Polaronic effects on laser dressed donor impurities in a quantum well

The effect of laser field on the binding energy in a GaAs/Ga1_1−xAl_xAs quantum well within the single band effective mass-approximation is investigated. Exciton binding energy is calculated as a function of well width with the renormalization of the semiconductor gap and conduction valence effective masses. The calculation includes the laser dressing effects on both the impurity Coulomb potential and the confinement potential. The valence-band anisotropy is included in our theoretical model. The 2D Hartree–Fock spatial dielectric function and the polaronic effects have been employed in our calculations. We investigate that reduction of binding energy in a doped quantum well due to screening effect and the intense laser field leads to semiconductor–metal transition.     

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.     

Energy band-gap engineering of graphene nanoribbons

We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the nonlinear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.     

Electronic specific heat of nanographite ribbons

The electronic specific heat of nanographite ribbons exhibits rich temperature dependence, mainly owing to the special band structures. The thermal property strongly depends on the geometric structures, the edge structure and the width. There is a simple relation between the ribbon width and the electronic specific heat for the metallic or semiconducting armchair ribbons. However, it is absent for the zigzag ribbons. The metallic armchair ribbons exhibit linear temperature dependence. The semiconducting armchair ribbons exhibit composite behavior of power and exponential functions. As for the zigzag ribbons, the temperature dependence of the specific heat is proportional to T^1−p. The value of p quickly increases from to 1 as the ribbon width gradually grows. The zigzag ribbons might be the first system which exhibits the novel temperature dependence. The nanographite ribbons differ from an infinite graphite sheet, which illustrates that the finite-size effects are significant.     

Magnetic behavior and domain structure in as-quenched, annealed, and stress-annealed CoFeCrSiB ribbons

Combination of magneto-optical (MO) vector magnetometry and magneto-optical Kerr microscopy is used to investigate the surface magnetic properties of amorphous CoFeCrSiB ribbons. Strongly inhomogeneous magnetic behavior of ribbons in as-quenched state is improved by field-annealing and stress-annealing processes that induce weak uniaxial longitudinal and transverse anisotropy. It was shown that values of coercive and anisotropy field increase with increasing annealing temperature. Inclination of easy axis from the ribbon axis is estimated by comparing the measured surface hysteresis loops with the Stoner–Wohlfarth model, and is supported also by the Kerr microscopy. Method with the current flowing through the ribbon is proposed for magnetic domains observations.     

X-ray and Mössbauer study of rapidly solidified Ce20Fe80 ribbons; effect of the quenching rate

Ce₂₀Fe₈₀ ribbons have been produced by planar flow casting under an He atmosphere at linear wheel velocities between 19 and 29 m s^–1. Analysis of ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 77–300 K shows that the ribbons are crystallized. For higher velocities, the ribbon is constituted of the two equilibrium phases CeFe₂ and Ce₂Fe₁₇, but, for lower velocities, there appears a third iron metallic phase, which can be explained by the quenching rate of the melt. A coherent hyperfine parameter set was deduced from fitting Mössbauer spectra in the whole temperature range.     

Effect of heat treatment on structure, magnetization and magnetostriction of Fe81Ga19 melt-spun ribbons

Structure, magnetization and magnetostriction of melt-spun Fe₈₁Ga₁₉ ribbons were investigated both before and after heat treatment. The matrix of melt-spun Fe₈₁Ga₁₉ ribbons kept a body-centered-cubic (bcc) structure (A2) at room temperature. [1 0 0] preferred orientation was formed during melt-spinning process and became stronger with the increase of the ribbon thickness. For the ribbons with a thickness of 110 μm, maximum saturation magnetostrictive strain of −189 ppm along ribbon length was obtained in the samples heat treated at 800 °C for 3 h and then quenched into water. This value was about 16% larger than that of melt-spun ones, which could be contributed to the single disordered A2 structure and the enhancement of [1 0 0]-oriented texture. However, when the ribbon samples were cooled at 2 and 0.5 °C/min after heat treatment at 800 °C for 3 h, a minor quantity of ordered D0₃ and L1₂ phase was found to precipitate in the A2 matrix, respectively, which resulted in the reduction of both magnetization and magnetostrictive strain.     

Band engineering of dichalcogenide MX2 nanosheets (M = Mo,W and X = S,Se) by out-of-plane pressure

The electronic structures of the two-dimensional transition-metal dichalcogenide nanosheets under different out-of-plane pressure were investigated by using the first principle calculations. The band-gaps of all the nanosheets (thickness = 2, 4 and 6 layers) decrease with increasing pressure and finally close, indicating a semiconductor–metal transition. The critical pressure for the semiconductor–metal transition is larger for the thinner nanosheets, and the band-gap closes faster for the Mo-containing nanosheets than the W-containing ones. By taking bilayer MoS₂ as an example, it was found that the physical mechanism of the band-gap variation relates to the charge accumulation and delocalization in the interlayer region.     

Insulator-semimetallic transition in quasi-1D charged impurity-infected armchair boron-nitride nanoribbons

We address control of electronic phase transition in charged impurity-infected armchair-edged boron-nitride nanoribbons (ABNNRs) with the local variation of Fermi energy. In particular, the density of states of disordered ribbons produces the main features in the context of pretty simple tight-binding model and Green's functions approach. To this end, the Born approximation has been implemented to find the effect of π-band electron-impurity interactions. A modulation of the π-band depending on the impurity concentrations and scattering potentials leads to the phase transition from insulator to semimetallic. We present here a detailed physical meaning of this transition by studying the treatment of massive Dirac fermions. From our findings, it is found that the ribbon width plays a crucial role in determining the electronic phase of disordered ABNNRs. The obtained results in controllable gap engineering are useful for future experiments. Also, the observations in this study have also fueled interest in the electronic properties of other 2D materials.     

Role of doubled-frequency absorption in two-photon response of the Si MSM structure

The physical mechanism of two-photon response was studied in this paper by measuring characteristics of the two-photon response of the Si metal–semiconductor–metal (MSM) structure sample. The two-photon response includes two-photon absorption (TPA) and doubled-frequency absorption (DFA). An experiment was designed to measure the photocurrent dependence on incident light power, the dependence of the photoelectric signal on the applied voltage and the relationship between the photoelectric current and the light-spot position. The experimental fact that two-photon response of the silicon sample is relative to the applied electric field shows that DFA is the main physical mechanism of two-photon response and establishes the foundation for fabricating high-sensitivity two-photon response Si photodetector.     

Characterization of crystalline texture and magnetic properties of anisotropic melt-spun Sm(CoZr)7Cy alloy

The effect of additive C on characterization of crystalline texture and magnetic properties of anisotropic melt-spun Sm(CoZr)₇ alloy has been investigated. A few percent of C addition was found to not only significantly enhance the coercivity of the ribbons but also affect the characterization of crystalline texture. The easy-magnetization c axis is changed from parallel to the ribbon plane for SmCo_6.5Zr_0.5 ribbons, to normal to the ribbon plane for the SmCo_6.5Zr_0.5C_0.5 one. Maximum energy product of SmCo_6.5Zr_0.5C_0.5 ribbons prepared at 5 m/s, pulse magnetized to 5 T and measured normal or parallel to the ribbon plane is 8.6 MGOe or 3.2 MGOe, respectively. The domain structure was studied by magnetic-force microscope. A strip-shaped domain can be observed on the surface of the SmCo_6.5Zr_0.5 ribbons and the walls lie straight and parallel. For C-doped ribbons, the domain walls form a maze domain pattern of grains with c axis normal to the ribbon plane. Scanning electron micrography analysis showed that a dendrite structure was present in the SmCoZr ribbon surface. C addition leads to the diminishing of the above dendrite and the grains become equiaxial. The characterization of crystalline texture in C-free and C-doped ribbons has been correlated with their microstructure. Received: 14 January 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Light scattering by the surface of amorphous alloy ribbons modified by annealing and cryogenic treatment

The modification of the microrelief and structure of the surface layers of ribbons of an amorphous metal alloy based on iron and cobalt after thermal treatment at elevated and cryogenic temperatures and under the action of an external magnetic field is studied by the method of light scattering. The parameters of the surface roughness were calculated from the experimentally found indicatrices of light scattering. It is shown that heating of the metal ribbons to T=650–750 K partially relieves stresses arising in the course of the ribbon preparation and reduces the surface roughness as compared to that of freshly prepared samples. Cryogenic treatment at T=78 K increases the surface roughness, and application of a magnetic field to a ribbon causes anisotropy in the surface layer due to the magnetostrictive effect.     

Microstructures and soft magnetic properties of Fe80P20-xSix (x = 4.5–6.5) amorphous ribbons

Fe-based amorphous ribbons with excellent soft magnetic properties and mechanical properties were prepared in the Fe–Si–P ternary system. Enhanced soft magnetic properties could be achieved through annealing treatment of the ribbons for 1 h at 325 °C, which is far below the glass transition temperatures (462–474 °C). Icosahedral medium-range ordering with a size range of around 2 nm occurred throughout the amorphous matrix during the low-temperature annealing treatment. The annealed ribbons exhibited improved magnetic saturation of over 185 emu/g while maintaining good mechanical flexibility. During icosahedral ordering, the distance between the Fe atoms and the coordination number within the amorphous ribbon can be optimised for achieving high magnetic saturation. However, nanocrystallisation of the SiP and Fe₂P transition phases embedded within the amorphous matrix occurred after the annealing treatment for 1 h at 385 °C, which caused deterioration of the soft magnetic properties and mechanical flexibility of the ribbons. Therefore, the combination of high magnetic saturation and mechanical flexibility of the amorphous ribbons could be optimised through low-temperature annealing treatment without any nanocrystallisation.     

A comparative investigation of an AB- and AA-stacked bilayer graphene sheet under an applied electric field: A density functional theory study

An AB- and AA-stacked bilayer graphene sheet(BLG) under an electric field is investigated by ab initio calculation.The interlayer distance between the two layers,band structures,and atomic charges of the system are investigated in the presence of different electric fields normal to the BLG.The AB- stacked BLG is able to tune the bandgap into 0.234 eV with the increase of the external electronic field to 1 V/nm,however,the AA-stacked BLG is not sensitive to the external electric field.In both the cases,the spacing between the BLG slightly change in terms of the electric field.The charges in the AB- stacked BLG are increased with the increase of the electric field,which is considered to be the reason that causes the bandgap opening in the AB- stacked BLG.     

Entanglement complexity of lattice ribbons

We consider a discrete ribbon model for double-stranded polymers where the ribbon is constrained to lie in a three-dimensional lattice. The ribbon can be open or closed, and closed ribbons can be orientable or nonorientable. We prove some results about the asymptotic behavior of the numbers of ribbons withn plaquettes, and a theorem about the frequency of occurrence of certain patterns in these ribbons. We use this to derive results about the frequency of knots in closed ribbons, the linking of the boundary curves of orientable closed ribbons, and the twist and writhe of ribbons. We show that the centerline and boundary of a closed ribbon are both almost surely knotted in the infinite-n limit. For an orientable ribbon, the expectation of the absolute value of the linking number of the two boundary curves increases at least as fast as n, and similar results hold for the twist and writhe.     

Giant magnetostriction in rapidly quenched Fe–Ga?

Substituting Fe by nonmagnetic Ga causes a dramatic increase of the magnetostriction. The reason for this effect is related to structure and also due to softening of the elastic properties. Of special interest is that in literature “giant” magnetostriction values (up to 2100 ppm) for rapidly quenched Fe–Ga (15–20% Ga) ribbons were reported. In this work, careful investigations using a strain gauge method as well as a capacitance cell were performed. Especially for the case applying an external field perpendicular to the ribbon plane, it is demonstrated that bending effects can occur and they are difficult to avoid without introducing any stress into the sample. This effect leads to large signals in the strain gauge of more than ±3000 ppm, which sign depends on the occurrence of strain or stress. Experiments on a 25-μm-thin Fe foil leads to similar results. Avoiding bending by gluing ribbons or thin foils or splat-cooled thin pure Ni on a thin plastic plate, gave magnetostriction values close to those of polycrystalline bulk materials.     

Nitride-based photodetectors: from visible to X-ray monitoring

The performance of nitride-based photodetectors is investigated beyond the usual near-UV (400–300 nm) and mid-UV (300–200 nm) operation ranges. The responses of metal–semiconductor–metal (MSM) photodiodes were analyzed in the vacuum–UV and soft X-ray regions. To interpret the results, the absorption properties and the attributes of each of the photons with energies for producing multiple electron–hole pairs were considered. The soft X-ray characterization showed that in-plane MSMs worked efficiently up to photon energies of 600 eV. Above this value, the absorption decrease makes the diffusion length and layer thickness become critical parameters for the detector behavior. To perform detection in the violet and near-UV, InGaN-based photoconductors were fabricated and spectrally characterized. The devices presented abrupt wavelength cut-offs, demonstrating that the InGaN compositional fluctuations were tolerable up to In contents of 10% for fabricating selective photodetectors. Back-face illumination allowed us to obtain bandpass detectors for these spectral ranges.     

Calculations of electric capacitance in carbon and BN nanotubes, and zigzag nanographite (BN, BCN) ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The electronic states with the opposite electric charges separated along both edges are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano-functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons, the electronic structure is always like that of semiconductors. The calculated capacitance is inversely proportional to the distance between the positive and negative electrodes.     

A new routine to fabricate Bi single crystalline tapering junction nanowire arrays

Large-scale tapering junction Bi nanowire arrays have been synthesized by pulsed electrodeposition within a porous anodic alumina membrane (AAM). Bi metal–semiconductor and semiconductor–semiconductor junction nanowires with different diameters were fabricated easily by modulating only the pulse time using only an ordinary AAM template. The morphology and structure of the tapering junction Bi nanowire arrays and individual nanowires are characterized by scanning electron microscopy and transmission electron microscopy. The voltage–current measurements show the intrinsic nonlinear and asymmetric characteristics of metal–semiconductor junction nanowires. PACS 81.05.Bx; 82.80.Fk; 73.63.Nm