Nonequilibrium Green's function method for phonon heat transport in quantum system.

Phonon heat transport property in quantum devices is of great interesting since it presents significant quantum behaviors. In the past few decades, great efforts have been devoted to establish the theoretical method for phonon heat transport simulation in nanostructures. However, modeling phonon heat transport from wavelike coherent regime to particlelike incoherent regime remains a challenging task. The widely adopted theoretical approach, such as molecular dynamics, semiclassical Boltzmann transport equation, captures quantum mechanical effects within different degrees of approximation. Among them, Non-equilibrium Green's function (NEGF) method has attracted wide attention, as its ability to perform full quantum simulation including many-body interactions. In this review, we summarized recent theoretical advances of phonon NEGF method and the applications on the numerical simulation for phonon heat transport in nanostructures. At last, the challenges of numerical simulation are discussed.

Effect of electrophilic substitution and destructive quantum interference on the thermoelectric performance in molecular devices.

Using density function theory combined with the non-equilibrium Green's function method, the thermoelectric properties of para-Xylene-based molecular devices are investigated. It is found that destructive quantum interference can be triggered in n-type of para-connected para-Xylene-based molecular device and can obviously enhance the thermoelectric performance of the devices. Moreover, bridge atom electrophilic substitution can significantly improve the thermoelectric properties of p-type monolayer molecular device. The ZT value of p-type monolayer molecular device with doped electrodes can be optimized to 2.2 at 300 K and 2.8 at 500 K, and n-type bilayer molecular device can achieve the value of 1.2 at 300 K and 2.0 at 500 K. These results offer the information to design the complete molecular thermoelectric device with p-type and n-type of components and to promote the thermoelectric properties of bilayer molecular junctions by employing destructive quantum interference effects.

Strain tuning of electronic properties of various dimension elemental tellurium with broken screw symmetry.

We present a systematical study of atomic structures and electronic properties of various dimension tellurium (Te) with broken intrinsical screw symmetry by applying reasonable strain. It is demonstrated that (i) bulk trigonal Te has degenerate Weyl nodes around the H point near the Fermi energy, and this degeneracy will be broken by introducing the selenium (Se) atom through creating the inner unsymmetrical strain, instead of external shear strain. (ii) 2D structures of tetragonal Te (t-Te) and 1T-MoS2-like Te (1T-Te) show direct and indirect band gap, respectively. Under the uniform biaxial compressive (BC) strain, monolayer of t-Te shows the direct-to-indirect band gap transition, while 1T-Te monolayer has a band gap transition firstly from indirect to direct and then from direct to indirect. Their effective masses of hole and electron can be effectively tuned by BC strain. (iii) One-dimensional (1D) structures of single helix, triangular Te and hexagonal Te nanowires display the obvious quantum confinement effect on the band structure and different sensitivity to the effect of uniaxial compressive strain.