Xiongwen Chen

Symmetry-dependent spin-charge transport and thermopower through a ZSiNR-based FM/normal/FM junction

We investigate the spin-dependent transport and spin thermopower for a zigzag silicene nanoribbon (ZSiNR) with two ends covered by ferromagnets (FMs) under the modulation of a perpendicular electric field, where we take the 6- and 7-ZSiNR to exemplify the effect of the even- and odd-N ZSiNRs, respectively. By using the nonequilibrium Greens function approach, it is demonstrated that a ZSiNR-based FM/normal/FM junction still shows an interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. The junction with even- or odd-N ZSiNR has very different transport and thermopower behavior, which is attributed to the different parity of π and [Formula: see text] band wavefunctions under the c 2 symmetry operation with respect to the centre axis between two edges, and is linked to the unique symmetry of the band structure for the ribbon. As a result, the magnetoresistance (MR) for the 6-ZSiNR junction with a 100% plateau around zero electron energy is observed, but the plateau is absent for the 7-ZSiNR one. In addition, the spin thermopower also displays the even-odd behaviour. The 6-ZSiNR junction is found to possess superior thermospin performance compared with the 7-ZSiNR one, and its spin thermopower can be improved by one order of magnitude in the absence of an electric field. As the strength of the field increases, the spin thermopower for the 6-ZSiNR junction dramatically decreases, while it notably enhances for the 7-ZSiNR one. Interestingly, the spin thermopower for both junctions is strongly dependent on the strength of magnetisation in FM, and it can be very pronounced with a maximum absolute value of 200 μV K(-1)by the optimisation of the parameters. However, with the increase in temperature, the spin thermopower for the 6-ZSiNR junction decreases, but the situation for the 7-ZSiNR one is opposite. Finally, the spin figure of merit for the 6-ZSiNR junction is found to be four orders of magnitude larger than that for the 7-ZSiNR one. This even-odd effect is common for N-SiNR, and the result can be regarded as an advance in the understanding of the characteristics of silicene and may be valuable for experimentally designing spin valve and heat spintronic devices based on silicene.

Robust and controllable electronic local transports in armchair silicene nanoribbons under a perpendicular electric field

We study the electronic local distribution and transports in pristine armchair-edge silicene nanoribbons (ASiNRs) based on the tight-binding approximation. By calculating the local densities of states at different sites and the bond current between two adjacent sites, we show that comparing to the pristine armchair-edge graphene nanoribbons, a similar “3j” rule and multiple low-electron transport channels exist in the pristine (3p + 2)-ASiNRs. However, differently, they are controllable to appear and disappear by applying an electric field perpendicular to the ribbon plane. Therefore, one can manipulate the semiconducting channels and realize the current switch “on/off,” unchanging their structures. Moreover, the results are robust against the edge-passivation and a few structural defects, which ensures their stability for the practical application in the silicene-based device.

Electronic structure, chemical bonding and optical properties of the nonlinear optical crystal ZnGeP2 by first-principles calculations

Using the plane wave pseudopotential method within density-functional theory, we have theoretically investigated the structural, electronic, chemical bonding and optical properties of the chalcopyrite semiconductor ZnGeP2. It is found that ZnGeP2 has an indirect band gap of 1.222 eV. The covalent character of the bonds in ZnGeP2 crystal is verified by Mulliken population. By analyzing the optical properties including the dielectric function, refractive index, extinction coefficient, reflectivity spectrum and absorption coefficient, we indicate that ZnGeP2 is a promising mid-IR optical material, which is in good agreement with the available experimental results.

Influence of cavity decay on unconventional geometric quantum phase gates with superconducting quantum-interference-device qubits inside a cavity

We propose a potential scheme for carrying out two-qubit unconventional geometric logic gates on two identical superconducting quantum-interference-device (SQUID) qubits coupled to a single-mode microwave field. The geometric logic gate operation is performed in two lower flux states, and the excited state |2 does not participate in the procedure. The SQUIDs undergo no transitions during gate operation. Thus, the decoherence due to energy spontaneous emission based on the levels of SQUIDs is suppressed. We present the two-qubit unconventional geometric logic gates in both an ideal cavity and a real cavity with decay. Discussions about the fidelity and the success probability of the proposed scheme as well as the experimental feasibility are given in detail.