Anisotropic charge dynamics in the quantum spin-liquid candidate κ\kappa-(BEDT-TTF)2_2Cu2_2(CN)3_3

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\kappa$-(BEDT-TTF)$_2$\-Cu$_2$(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 $\rho(T)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane 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.Comment: 14 pages, 13 figure

Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors

An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 in the Pressure-Temperature phase diagram between T=0 and 20 K and a theoretical description based on the weak coupling renormalization group method are reported. The analysis of the data reveals a high temperature domain (T\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the resistivity is dominated by a linear-in temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spin-lattice relaxation due to antiferromagnetic fluctuations. In addition, a tight connection is clearly established between linear transport and T_c. We propose a theoretical description of the anomalous resistivity based on a weak coupling renormalization group determination of electron-electron scattering rate. A linear resistivity is found and its origin lies in antiferromagnetic correlations sustained by Cooper pairing via constructive interference. The decay of the linear resistivity term under pressure is correlated with the strength of antiferromagnetic spin correlations and T_c, along with an unusual build-up of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts

Magnetoresistance in the SDW state of (TMTSF)₂PF₆ above T*≈ 4K-Novel effect due to the Landau quantization

Magnetoresistance in the spin-density wave (SDW) state of (TMTSF) 2PF6 is known to exhibit a rich variety of the angular dependencies when a magnetic field B is rotated in the b'-c*, a-b' and a-c* planes. In the presence of a magnetic field the quasiparticle spectrum in the SDW with imperfect nesting is quantized. In such a case the minimum quasiparticle energy depends both on the magnetic-field strength \B\ and the angle between the field and the crystal direction a, b' or c*. This approach describes rather satisfactory the magnetoresistance above T* approximate to 4K

Unconventional spin density wave in (TMTSF)₂PF₆ below T=4.2 K

It is well documented that SDW in (TMTSF)(2)PF6 undergoes another phase transition at T* approximate to 4 K, though the nature of the new phase has not been identified. In particular the angular dependence of the magnetoresistance for Bparallel to(a, b') plane below 4.2 K is dramatically different from the one above 4.2 K. We propose that below T* the new phase unconventional SDW (USDW) appears which modifies drastically the quasiparticle spectrum. The present model describes the observed angular dependence of the magnetoresistance reasonably well, which cannot be interpreted in terms of the conventional SDW

Modalities of self-organized charge response in low dimensional systems

We present modalities of self-organized charge response in low dimensional systems, like diverse organic and quantum spin systems, studied by the low-frequency (10 mHz – 1 MHz) dielectric spectroscopy. Density wave structures with the order of commensurability N = 4 can be recognized as phasons in a random impurity potential, whereas those with N = 3 can be viewed as topological defects like charge domain wall pairs in the background domain structure

Galvanomagnetic properties in the spin-density-wave phase of (TMTSF)2PF6

We have measured the magnetoresistance and the Hall effect in the spin-density-wave (SDW) state of (TMTSF)2PF6 down to 2K and in magnetic fields up to 9T in order to gain an additional insight into the nature of the possible subphases in the SDW state. We have found that the temperature dependent magnetoresistance anisotropy changes below 4K ; this change being maximal for the current in the lowest conductivity direction. The Hall resistivity has showed different magnetic field dependencies for 7>4K and T<4K. The resistivity and the Hall resistivity were also investigated as the function of the electric field. The backflow coefficient a does not change below 4K