Deep defects generated in n-conducting ZnO:TM thin films

The ferromagnetism in highly transparent and intrinsically n-type conducting zinc oxide doped with 3d transition metals (TM), is predicted to be defect mediated. We investigate the generation of deep defects in n-conducting 1 μm thick ZnO:TM films (TM=Co, Mn, Ti) with a nominal TM content of 0.02, 0.20 and 2.00 at.% grown by pulsed laser deposition on a-plane sapphire substrates using deep level transient spectroscopy. We find that a defect level is generated, independent of the TM content, located 0.31 and 0.27 eV below the conduction band minimum of ZnO:Mn and ZnO:Ti, respectively. Different defect levels are generated in dependence on the Co content in ZnO:Co. This work shows that an optimization of defect-related ferromagnetism in n-conducting ZnO:TM thin films will only be possible if the preparation sensitive formation of deep defects is controlled in the same time.     

Transition-metal-ethylene complexes as high-capacity hydrogen-storage media

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of approximately 14 wt %. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.     

First-principles study of transition metal linear monoatomic chains adsorption on boron nitride nanotube

Structural, electronic and magnetic properties of six 3d transition metals (TM=V, Cr, Mn, Fe, Co and Ni) linear monoatomic chains adsorbed on the (5,5) boron nitride nanotube (BNNT) at five different sites have been investigated by first-principle calculations. The results indicate all TM chains can be spontaneously adsorbed on the outer surface of the BNNT. The stable adsorption sites are different for different TM chains. All TM chains can be adsorbed on the N site, while the adsorption on the Z site is unstable. The dispersion character occurs in energy band curves of stable TM/BNNT systems and bring about the band gap disappearance in comparison with that of pure (5,5) BNNT. Interestingly, the TM/BNNT systems with nearly half-filled 3d metals V and Cr at H and N sites, as well as Mn at A site show a half-metal character and are usable in spintronics devices. The different electronic properties of BNNT can also be achieved through decorations of the same TM chain on different sites. The TM chain adsorbed BNNT systems exhibit high stability, promising electronic properties and high magnetic moments, which may be useful for a wide variety of next-generation nanoelectronic device components.     

Spin gap and string order parameter in the ferromagnetic spiral staircase heisenberg ladder: a quantum Monte Carlo study

We consider a spin-1/2 ladder with a ferromagnetic rung coupling J perpendicular and inequivalent chains. This model is obtained by a twist (theta) deformation of the ladder and interpolates between the isotropic ladder (theta=0) and the SU(2) ferromagnetic Kondo necklace model (theta = pi). We show that the ground state in the (theta, J perpendicular) plane has a finite string order parameter characterizing the Haldane phase. Twisting the chain introduces a new energy scale, which we interpret in terms of a Suhl-Nakamura interaction. As a consequence we observe a crossover in the scaling of the spin gap at weak coupling from delta/J parallel proportional, variant J perpendicular/J parallel for theta < theta c approximately 8 pi/9 to delta/J parallel proportional, variant (J perpendicular/J parallel)2 for theta > theta c. Those results are obtained on the basis of large scale quantum Monte Carlo calculations.     

Anticharmed pentaquark from B decays

We explore the possibility of observing the anticharmed pentaquark state from the theta(c)npi(+) decay of B mesons produced in B-factory experiments. We first show that the observed branching ratio of the B(+) to lambda(-)(c)p pi(+), as well as its open histograms, can be remarkably well explained by assuming that the decay proceeds first through the pi(+) D(0) (or D(*0)) decay, and then through the subsequent decay of the virtual D(0) or D(*0) mesons to lambda(-)(c)p. We then note that the theta(c)can be similarly produced when the virtual D(0) or D(*0) decay into an antinucleon and a theta(c). Combining the present theoretical estimates for the ratio g(DNlambda(c))/g(DNtheta(c)) approximately 13 and g(D*Ntheta(c)) approximately 1/3g(DNtheta(c)), we find that the anticharmed pentaquark theta(c), which was predicted to be bound by several model calculations, can be produced via B(+)--> theta(c)npi(+), and be observed from the B-factory experiments through the weak decay of theta(c)--> pK(+) pi(-) pi(-).     

Trends in hydrogen chemisorption on transition metals

A systematic study of H adsorption on the close-packed surfaces of the transition metals in the 3d and 4d series is presented. The effect of the TM d band on the chemisorption bond is investigated, by embedding a cluster of TM muffin tins at the surface of an effective jellium-like medium. It is found that the broad and incomplete H/jellium resonance is narrowed, shifted down and made to contain more electrons as a result of hybridization with the TM d states. These effects are larger in the case of the 4d metals, thus indicating a greater participation in the chemisorption bond of the d electrons for these metals than for the 3d metals. Calculation of one-electron energy differences on going from the H on jellium system to that of H on the TM cluster are presented. Trends for the one-electron energy differences are compared to trends in experimental chemisorption energy. H adsorption in the three-fold hollow site with no secondlayer TM atom below the H site is favoured for the hcp metals, while no discernible preference between the two hollow sites is recorded for the fcc metals, with the exception of Rh where the site with no second-layer TM atom below is preferred.     

Absorption enhancement of an electric quadrupole transition of cesium atoms in an evanescent field

We have observed for the first time reflection spectra of an electric quadrupole transition for the cesium atom (6 (2)S(1/2)-5 (2)D(5/2)) line at an angle of incidence from theta(c)-11.9 to theta(c)+107.5 mrad, where theta(c) is the critical angle for total reflection. From a comparison with the calculated absorption in the attenuated total reflection, the oscillator strengths for s and p polarizations were found to increase with an increase in the angle of incidence by a factor up to 1.5 at theta(c)+83.8 mrad and 2.4 at theta(c)+107.5 mrad, respectively, in the experiment. The dependences of the observed enhancement on the angle of incidence were in good agreement with the calculated ones for the oscillator strength of the quadrupole transition in the evanescent light.     

New scientific opportunities for high pressure research by energy-dispersive XMCD

ABSTRACT

XMCD under pressure is used to study the magnetic properties of the transition metal (TM) systems for over 15 years. We present the technique and how it has been developed. The energy dispersive XAS spectrometer is particularly suited for these studies. The effect of pressure on TM magnetism is discussed. Recent studies performed at different edges illustrate the information that can be obtained through XMCD. Finally, some results obtained on TMs are presented, either at the L_II,III edges of 5d metals or at the K edge of 3d metals, which correspond to the energy ranges that can be probed when using diamond anvil cells for high pressure. Different cases are treated: pure 3d metals, alloys, magnetic insulator and inorganic compounds.     

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co,Ni, Cu,Rh, Pd and Ag

We present a muffin tin based calculation on (TM)₃H, (TM)₇H and (TM)₁₉H clusters embedded at the surface of an effective jellium-like medium whose potential is treated in scattering length approximation. We consider the changes occurring when the d-like perturbation of the TM muffin tins is switched on. The broad chemisorption-induced resonance seen for H on the effective jellium surface is narrowed and shifted down in energy. Furthermore the occupation of this resonance is increased from about 1.1 electrons to about 1.4 (on 3d metals) or 1.8 (on 4d metals), due to d-like states dropping down from the d band to form a relatively welldefined “bonding state”. An antibonding state containing about 0.4 electrons is formed at the top of the d band. The results are compared with other calculations and with photoemission data. Implications for the metal-hydrogen distance and (for Ni) the demagnetizing effect of hydrogen chemisorption are discussed. We use the change in total single particle energy when the d-like perturbation is switched on to estimate trends in chemisorption energy along the 3d and 4d series. In the 3d case experimental data is available on the difference in chemisorption energy between Ni and Cu which is in reasonable agreement with our estimate.     

Free energy and torque for superconductors with different anisotropies of H(c2) and lambda

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma(H)=H(c2,a)/H(c2,c), different from the anisotropy of the penetration depth gamma(lambda)=lambda(c)/lambda(a). With increasing difference between gamma(H) and gamma(lambda), the equilibrium orientation of the crystal relative to the applied field may shift from theta=pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta=0 for large enough gamma(H). These effects are expected to take place in MgB2.     

First principle study of the structural,electronic and magnetic properties of Fe,Co, Ni atomic nanochains encapsulated in single walled and double walled beryllium oxygen nanotubes

By using first-principles calculations within the density function theory, the structural, electronic and magnetic properties of transition metals TM (TM=Fe, Co and Ni) atomic chains wrapped in the single walled and double walled BeO nanotubes are investigated. It is found that all these TM chains @ BeONTs systems are ferromagnetic (FM) and a spin splitting between spin up and down is observed. The high magnetic moment and spin polarization of the TM @ BeONTs systems imply that it can be used as magnetic nanostructure and future applications in permanent magnetism, magnetic recording, and spintronics.     

Elimination of GeO2 and Ge3N4 interfacial transition regions and defects at n-type Ge interfaces: A pathway for formation of n-MOS devices on Ge substrates

The contribution from relatively low-K SiON interfacial transition regions (ITRs) between Si and transition metal (TM) gate dielectrics places a significant limitation on equivalent oxide thickness (EOT) scaling for Si complementary metal-oxide-semiconductor (CMOS) devices. This limitation is equally significant and limiting for Ge CMOS devices. Low-K Ge-based ITRs in Ge devices have also been shown to limit performance and reliability, particular for n-MOS field effect transistors. This article identifies the source of significant electron trapping at interfaces between n-Ge or inverted p-Ge, and Ge oxide, nitride and oxynitride ITRs. This is shown to be an interfacial band alignment issue in which native Ge ITRs have conduction band offset energies smaller than those of TM dielectrics, and trap electrons for negative Ge substrate bias. This article also describes a novel remote plasma processing approach for effectively eliminating any significant native Ge ITRs and using a plasma-processing/annealing process sequence for bonding TM gate dielectrics directly to the Ge substrate surface.     

Direct evidence of open ray orbits in a square two-dimensional resonator of dye-doped polymers

Lasing has been observed in optically pumped 4-dicyanomethylene-2-methyl-6-( p-dimethylaminostyrl)-4H-pyran-doped poly(methyl methacrylate) square-shaped micropillars that allow four-bounce closed and open ray orbits with internal incident angle theta(inc) > theta(c) (the critical angle for total internal reflection) and with the associated surface waves that emit at the four corners. We also detect strongly TE-polarized and spatially varying emission from the square sidewalls that is due to leaky open ray orbits with theta(inc) near but less than theta(c) for two of the four bounces. By selectively pumping the square microcavity with a stripe-shaped beam, we excite different four-bounce ray orbits.     

Anisotropy of the Sommerfeld coefficient in magnesium diboride single crystals

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B-->0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis) and gammapi(B,theta)=gammapi(B)=B/Bpi(B). The "critical field" of the pi band Bpi is fully isotropic but field dependent increasing from approximately 0.25 T for B< or =0.1 T up to 3 T approximately Bc2c for B-->3 T. Because of the coupling of the two bands, superconductivity survives in the pi band up to 3 T but is totally destroyed above for any orientation of the field.     

Antibacterial tourmaline nanoparticles/polyurethane hybrid mat decorated with silver nanoparticles prepared by electrospinning and UV photoreduction

In this study, tourmaline (TM) nanoparticles (NPs) were incorporated in a polyurethane (PU) matrix by electrospinning and silver (Ag) nanoparticles in the form of wire-like structure were further decorated on the TM/PU nanofibrous mat by photoreduction under ultraviolet light irradiation. The incorporation of TM NPs has increased the conductivity of the solution, thus forming thinner fiber diameters compared to neat PU, but with improved tensile strength. Wire-like, agglomerated Ag NPs were decorated on the TM/PU matrix, and exhibited high bactericidal activity depending on the Ag content. The present antibacterial Ag/TM/PU hybrid mat has potential application in water treatment.     

Scalings of domain wall energies in two dimensional Ising spin glasses

We study domain wall energies of two dimensional spin glasses. The scaling of these energies depends on the model's distribution of quenched random couplings, falling into three different classes. The first class is associated with the exponent theta approximately -0.28; the other two classes have theta=0, as can be justified theoretically. In contrast to previous claims, we find that theta=0 does not indicate d=d(c)(l) but rather d< or =d(c)(l), where d(c)(l) is the lower critical dimension.     

Conformally invariant fractals and potential theory

The multifractal (MF) distribution of the electrostatic potential near any conformally invariant fractal boundary, like a critical O(N) loop or a Q-state Potts cluster, is solved in two dimensions. The dimension &fcirc;(straight theta) of the boundary set with local wedge angle straight theta is &fcirc;(straight theta) = pi / straight theta-25-c / 12 (pi-straight theta)(2) / straight theta(2pi-straight theta), with c the central charge of the model. As a corollary, the dimensions D(EP) of the external perimeter and D(H) of the hull of a Potts cluster obey the duality equation (D(EP)-1) (D(H)-1) = 1 / 4. A related covariant MF spectrum is obtained for self-avoiding walks anchored at cluster boundaries.     

Vegetation against dune mobility

Vegetation is the most common and most reliable stabilizer of loose soil or sand. This ancient technique is for the first time cast into a set of equations of motion describing the competition between aeolian sand transport and vegetation growth. Our set of equations is then applied to study quantitatively the transition between barchans and parabolic dunes driven by the dimensionless fixation index theta which is the ratio between the dune characteristic erosion rate and vegetation growth velocity. We find a fixation index theta(c) below which the dunes are stabilized, characterized by scaling laws.     

Oxidation protection of copper surfaces using self-assembled monolayers of octadecanethiol

Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto clean surfaces of face centred cubic (fcc) metals have been studied extensively for their ability to control the chemical functionality of the surface and as a means of preventing the oxidation and corrosion of the substrate metal. However, in many cases it has been found that on reactive substrates such as copper, it is difficult to prepare SAMs without the incorporation of some oxygen into the structure. In this work, self-assembled monolayers of octadecanethiol (ODT) were formed on copper foil substrates using a series of etching treatments to remove the native oxide layer prior to deposition of the ODT coating from a modified solution. X-ray photoelectron spectroscopy was used to analyse the SAMs and showed that monolayers with no detectable oxygen content could be produced. The effect of exposing the samples to air at different temperatures was monitored to examine the rate of the oxidation process, which was found to vary strongly with temperature. Samples stored at room temperature were found to oxidise relatively quickly, while those kept in a refrigerator were slower. Storing samples in a freezer dramatically reduced the oxidation of the copper, such that samples kept for 10 weeks still did not show any clear evidence of oxygen incorporation.     

Electric field gradients in cubic alloys

The theoretical investigations of electric field gradient (EFG) and asymmetry parameter (ν) for cubic dilute alloys are reviewed. The valence and size EFG's and the Sternheimer antishielding factor are discussed in detail. The calculations of EFG and ν for simple and transition metal (TM) dilute alloys of aluminium and copper, TM dilute alloys of vanadium and trinary dilute alloys of noble metals are summarized. It is emphasized that the size EFG is as important as the valence EFG to calculate the total EFG and ν, which are compared with the experimental values. An epilogue for further investigations is added.