Spin-polarized currents induced in antiferromagnetic polymer multilayered field-effect transistors.

A theoretical construction of an antiferromagnetic polymer multilayered field-effect transistor with polymers stretched between the source and drain contacts was undertaken. The model employed a quantum approach to the on-chain spin-charge distribution, which was self-consistently coupled with the charge distribution controlled by the gate voltage. Contrary to standard field-effect transistors, we found that the current firstly increased superlinearly with the drain voltage, then it achieved the maximum for drain voltages notably lower than the gate voltage, and after that, it decreased with the drain voltage with no saturation. Such effects were coupled with the formation of the current spin-polarization ratio, where the on-chain mobility of respective spin-polarized charges was significantly dependent on the applied drain voltage. These effects arise from competition among the antiferromagnetic coupling, the intra-site spin-dependent Coulomb interaction, and the applied drain and gate voltages, which strongly influence the on-chain spin-charge distribution, varying from an alternating spin configuration to a spin-polarized configuration at both ends of the chain. Substantial control over the magnitude of spin-polarized currents was achieved by manipulating gate and drain voltages, showcasing the feasibility of practical applications in spintronics.

Formation of spin-polarized current in antiferromagnetic polymer spintronic field-effect transistors.

We have theoretically investigated the feasibility of constructing a spintronic field-effect transistor with the active channel made of a polymer chain with the antiferromagnetic coupling oriented in the source-to-drain direction. We found two different device function regimes controlling the on-chain spin-charge carrier density by tuning the gate voltage. At higher charge carrier densities, the source-drain current linearly increases with decreasing charge carrier densities. In this regime, no polymer spin-polarized current is observed. Upon reaching a critical gate voltage, the current decreases with decreasing charge densities. It is accompanied by the formation of spin-polarized current, generated by an on-chain process, which can be related to spin-charge spatial distribution symmetry breaking caused either by an application of the source-to-drain voltage (higher spin polarization near the drain), or the breakdown of the Peierls dimerization near chain ends. Numerical simulation of the transistor characteristics suggests that the design of a polymer spintronic field-effect transistor is in principle feasible.

Field-Free Magnetoplasmon-Induced Ultraviolet Circular Dichroism Switching in Premagnetized Magnetic Nanowires.

Broadband modulation of magnetic circular dichroism (MCD) using a relatively low magnetic field or by producing a field-free magnetoplasmonic effect in the remnant magnetic state was achieved by the integration of the noble metals (NMs) Au and Ag and the perpendicular magnetic anisotropy of Co with ZnO nanowires (NWs) used as the template. The samples containing NMs revealed MCD sign reversals and enhancements when compared with the original Co/ZnO NWs. The magnetoplasmonic effect of Au close to the visible light spectrum could induce the CD change in the visible region. Notably, the ultraviolet (UV) CD in Ag/Co/ZnO NWs is 12.5 times larger under a magnetic field (∼0.2 T) and 10 times greater in the remnant state (field-free) than those of the original Co/ZnO NWs because of the magnetoplasmonic effect of Ag in the UV spectrum. These results are attributable to the coupling of the remnant magnetic state of Co magnetization, the magnetoplasmons of the NMs, and the excitons of the ZnO NWs. The findings are potentially applicable in magneto-optical recording, biosensing, and energy contexts involving magnetoplasmonic functionalization.

Enhanced Photodegradation in Metal Oxide Nanowires with Co-Doped Surfaces under a Low Magnetic Field.

This work demonstrated the enhanced photodegradation (PD) resulting from Co-rich doping of ZnO nanowire (NW) surfaces (Co2+/ZnO NWs) prepared by combining Co sputtering on ZnO NWs and immersion in deionized water to exploit the hydrophilic-hydrophobic transitions on the ZnO surfaces resulting from Co atom diffusion. Because of the controllable spin-dependent density of states (DOS) induced by Co2+, the PD of methylene blue dye can be enhanced by approximately 90% (when compared with bare ZnO NWs) by using a conventional permanent magnet with a relatively low magnetic field strength of approximately 0.15 T. The reliability of spin polarization-modulation attained through surface doping, based on the magnetic response observed from X-ray absorption measurements and magnetic circular dichroism, provides an opportunity to create highly efficient catalysts by engineering surfaces and tailoring their spin-dependent DOS.

Gate voltage impact on charge mobility in end-on stacked conjugated oligomers.

We present a model of the charge transport in thin film organic field-effect transistors with the active channel made of linear conjugated chains stacked on the substrate with end-on-orientation. The transport was simulated in a box consisting of 25 polymer chains, in which the delocalized quantum orbital eigenstates of the on-chain hole distribution were calculated. The inter-chain charge transfer was solved semi-classically. The full self-consistent distribution of charge density and electric field was determined for various applied gate and source-drain voltages. We found that the dependence of charge mobility on gate voltage is not monotonic: it first increases with increasing gate voltage for a limited interval of the latter, otherwise it decreases with the gate voltage. Next, we found formation of the second resonant peak for higher gate voltages. The mobility dependence on the gate voltage confirmed that the current flowing through the active semiconductor layer should be described not only as the hole transfer between adjacent repeat units of the neighbouring chains, but also as the transfer of coherences among on-chain repeat units. The presented model can also give a new insight into the charge transport in organic field-effect transistors with a novel vertical architecture.

Effects of Strontium incorporation to Mg-Zn-Ca biodegradable bulk metallic glass investigated by molecular dynamics simulation and density functional theory calculation.

Molecular dynamics (MD) simulation and density functional theory (DFT) calculations were used to predict the material properties and explore the improvement on the surface corrosion resistance for the Mg66Zn30Ca3Sr1 bulk metallic glass (BMG). The Mg66Zn30Ca4 BMG was also investigated to realize the influence of the addition of Sr element on the material behaviors of Mg66Zn30Ca4. The Mg-Zn-Ca-Sr parameters of the next nearest-neighbor modified embedded atom method (2NN MEAM) potential were first determined by the guaranteed convergence particle swarm optimization (GCPSO) method based on the reference data from the density functional theory (DFT) calculation. Besides, using the 2NN MEAM parameters of the Mg-Zn-Ca-Sr system, the structures of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 were predicted by the simulated-annealing basin-hopping (SABH) method. The local atomic arrangements of the predicted BMG structures are almost the same as those measured in some related experiments from a comparison with the calculated and experimental X-ray diffraction (XRD) profiles. Furthermore, the HA index analysis shows that the fractions of icosahedra-like local structures are about 72.20% and 72.73% for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively, indicating that these two BMG structures are entirely amorphous. The uniaxial tensile MD simulation was conducted to obtain the stress-strain relationship as well as the related mechanical properties of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Consequently, the predicted Young's moduli of both BMGs are about 46.4 GPa, which are very close to the experimental values of 48.8 ± 0.2 and 49.1 ± 0.1 GPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1, respectively. However, the predicted strengths of Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1 are about 850 and 900 MPa, both are slightly higher than the measured experimental values about 747 ± 22 and 848 ± 21 MPa for Mg66Zn30Ca4 and Mg66Zn30Ca3Sr1. Regarding the thermal properties, the predicted melting temperature of Mg66Zn30Ca3Sr1 by the square displacement (SD) profile is about 620 K, which is very close to the experimental melting temperature of about 613 K. The self-diffusion coefficients of Mg, Zn, Ca, and Sr elements were also calculated for temperatures near their melting points by means of the Einstein equation. The methodology can determine the diffusion barriers for different elements by utilizing these diffusion coefficients resulting in a fact that the diffusion barriers of Ca and Sr elements of Mg66Zn30Ca3Sr1 are relatively high. For the electronic properties predicted by the DFT calculation, the projected density of states (PDOS) profiles of surface Mg, Zn, Ca, and Sr elements clearly show that the addition of Sr into Mg66Zn30Ca4 effectively reduces the s and p orbital states of surface Mg and Zn elements near the Fermi level, particularly the p orbits, which suppresses the electron transfer as well as increases the surface corrosion resistance of Mg66Zn30Ca4. Consequently, this study has provided excellent 2NN MEAM parameters for the Mg, Zn, Ca, and Sr system by the GCPSO method to predict real BMG structures as well as by means of the DFT calculation to explore the electronic properties. Eventually, through our developed numerical processes the material properties of BMGs with different compositions can be predicted accurately for the new BMG design.

Theoretical model investigating the magnetic properties of cobalt-doped ZnO.

We propose a theoretical model to investigate the magnetic properties of cobalt-doped ZnO (ZnO:Co) thin films qualitatively. The model was built on the dilute Co dopants in the host of ZnO forming the magnetic Co+2 ions and the energy level of the magnetic ions crossing the band edge of ZnO resulting in a magnetic interaction between the Co+2 spins and the spins of the electrons from the conduction band of ZnO. The mechanism of the ferromagnetism revealed in the studied system is proposed here to be induced not only by the mediated conducting electrons via spin interactions but also by the Coulomb excitations, arising from the electrons localized by the oxygen vacancies. This approach of including Coulomb excitation in the modified carrier-mediated model could explain well the magnetic properties of ZnO:Co and solves the drawback of the carrier-mediated model in interpreting the appearance of ferromagnetism in the insulating ZnO:Co. We propose that the Coulomb excitations induced by the electrons captured by the oxygen vacancies are an essential element in the magnetic ZnO, which reveals the fact that the bound magnetic polaron model without considering the Coulomb excitation is insufficient to describe the magnetic properties of ZnO.

Observing the three-dimensional terephthalic acid supramolecular growth mechanism on a stearic acid buffer layer by molecular simulation methods.

The terephthalic acid (TPA) supramolecular growth mechanisms on the stearic acid (STA) buffer layer, such as the phase separation and layer-by-layer (LBL) mechanisms, were considered by molecular simulations. The electrostatic surface potential (ESP) charges obtained by the semi-empirical ab initio package VAMP with PM6 were used with the Dreiding force field. The stochastic tunneling-basin hopping-discrete molecular dynamics method (STUN-BH-DMD) was first used to construct the most stable STA buffer layers (STA100, STA120, and STA140) on graphene. At STA100 and STA120, the STA molecule stacking along their long axis is the major mechanism to obtain the stable STA buffer layer. At STA140, the hydrogen bond network between the terminal COOH groups of STA molecules makes the STA buffer layer the most stable, leading to a higher disintegration temperature among all STA coverages. In the early growth of the TPA supramolecule, TPA molecules were first adsorbed by the holes between STA piles. At STA100 and STA120, the subsequent TPA molecules were adsorbed by the TPA molecules within the holes, leading to the phase separation growth. At STA140, the TPA supramolecule tends to grow by the LBL mechanism.

Extraction of magnetic circular dichroism effects from blended mixture of magnetic linear dichroism signals in the cobalt/Scotch tape system.

Circular dichroism (CD) signals revealed in some materials may arise from different origins during measurements. Magnetic field dependent CD (MCD) emanating from the spin-polarized band provides direct insight into the spin-spin interband transitions in magnetic materials. On the contrary, natural CD effects which are artefactual signals resulting from the linear polarization (LP) components during the polarization modulation with a photo-elastic modulator in anisotropic polymer systems were usually observed. There is no simple method to reliably distinguish MCD effect due to spin polarized band structures from natural CD effect, which limits our understanding of the magnetic material/polymer hybrid structures. This paper aims to introduce a general strategy of averaging out the magnetic linear dichroism (MLD) contributions due to the anisotropic structure and disentangling MCD signal(s) from natural MCD signal(s). We demonstrate the effectiveness of separating MCD from natural MCD using rotational MCD measurement and presented the results of a sample with Co thin film on polymer Scotch tape (unplasticized polyvinyl chloride) glued on a quartz substrate. We demonstrate that the proposed method can be used as an effective tool in disentangling MCD and natural MCD effects, and it opens prospects to study the magnetic material /polymer hybrid systems.

Theoretical investigation of Hall sign change in SrRuO3.

The appearance of Hall sign change in perovskite SrRuO3 thin films at Curie temperature was confirmed from our fabricated samples and the result was simulated by our proposed theoretical model. In particular, our simulation results are consistent with experimental results mainly due to the introduction of an impurity band in a two-band model. We found the other important factors in our theory responsible for observed consistency Hall measurements are the itinerant carrier density and its intrinsic carrier type. Eventually the theory possibly interprets the mechanism of Hall sign change.

Helical Majorana fermions in d(x2-y2) + id(xy)-wave topological superconductivity of doped correlated quantum spin Hall insulators.

There has been growing interest in searching for exotic self-conjugate, charge-neutral low-energy fermionic quasi-particles, known as Majorana fermions (MFs) in solid state systems. Their signatures have been proposed and potentially observed at edges of topological superconcuctors with non-trivial topological invariant in the bulk electronic band structure. Much effort have been focused on realizing MFs in odd-parity superconductors made of strong spin-orbit coupled materials in proximity to conventional superconductors. In this paper, we propose a novel mechanism for realizing MFs in 2D spin-singlet topological superconducting state induced by doping a correlated quantum spin Hall (Kane-Mele) insulator. Via a renormalized mean-field approach, the system is found to exhibits time-reversal symmetry (TRS) breaking d(x2-y2) + id(xy)-wave (chiral d-wave) superconductivity near half-filling in the limit of large on-site repulsion. Surprisingly, however, at large spin-orbit coupling, the system undergoes a topological phase transition and enter into a new topological phase protected by a pseudo-spin Chern number, which can be viewed as a persistent extension of the quantum spin Hall phase upon doping. From bulk-edge correspondence, this phase is featured by the presence of two pairs of counter-propagating helical Majorana modes per edge, instead of two chiral propagating edge modes in the d + id' superconductors.

Observation of the amorphous zinc oxide recrystalline process by molecular dynamics simulation.

The detailed structural variations of amorphous zinc oxide (ZnO) as well as wurtzite (B4) and zinc blende (B3) crystal structures during the temperature elevation process were observed by molecular dynamics simulation. The amorphous ZnO structure was first predicted through the simulated-annealing basin-hopping algorithm with the criterion to search for the least stable structure. The density and X-ray diffraction profiles of amorphous ZnO of the structure were in agreement with previous reports. The local structural transformation among different local structures and the recrystalline process of amorphous ZnO at higher temperatures are observed and can explain the structural transformation and recrystalline mechanism in a corresponding experiment [Bruncko et al., Thin Solid Films 520, 866-870 (2011)].

Substrate coupling effect to transport in nanographene ribbon.

Due to the components of electronics being developed on substrates, the substrate coupling to the conduction channel of electronics should be reasonably considered. We propose a nanographene ribbon coupling with substrates to study the variation in conductivity and magnetoresistance.

Temperature sensing using individual single-walled carbon nanotubes.

Molecular dynamics simulations and quantum transport theory are employed to study the temperature-dependent electrical properties of individual (12,0) zigzag and (5,5) armchair carbon nanotubes deposited on silicon substrates. The results demonstrate that the magnitude of the leakage current depends on the length of the nanotube. Furthermore, the leakage current is generated periodically along the length of the nanotube. Finally, the results indicate that given an appropriate value of the applied bias voltage, the induced current varies linearly with the temperature over specific temperature ranges. As a result, the temperature can be inversely derived from the measured current signal. Overall, the results show that the (12,0) zigzag and (5,5) armchair carbon nanotubes are suitable for temperature sensing applications over temperature ranges of 200-420 K and 300-440 K, respectively.

Investigation on the structural variation of Co-Cu nanoparticles during the annealing process.

This study uses molecular dynamics simulations to investigate the crystalline process of Co-Cu nanoparticles of high and low Co concentrations (5 and 25%) during the annealing process. The modified many-body tight-binding potential involving magnetic contribution is adopted to accurately model the Cu-Cu, Co-Co, and Co-Cu pair interactions. The Co-Co bond length increases, while the Co-Cu bond length decreases as the temperature gradually drops from 2000 K to the upper melting point. During that process, the Cu-Cu bond length remains constant and the value of the first peak of the radial distribution function (RDF) increases, which indicates that Cu atoms increase their short-range order by mutual rearrangement. At temperatures lower than the upper melting point, the bond length of each pair decreases while the value of the first peak increases as the temperature is continuously reduced. Because the kinetic energy of an individual atom is not enough for rearrangement, the variations of bond length and the first RDF peak can be attributed to the shrinking effect.