We study the quantum transport behavior of a normal-superconductor-normal junction based on a type-II Weyl semimetal, which is arranged in the tilting direction of the Weyl semimetal. We find that both the crossed Andreev reflection and normal reflection are forbidden, while there will be double Andreev reflections and double electron transmissions for the incident electron from the semimetal side. Andreev reflections and transmissions occur both in the retro and specular directions simultaneously, symmetric about the normal of the interface but with different amplitudes, depending on the angle and energy of incident electrons. These transport processes make the junction here quite different from that based on the normal metal or graphene. In addition, the differential conductance is studied for experimental signatures. We find that the conductance is almost unaffected by the electrostatic potential in the normal region and it is enhanced with increasing junction length.

By introducing the next-nearest-neighboring (NNN) intersite coupling, we investigate the eigenenergies of the PT -symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model with two conjugated imaginary potentials at the end sites. It is found that with the strengthening of NNN coupling, the particle-hole symmetry is destroyed. As a result, the bonding band is first narrowed and then undergoes the top-bottom reversal followed by the its width's increase, whereas the antibonding band is widened monotonously. In this process, the topological state extends into the topologically-trivial region, and its energy departs from the energy zero point, accompanied by the emergence of one new topological state in this region. All these results give rise to the complication of the topological properties and the manner of PT -symmetry breaking. It can be concluded that the NNN coupling takes important effects to the change of the topological properties of the non-Hermitian SSH system.

We theoretically perform a comprehensive analysis about the influences of the space-inversion symmetry breaking in graphene-based materials on the Andreev reflections (AR) in the normalmetal/superconductor (NS) and NSN heterojunctions. It is found that in the NS junction, the AR can be suppressed or be enhanced by the enhancement of space-inversion symmetry breaking, depending on the relationship among the coherence parameters. Following this result, the AR properties in the NSN structure are evaluated. It is readily observed that the local AR can be weakened for low spaceinversion symmetry breaking, and can be enhanced for high space-inversion symmetry breaking. Alternatively, the efficiency of the crossed AR can be improved to a great degree, with the increase of space-inversion symmetry breaking. One can therefore understand the special role of space-inversion symmetry in modulating the AR, especially for the enhancement of crossed AR. ferromagnetic leads are introduced to couple to the QDs, spin-polarized Andreev transport can be observed [34].On the other hand, graphene-based Cooper pair splitters have received a lot of attentions, after the successful fabrication of graphene. Graphene has the advantage of the controllability of its-based mesoscopic circuits, and it can be superconducting by proximity to a superconductor on top of it [35][36][37]. This has attracted vast studies in the electronic transport through the NS heterostructures based on graphene [38][39][40][41]. It is also known that graphene has the linear dispersion relation and the conduction and valence bands touch each other at the Dirac point, making it a gapless semiconductor. Hence in graphene, the carrier type (electron: n-type; hole: p-type) and its density can be easily tuned in a controllable manner by local electrostatic gates or chemical doping. Many groups have demonstrated that the two-dimensional crystals, e.g. graphene, are possible areas for CAR processes where the magnitude of the CAR conductance can be enhanced in NSN hybrid structures. By building the NSN structure with the n-type and p-type graphene coupled to the superconducting part, channels for the electron transmission (ET) and local Andreev reflection (LAR) processes have been found to be seriously suppressed, leading to the enhancement of CAR process [42]. Other researches try to improve the CAR efficiency by proposing a graphene-based NSN spin valve [43]. This shows a spin-switch effect between pure CAR process and pure ET through the reverse of the magnetization direction in the ferromagnetic layers. Besides, a pure CAR process has been observed in a magnetized zigzag graphene nanoribbon junction with an even zigzag chain number [44].It is well known that besides the physical real spin index in graphene, one additional intrinsic degree of freedom also affects the properties of graphene, due to the existence of two types of carbon atoms (labeled as A and B) in the honeycomb lattice of graphene. When A and B atoms are different in energy, the space-inversion symmetry ...

We theoretically investigate the band structure of the parity-time ( P T )-symmetric non-Hermitian bilayer honeycomb photonic lattice by considering the co-existence of onsite energy detuning and P T -symmetric imaginary potentials. It is found that P T symmetry causes the system’s energy spectra to display salient features, which include various energy-band degeneracies and deformation of the band structure in the vicinity of the Dirac point. Also, the P T -symmetry breaking can be modulated by introducing strain to the system. The findings in this work can help in understanding the band structures of P T -symmetric bilayer honeycomb photonic-lattice systems.

We investigate the quantum transmission through the n-p-n heterojunction of massive 8-Pmmn borophene. It is found that the Dirac mass of the electron interacts nontrivially with the anisotropy of the 8-Pmmn borophene, leading to the occurrence of new transmission behaviors in this n-p-n heterojunction. Firstly, the effective energy range of nonzero transmission can be reduced but deviates from the mass amplitude, which induces the further controllability of the transmission property. Secondly, even if the equal-energy surfaces in the p and n parts do not encounter in the k-space, finite transmission is allowed to occur as well. In addition, the existence of Dirac mass can change the reflection manner from the retroreflection to the specular reflection under appropriate conditions. The findings in this work can be helpful in describing the quantum transport properties of the heterojunction based on 8-Pmmn borophene.