We propose a relation which predicts the low-frequency thermal conductivity of a one-dimensional (1D) system from the thermal conductivity and bulk viscosity at higher frequency. Our theory is based on the assumption that "ballistic" transport by sound waves dominates the heat transport. For a system with equal heat capacities (c(p) = c(v)) this relation is particularly simple. We test the prediction by simulating a chain of particles with quartic interparticle potentials under zero pressure conditions. As the frequency omega --> 0 the theory predicts that the energy current power spectrum diverges as omega(-1/2), not seen in previous simulations. Because we simulate very long chains to long times we do observe the crossover into this regime. The bulk viscosity of a 1D chain has been determined via simulation. It is found to be finite for our system, in contrast to the thermal conductivity which is infinite.

We examine the thermal conductivity and bulk viscosity of a onedimensional (1D) chain of particles with cubic-plus-quartic interparticle potentials and no on-site potentials. This system is equivalent to the FPUαβ system in a subset of its parameter space. We identify three distinct frequency regimes which we call the hydrodynamic regime, the perturbative regime and the collisionless regime. In the lowest frequency regime (the hydrodynamic regime) heat is transported ballistically by long wavelength sound modes. The model that we use to describe this behaviour predicts that as ω → 0 the frequency dependent bulk viscosity,ζ(ω), and the frequency dependent thermal conductivity,κ(ω), should diverge with the same power law dependence on ω. Thus, we can define the bulk Prandtl number, P r ζ = kBζ(ω)/(mκ(ω)), where m is the particle mass and kB is Boltzmann's constant. This dimensionless ratio should approach a constant value as ω → 0. We use mode-coupling theory to predict the ω → 0 limit of P r ζ . Values of P r ζ obtained from simulations are in agreement with these predictions over a wide range of system parameters. In the middle frequency regime, which we call the perturbative regime, heat is transported by sound modes which are damped by four-phonon processes. This regime is characterized by an intermediate-frequency plateau in the value ofκ(ω). We find that the value ofκ(ω) in this plateau region is proportional to T −2 where T is the temperature; this is in agreement with the expected result of a four-phonon Boltzmann-Peierls equation calculation. The Boltzmann-Peierls approach fails, however, to give a nonvanishing bulk viscosity for all FPU-αβ chains. We call the highest frequency regime the collisionless regime since at these frequencies the observing times are much shorter than the characteristic relaxation times of phonons.

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A one-dimensional system of particles is examined in which even numbered particles are bound to adjacent even particles by harmonic spring forces, while odd particles are free. Even and odd particles collide elastically. This is a momentum conserving modification of the famous "ding-a-ling" model. Molecular-dynamics simulations are carried out and the current power spectra are obtained. The energy current power spectrum has zero slope at low frequencies. This implies that the thermal conductivity κ is finite and independent of system length L , for L sufficiently large. Steady-state simulations provide further evidence that κ is independent of L at large values of L . The relevance of this result to the proof by Prosen and Campbell that momentum conservation with nonvanishing pressure implies an infinite thermal conductivity is discussed.