Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.     

Spatially resolved inhomogeneous ferromagnetism in (Ga,Mn)as diluted magnetic semiconductors: a microscopic study by muon spin relaxation

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-microSR) as well as by transport and magnetization measurement techniques. LE-microSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.     

Spin injection and relaxation in a mesoscopic superconductor

We study spin transport in a superconducting nanowire using a set of closely spaced magnetic tunnel contacts. We observe a giant enhancement of the spin accumulation of up to 5 orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements, combined with our theoretical model, allow us to distinguish the individual spin-flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.     

Growth and characterization of type-II ZnO/ZnTe core-shell nanowire arrays for solar cell applications

Large area, well-aligned type-II ZnO/ZnTe core-shell nanowire arrays have been fabricated on an a-plane sapphire substrate. The ZnO nanowires were grown in a furnace by chemical vapor deposition with gold as catalyst and then were coated with a ZnTe shell on the ZnO nanowires surface by a metal-organic chemical deposition chamber. The morphology and size distribution of the ZnO/ZnTe core-shell nanowire arrays were studied by scanning electron microscopy (SEM) and the crystal structure was examined by x-ray diffraction (XRD). Transmission measurement was used to study the optical properties of the core-shell nanowires. The results indicated that the ZnO/ZnTe core-shell nanowire arrays have good crystalline quality. In addition, it was found that the nanowire arrays have good light absorption characteristics and these properties make it suitable for making photovoltaic devices.     

Angular dependence of shot noise in the presence of Rashba spin–orbit coupling in semiconductor spintronics junctions

We investigate spin-dependent current and shot noise, taking into account the Rashba spin–orbit coupling (RSOC) effect in double diluted magnetic semiconductor (DMS) barrier resonant tunneling diodes. The calculation is based on an effective mass approach. The magnetization of DMS is calculated by the mean-field approximation in low magnetic field. The spin-splitting of DMS depends on the sp–d exchange interaction. We also examine the dependence of transport properties of CdTe/CdMnTe heterostructures on applied voltage and relative angle between the magnetization of two DMS layers. It is found that the RSOC has great different influence on the transport properties of tunneling electrons with spin-up and spin-down, which have different contributions to the current and the shot noise. Also, we can see that the RSOC enhances the spin polarization of the system, which makes the nanostructure a good candidate for new spin filter devices. Thus, these numerical results may shed light on the next applications of quantum multilayer systems and make them a good choice for future spintronics devices.     

Spintronics: electron spin coherence,entanglement, and transport

The prospect of building spintronic devices in which electron spins store and transport information has attracted strong attention in recent years. Here we present some of our representative theoretical results on three fundamental aspects of spintronics: spin coherence, spin entanglement, and spin transport. In particular, we discuss our detailed quantitative theory for spin relaxation and coherence in electronic materials, resolving in the process a long-standing puzzle of why spin relaxation is extremely fast in Al (compared with other simple metals). In the study of spin entanglement, we consider two electrons in a coupled GaAs double-quantum-dot structure and explore the Hilbert space of the double dot. The specific goal is to critically assess the quantitative aspects of the proposed spin-based quantum dot quantum computer architecture. Finally, we discuss our theory of spin-polarized transport across a semiconductor/metal interface. In particular, we study Andreev reflection, which enables us to quantify the degree of carrier spin polarization and the strength of interfacial scattering.     

Magnetism of Fe,Co, and Ni nanowires encapsulated in carbon nanotubes

We have investigated the electronic and magnetic properties of Fe, Co, and Ni nanowires encapsulated in carbon nanotubes (CNTs) using spin polarized ab initio calculation. The incorporated systems with hollow region between the nanowire and the C shell have the enhanced magnetic moments compared to the ferromagnetic nanowires tightly wrapped by CNTs. The Co nanowire encapsulated in CNTs is a strong ferromagnet and has high spin polarization regardless of the distance between the nanowire and the C shell. The results show that the Co-filled CNTs are useful for spin polarized transport nanodevice.     

An investigation of competition on the dynamic magnetic properties of the core/shell nanowire

In this study, we investigate the dynamic magnetic properties of Ising-type core/shell nanowires (NW) for different spin systems. The model of NW X_(Spin-1/2)@Y with Y = Spin-1/2, Spin-1 and Spin-3/2 are considered for discussing an effect of the nature of shell particle on the dynamic properties. The mean-field theory and Glauber-type stochastic dynamics have been successfully applied to this, and the results of the dynamic magnetic properties for the core/shell NW are obtained. Different shell spin states are employed to the analysis of dynamic magnetic behavior for core/shell NW. Results of numerical calculation for the magnetization and coercivity curves are discussed for the effect of shell particles, shell interaction and oscillating field frequency. All results present that dynamic magnetic properties of the NW strongly dependent on the shell particle and the shell interaction.     

Electronic structure and hot carrier relaxation in ⟨001⟩ anatase TiO2 nanowire

We present an electronic structure and non-adiabatic excited state dynamics study of ?001? anatase TiO₂ nanowire (NW) by combining density matrix formalism and ab initio electronic structure calculations. Our results show that quantum confinement increases the energy gap as the dimension of TiO₂ is reduced from the bulk to a NW with a diameter of several nanometres and that the probability of electronic transitions induced by lattice vibrations for the NW follows band gap law. The electron non-radiative relaxation to the bottom of the conduction band is involving Ti 3d orbitals, while the hole non-radiative relaxation of holes to the top of the valence band occurs by subsequent occupation of O 2p orbitals.     

Investigation of mechanical properties and deformation behavior of single-crystal Al-Cu core-shell nanowire generated using non-equilibrium molecular dynamics simulation

Molecular dynamics (MD) simulation studies were carried out to generate a cylindrical single-crystal Al-Cu core-shell nanowire and its mechanical properties like yield strength and Young’s modulus were evaluated in comparison to a solid aluminum nanowire and hollow copper nanowire which combines to constitute the core-shell structure respectively. The deformation behavior due to changes in the number of Wigner-Seitz defects and dislocations during the entire tensile deformation process was thoroughly studied for the Al-Cu core-shell nanowire. The single-crystal Al-Cu core-shell nanowire shows much higher yield strength and Young’s modulus in comparison to the solid aluminum core and hollow copper shell nanowire due to tangling of dislocations caused by lattice mismatch between aluminum and copper. Thus, the Al-Cu core-shell nanowire can be reinforced in different bulk matrix to develop new type of light-weight nanocomposite materials with greatly enhanced material properties.     

Magnetic properties of epitaxial oxide heterostructures

Diluted Magnetic Semiconductors (DMS) are of great interest as injection sources for spin-polarized currents into semiconductors. Epitaxial devices have been synthesized with an intermediate spacer layer of the same semiconductor (zinc oxide, ZnO) used to produce the DMS material (ZnCoO) ensuring a homoepitaxial junction to help reduce the interface states and conduction mismatch. We observe a large magnetoresistance of about 32% in the devices at low temperatures. The present work suggests that spin polarized transport could be achieved with DMS materials acting as the source of injected spins into a non-magnetic host.     

Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

碳纳米管-硅纳米线复合结构的形成和热稳定性

通过分子动力学方法模拟了在碳纳米管内填充一定数目的半导体元素硅形成碳纳米管-硅纳米线复合结构的过程,并采用Lindemann指数研究了这种复合结构的热稳定性．计算结果表明,当考虑碳纳米管和硅纳米线轴向方向的周期性边界条件之后,在C（13,0）和C （14,0）碳纳米管内能够形成亚稳结构的硅纳米线Si₁₆NW和Si₂₀NW,从而获得一种碳纳米管-硅纳米线的新型复合结构．通过计算这种复合结构的Lindemann指数,可以看到由于碳纳米管的保护作用,碳纳米管包裹的硅纳 关键词： 复合结构 纳米线 碳纳米管 分子动力学     

Plastic relaxation of mixed dislocation in axial nanowire heterostructures using Peach–Koehler approach

In this Letter, the plastic relaxation introduced by typical 60° mixed dislocation in zinc‐blende axial nanowire (NW) heterostructures is evaluated by numerical Peach–Koehler approach which reflects the interaction between coherent strain field and misfit dislocation. Cylindrical NW epilayers grown on NW and bulk substrate are separately modeled. We reveal that straight 60° dislocation with Burgers vector (a /2) 〈101〉 would always generate at NW center orienting [111], while aspect ratio dependent off‐center equilibrium position should be expected for NW epilayer along [001]. Moreover, the critical diameters, below which coherency is maintained, are predicted based on energy balance criterion for axial NW heterostructures along predominating growth direction [111]. The results are compared with existing theory and experimental observations. Our work provides a necessary quantitative complement to the understanding of plastic relaxation and coherency limit in axial NW heteroepitaxy. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nuclear spin relaxation in paramagnetic systems (S>/=1) under fast rotation conditions

A new theoretical model for nuclear spin relaxation in paramagnetic systems in solution has been developed. Fast rotational motion is included in the model, both as a source of modulation of the static zero-field splitting, which provides a mechanism for electron spin relaxation, and as an origin of the stochastic variation of the electron spin-nuclear spin dipole-dipole interaction leading to nuclear spin relaxation. At the limit of low magnetic field, the model is essentially identical to the earlier formulations from our laboratory, but new closed-form expressions are given for the inner- and outer-sphere relaxation at the high-field limit. Numerical comparisons with a general theory are reported for the inner-sphere case. In addition, some nuclear magnetic relaxation dispersion (NMRD) profiles from the literature are considered for systems where experiments have been done with both low-molecular weight paramagnetic complexes and their adducts with proteins. Previously developed theories are used to interpret data for the slowly rotating protein adducts, and good fits of the fast-rotating counterparts are obtained by further adjustment of one or two additional parameters.     

Zeeman energy and spin relaxation in a one-electron quantum dot

We have measured the relaxation time, T1, of the spin of a single electron confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic field, applied parallel to the two-dimensional electron gas in which the quantum dot is defined, Zeeman splitting of the orbital states is directly observed by measurements of electron transport through the dot. By applying short voltage pulses, we can populate the excited spin state with one electron and monitor relaxation of the spin. We find a lower bound on T1 of 50 micros at 7.5 T, only limited by our signal-to-noise ratio. A continuous measurement of the charge on the dot has no observable effect on the spin relaxation.     

Generalized Elliott-Yafet theory of electron spin relaxation in metals: origin of the anomalous electron spin lifetime in MgB2

The temperature dependence of the electron-spin relaxation time in MgB2 is anomalous as it does not follow the resistivity above 150 K; it has a maximum around 400 K and decreases for higher temperatures. This violates the well established Elliot-Yafet theory of spin relaxation in metals. The anomaly occurs when the quasiparticle scattering rate (in energy units) is comparable to the energy difference between the conduction and a neighboring bands. The anomalous behavior is related to the unique band structure of MgB2 and the large electron-phonon coupling. The saturating spin relaxation is the spin transport analogue of the Ioffe-Regel criterion of electron transport.     

Dye-sensitized solar cells based on ZnO nanowire array/TiO2 nanoparticle composite photoelectrodes with controllable nanowire aspect ratio

The ZnO nanowire (NW) array/TiO₂ nanoparticle (NP) composite photoelectrode with controllable NW aspect ratio has been grown from aqueous solutions for the fabrication of dye-sensitized solar cells (DSSCs), which combines the advantages of the rapid electron transport in ZnO NW array and the high surface area of TiO₂ NPs. The results indicate that the composite photoelectrode achieves higher overall photoelectrical conversion efficiency (η) than the ZnO NW alone. As a result, DSSCs based on the ZnO NW array/TiO₂ NP composite photoelectrodes get the enhanced photoelectrical conversion efficiency, and the highest η is also achieved by rational tuning the aspect ratio of ZnO NWs. With the proper aspect ratio (ca. 6) of ZnO NW, the ZnO NW array/TiO₂ NP composite DSSC exhibits the highest conversion efficiency (5.5 %). It is elucidated by the dye adsorption amount and interfacial electron transport of DSSCs with the ZnO NW array/TiO₂ NP composite photoelectrode, which is quantitatively characterized using the UV-Vis absorption spectra and electrochemical impedance spectra. It is evident that the DSSC with the proper aspect ratio of ZnO NW displays the high dye adsorption amount and fastest interfacial electron transfer.     

Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Equilibrium ensembles of quantum dots in atomically inhomogeneous pentagonal nanowires

A theoretical model has been proposed for describing the relaxation of elastic stresses in an atomically inhomogeneous pentagonal nanowire due to the formation of quantum dots in the form of precipitates of the second phase. Quantum dots have been considered as finite-height coaxial cylindrical inclusions subjected to intrinsic axial dilatation and located along the axis of the nanowire. The optimum shape and sizes of the quantum dots have been calculated for specified nanowire parameters. It has been shown that such quantum dots can form different equilibrium periodic structures in the pentagonal nanowires.