Modification of plasmonic properties in several transition metal-doped graphene studied by the first principles method.

Graphene doped with different transition metal (TM) atoms, namely, Co, Ni, Cu, Zn, and Au, have been investigated through first-principles calculations. The TM atom forms a substitutional defect, replacing one carbon atom in the graphene basal plane, which considerably can be obtained through wet or dry chemical processes as reported elsewhere. The calculation results showed that TM atom substitution leads to the opening of a band gap and the emergence of mid-gap states with the Fermi energy in the middle of it. The effects on optical properties were seen from the calculated optical absorption and Electron Energy Loss Spectroscopy (EELS) spectra. Two EELS bands are seen in the far UV region corresponding to the π and (π + σ) plasmons but the influence of the substituted TM effects on the plasmon frequency is small. On the other hand, as the Fermi energy level appears in the middle of the mid-gap state band while the real part of its dielectric permittivity at low photon energy is negative, these TM-doped graphene have a metal-like characteristic. Hence, plasmon wave excitation can be expected at the THz region which is dependent on the dopant TM atom. The plasmon excitation in these TM-doped graphene is thus principally similar to the plasmonic excitation in pure graphene by electric or magnetic fields, where the Fermi energy level is shifted from the graphene Dirac point leading to the possibility of an intraband transition.

Effects of surface and twinning energies on twining-superlattice formation in group III-V semiconductor nanowires: a first-principles study.

The formation of twin plane superlattices in group III-V semiconductor nanowires (NWs) is analyzed by considering two dimensional nucleation using surface and twinning energies, obtained by performing electronic structure calculations within density functional theory. The calculations for GaP, GaAs, InP, and InAs demonstrate that surface energies strongly depend on the growth conditions such as temperature and pressure during the epitaxial growth. Furthermore, the calculated twinning energies are found to be much smaller than previously estimated values by the dissociation width of edge dislocations, which lead to smaller segment lengths. We also find that the nonlinear relationship between segment length and NW diameter depending on constituent elements is due to the difference in twinning energies. These results imply that twinning formation as well as surface stability are crucial for the formation of twin plane superlattices in group III-V semiconductor NWs.

Tunable inverse spin Hall effect in nanometer-thick platinum films by ionic gating.

Electric gating can strongly modulate a wide variety of physical properties in semiconductors and insulators, such as significant changes of conductivity in silicon, appearance of superconductivity in SrTiO3, the paramagnet-ferromagnet transition in (In,Mn)As, and so on. The key to such modulation is charge accumulation in solids. Thus, it has been believed that such modulation is out of reach for conventional metals where the number of carriers is too large. However, success in tuning the Curie temperature of ultrathin cobalt gave hope of finally achieving such a degree of control even in metallic materials. Here, we show reversible modulation of up to two orders of magnitude of the inverse spin Hall effect-a phenomenon that governs interconversion between spin and charge currents-in ultrathin platinum. Spin-to-charge conversion enables the generation and use of electric and spin currents in the same device, which is crucial for the future of spintronics and electronics.

Energy gap opening by crossing drop cast single-layer graphene nanoribbons.

Band gap opening of a single-layer graphene nanoribbon (sGNR) sitting on another sGNR, fabricated by drop casting GNR solution on Au(111) substrate in air, was studied by means of scanning tunneling microscopy and spectroscopy in an ultra-high vacuum at 78 K and 300 K. GNRs with a width of ∼45 nm were prepared by unzipping double-walled carbon nanotubes (diameter ∼15 nm) using the ultrasonic method. In contrast to atomically-flat GNRs fabricated via the bottom-up process, the drop cast sGNRs were buckled on Au(111), i.e., some local points of the sGNR are in contact with the substrate (d âˆ¼ 0.5 nm), but other parts float (d âˆ¼ 1-3 nm), where d denotes the measured distance between the sGNR and the substrate. In spite of the fact that the nanoribbons were buckled, dI/dV maps confirmed that each buckled sGNR had a metallic character (∼3.5 Go) with considerable uniform local density of states, comparable to a flat sGNR. However, when two sGNRs crossed each other, the crossed areas showed a band gap between -50 and +200 meV around the Fermi energy, i.e., the only upper sGNR electronic property changed from metallic to p-type semiconducting, which was not due to the bending, but the electronic interactions between the up and down sGNRs.

Correlation of the Dzyaloshinskii-Moriya interaction with Heisenberg exchange and orbital asphericity.

Chiral spin textures of a ferromagnetic layer in contact to a heavy non-magnetic metal, such as Néel-type domain walls and skyrmions, have been studied intensively because of their potential for future nanomagnetic devices. The Dyzaloshinskii-Moriya interaction (DMI) is an essential phenomenon for the formation of such chiral spin textures. In spite of recent theoretical progress aiming at understanding the microscopic origin of the DMI, an experimental investigation unravelling the physics at stake is still required. Here we experimentally demonstrate the close correlation of the DMI with the anisotropy of the orbital magnetic moment and with the magnetic dipole moment of the ferromagnetic metal in addition to Heisenberg exchange. The density functional theory and the tight-binding model calculations reveal that inversion symmetry breaking with spin-orbit coupling gives rise to the orbital-related correlation. Our study provides the experimental connection between the orbital physics and the spin-orbit-related phenomena, such as DMI.

Spatially Resolved Magnetic Anisotropy of Cobalt Nanostructures on the Au(111) Surface.

Understanding the origin of perpendicular magnetic anisotropy in surface-supported nanoclusters is crucial for fundamental research as well as data storage applications. Here, we investigate the perpendicular magnetic anisotropy energy (MAE) of bilayer cobalt islands on Au(111) substrate using spin-polarized scanning tunneling microscopy at 4.6 K and first-principles theoretical calculations. Au(111) substrate serves as an excellent model system to study the effect of nucleation site and stacking sequence on MAE. Our measurements reveal that the MAE of bilayer islands depends strongly on the crystallographic stacking of the two Co layers and nucleation of the third layer. Moreover, the MAE of Co atoms on Au(111) is enhanced by a factor of 1.75 as compared to that reported on Cu(111). Our first-principles calculations attribute this enhancement to the large spin-orbit coupling of the Au atoms. Our results highlight the strong impact of nanometer-scale structural changes in Co islands on MAE and emphasize the importance of spatially resolved measurements for the magnetic characterization of surface-supported nanostructures.

Nitridation of Al2O3 surfaces: chemical and structural change triggered by oxygen desorption.

We present theoretical investigations that clarify elemental nitridation processes of corundum Al2O3(0001) and (1102) surfaces. The calculations within the density functional theory framework reveal that the structures with substitutional N atoms beneath the surface are stabilized under nitridation conditions. We also find that the desorption of O atoms at the topmost layer induces outward diffusion of O atoms as well as inward diffusion of N atoms, leading to the transformation into AlN films. The kinetic Monte Carlo simulations in conjunction with density functional theory results indeed observe a dependence of these chemical and structural changes on temperature and pressure.

Band alignment tuning in twin-plane superlattices of semiconductor nanowires.

The band alignments of twin-plane superlattices in semiconductor nanowires are systematically investigated on the basis of density functional calculations. Our calculations demonstrate that for nanowires with small diameters the quantum-confinement effect is prominent within wurtzite structure regions and the energy gap in wurtzite-structured nanowires is remarkably larger than that including zinc-blende structure. This results in the straddling band alignment, in which both electrons and holes are confined in zinc-blende structure region. The analysis using a simple tight-binding methods also clarifies that the straddling band alignments can be realized when the diameters of nanowires are less than 4-8 nm, leading to full control of band alignments by varying the nanowire diameter. Our results provide the ability of band-alignment tuning and open new possibilities for band engineering.

Giant modification of the magnetocrystalline anisotropy in transition-metal monolayers by an external electric field.

Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.

The effects of the extent of spinal block on the BIS score and regional cerebral oxygen saturation in elderly patients: A prospective, randomized, and double-blinded study.

BACKGROUND: Patients may become sedated with spinal anesthesia; however, the effect of the extent of spinal block on the Bispectral index (BIS), a processed electroencephalographic variable, has not been fully investigated. We evaluated the influence of the extent of spinal block on BIS values and on regional cerebral oxygen saturation (rSO(2)) in elderly patients. METHODS: A prospective, randomized, double-blinded study was performed in 55 ASA II patients undergoing urological surgery. The patients were randomly allocated into one of two groups to receive 2.7 ml of 0.5% hyperbaric bupivacaine or 1.5 ml, and then divided into two groups according to level of spinal blockade: high spinal group (Th6 and above) or low spinal group (Th12 and below). Systolic blood pressure (SBP), heart rate (HR), cardiac output (CO), stroke volume (SV), BIS values, and rSO(2) were measured for 30 min. CO and SV were evaluated using impedance cardiograph methods. RESULTS: The level of spinal blockade was Th4.7 +/- 1.0 in high spinal group (n = 20) and L2.5 +/- 2.2 in low spinal group (n = 20). High spinal anesthesia produced a significant decrease in SBP (p < 0.01) and SV (p < 0.01), but had no effect on CO. High spinal anesthesia significantly decreased BIS values (p < 0.01) without affecting rSO(2). There was relationship between level of spinal blockade and BIS values (r = 0.566). In contrast, no changes in above parameters were found in low spinal group. CONCLUSIONS: This study provides evidence that the extent of spinal block may have significant influence on BIS values without affecting rSO(2) in elderly patients.

Noncollinear magnetism and exchange interaction in spin-spiral structures of thin film Fe(110).

Spin-spiral structures in a free-standing Fe(110) monolayer are determined by the first-principles film full-potential linearized augmented plane wave method with intra-atomic noncollinear magnetism. The results obtained predict that the spin-spiral structures are energetically favourable over the collinear ferromagnetic state. The interatomic exchange parameters, which are evaluated from the formation energy of the spin-spiral structures, indicate that a competition between the nearest-neighbour ferromagnetic interaction and the long-distant antiferromagnetic interactions leads to the stabilization of the spin-spiral structures. In addition, the spin-orbit coupling is found to play an important role in determining the magnetic ground state.

Half-metallic exchange bias ferromagnetic/antiferromagnetic interfaces in transition-metal chalcogenides.

To investigate half-metallic exchange bias interfaces, magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in the zinc blende transition-metal chalcogenides, and with compensated and uncompensated AFM interfaces, were determined by the full-potential linearized augmented plane-wave method. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity. More striking still, in the compensated AFM interface the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but the Mn moments strongly cant to induce a net moment so as to retain the half-metallicity. These findings may offer a key ingredient for exchange biased spintronic devices with 100% spin polarization, having a unidirectional anisotropy to control and manipulate spins at the nanoscale.

Atomically sharp magnetic domain wall in thin film Fe(110): a first principles noncollinear magnetism study.

Magnetic domain wall structures in an Fe (110) monolayer are determined by the highly precise first principles full-potential linearized augmented plane-wave method including intra-atomic noncollinear magnetism. The self-consistent results demonstrate that the magnetization changes from one orientation to the opposite (180 degrees ) orientation within an 8 A width without any abrupt rotation. This narrow domain wall is found to arise from band effects. Our results are consistent with and support domain walls having a 6 A width recently observed in spin-polarized scanning tunneling microscopy experiments.