Qing-Rong Zheng

T-carbon: a novel carbon allotrope.

A structurally stable crystalline carbon allotrope is predicted by means of the first-principles calculations. This allotrope can be derived by substituting each atom in diamond with a carbon tetrahedron, and possesses the same space group Fd3m as diamond, which is thus coined as T-carbon. The calculations on geometrical, vibrational, and electronic properties reveal that T-carbon, with a considerable structural stability and a much lower density 1.50  g/cm3, is a semiconductor with a direct band gap about 3.0 eV, and has a Vickers hardness 61.1 GPa lower than diamond but comparable with cubic boron nitride. Such a form of carbon, once obtained, would have wide applications in photocatalysis, adsorption, hydrogen storage, and aerospace materials.

Hinge-like structure induced unusual properties of black phosphorus and new strategies to improve the thermoelectric performance

We systematically investigated the geometric, electronic and thermoelectric (TE) properties of bulk black phosphorus (BP) under strain. The hinge-like structure of BP brings unusual mechanical responses such as anisotropic Youngs modulus and negative Poissons ratio. A sensitive electronic structure of BP makes it transform among metal, direct and indirect semiconductors under strain. The maximal figure of merit ZT of BP is found to be 0.72 at 800 K that could be enhanced to 0.87 by exerting an appropriate strain, revealing BP could be a potential medium-high temperature TE material. Such strain-induced enhancements of TE performance are often observed to occur at the boundary of the direct-indirect band gap transition, which can be attributed to the increase of degeneracy of energy valleys at the transition point. By comparing the structure of BP with SnSe, a family of potential TE materials with hinge-like structure are suggested. This study not only exposes various novel properties of BP under strain, but also proposes effective strategies to seek for better TE materials.

Erratum: Family of boron fullerenes: General constructing schemes, electron counting rule, and ab initio calculations

A set of general constructing schemes is unveiled to predict a large family of stable boron monoelemental, hollow fullerenes with magic numbers 32+8k (k>=0). The remarkable stabilities of these new boron fullerenes are then studied by intense ab initio calculations. An electron counting rule as well as an isolated hollow rule are proposed to readily show the high stability and the electronic bonding property, which are also revealed applicable to a number of newly predicted boron sheets and nanotubes.

Octagraphene as a versatile carbon atomic sheet for novel nanotubes, unconventional fullerenes, and hydrogen storage

We study a versatile structurally favorable periodic sp2-bonded carbon atomic planar sheet with C4v symmetry by means of the first-principles calculations. This carbon allotrope is composed of carbon octagons and squares with two bond lengths and is thus dubbed as octagraphene. It is a semimetal with the Fermi surface consisting of one hole and one electron pocket, whose low-energy physics can be well described by a tight-binding model of π-electrons. Its Youngs modulus, breaking strength, and Poissons ratio are obtained to be 306 N/m, 34.4 N/m, and 0.13, respectively, which are close to those of graphene. The novel sawtooth and armchair carbon nanotubes as well as unconventional fullerenes can also be constructed from octagraphene. It is found that the Ti-absorbed octagraphene can be allowed for hydrogen storage with capacity around 7.76 wt. %.

Tinselenidene: a Two-dimensional Auxetic Material with Ultralow Lattice Thermal Conductivity and Ultrahigh Hole Mobility

By means of extensive ab initio calculations, a new two-dimensional (2D) atomic material tin selenide monolayer (coined as tinselenidene) is predicted to be a semiconductor with an indirect gap (~1.45 eV) and a high hole mobility (of order 10000 cm2V−1S−1), and will bear an indirect-direct gap transition under a rather low strain (<0.5 GPa). Tinselenidene has a very small Young’s modulus (20–40 GPa) and an ultralow lattice thermal conductivity (<3 Wm−1K−1 at 300 K), making it probably the most flexible and most heat-insulating material in known 2D atomic materials. In addition, tinseleniden has a large negative Poisson’s ratio of −0.17, thus could act as a 2D auxetic material. With these intriguing properties, tinselenidene could have wide potential applications in thermoelectrics, nanomechanics and optoelectronics.

Spin current and current-induced spin transfer torque in ferromagnet–quantum dot–ferromagnet coupled systems

Based on Keldyshs nonequilibrium Green function method, the spin-dependent transport properties in a ferromagnet\char21{}quantum dot (QD)\char21{}ferromagnet coupled system are investigated. It is shown the spin current shows quite different characteristics from its electrical counterpart, and by changing the relative orientation of both magnetizations, it can change its magnitude and even sign. The current-induced spin transfer torque (CISTT) is uncovered to be greatly enhanced when the bias voltage meets with the discrete levels of the QD at resonant positions. The relationship between the CISTT, the electrical current, and the spin current, is also addressed.

Time-dependent spin-polarized transport through a resonant tunneling structure with multiterminals

The spin-dependent transport of the electrons tunneling through a resonant tunneling structure with ferromagnetic multi-terminal under dc and ac fields is explored by means of the nonequilibrium Green function technique. A general formulation for the time-dependent current and the time-averaged current is established. As its application the systems with two and three terminals in noncollinear configurations of the magnetizations under dc and ac biases are investigated, respectively. The asymmetric factor of the relaxation times for the electrons with different spin in the central region is uncovered to bring about various behaviours of the TMR. The present three-terminal device is different from that discussed in literature, which is coined as a spin transistor with source. The current-amplification effect is found. In addition, the time-dependent spin transport for the two-terminal device is studied. It is found that the photonic sidebands provide new channels for the electrons tunneling through the barriers, and give rise to new resonances of the TMR, which is called as the photon-asisted spin-dependent tunneling. The asymmetric factor of the relaxation times is observed to lead to additional resonant peaks besides the photon-asisted resonances.

Strain-induced Dirac cone-like electronic structures and semiconductor–semimetal transition in graphdiyne

By means of first-principles calculations combined with the tight-binding approximation, the strain-induced semiconductor-semimetal transition in graphdiyne is discovered. It is shown that the band gap of graphdiyne increases from 0.47 eV to 1.39 eV with increasing the biaxial tensile strain, while the band gap decreases from 0.47 eV to nearly zero with increasing the uniaxial tensile strain, and Dirac cone-like electronic structures are observed. The uniaxial strain-induced changes of the electronic structures of graphdiyne come from the breaking of geometrical symmetry that lifts the degeneracy of energy bands. The properties of graphdiyne under strains are found to differ remarkably from that of graphene.

Effect of spin-flip scattering on electrical transport in magnetic tunnel junctions

Abstract By means of the nonequilibrium Green function technique, the effect of spin-flip scatterings on the spin-dependent electrical transport in ferromagnet–insulator–ferromagnet (FM–I–FM) tunnel junctions is investigated. It is shown that Jullieres formula for the tunnel conductance must be modified when including the contribution from the spin-flip scatterings. It is found that the spin-flip scatterings could lead to an angular shift of the tunnel conductance, giving rise to the junction resistance not being the largest when the orientations of magnetizations in the two FM electrodes are antiparallel, which may offer an alternative explanation for such a phenomenon observed previously in experiments in some FM–I–FM junctions. The spin-flip assisted tunneling is also observed.

Face-centered-cubic B 80 metal: Density functional theory calculations

By means of ab initio calculations within the density functional theory, we have found that