K.L. Yao

Molecular spin valve and spin filter composed of single-molecule magnets

We presented a theoretical study on the spin-polarized transport of one single-molecule magnet. The results show it has spin filter behavior, and can also be used as a molecular spin valve with large abnormal magnetoresistance, which is an excellent candidate for spintronics of molecular devices. It is also found that the steplike features with negative differential resistance in the I-V curve indeed originate from the discreteness and narrow features in the density of states of the molecular levels.

Half-metallic ferromagnetism in wurtzite ScM (M=C, Si, Ge, and Sn): Ab initio calculations

Using the full potential linearized augment plane wave method with the modified Becke and Johnson (mBJ) potential, the half-metallicity and electronic structure for the wurtzite ScM (M = C, Si, Ge, and Sn) compounds are investigated. The ScM series compounds are found to be excellent half-metallic ferromagnets (HMFs) with large half-metallic gaps (0.76–0.33 eV). The magnetic moments are 2.00 μB per cell, and p-d hybridization mechanism plays crucial role in forming the half-metallic ferromagnetism. The ferromagnetic ground states, larger half metallic gaps, the robust half-metallicity with respect to the lattice compression, negative cohesive energy, and heat of formation indicate ScM compounds would be promising HMFs.

First-principles calculations: half-metallic Au–V(Cr) quantum wires as spin filters

The half-metallic behavior of Au-V(Cr) quantum wires adsorbed on an armchair (5, 5) boron nitride nanotube is obtained by performing spin-polarized density functional calculations. The density of states shows a metallic property at the Fermi level for the majority spin channel and a semiconductor gap in the minority spin channel. The half-metallic behavior of the quantum wire/nanotube complex originates from the half-metallic behavior of the free-standing Au-V(Cr) quantum wires. The calculations of spin-polarized transport indicate that such a one-dimensional half-metallic magnet can be used as a spin filter.

Experimental realization of a broadband conformal mapping lens for directional emission

We experimentally realize a transformation lens for directional emission at microwave frequencies. The refractive index of the lens, based on the design of conformal mapping, has variation from 1 to 4 inside a semi-circular geometry. By drilling subwavelength holes in dielectric plates inhomogeneously, we design the lens sample. The scanning measurements of the electric field of the lens demonstrate the high directivity of emission, within a broadband working frequency region of 7-11 GHz.

Conformal transformations to achieve unidirectional behavior of light

The method of optical conformal mapping is used to design two isotropic devices through which light travels in a unidirectional manner. The first device is a directional emitter. By setting a line current source in a properly tuned refractive index profile, fields can be radiated in only one direction or its opposite without using any reflector or metallic structure. The second proposal is a dual-functional device. It works not only as a directional emitter for an embedded source but also as a quasi-diode for beams, thus having potential on-chip applications. Functionalities of the two designs are verified by finite- element-based simulations. We further investigate the spatial dependence of the refractive index near singularities, and corresponding optimization is proposed in the interests of experimental consideration. Numerical results show that the one-way property is well preserved after the parameter reduction.

Collimating lenses from non-Euclidean transformation optics

Based on non-Euclidean transformation optics, we design a thin metamaterial lens that can achieve wide-beam radiation by embedding a simple source (a point source in the three-dimensional (3D) case or a line current source in the 2D case). The scheme is performed on a layer-by-layer geometry to convert curved surfaces in the virtual space into flat sheets, which pile up and form the entire lens in the physical space. Compared to previous designs, the lens does not have extreme material parameters. Simulation results confirm its functionality.

Generalized laws of reflection and refraction from transformation optics

Based on transformation optics, we introduce another set of generalized laws of reflection and refraction (different from that of Yu N. et al., Science, 334 (2011) 333), through which a transformation media slab is derived as a meta-surface, producing anomalous reflection and refraction for all polarizations of incident light. For easy implementation, we also propose a reduced version.

STUDIES ON THE FERROMAGNETIC STATE IN QUASI-ONE-DIMENSIONAL ORGANIC POLYMER FERROMAGNETS WITH COMPETITION BETWEEN ELECTRON-ELECTRON INTERACTION AND ELECTRON-PHONON COUPLING

The possible ferromagnetic ground state of a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets is investigated in detail. Within the self-consistent-field Hartree-Fock approximation, allowing for full. lattice relaxation, the system is studied self-consistently. We find a critical interaction, which is determined by the competition between the electron-electron interaction and the electron-phonon coupling. When electron-electron interaction is larger than the critical value, the ground state of the system is a stable ferromagnetic state. It is also found that in this stable ferromagnetic state the pi-electrons SDW will become very strong, and almost zero dimerization happens for the main chain.

Noncollinear magnetic ground state of PrFeAsO

Noncollinear magnetic investigations of the ground state in PrFeAsO have been performed by the density-functional theory. We calculated the total energy and made structure optimization, and the electronic density of states of PrFeAsO was analyzed. There are three different magnetic structures in PrFeAsO defined by experiments. Based on these magnetic structures, we studied four collinear and four noncollinear cases. The ground state is found to take the ordering proposed by Zhao, in which the FeAs plane is of stripe antiferromagnetism and Pr spins are perpendicular to Fe spins. The electronic density of states indicates that for PrFeAsO the increase of the electron Coulomb interaction leads to a decrease in conductivity.

First-principles studies on the spin distribution on the dihalide-bridged polymer antiferromagnet: Cu(thiazole)2Br2 and M(thiazole)2Cl2?(M = Cu,Fe)

First-principles calculations have been performed to study the electronic band structure and the magnetic properties for the polymeric compound of Cu(thiazole)(2)Br-2 and M(thiazole)(2)Cl-2 (M = Cu, Fe). The relative stability of the ground state, density of states and electronic band structure were examined. Total energy calculations reveal that the antiferromagnetic phase is a stable ground state for Cu(thiazole)(2)Br-2 and M(thiazole)(2)Cl-2 (M = Cu, Fe). It is noteworthy that we predict the compound Fe(thiazole)(2)Cl-2 is a metallic antiferromagnet and has a half-metallic (HM) ferromagnetic metastable state. But the spin-up and spin-down DOS in the ferromagnetic and anti ferromagnetic phases of the compounds Cu(thiazole)(2)Cl-2 and Cu(thiazole)(2)Br-2 all opened a large bandgap. The spin magnetic moments of Cu(thiazole)(2)Br-2 and M(thiazole)(2)Cl-2 (M = Cu, Fe) are mainly assembled at the copper and iron atoms, with a little contribution from the chlorine, bromine, nitrogen, sulfur, and carbon atoms.