BEDT-TTF organic superconductors: the entangled role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency.Comment: 4 pages, including 4 figures. Published version, with Ref. 17 corrected after publicatio

Multiple logistic regressions: controlling factors in applications to soil class prediction.

Métodos mais eficazes para determinação do padrão de distribuição de classes de solo na paisagem precisam ser avaliados visando suprir a demanda por mapas de solo em escalas regional e global. Neste estudo, Regressões Logísticas Múltiplas foram utilizadas como modelos preditores em uma aplicação de Mapeamento Digital de Solos. Os modelos foram gerados utilizando um mapa de solos existente como variável dependente e atributos de terreno como variáveis independentes, o que possibilitou determinar a probabilidade de encontrar classes de solo na paisagem no primeiro e no segundo nível categórico do SiBCS. A qualidade dos mapas preditos foi verificada por meio da matriz de contingência. A classe dos Argissolos foi predita corretamente, em relação ao mapa original, em aproximadamente 85 %. As classes de solos hidromórficos (Planossolos e Gleissolos) foram preditas corretamente em 75 %. Houve confundimento dos modelos para as classes que ocupam posições muito semelhantes na paisagem. Foi verificado também que classes de solo pouco representativas na paisagem não são adequadamente espacializadas em razão da sensibilidade dos modelos logísticos à proporção relativa das amostras usadas para treinar os modelos

Nonlinear equation for anomalous diffusion: unified power-law and stretched exponential exact solution

The nonlinear diffusion equation $\frac{\partial \rho}{\partial t}=D \tilde{\Delta} \rho^\nu$ is analyzed here, where $\tilde{\Delta}\equiv \frac{1}{r^{d-1}}\frac{\partial}{\partial r} r^{d-1-\theta} \frac{\partial}{\partial r}$, and $d$, $\theta$ and $\nu$ are real parameters. This equation unifies the anomalous diffusion equation on fractals ($\nu =1$) and the spherical anomalous diffusion for porous media ($\theta=0$). Exact point-source solution is obtained, enabling us to describe a large class of subdiffusion ($\theta > (1-\nu)d$), normal diffusion ($\theta= (1-\nu)d$) and superdiffusion ($\theta < (1-\nu)d$). Furthermore, a thermostatistical basis for this solution is given from the maximum entropic principle applied to the Tsallis entropy.Comment: 3 pages, 2 eps figure

Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution

In this work we incorporate, in a unified way, two anomalous behaviors, the power law and stretched exponential ones, by considering the radial dependence of the $N$-dimensional nonlinear diffusion equation $\partial\rho /\partial{t}={\bf \nabla} \cdot (K{\bf \nabla} \rho^{\nu})-{\bf \nabla}\cdot(\mu{\bf F} \rho)-\alpha \rho ,$ where $K=D r^{-\theta}$, $\nu$, $\theta$, $\mu$ and $D$ are real parameters and $\alpha$ is a time-dependent source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion equation on fractals ($\nu =1$) and the spherical anomalous diffusion for porous media ($\theta=0$). An exact spherical symmetric solution of this nonlinear Fokker-Planck equation is obtained, leading to a large class of anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.

Heat Conduction in κ\kappa-(BEDT-TTF)2_2Cu(NCS)2_2

The first study of thermal conductivity, $\kappa$, in a quasi-two-dimensional organic superconductor of the $\kappa$-(BEDT-TTF)$_2$X family reveals features analogous to those already observed in the cuprates. The onset of superconductivity is associated with a sudden increase in $\kappa$ which can be suppressed by the application of a moderate magnetic field. At low temperatures, a finite linear term - due to a residual electronic contribution- was resolved. The magnitude of this term is close to what is predicted by the theory of transport in unconventional superconductors.Comment: 5 pages, 4 figures include

Thermal Conductivity of superconducting (TMTSF)_2ClO_4: evidence for a nodeless gap

We report on the first measurements of thermal conductivity in the superconducting state of (TMTSF)_2ClO_4. The electronic contribution to heat transport is found to decrease rapidly below T_c, indicating the absence of low-energy electronic excitations. We argue that this result provides strong evidence for a nodeless superconducting gap function but does not exclude a possible unconventional order parameter.Comment: 4 pages including 4 figures, submitted to Phys. Rev. Let

Correlation gap in the optical spectra of the two-dimensional organic metal (BEDT-TTF)_4[Ni(dto)_2]

Optical reflection measurements within the highly conducting (a,b)-plane of the organic metal (BEDT-TTF)_4[Ni(dto)_2] reveal the gradual development of a sharp feature at around 200 cm as the temperature is reduced below 150 K. Below this frequency a narrow Drude-like response is observed which accounts for the metallic behavior. Since de Haas-von Alphen oscillations at low temperatures confirm band structure calculations of bands crossing the Fermi energy, we assign the observed behavior to a two-dimensional metallic state in the proximity of a correlation induced metal-insulator transition.Comment: 4 pages, 2 figure

Logarithmic diffusion and porous media equations: a unified description

In this work we present the logarithmic diffusion equation as a limit case when the index that characterizes a nonlinear Fokker-Planck equation, in its diffusive term, goes to zero. A linear drift and a source term are considered in this equation. Its solution has a lorentzian form, consequently this equation characterizes a super diffusion like a L\'evy kind. In addition is obtained an equation that unifies the porous media and the logarithmic diffusion equations, including a generalized diffusion equation in fractal dimension. This unification is performed in the nonextensive thermostatistics context and increases the possibilities about the description of anomalous diffusive processes.Comment: 5 pages. To appear in Phys. Rev.