Thermal rectification and negative differential thermal conductance in harmonic chains with nonlinear system-bath coupling

The effect of temperature oscillations on energy storage rectification in harmonic systems

Rectification, the preferential transport of a current in one direction through a system, has garnered significant attention in molecules because of its importance for controlling thermal and electronic currents at the nanoscale. Here, we report the presence of energy storage rectification effects in a molecular chain. This phenomenon is generated by subjecting a harmonic molecular chain to an oscillating temperature gradient and showing that the energy absorption rate of the system depends on the direction of the gradient. We examine how the energy storage rectification ratios in the chain are affected by the oscillating gradient, asymmetry in the chain, and the system parameters. We find that energy storage rectification can be observed in harmonic lattice structures with time-dependent temperatures and that, correspondingly, anharmonicity is not required to generate this rectification mechanism in such systems.

Phase diffusion and fluctuations in a dissipative Bose-Josephson junction

We analyze the phase diffusion, quantum fluctuations and their spectral features of a one-dimensional Bose-Josephson junction (BJJ) nonlinearly coupled to a bosonic heat bath. The phase diffusion is considered by taking into account of random modulations of the BJJ modes causing a phase loss of initial coherence between the ground and excited states, whereby the frequency modulation is incorporated in the system-reservoir Hamiltonian by an interaction term linear in bath operators but nonlinear in system (BJJ) operators. We examine the dependence of the phase diffusion coefficient on the on-site interaction and temperature in the zero- and π-phase modes and demonstrate its phase transition-like behavior between the Josephson oscillation and the macroscopic quantum self-trapping (MQST) regimes in the π-phase mode. Based on the thermal canonical Wigner distribution, which is the equilibrium solution of the associated quantum Langevin equation for phase, coherence factor is calculated to study phase diffusion for the zero- and π-phase modes. We investigate the quantum fluctuations of the relative phase and population imbalance in terms of fluctuation spectra which capture an interesting shift in Josephson frequency induced by frequency fluctuation due to nonlinear system-reservoir coupling, as well as the on-site interaction-induced splitting in the weak dissipative regime.

Universal Kardar-Parisi-Zhang transient diffusion in nonequilibrium anharmonic chains

The well known nonlinear fluctuating hydrodynamics theory has grouped diffusions in anharmonic chains into two universality classes: one is the Kardar-Parisi-Zhang (KPZ) class for chains with either asymmetric potential or nonzero static pressure and the other is the Gaussian class for chains with symmetric potential at zero static pressure, such as Fermi-Pasta-Ulam-Tsingou (FPUT)-β chains. However, little is known of the nonequilibrium transient diffusion in anharmonic chains. Here, we reveal that the KPZ class is the only universality class for nonequilibrium transient diffusion, manifested as the KPZ scaling of the side peaks of momentum correlation (corresponding to the sound modes correlation), which was completely unexpected in equilibrium FPUT-β chains. The underlying mechanism is that the nonequilibrium soliton dynamics cause nonzero transient pressure so that the sound modes satisfy approximately the noisy Burgers equation, in which the collisions of solitons was proved to yield the KPZ dynamic exponent of the soliton dispersion. Therefore, the unexpected KPZ universality class is obtained in the nonequilibrium transient diffusion in FPUT-β chains and the corresponding carriers of nonequilibrium transient diffusion are attributed to solitons.

Computational predictions of quantum thermal transport across nanoscale interfaces

Interfaces are essential elements in nanoscale devices and their properties can differ significantly from their bulk counterparts. Because interfaces often act as bottlenecks in heat dissipation, the prediction and control of the interfacial thermal conductance is critical to the design of nanoscale devices. In this review, we examine the recent advances in quantum interfacial thermal transport from a theoretical and computational perspective. We discuss in detail recent advances in the Atomistic Green's Function method which is an important tool for predicting interfacial thermal transport. We also discuss recent progress in the understanding of interfacial transport mechanisms, including the role of interfacial modes, the role of anharmonic phonon-phonon coupling, the role of electron-phonon interaction, and the ways to tune the interfacial thermal conductance. Finally, we give an overview of the challenges and opportunities in this research field.

Thermal rectification in three-dimensional mass-graded anharmonic oscillator lattices

In this work we study the thermal rectification efficiency, i.e., asymmetric heat flow, of a three-dimensional mass-graded anharmonic lattice of length N and width W by means of nonequilibrium molecular dynamics simulations. The obtained rectification, which is of the same order of magnitude as that of the corresponding one-dimensional lattice, saturates at low values of the aspect ratio W/N, consistent with the already known behavior of the corresponding heat fluxes of the homogeneous system under analogous conditions. The maximum rectification is obtained in the temperature range wherein no rectification could be obtained in other one-dimensional systems, as well as in the corresponding one-dimensional instance of the model studied herein.

The effect of structural asymmetry on thermal rectification in nanostructures

Three SWCNT-graphene nanostructure-based models are designed to probe the thermal rectification caused by the structural asymmetry in the boundary thermal contacts, the device, and the whole system, respectively. We find that both the asymmetry of entire system and the asymmetry of the device are not necessary condition for the existence of thermal rectification, and the asymmetry in boundary thermal contacts is more important than the asymmetry in device toward determining both the magnitude and the direction of thermal rectification. Interestingly, notable thermal rectification can exist in the systems with overall structural symmetry when the boundary thermal contacts are structurally asymmetric. Moreover, nanostructures with a structurally symmetric device and structurally asymmetric boundary thermal contacts can still display significant thermal rectification. These findings could offer insight into the future design and performance improvement of nanostructured thermal rectifiers.

Solitons as candidates for energy carriers in Fermi-Pasta-Ulam lattices

Currently, effective phonons (renormalized or interacting phonons) rather than solitary waves (for short, solitons) are regarded as the energy carriers in nonlinear lattices. In this work, by using the approximate soliton solutions of the corresponding equations of motion and adopting the Boltzmann distribution for these solitons, the average velocities of solitons are obtained and are compared with the sound velocities of energy transfer. Excellent agreements with the numerical results and the predictions of other existing theories are shown in both the symmetric Fermi-Pasta-Ulam-β lattices and the asymmetric Fermi-Pasta-Ulam-αβ lattices. These clearly indicate that solitons are suitable candidates for energy carriers in Fermi-Pasta-Ulam lattices. In addition, the root-mean-square velocity of solitons can be obtained from the effective phonons theory.