lattice of S = 1/2 dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasi-degenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound κ-(BEDT-TTF)2Cu2(CN)3.PACS numbers: 75.10. Kt, 77.22.Gm, Electronic ferroelectricity and multiferroicity attracts great attention of condensed matter physicists due to their fundamental and technological importance. 1-3 They are identified in systems with strong electronic correlations such as transition-metal oxides and low-dimensional charge-transfer molecular solids. In the latter category, electric polarization arises from valence instability and charge ordering. In both cases, breaking the inversionsymmetry results in the concurrence of non-equivalent charge-sites and bonds. 4 There is no doubt that electron correlations are fundamental for stabilizing the ferroelectric ground state, nevertheless, experimental evidence indicates that the delicate interplay of Coulomb forces and structural changes within the coupled molecular-anion system have to be taken into account. Along these lines a solid understanding of electronic ferroelectricity was achieved for the families of quasi-one-dimensional organic charge-transfer salts: (TMTTF) 2 X and TTF-X, but also some layered (BEDT-TTF) 2 X systems. [5][6][7] However, no consensus has been reached yet on the origin of the ferroelectric signatures detected in the strongly dimerized κ-(BEDT-TTF) 2 X salts. [8][9][10][11][12] In these compounds, the BEDT-TTF dimers are arranged in a triangular lattice with a relatively high geometrical frustration. In some of them, indications of charge-ordering phenomena have been reported, but in-depth studies are missing 13,14 . On the other hand, the Mott dimer insulators κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl and κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , called κ-CuCN, have been thoroughly studied because they are discussed as prototypes of a molecular multiferroic and quantum spin liquid (QSL) systems. 9,15 It turns out to be extremely challenging to reconcile the idea of quantum electric dipoles on molecular dimers interacting via dipolar-spin coupling [16][17][18][19] with the experimentally evidenced absence of any considerable charge imbalance. So far no global structural changes and no charge disproportionation between molecular dimer sites larger than 2δ ρ ≈ ±0.01e that could break the symmetry have been found. 20,21 In the case of the QSL κ-...
The charge response in the spin chain/ladder compound Sr14−xCaxCu24O41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x = 0 to 10 K for x = 9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.
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