S. Tomić

The pressure-temperature phase diagram of the organic conductor (2,5 DM-DCNQI)2Cu

The authors show that it is possible to suppress the ambient-pressure metallic phase in (2,5 DM-DCNQI)2Cu by applying a pressure-even one as low as 100 bar. On increasing the pressure up to 8.5 kbar, the metal-insulator transition temperature increases and its strong first-order character becomes weaker, whereas the activation energy stays constant. In addition, a re-entrance of the metallic phase is observed at very low temperatures up to pressures of about 300 bar.

Dielectric relaxation of DNA aqueous solutions

We report on a detailed characterization of complex dielectric response of Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40 Hz-110 MHz). Results reveal two broad relaxation modes of strength 20<Deltaepsilon(LF)<100 and 5<Deltaepsilon(HF)<20, centered at 0.5 kHz<nu(LF)<70 kHz and 0.1 MHz<nu(HF)<15 MHz. The characteristic length scale of the low-frequency (LF) process, 50<L(LF)<750 nm, scales with DNA concentration as c(DNA)(-0.29+/-0.04) and is independent of the ionic strength in the low added salt regime. Conversely, the measured length scale of the LF process does not vary with DNA concentration but depends on the ionic strength of the added salt as I(s)(-1) in the high added salt regime. On the other hand, the characteristic length scale of the high-frequency (HF) process, 3<L(HF)<50 nm, varies with DNA concentration as c(DNA)(-0.5) for intermediate and large DNA concentrations. At low DNA concentrations and in the low added salt limit the characteristic length scale of the HF process scales as c(DNA)(-0.33). We put these results in perspective regarding the integrity of the double stranded form of DNA at low salt conditions as well as regarding the role of different types of counterions in different regimes of dielectric dispersion. We argue that the free DNA counterions are primarily active in the HF relaxation, while the condensed counterions play a role only in the LF relaxation. We also suggest theoretical interpretations for all these length scales in the whole regime of DNA and salt concentrations and discuss their ramifications and limitations.

Electrodynamic response of the charge ordering phase: Dielectric and optical studies ofα-(BEDT-TTF)2I3

We report on the anisotropic response, the charge and lattice dynamics of normal and charge-ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(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 nu27(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 Deltaepsilon_LD ~= 5000 and Deltaepsilon_SD ~= 400, centered at 1 kHz < f_LD < 100 MHz and f_SD ~= 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.

Pressure-Induced Metal-to-Insulator Phase Transitions in the Organic Conductor (2,5 DM-DCNQI)2Cu

The electrical resistivity along the high-conductivity axis has been measured at pressures up to 9 kbar in the novel organic conductor (2,5 DM-DCNQI)2Cu. Deviations from the usual linear behaviour of the conductivity under pressure at room temperature have been observed. Moreover, at lower temperatures metal-to-insulator phase transitions have been induced with values of transition temperatures depending on the applied pressure. The obtained results are discussed and compared with corresponding data for other organic compounds. A tentative interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF)2PF6

Abstract:The phase diagram of the organic superconductor (TMTSF)2PF6has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between TSDW and TSC and the superconducting critical current. The onset temperature TSC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below TSC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with TSC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings.

Pressure-temperature phase diagram of the organic conductor (DM-DCNQI)2Cu

Abstract We present an experimental study of the pressure-temperature phase diagram of the organic conductor (DM-DCNQI) 2 Cu. We show that it is possible to suppress the ambient pressure metallic phase by applying pressure — even as low as 100 bar. On increasing the pressure, the metal-insulator transition temperature increases and its strong first-order character becomes weaker, whereas the activation energy in the insulating phase stays constant. In addition, a re-entrance of the metallic phase is observed at very low temperatures up to pressures of about 300 bar. The results obtained are discussed and compared with corresponding data for other organic compounds. An interpretation of this novel and unusual phase diagram is proposed in terms of a mixed valence state for copper ions.

Ferroelectricity in molecular solids : a review of electrodynamic properties

In conventional ferroelectrics the polarization is induced either by the relative displacement of positive and negative ions due to a lattice distortion or by the collective alignment of permanent electric dipoles. Strongly correlated materials with the inversion-symmetry-broken ground states feature electronic ferroelectricity, a phenomenon which has recently caught the attention of condensed matter physicists due to its great fundamental and technological importance. The discovery of electronic ferroelectricity in one and two-dimensional molecular solids is an exciting development because they show a rich variety of nonlinear properties and complex electrodynamics, including nontrivial emergent excitations. We summarize key experimental results, sketch the current theoretical understanding and outline promising prospects of this phenomenon which have great potential for future electronic devices.

Suppression of the charge-density-wave state in Sr14Cu24O41 by calcium doping

The charge response in the spin chain and/or 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 interladder 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.

Charge-density wave formation in Sr14-xCaxCu24O41

The electrodynamic response of the spin-ladder compound Sr

Anisotropic charge dynamics in the quantum spin-liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3

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