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Physical properties of polycrystalline ferroelectrics including the contributions of the fixed dipolar defects and the average grain size in the Potts-Ising model are simulated by using the Monte Carlo method. Domain pattern, hysteresis loop and switching current of the polarization reversal process are obtained. Two processes are considered in our simulation. In the first one, the grain texture of ferroelectric ceramics are produced from the Potts model, and then the Ising model is implemented in the obtained polycrystalline texture to produce the domain pattern, hysteresis loop and switching current. It is concluded that the defect has the ability to decrease the remnant polarization Pr as well as the coercive field Ec. The back switching is obviously observed after the electric field is off, and it shows some variation after introducing the fixed dipolar defect. Meanwhile, the spike of the switching current is found to lower with the increasing defect concentration and the decreasing average grain size.

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3(0001) and ZnO(0001) substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450°C. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction (RHEED) and ex-situ high resolution x-ray diffraction (HRXRD). The epitaxial thin films are single crystal at film thickness smaller than 500 nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co2+ substituting Zn2+ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures TC above room temperature.

We investigate the re-exciting process of a soliton, a polaron and a bipolaron in conjugated polymers. For a soliton case, the processes S0+h?S?+P and S?+h?S0+P? will take place, where S0, S? and P? express a neutral soliton, charged solitons and charged polarons, respectively. For a polaron case, the processes P?+h?P+BP?2 will occur, where BP?2 expresses a charged bipolaron with two electronic units. For a bipolaron case, the processes BP?2+h?P?+P? and BP?2+h?BP?2+E0 are obtained.

We investigate the variations from as-deposited Zn1−xCoxO magnetic semiconductors to the post-annealed Co–ZnCoO granular composite. The as-deposited Zn1−xCoxO magnetic semiconductor deposited under thermal non-equilibrium conditions is composed of Zn1−xCoxO nanograins of high Co concentration. The room-temperature ferromagnetism with high magnetization and large negative magnetoresistance are found in the as-deposited samples. By annealing, the samples become of granular composite consisting of the Co metal grains and the remanent Zn1−xCoxO matrix. Although the magnetization is enhanced after annealing, the spin-dependent negative magnetoresistance disappears at room temperature. The magnetoresistance observed in the annealed samples in the high field region has no relation with the ferromagnetism, which in turn indicates that the room-temperature ferromagnetism and large negative magnetoresistance observed in the as-deposited are the intrinsic properties of the Zn1−xCoxO magnetic semiconductor.

We prepare 2×(NiFe/CoZnO)/ZnO/(CoZnO/Co)×2 spin valve structures used for spin injection by sputtering and photolithography. In the junctions, the free magnetic layer 2×(NiFe/CoZnO) and the fixed magnetic layer (CoZnO/Co)×2 are used to realize the spin valve functions in the external switch magnetic field. Since the wide gap semiconductor ZnO layer is located between the two magnetic semiconductor layers CoZnO, the electrical spin injection from the magnetic semiconductor CoZnO into the non-magnetic semiconductor ZnO is realized. Based on the measured magnetoresistance and the Schmidt model, the spin polarization ratio in the ZnO semiconductor is deduced to be 11.7% at 90 K and 7.0% at room temperature, respectively.

Reverse polarization in polythiophene under an applied electric field has been studied in the framework of the tight-binding model. It is found that the applied electronic field has a great influence on the excited states of polythiophene. The effect of the heteroatoms on the polarization has been calculated and analysed carefully. It is indicated that a reverse polarization of biexcitons in polythiophene will be observed more easily. The heteroatoms increase this reversed polarization strength apparently.

Polaron tunnelling is studied in xPA/nPPP/xPA (PA for polyacetylene and PPP poly (p-phenylene)) triblock copolymer, which has a well–barrier–well structure. An extended tight-binding Hamiltonian including external electric field is adopted. Without electric field, the injected electrons would not extend over the whole copolymer chain but instead be confined in the segments of PA. This is different from the behaviour of the traditional semiconductors. It is found that the polaron can transfer to the potential barrier-PPP segment when the applied electric field reaches a certain value. The critical polaron tunnelling electric fields depend upon the lengths of PPP segments.

A tight-binding calculation to describe the triblock copolymer xPA(polyacetylene)/nPPP (poly(p-phenylene))/yPA or xPPP/nPA/yPPP is presented. The interfacial coupling between homopolymer segments is attributed to the hopping of π-electrons and the coupling of σ-bonds. The dependence of the band gap of triblock copolymers on the interfacial couplings or on the composite segment lengths is studied. The influence of composite segment lengths on the electron density is also studied. For nPPP/xPA/nPPP structures, the band gap varies with PA segment length over a wide range of 1.32-2.74eV. For nPA/xPPP/nPA structures, the band gap is invariant with PPP segment length. It is found that a spontaneous tunnelling phenomenon could take place in nPA/xPPP/nPA structures. Furthermore, the polaron caused by doping an electron into nPA/xPPP/nPA will tend to be confined in one of the well (PA) parts. This kind of confinement may increase the electron-hole recombination probability.

The persistent current and transmission of two connected Aharonov–Bohm rings coupled to external leads are solved analytically. The conditions on which we may observe persistent current, zero or perfect transmission as well as Fano resonance are studied.

The electronic properties of xPA/nPPP/yPA sandwiched copolymers with a well-barrier-well structure have been studied by using a tight-binding calculation. It was found that the electronic properties of the neutral states of these sandwiched copolymers are sensitive to the constitutions of PPP and PA monomers and the interface coupling between PA and PPP. It is verified that the quantum tunnelling effect will occur at the lowest conductive state of xPA/nPPP/xPA copolymers.

Magnetic multilayers [Co-Nb(1.5 or 3.6 nm)/Pd(x nm)]40 were fabricated by radio frequency sputtering synthesis. The structures and magnetic properties of the Co-Nb/Pd multilayers were studied by x-ray diffraction, vibrating sample magnetometer, torque magnetometer and ferromagnetic resonance measurements. The oscillatory dependence of the phase structure, the strain, the ferromagnetic interlayer coupling and the spacer polarization of multilayers on the increase of Pd spacer thickness was observed. The relationship among phase structure, strain, interlayer coupling and the polarization of Pd spacers was discussed. The magnetic polarization of Pd spacers will enhance the ferromagnetic coupling between magnetic layers and will restrain antiferromagnetic coupling to occur. The spin-dependent electron transport along the normal direction of multilayers and its exchange interaction play key roles in causing these oscillatory behaviors.

Magnetic multilayers [Co(1.5 nm)/Pd(x nm)]40 grown by rf sputtering were studied by x-ray diffraction, vibrating sample magnetometer, torque magnetometer and magnetoresistance measurements. The dependences of the interfacial roughness and the structure perfectness of Co/Pd multilayers on the spacer Pd layer thickness have been revealed from x-ray diffraction data. An oscillatory variation of saturation magnetization versus the thickness of the Pd spacer was observed. A similar oscillatory behavior in the dependence of its magnetic anisotropy on the thickness of the Pd spacer was observed. It is suggested from these two phenomena that the magnetic polarization of Pd spacers oscillates between ferromagnetic and antiferromagnetic polarization with the increase of Pd spacer thickness under a RKKY-like interaction. Evidence is found for the antiferromagnetic polarization of the inner-part of the Pd spacer. The antiferromagnetic coupling between two Co layers and giant magnetoresistance effect in Co/Pd multilayer system were not observed. The relationship among structure, interlayer coupling and magnetic polarization effect of the Pd spacer was discussed.

The stopping power of an electron gas for hydrogen ions is calculated based on the full self-consistent nonlinear density-functional frame. Three different configurations of hydrogen ions are considered and a criterion to test the validity of the self-consistent calculation is proposed. Comparison of calculation with available experimental data shows good agreement.

An oscillatory behavior in specific saturation magnetization of Fe/Pd multilayers is observed. The conversion electron Mossbauer spectroscopy and other experiments indicate that this behavior is caused by the magnetic polarization of Pd spacers, which alternates between positive and negative polarization with respect to the magnetic moments of Fe layers. The interlayer coupling between two Fe layers always keeps ferromagnetic. Then, the interlayer couplings in ferromagnetic metal/nearly ferromagnetic metal multilayers were calculated by using the model of Barnas. The results of calculation prove that the above phenomenon might appear under some condition. The influence of magnetic polarization potential of spacers and interface chemical potential on the interlayer couplings is also discussed.

[ZnO(0.5 nm)/Co(0.6 nm)]60 multilayers were fabricated by magnetron sputtering. The magnetic, transport and optical properties were studied. The ferromagnetic state at low temperature was demonstrated. Most of the grains in the samples will be unblocked when temperature is below 50 K. The measured magneto-resistance at 4.8 K is higher than 30%. This value is larger than any other reported data. The transport mechanism may come down to a combination of tunnelling conduction with the second-order hopping. Some small peaks on transmittance curves were found, the matter is still open.

Chemical adsorption and desorption of hydrogen atoms on single-walled carbon nanotubes (SWNTs) are investigated by using molecular dynamics simulations. It is found that the adsorption and desorption energy of hydrogen atoms depend on the hydrogen coverage and the diameter of the SWNTs. Hydrogen-adsorption geometry at the coverage of 1.0 is more energetically stable. The adsorption energy decreases with the increasing diameter of the armchair tubes. The adsorption and desorption energy of hydrogen atoms can be modified reversibly by externally radial deformation. The averaged C-H bond energy on the high curvature sites of the deformed tube increases with increasing radial deformation, while that on the low curvature sites decreases.

Nanogranular Fe0.35/(In2O3)0.65 films with complex magnetic structure were prepared by the rf sputtering method. An extra-large magnetoresistance up to 506% was obtained at 2.2 K, which is two orders of magnitude larger than that obtained at room temperature. This is related to the interaction with the impurities influencing the local magnetization, which is quite different from the spin-dependent tunnelling effect at room temperature. The interspacing Fe atoms dispersed in the In2O3 matrix play an important role in the transportation properties of carriers at low temperature.

A molecular dynamics simulation method is used to study the growth of narrow single-wall carbon nanotubes. It is found that the growth temperature and the species of small carbon clusters in the feedstock are important for the quality of the nanotubes grown. There is a temperature range of 1000-1500 K in which the narrow armchair single-wall carbon nanotubes can grow rapidly via adduction of C2 dimers, even without the existence of catalysts. The narrow zigzag tubes cannot keep open-ended growth. If the feedstock consists of various species of carbon clusters, the tubes cannot keep open-ended growth. At higher temperatures, the narrow nanotubes close rapidly in the noncatalytic environment, and the products grown are fullerene-like capsules. PACS: 61. 46. +w, 36. 40. -c

A material asymmetry Co/Cu/Fe junction structure has been prepared for studying the spin-polarized electron injection at 77K. The sample performance was demonstrated to be analogous to that of a bipolar transistor. The maximal value of the output pulse voltage between Cu and Fe layers could reach the order of several μV when the bias current between Co and Cu layers was 10μA. The interface roughness, photograph of material, magnetic loop and injection characteristic curves have been measured. Some important points on this topic have been discussed.

The influence of surface polarity on the structural properties of BiFeO3 (BFO) thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 (STO) (100) and polar (111) surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar (001) or polar (111) surfaces perform very differently. BFO/STO (001) is a fully strained tetragonal phase with orientation relationship (001)[100]BFO||(001)[100]STO, while BFO/STO (111) is a rhombohedral phase. BFO/STO (111) has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO (001) and STO (111) are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.

We give a brief introduction to the oxide (ZnO, TiO2, In2O3, SnO2, etc.)-based magnetic semiconductors from fundamental material aspects through fascinating magnetic, transport, and optical properties, particularly at room temperature, to promising device applications. The origin of the observed ferromagnetism is also discussed, with a special focus on first-principles investigations of the exchange interactions between transition metal dopants in oxide-based magnetic semiconductors.

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO3(STO)(110) substrates. Inclined ZnO and cobalt-doped ZnO (ZnCoO) thin films are grown on unetched STO(110) substrates using oxygen plasma assisted molecular beam epitaxy, with the c-axis 42? inclined from the normal STO(110) surface. The growth geometries are ZnCoO[100]//STO[11?0] and ZnCoO[11?1]//STO[001]. The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D0X emissions associated with A0X emissions, and the characteristic emissions for the 2E(2G)?4A2(4F) transition of Co2+ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film, indicating the incorporation of Co2+ ions at the lattice positions of the Zn2+ ions. The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.

This paper proposes a universal spin-dependent variable range hopping theoretical model to describe various experimental transport phenomena observed in wide-band-gap oxide ferromagnetic semiconductors with high transition metal concentration. The contributions of the 'hard gap' energy, Coulomb interaction, correlation energy, and exchange interaction to the electrical transport are considered in the universal variable range hopping theoretical model. By fitting the temperature and magnetic field dependence of the experimental sheet resistance to the theoretical model, the spin polarization ratio of electrical carriers near the Fermi level and interactions between electrical carriers can be obtained.

Fe-doped In2O3 films are grown epitaxially on YSZ (100) substrates by pulsed laser deposition. The in-situ reflection high-energy electron diffraction, the atomic force microscopy, and the x-ray diffraction patterns show that the films have a well defined cubic structure epitaxially oriented in the (100) direction. Room temperature ferromagnetism is observed by an alternating gradient magnetometer. Strong perpendicular magnetic anisotropy with a remnant magnetization ratio of 0.83 and a coercivity of 2.5kOe is revealed. Both the structural and the magnetic measurements suggest that this ferromagnetism is an intrinsic property deriving from the spin-orbit coupling between the diluted Fe atoms.

Zn1−xFexO inhomogeneous oxide magnetic semiconductor films with high Fe concentration are prepared by sputtering, and fast annealing is carried out at different temperatures. It is found that magnetic properties are greatly modulated by controlling the composition inhomogeneity and subsequently fast annealing. Both ferromagnetic and paramagnetic components are found to coexist in the as-deposited Zn1−xFexO magnetic semiconductor. In particular, the antiferromagnetic coupling between the neighbouring local ferromagnetic regions is found in the as-deposited Zn0.23Fe0.77 O film, and the antiferromagnetic coupling strength increases with increasing temperature from 110 K to 300 K. We believe that this unusual antiferromagnetic coupling is mediated by thermally activated hopping carriers.