Novel superconducting phenomena in quasi-one-dimensional Bechgaard salts

It is the saturation of the transition temperature Tc in the range of 24 K for known materials in the late sixties which triggered the search for additional materials offering new coupling mechanisms leading in turn to higher Tc's. As a result of this stimulation, superconductivity in organic matter was discovered in tetramethyl-tetraselenafulvalene-hexafluorophosphate, (TMTSF)2PF6, in 1979, in the laboratory founded at Orsay by Professor Friedel and his colleagues in 1962. Although this conductor is a prototype example for low-dimensional physics, we mostly focus in this article on the superconducting phase of the ambient-pressure superconductor (TMTSF)2ClO4, in which the superconducting phase has been studied most intensively among the TMTSF salts. We shall present a series of experimental results supporting nodal d-wave symmetry for the superconducting gap in these prototypical quasi-one-dimensional conductors.Comment: Review article with 35 pages and 19 figures. Title, text, figures, and references are modified. To be published in Compte Rendu de Physique. Comments are welcom

Quasi one-dimensional organic conductors: from Fr\"ohlich conductivity and Peierls insulating state to magnetically-mediated superconductivity, a retrospective

It is indisputable that the search for high-temperature superconductivity has stimulated the work on low-dimensional organic conductors at its beginning. Since the discovery of true metal-like conduction in molecular compounds more than 50 years ago, it appeared that the chemical composition and the quasi one-dimensional crystalline structure of these conductors were determining factors for their physical properties; materials with incommensurate conduction band filling favoring the low-dimensional electron-phonon diverging channel and the establishment of the Peierls superstructure and more rarely superconductivity at low temperature, while those with commensurate band filling favor either magnetic insulating or superconducting states depending on the intensity of the coupling between conductive chains. In addition, the simple structures of these materials have allowed the development of theoretical models in close cooperation with almost all experimental findings. Even though these materials have not yet given rise to true high-temperature superconductivity, the wealth of their physical properties makes them systems of choice in the field of condensed matter physics due to their original properties and their educational qualities. Research efforts continue in this field. The present retrospective, which does not attempt to be an exhaustive review of the field, provides a set of experimental findings alluding to the theoretical development while a forthcoming article will address in more details the theoretical aspect of low dimensional conductors and superconductors.Comment: 96 pages, 131 figure

Correspondence between configurational temperature and molecular kinetic temperature thermostats

Molecular fluids undergoing shear flow are often modeled using a homogeneous nonequilibrium molecular dynamics algorithm. To reach a steady state, this method must be used in conjunction with a thermostating mechanism which duplicates the heat dissipation in the experimental setup (e.g., by conduction to the shearing boundaries). The most commonly used type of thermostat involves fixing the center of mass kinetic (c.m.) temperature. Though perfectly valid, this approach does not seem to be the most realistic for a molecular fluid since heat is removed only through the 3 degrees of freedom of the center of mass for each molecule. The second type of thermostat involves fixing the “atomic” kinetic temperature and therefore takes into account all degrees of freedom. However, since the streaming velocity of atoms within their constituent molecules is unknown, the implementation of such a thermostat is problematic and relies on incorrect assumptions on the streaming velocity of atoms. The recently developed configurational temperature thermostat requires no assumption on the streaming velocity of atoms and takes into account all degrees of freedom. Using a configurational temperature thermostat to thermostat homogeneous shear flow thus seems to be a more realistic approach than the c.m. kinetic thermostat. In this work, we apply this configurational temperature thermostat to the study of linear alkanes (C₁₀ and C₂₀) undergoing shear flow. The results so obtained are compared with those obtained using a c.m. kinetic thermostat. Our aims are (1) to test the influence of the total number of degrees of freedom of the system, (2) to make a connection between the results obtained with the two types of thermostats. By carefully examining the energies of the internal modes, we have been able to characterize the loss of accuracy of a c.m. kinetic thermostat at high shear rates and for high molecular weight compounds. Finally, we establish a correspondence between the two types of thermostats by showing that, for the internal modes, a simulation at a fixed c.m. kinetic temperature is equivalent to a simulation at a fixed but higher configurational temperature

Configurational temperature profile in confined fluids. I. Atomic fluid

Two configurational expressions for the temperature are applied to the calculation of temperature profiles within a confined atomic fluid in a narrow slit pore. The configurational temperatures profiles so obtained are compared to the kinetic temperature, calculated from the equipartition principle, in equilibrium (EMD), and nonequilibrium molecular dynamics (NEMD) simulations of planar Poiseuille flow. We show that one of the configurational expressions exhibits a system-size dependence which prevents its application to the determination of high-resolution temperature profiles. The other expression yields good agreement with the kinetic temperature profile in both equilibrium and nonequilibrium systems

Comparison of thermostatting mechanisms in NVT and NPT simulations of decane under shear

Nonequilibrium molecular dynamics (NEMD) simulations play a major role in characterizing the rheological properties of fluids undergoing shear flow. However, all previous studies of flows in molecular fluids either use an “atomic” thermostat which makes incorrect assumptions concerning the streaming velocity of atoms within their constituent molecules, or they employ a center of mass kinetic (COM) thermostat which only controls the temperature of relatively few degrees of freedom (3) in complex high molecular weight compounds. In the present paper we show how recently developed configurational expressions for the thermodynamic temperature can be used to develop thermostatting mechanisms which avoid both of these problems. We propose a thermostat based on a configurational expression for the temperature and apply it to NEMD simulations of decane undergoing Couette flow at constant volume and at constant pressure. The results so obtained are compared with those obtained using a COM kinetic thermostat. At equilibrium the properties of systems thermostatted in the two different ways are of course equivalent. However, we show that the two responses differ far from equilibrium. In particular, we show that the increase in the potential energy of the internal modes with increasing shear is only observed with a Gaussian isokinetic COM thermostat in both NVT and NPT simulations. There is no such increase with the configurational thermostat, which, unlike the Gaussian isokinetic COM thermostat, correctly accounts for the internal degrees of freedom of the molecular fluid

Configurational temperature profile in confined fluids. II. Molecular fluids

In an earlier paper, we applied configurational expressions of the temperature to the calculation of temperature profiles within a confined atomic fluid. This paper focuses on the application of these expressions to confined molecular fluids using ethane and hexane as examples. We first give configurational expressions for the temperature for these constrained systems. The configurational temperature profiles so obtained are compared to the kinetic temperature calculated using the equipartition principle, in equilibrium systems. These expressions are then used in nonequilibrium molecular dynamics (NEMD) simulations of fluids undergoing planar Poiseuille flow. We show that these configurational expressions provide a direct and accurate determination of the temperature profile for these systems

Suppression of superconductivity by non-magnetic disorder in the organic superconductor (TMTSF)2(ClO4)(1-x)(ReO4)x

We present a study of the superconducting properties (Tc and Hc2) in the solid solution (TMTSF)2(ClO4)(1-x)(ReO4)x with a ReO-4 nominal concentration up to x = 6%. The dramatic suppression of Tc when the residual resistivity is increased upon alloying with no modification of the Fermi surface is the signature of non-conventional superconductivity . This behaviour strongly supports p or d wave pairing in quasi one dimensional organic superconductors. The determination of the electron lifetime in the normal state at low temperature confirms that a single particle Drude model is unable to explain the temperature dependence of the conductivity and that a very narrow zero frequency mode must be taken into account for the interpretation of the transport properties.Comment: Received 26 January 2004 / Received in final form 17 June 2004 / Published online 3 August 200

On the effects of assuming flow profiles in nonequilibrium simulations

Atomic simulation methods modelling fluid flows often incorporate in the equations of motion the steady state flow profile predicted by Navier–Stokes equations. We show in this work that this may lead to significant errors such as spurious shear induced ordering, unphysical steady state flow profiles or artificial dampening of thermal motion even at shear rates regarded as low in simulation applications. Our results also suggest that nonequilibrium molecular dynamics coupled with the recently developed configurational thermostat, which makes no assumption at all on the flow profile, provides a much more realistic way to study these phenomena

Non-Newtonian behavior in simple fluids

Using nonequilibrium molecular dynamics simulations, we study the non-Newtonian rheology of a microscopic sample of simple fluid. The calculations were performed using a configurational thermostat which unlike previous nonequilibrium molecular dynamics or nonequilibrium Brownian dynamics methods does not exert any additional constraint on the flow profile. Our findings are in agreement with experimental results on concentrated "hard sphere"-like colloidal suspensions. We observe: (i) a shear thickening regime under steady shear; (ii) a strain thickening regime under oscillatory shear at low frequencies; and (iii) shear-induced ordering under oscillatory shear at higher frequencies. These results significantly differ from previous simulation results which showed systematically a strong ordering for all frequencies. They also indicate that shear thickening can occur even in the absence of a solvent.J.D. acknowledges support from the Research School of Chemistry ~ANU! through a visiting fellowship