Spin-polarization and magnetoresistance in graphene-based resonant-tunnelling structures

Spin-dependent transport of relativistic electrons through graphene based double barrier (well) structures with ferromagnetic electrodes have been theoretically investigated. Electron transmission with different spin states is strongly influenced by the incident wave vector, the height (depth) of the barrier and the separation between them. When the angle of the incident electrons is varied from zero to ±π/2, spin polarization varies from zero to 100% with characteristic oscillations that indicate spin anisotropy. Due to Klein tunnelling, spin-polarization is always zero for normal incident electrons; high spin-polarization only occurs at large incident angles. Because the resonance features in the spin-dependent transmission result from resonant electron states in wells or hole states in barriers, the conductance can reach e²/h in this resonant-tunnelling structure.     

Coherent quantum transport in two-dimensional electron gas/superconductor double junctions with Rashba spin-orbit coupling

Based on the extended Blonder-Tinkham-Klapwijk (BTK) approach, we have investigated the coherent quantum transport in two-dimensional electron gas/superconductor (2DEG/SC) double tunneling junctions in the presence of the Rashba spin-orbit coupling (RSOC). It is found that all the reflection coefficients in BTK theory as well as conductance spectra oscillate with the external voltage and energy. The oscillation feature of conductance can be tuned largely by the RSOC for low insulating barriers, while for high insulating barriers it is almost independent of the RSOC. These phenomena are essentially different from those found in ferromagnet/superconductor double tunneling junctions.     

Ballistic transport in bilayer nano-graphite ribbons under gate and magnetic fields

This study uses the tight-binding model to examine the ballistic transport of short and infinitely long bilayer nano-graphite ribbons for different stacked structures, AA and AB, under perpendicularly applied gate and magnetic fields. In the small bias region, the conduction of the AB-stacked ribbon is better than for the AA. Under a gate field with small bias, the AB-stacked ribbon exhibits a significant current peak at the zero gate field point, similar to the graphene ribbon. On the contrary, this current peak is not found in the AA-stacked case. Under a perpendicular magnetic field with small bias, the magnetoresistance ratio in both stacked graphene ribbons are proportional to the square of the magnetic field.     

Conductance through analytic constrictions

We study the dependence of the intrinsic conductance of a nanocontact on its shape by using the recursion-transfer-matrix method. Hour-glass, torus, and spherical shapes are defined through analytic potentials, the latter two serving as rough models for ring-like and spherical molecules, respectively. The sensitivity of the conductance to geometric details is analyzed and discussed. Strong resonance effects are found for a spherical contact weakly coupled to electron reservoirs.     

Spin filtering through ballistic nanojunctions, the role of geometry and of spin orbit interaction

We suggest a spin filter scheme using T-stub nanometric crossjunctions patterned in two dimensional electron gases (2DEGs) in the presence of spin orbit interaction (SOI). We compare the effects of SOI arising from vertical confinement of charge carriers in the well, Rashba or α-SOI, with SOI generated by lateral confinement of the wire, β-SOI. We show that β coupling can be more effective in generating a spin polarized current as compared to α-SOI. We also compare the efficiency of the T-stub filter with the one of the X shaped cross junction.     

Numerical investigation of spin-torque using the Heisenberg model

We present numerical calculations of the spin transfer torque resulting in current-induced domain wall motion. Rather than the conventional micromagnetic finite difference or finite element method, we use an atomistic/classical Heisenberg spin model approach, which is well suited to study geometrically confined domain walls. We compute the behaviour of domain walls in a one dimensional chain when currents are injected using adiabatic and non-adiabatic spin torque terms. Our results are compared to analytical calculations and are found to agree very well for small current densities. At larger current densities deviations are observed, which can be attributed to the approximations used in the analytical calculations.     

Energy level alignment at interfaces between organic semiconductors and clean ferromagnetic La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin film contacts for spin injection

We have studied the energy level alignment at interfaces between clean ferromagnetic La_0.7Sr_0.3MnO₃ and two archetype organic semiconductors, α-sexithiophene and copper-phthalocyanine, by combined X-ray and ultraviolet photoelectron spectroscopy. We observe the formation of a large interface dipole at both studied interfaces and small hole injection barriers. In addition, our results indicate a small chemical interaction between the organic materials and the La_0.7Sr_0.3MnO₃ surface which leads to a pinning of the Fermi level and the relatively small hole injection barriers.     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Current-induced conformational switching in single-molecule junctions

Current-induced conformational switching in single-molecule junctions constitutes a fundamental process in molecular electronics. Motivated by recent experiments on azobenzene derivatives, we study this process for molecules which exhibit two (meta)stable conformations in the neutral state but only a single stable conformation in the ionic state. We derive and analyze appropriate Fokker–Planck equations obtained from a density-matrix formalism starting from a generic model and present comprehensive analytical and numerical results for the switching dynamics in general and the quantum yield in particular.     

Laser-assisted local patterning of ZnO-based spots for mirror-less lasing

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.     

Electronic and structural properties of graphene-based transparent and conductive thin film electrodes

We demonstrate that graphene-based transparent and conductive thin films (GTCFs), fabricated by thermal reduction of graphite oxide, have very similar electronic and structural properties as highly oriented pyrolytic graphite (HOPG). Electron spectroscopy results suggest that the GTCFs are also semi-metallic and that the individual graphene sheets of the film are predominantly oriented parallel to the substrate plane. These films may therefore be considered as a technologically relevant analogue to HOPG electrodes, which cannot be easily processed into thin films.     

Spin-orbit berry phase in a quantum loop

We have found a manifestation of spin-orbit Berry phase in the conductance of a mesoscopic loop with Rashba spin-orbit coupling placed in an external magnetic field perpendicular to the loop plane. In detail, the transmission probabilities for a straight quantum wire and for a quantum loop made of the same wire have been calculated and compared with each other. The difference between them has been investigated and identified with a manifestation of spin-orbit Berry phase. The non-adiabaticity effects at small radii of the loop have been found as well.     

Another tetraquark structure in the <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">χ</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>1</Subscript> invariant mass distribution

In this article, we assume that there exists a scalar hidden charm tetraquark state in the π ⁺ χ _c1 invariant mass distribution, and we study its mass using the QCD sum rules. The numerical result M _Z =(4.36±0.18) GeV is consistent with the mass of the Z(4250). The Z(4250) may be a tetraquark state, but other possibilities, such as a hadro-charmonium resonance and a molecular state, are not excluded.     

Possible tetraquark states in the <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">χ</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>1</Subscript> invariant-mass distribution

In this article, we assume that there exist hidden charmed tetraquark states with spin–parity J ^P=1⁻, and we calculate their masses with the QCD sum rules. The numerical result indicates that the masses of the vector hidden charmed tetraquark states are about M _Z =(5.12±0.15) GeV or M _Z =(5.16±0.16) GeV, which are inconsistent with the experimental data on the π ⁺ χ _c1 invariant-mass distribution. The hidden charmed mesons Z ₁, Z ₂ or Z may be scalar hidden charmed tetraquark states, hadro-charmonium resonances or molecular states.     

Ultrafast laser based “green” synthesis of non-toxic nanoparticles in aqueous solutions

Inorganic nanoparticles offer novel promising properties for biological sensing and imaging, as well as in therapeutics. However, these applications are often complicated by the possible toxicity of conventional nanomaterials, arising as a result of inadequate purification procedures of nanoparticles obtained via synthetic pathways using toxic or non-biocompatible substances. We review novel femtosecond laser-assisted methods, which enable the preparation of metal nanomaterials in clean, biologically friendly aqueous environment (“green” synthesis) and thus completely solve the toxicity problem. The proposed methods, including laser ablation and fragmentation, make possible the production of stable metal colloids of extremely small size (∼2 nm) with a low coefficient of variation (15–25%). Those nanoparticles exhibit unique surface chemistry and can be used for bio-imaging, cancer treatment and nanoparticle-enhanced Raman spectroscopy.     

Bound states in the continuum in two-dimensional serial structures

We investigate the occurrence of bound states in the continuum (BICs) in serial structures of quantum dots coupled to an external waveguide, when some characteristic length of the system is changed. By resorting to a multichannel scattering-matrix approach, we show that BICs do actually occur in two-dimensional serial structures, and that they are a robust effect. When a BIC is produced in a two-dot system, BICs also occur for several coupled dots. We also show that the complex dependence of the conductance upon the geometry of the multi-dot system allows for a simple picture in terms of the resonance pole motion in the multi-sheeted Riemann energy surface. Finally, we show that in correspondence to zero-width states for the open system one has a multiplet of degenerate eigenenergies for the associated closed serial system, thereby generalizing results previously obtained for single dots and two-dot structures.     

Non-commutative harmonic oscillator in magnetic field and continuous limit

The spectra of a charged harmonic oscillator minimally coupled to a perpendicular magnetic field in the non-commutative plane are studied by using the path integral formulation. We get the spectra in a mapping-independent way. Interestingly, we find that the spectra have no continuous limit when the dimensionless parameter tends to zero. In order to get a finite result, a truncation is inevitable. Finally, we give a reasonable explanation of truncation from the constrained theory point of view.     

Spin precession and electron spin polarization wave in [001]-grown quantum wells

We theoretically study the spatial behaviors of spin precessions modulated by an effective magnetic field in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field, we find some laws of spin precession in the system, by which we explain some previous phenomena of spin precession, and predict a controllable electron spin polarization wave in [001]-grown quantum wells. The shape of the wave, like water wave, mostly are ellipse-like or circle-like, and the wavelength is anisotropic in the quantum wells with two unequal coupling strengths of the Rashba and Dresselhaus interactions, and is isotropic in the quantum wells with only one spin orbit interaction.     

Effect of vacuum magnetic field on the gravitodynamic stability of liquid jet

Summary  The MHD stability of a liquid jet (radiusR ₀ and density ϱ) under the influence of self-gravitating force has traditionally been studied using a normal-mode type of analysis. The dispersion relation is obtained and studied analytically and numerically. The axial magnetic fields inside and outside the jet have always stabilizing effects. The transverse magnetic field has a destabilizing effect. It is found that the largest doman of instability is fastly shrinking (as α> ?) with increasingH ₀ /H _G values,H _G 4(4πGϱ² R ² ₀ μ)^1/2, whereG is the gravitational constant. Some reported works are recovered as limiting cases from the present work.