An {\it ab initio} study of the magnetic and electronic properties of Fe, Co, and Ni nanowires on Cu(001) surface

Magnetism at the nanoscale has been a very active research area in the past decades, because of its novel fundamental physics and exciting potential applications. We have recently performed an {\it ab intio} study of the structural, electronic and magnetic properties of all 3$d$ transition metal (TM) freestanding atomic chains and found that Fe and Ni nanowires have a giant magnetic anisotropy energy (MAE), indicating that these nanowires would have applications in high density magnetic data storages. In this paper, we perform density functional calculations for the Fe, Co and Ni linear atomic chains on Cu(001) surface within the generalized gradient approximation, in order to investigate how the substrates would affect the magnetic properties of the nanowires. We find that Fe, Co and Ni linear chains on Cu(001) surface still have a stable or metastable ferromagnetic state. When spin-orbit coupling (SOC) is included, the spin magnetic moments remain almost unchanged, due to the weakness of SOC in 3$d$ TM chains, whilst significant orbital magnetic moments appear and also are direction-dependent. Finally, we find that the MAE for Fe, and Co remains large, i.e., being not much affected by the presence of Cu substrate.Comment: 4 pages, 2 figure

Band structure of honeycomb photonic crystal slabs

Two-dimensional (2D) honeycomb photonic crystals with cylinders and connecting walls have the potential to have a large full band gap. In experiments, 2D photonic crystals do not have an infinite height, and therefore, we investigate the effects of the thickness of the walls, the height of the slabs and the type of the substrates on the photonic bands and gap maps of 2D honeycomb photonic crystal slabs. The band structures are calculated by the plane wave expansion method and the supercell approach. We find that the slab thickness is a key parameter affecting the band gap size while on the other hand the wall thickness hardly affact the gap size. For symmetric photonic crystal slabs with lower dielectric claddings, the height of the slabs needs to be sufficiently large to maintain a band gap. For asymmetric claddings, the projected band diagrams are similar to that of symmetric slabs as long as the dielectric constants of the claddings do not differ greatly.Comment: Accepted for publication in Journal of Applied Physic

A non-variational approach to nonlinear stability in stellar dynamics applied to the King model

In previous work by Y. Guo and G. Rein, nonlinear stability of equilibria in stellar dynamics, i.e., of steady states of the Vlasov-Poisson system, was accessed by variational techniques. Here we propose a different, non-variational technique and use it to prove nonlinear stability of the King model against a class of spherically symmetric, dynamically accessible perturbations. This model is very important in astrophysics and was out of reach of the previous techniques

Systematic {\it ab initio} study of the magnetic and electronic properties of all 3d transition metal linear and zigzag nanowires

It is found that all the zigzag chains except the nonmagnetic (NM) Ni and antiferromagnetic (AF) Fe chains which form a twisted two-legger ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state. The electronic spin-polarization at the Fermi level in the FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above. In the zigzag structure, the AF state is more stable than the FM state only in the Cr chain. It is found that the shape anisotropy energy may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Remarkably, the axial magnetic anisotropy in the FM Ni linear chain is gigantic, being ~12 meV/atom. Interestingly, there is a spin-reorientation transition in the FM Fe and Co linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co and Ni linear chains

Anomalous Nernst and Hall effects in magnetized platinum and palladium

We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in proximity-induced ferromagnetic palladium and platinum which is widely used in spintronics, within the Berry phase formalism based on the relativistic band structure calculations. We find that both the anomalous Hall ($\sigma_{xy}^A$) and Nernst ($\alpha_{xy}^A$) conductivities can be related to the spin Hall conductivity ($\sigma_{xy}^S$) and band exchange-splitting ($\Delta_{ex}$) by relations $\sigma_{xy}^A =\Delta_{ex}\frac{e}{\hbar}\sigma_{xy}^S(E_F)'$ and $\alpha_{xy}^A = -\frac{\pi^2}{3}\frac{k_B^2T\Delta_{ex}}{\hbar}\sigma_{xy}^s(\mu)"$, respectively. In particular, these relations would predict that the $\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since the $\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both $\sigma_{xy}^A$ and $\alpha_{xy}^A$ are approximately proportional to the induced spin magnetic moment ($m_s$) because the $\Delta_{ex}$ is a linear function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic platinum and palladium would be gigantic, being up to ten times larger than, e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be significant.Comment: Accepted for publication in the Physical Review

Systematic {\em ab initio} study of the phase diagram of epitaxially strained SrTiO3_3

We use density-functional theory with the local-density approximation to study the structural and ferroelectric properties of SrTiO$_3$ under misfit strains. Both the antiferrodistortive (AFD) and ferroelectric (FE) instabilities are considered. The rotation of the oxygen octahedra and the movement of the atoms are fully relaxed within the constraint of a fixed in-plane lattice constant. We find a rich misfit strain-induced phase transition sequence and is obtained only when the AFD distortion is taken into account. We also find that compressive misfit strains induce ferroelectricity in the tetragonal low temperature phase only whilst tensile strains induce ferroelectricity in the orthorhombic phases only. The calculated FE polarization for both the tetragonal and orthorhombic phases increases monotonically with the magnitude of the strains. The AFD rotation angle of the oxygen octahedra in the tetragonal phase increases dramatically as the misfit strain goes from the tensile to compressive strain region whilst it decreases slightly in the orthorhombic (FO4) phase. This reveals why the polarization in the epitaxially strained SrTiO$_3$ would be larger when the tensile strain is applied, since the AFD distortion is found to reduce the FE instability and even to completely suppress it in the small strain region. Finally, our analysis of the average polar distortion and the charge density distribution suggests that both the Ti-O and Sr-O layers contribute significantly to the FE polarization

Entanglement changing power of two-qubit unitary operations

We consider a two-qubit unitary operation along with arbitrary local unitary operations acts on a two-qubit pure state, whose entanglement is C_0. We give the conditions that the final state can be maximally entangled and be non-entangled. When the final state can not be maximally entangled, we give the maximal entanglement C_max it can reach. When the final state can not be non-entangled, we give the minimal entanglement C_min it can reach. We think C_max and C_min represent the entanglement changing power of two-qubit unitary operations. According to this power we define an order of gates.Comment: 11 page

Microscopic origin of light emission in Al_yGa_{1-y}N/GaN superlattice: Band profile and active site

We present first-principles calculations of AlGaN/GaN superlattice, clarifying the microscopic origin of the light emission and revealing the effect of local polarization within the quantum well. Profile of energy band and distributions of electrons and holes demonstrate the existence of a main active site in the well responsible for the main band-edge light emission. This site appears at the position where the local polarization becomes zero. With charge injection, the calculated optical spectra show that the broadening of the band gap at the active site leads to the blueshift of emission wavelength

Flat galaxies with dark matter halos - existence and stability

We consider a model for a flat, disk-like galaxy surrounded by a halo of dark matter, namely a Vlasov-Poisson type system with two particle species, the stars which are restricted to the galactic plane and the dark matter particles. These constituents interact only through the gravitational potential which stars and dark matter create collectively. Using a variational approach we prove the existence of steady state solutions and their nonlinear stability under suitably restricted perturbations.Comment: 39 page

Abrupt enhancement of non-centrosymmetry and appearance of the spin-triplet superconducting state in Li_2(Pd_{1-x}Pt_{x})_3B beyond x=0.8

We report synthesis, ^{195}Pt, ^{11}B and ^{7}Li NMR measurements, and first-principle band calculation for non-centrosymmetric superconductors Li_{2}(Pd_{1-x}Pt_{x})_{3}B (x=0, 0.2, 0.5, 0.8, 0.84, 0.9 and 1). For 0 \leq x \leq 0.8, the spin-lattice relaxation rate 1/T_1 shows a clear coherence peak just below T_c, decreasing exponentially at low temperature, and the Knight shift ^{195}K decreases below T_c. For x=0.9 and 1.0, in contrast, 1/T_1 shows no coherence peak but a T^3 variation and ^{195}K remains unchanged across T_c. These results indicate that the superconducting state changes drastically from a spin-singlet dominant to a spin-triplet dominant state at x=0.8. We find that the distortion of B(Pt,Pd)_6 increases abruptly above x=0.8, which leads to an abrupt enhancement of the asymmetric spin-orbit coupling as confirmed by band calculation. Such local structure distortion that enhances the extent of inversion-symmetry breaking is primarily responsible for the pairing symmetry evolution. The insight obtained here provides a new guideline for searching new NCS superconductors with large spin-triplet component.Comment: to appear in Phys. Rev. B (Rapid Commun.); contact the authors for high-resolution figure