With the dimension of materials shrinking into nanoscale, there has been growing interest in the Kapitza resistance which inhibits overall thermal transport. In this work, using the non-equilibrium Green's function method, we systematically investigate the optimized interfacial couplers with various gradient materials for phonon transport across one-dimensional atomic hetero-junction models. Relative to the optimized homogenous couplers, the mass-graded or coupling-graded structures are found to be applicable to improve the interfacial thermal conductance of two lead materials with both mismatched impedance and mismatched cutoff frequencies. For the couplers with both geometric graded mass and geometric graded coupling, the interfacial thermal conductance can be maximum enhanced (nearly up to sixfold enhancement on interfacial thermal conductance compared to the optimized homogenous case). The underlying mechanism of phonon transport enhancement by the optimized coupler is investigated by the phonon transmission coefficient: on the one hand, this kind of coupler is able to maximally suppress the destructive interference for transmitted phonon waves; on the other hand, the constructive interference for the transmitted phonon is also largely improved. Our findings may offer guidance for advanced thermal interface materials design.

Interfacial thermal resistance (ITR, or Kapitza resistance) is the bottleneck that limits the further growth of density for integrated circuit. In this paper, we study the interfacial thermal coupling between two nonlinear systems by using a onedimensional FPUβ heterojunction model through molecular dynamics simulation. It is found that the ITR first decreases rapidly and then increases slowly with the increase of interface coupling coefficient (ICC). When the nonlinearity is weak, the optimal ICC can be explained by selfconsistent phonon theory and effective phonon theory. We also find a double scale behavior in heterojunctions. The study of optimal interfacial thermal coupling for two nonlinear systems has potential applications in reducing the ITR between real materials.

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In modern information technology, as integration density increases rapidly and the dimension of materials reduces to nanoscale, interfacial thermal transport (ITT) has attracted widespread attention of scientists. This review introduces the latest theoretical development in ITT through one-dimensional (1D) atomic junction model to address the thermal transport across an interface. With full consideration of the atomic structures in interfaces, people can apply the 1D atomic junction model to investigate many properties of ITT, such as interfacial (Kapitza) resistance, nonlinear interface, interfacial rectification, and phonon interference, and so on. For the ballistic ITT, both the scattering boundary method (SBM) and the non-equilibrium Green's function (NEGF) method can be applied, which are exact since atomic details of actual interfaces are considered. For interfacial coupling case, explicit analytical expression of transmission coefficient can be obtained and it is found that the thermal conductance maximizes at certain interfacial coupling (harmonic mean of the spring constants of the two leads) and the transmission coefficient is not a monotonic decreasing function of phonon frequency. With nonlinear interaction-phonon-phonon interaction or electron-phonon interaction at interface, the NEGF method provides an efficient way to study the ITT. It is found that at weak linear interfacial coupling, the nonlinearity can improve the ITT, but it depresses the ITT in the case of strong-linear coupling. In addition, the nonlinear interfacial coupling can induce thermal rectification effect. For interfacial materials case which can be simulated by a two-junction atomic chain, phonons show interference effect, and an optimized thermal coupler can be obtained by tuning its spring constant and atomic mass.

There has been growing interest in revealing exotic properties of chiral phonons since they were found in honeycomb AB lattice, and very recently they were experimentally verified in a tungsten diselenide monolayer (2018 Science 359 579). In this work, we manipulate phonon chirality through interface transmission via a one-dimensional atomic junction model by using the scattering boundary method. Due to the difference of phase change between two transverse directions induced by the anisotropy at interface coupling, the phonon polarization can be tuned between circular and linear in the highfrequency range. In a double-junction atomic model with an anisotropic center, we find that the phase change accumulates when the phonon transmits through the interface material thus the phonon can be tuned between different chirality in the medium frequency range. The phase change is found to linearly depend on the width of the interface material, while the transmission coefficient vibrates. To obtain the same value of the transmission coefficients along the two transverse directions and thus to keep the outgoing phonon circularly polarized, we can connect two interface materials with opposite anisotropy, where the phase-change difference for chirality tuning can be adjusted by the difference of widths of the two materials. Therefore, by using the atomic junction model, we find that the phonon chirality can be effectively tuned through interfaces, which is helpful for the manipulation and application of chiral phonons.

There has been a growing interest in the phase of phonon, due to the theoretical prediction (Phys. Rev. Lett. 115.11 (2015)) and the experimental observation (Science 359.6379 (2018)) of chiral phonons, which have different phases in different components. While half-wave loss is a well-known concept in optics, in this work, a series of plateaus of quarters-wave loss are first found for the reflected phonon across an interface by using an atomic junction model. These plateaus can be understood by the Smatrix in the system with time-reversal symmetry. If a phonon wave propagates from a low acousticimpedance material (or a low cutoff frequency material) to a higher one in the long-wave limit (or in the high frequency limit), a half-wave loss takes place for the reflected phonon; however, the plateau of half-wave loss for reflected phonon occurs in the whole frequency domain if phonon transfers to a material with a larger spring constant. Besides the half-wave loss, we also observe plateaus of quarterwave (three-quarters-wave) loss in long wave limit when the two leads with identical acoustic impedance are coupled by a weak (strong) coupling in comparison with the optimum thermal coupling. The quarters-wave loss for phonons can be applied to chiral phonon manipulation and other phononics devices.

With the rapidly increasing integration density and power density in nanoscale electronic devices, the thermal management concerning heat generation and energy harvesting becomes quite crucial. Since phonon is the major heat carrier in semiconductors, thermal transport due to phonons in mesoscopic systems has attracted much attention. In quantum transport studies, the nonequilibrium Green's function (NEGF) method is a versatile and powerful tool that has been developed for several decades. In this review, we will discuss theoretical investigations of thermal transport using the NEGF approach from two aspects. For the aspect of phonon transport, the phonon NEGF method is briefly introduced and its applications on thermal transport in mesoscopic systems including onedimensional atomic chains, multi-terminal systems, and transient phonon transport are discussed. For the aspect of thermoelectric transport, the caloritronic effects in which the charge, spin, and valley degrees of freedom are manipulated by the temperature gradient are discussed. The timedependent thermoelectric behavior is also presented in the transient regime within the partitioned scheme based on the NEGF method.