Diffuson

Abstract: Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite ...

Abstract: We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 to 1636 nm grown via sputter deposition. While exhibiting a constant value up to $\ensuremath{\sim}100$ nm, the thermal conductivity increases with film thickness thereafter. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of $\ensuremath{\sim}1.8$ THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.

Abstract: We report here observations of the Zeeman effect in the 18-cm lines of OH in the envelope regions surrounding four molecular cloud cores toward which detections of B(LOS) have been achieved in the same lines, and evaluate the ratio of mass to magnetic flux, M/Phi, between the cloud core and envelope. This relative M/Phi measurement reduces uncertainties in previous studies, such as the angle between B and the line of sight and the value of [OH/H]. Our result is that for all four clouds, the ratios R of the core to the envelope values of M/Phi are less than 1. Stated another way, the ratios R' of the core to the total cloud M/Phi are less than 1. The extreme case or idealized (no turbulence) ambipolar diffusion theory of core formation requires the ratio of the central to total M/Phi to be approximately equal to the inverse of the original subcritical M/Phi, or R' > 1. The probability that all four of our clouds have R' > 1 is 3 x 10^{-7}; our results are therefore significantly in contradiction with the hypothesis that these four cores were formed by ambipolar diffuson. Highly super-Alfvenic turbulent simulations yield a wide range of relative M/Phi, but favor a ratio R < 1, as we observe. Our experiment is limited to four clouds, and we can only directly test the predictions of the extreme-case "idealized" models of ambipolar-diffusion driven star formation that have a regular magnetic field morphology. Nonetheless, our experimental results are not consistent with the "idealized" strong field, ambipolar diffusion theory of star formation.

Abstract: A model for the thermal conductivity of bulk solids is proposed in the limit of diffusive transport mediated by diffusons as opposed to phonons. This diffusive thermal conductivity, κdiff, is determined by the average energy of the vibrational density of states, ℏωavg, and the number density of atoms, n. Furthermore, κdiff is suggested as an appropriate estimate of the minimum thermal conductivity for complex materials, such that (at high temperatures): . A heuristic finding of this study is that the experimental ωavg is highly correlated with the Debye temperature, allowing κdiff to be estimated from the longitudinal and transverse speeds of sound: . Using this equation to estimate κmin gives values 37% lower than the widely-used Cahill result and 18% lower than the Clarke model for κmin, on average. This model of diffuson-mediated thermal conductivity may thus help explain experimental results of ultralow thermal conductivity.