[1] We examine statistics of rapid spatial variations of the magnetic field in simulations of magnetohydrodynamic (MHD) turbulence, by analyzing intermittency properties, and by using classical methods for identifying discontinuities. The methods identify similar structures, and give very similar event distribution functions. When the results are scaled to the correlation length, the average waiting times agree with typically reported waiting times between solar wind discontinuities. Thus discontinuities may be related to flux tube boundaries and intermittent structures that appear spontaneously in MHD turbulence.

Motivated by the growing evidence of the importance of charge fluctuations in the pseudogap phase in high-temperature cuprate superconductors, we apply a large-N expansion formulated in a path integral representation of the two-dimensional t-J model on a square lattice. We study all possible charge instabilities of the paramagnetic state in leading order of the 1/N expansion. While the d-wave charge density wave (flux phase) becomes the leading instability for various choices of model parameters, we find that a d-wave Pomeranchuk (electronic nematic phase) instability occurs as a next leading one. In particular, the nematic state has a strong tendency to become inhomogeneous. In the presence of a large second nearest-neighbor hopping integral, the flux phase is suppressed and the electronic nematic instability becomes leading in a high doping region. Besides these two major instabilities, bond-order phases occur as weaker instabilities close to half-filling. Phase separation is also detected in a finite temperature region near half-filling.Comment: 28 pages, 6 figure

We observe charge-order fluctuations in the quasi-two-dimensional organic superconductor β ′′ -(BEDT-TTF)2SF5CH2CF2SO3 both by means of vibrational spectroscopy, locally probing the fluctuating charge order, and investigating the in-plane dynamical response by infrared reflectance specctroscopy. The decrease of effective electronic interaction in an isostructural metal suppresses both charge-order fluctuations and superconductivity, pointing on their interplay. We compare the results of our experiments with calculations on the extended Hubbard model. PACS numbers: 74.70.Kn, 74.25.Gz, 71.30.+h, 71.10.Hf From a naive point of view, superconducting and ordered insulating states are incompatible. This is true, for example, for the competition of charge-density wave and superconductivity [1], or stripes in cuprates [2,3]. However, experimental and theoretical studies on materials with strong electronic correlations suggest that fluctuations of an ordered state may mediate superconductivity. Prime candidates for this mechanism are magnetic order in heavy fermions [4,5] and high-temperature superconductors [6], incommensurate charge-density waves in dichalcogenides [7], or fluctuating charge order in quasitwo-dimensional organic conductors [8][9][10][11][12][13][14].In BEDT-TTF-based 1/4-filled conductors the ground state can be tuned by modifying effective electronic correlations via changing the bandwidth [15,16] (Fig. 1a): A charge-ordered insulating state is observed when the effective Coulomb repulsion is large enough [16][17][18][19][20], while compounds with weaker effective electronic correlations are metallic [21]. In the metallic state close to the metalinsulator phase boundary, charge fluctuations are observed [9,20,22], while the response of coherent carriers is still present. These experimental results are in agreement to calculations on the extended Hubbard model. It is the minimum model that can describe a metalinsulator transition in quasi-two-dimensional molecular conductors with 1/4-filled conduction band [7]. It takes into account the effective on-site U/t and inter-site V /t Coulomb repulsion, where t is the hopping integral related to the bandwidth. This model predicts that fluctuations of checker-board charge order (CO) can act as an attractive interaction of quasiparticles forming Cooper pairs and lead to a superconducting state [8,23]. In this Letter we present an experimental evidence for a bandwidth tuned CO fluctuations in the normal state of β ′′ family of quasi-2D organic conductors. We see an unambiguous relation between the presence of CO fluctuations and superconductivity and discuss its origin. The studied materials are layered compounds, where a slightly anisotropic quasi-two-dimensional conducting electronic system of the (ab) plane is created by the overlap of the neighboring BEDT-TTF (bis-(ethylenedithio)tetrathiafulvalene) molecules [24] (Fig. 1b). The bandwidth is tuned by changing the size of the anion, by so called 'chemical pressure'. β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 (...