Spin Filtering Efficiency Research Articles

Tuning spin filtering and spin rectifying behaviors of zigzag silicon carbon nanoribbons by edge dual-hydrogenation

Abstract Using non-equilibrium Green's function combined with density functional theory, we investigate spin-resolved transport properties of zigzag silicon carbon nanoribbons (zSiCNRs) with the different edge hydrogenations. The dual-hydrogenation on edge C or Si atoms all can break the magnetic degeneracy of zSiCNRs with mono-hydrogenation, and change it from the initial metallicity to halfmetallic behavior under the ferromagnetic state. Under the parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms of zSiCNR devices can induce and reverse a prefect spin filtering behavior with nearly 100% spin filtering efficiency. Under the anti-parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms can enhance the spin rectifying behavior of zSiCNR devices obviously with the substantial increases in corresponding spin rectifying ratios of α-spin or β-spin currents. The above findings are very useful for its functional application in silicon carbon spin-dependent nanodevices.

Exploring how hydrogen at gold–sulfur interface affects spin transport in single-molecule junction**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674092, 11804093, and 61764005), the Natural Science Foundation of Hunan Province, China (Grant No. 2019JJ40006), the Scientific Research Fund of the Education Department of Hunan Province, China (Grant No. 18B368), the Science and Technology Development Plan Project

Very recently, experimental evidence showed that the hydrogen is retained in dithiol-terminated single-molecule junction under the widely adopted preparation conditions, which is in contrast to the accepted view [Nat. Chem. 11 351 (2019)]. However, the hydrogen is generally assumed to be lost in the previous physical models of single-molecule junctions. Whether the retention of the hydrogen at the gold—sulfur interface exerts a significant effect on the theoretical prediction of spin transport properties is an open question. Therefore, here in this paper we carry out a comparative study of spin transport in M-tetraphenylporphyrin-based (M = V, Cr, Mn, Fe, and Co; M-TPP) single-molecule junction through Au–SR and Au–S(H)R bondings. The results show that the hydrogen at the gold–sulfur interface may dramatically affect the spin-filtering efficiency of M-TPP-based single-molecule junction, depending on the type of transition metal ions embedded into porphyrin ring. Moreover, we find that for the Co-TPP-based molecular junction, the hydrogen at the gold–sulfur interface has no obvious effect on transmission at the Fermi level, but it has a significant effect on the spin-dependent transmission dip induced by the quantum interference on the occupied side. Thus the fate of hydrogen should be concerned in the physical model according to the actual preparation condition, which is important for our fundamental understanding of spin transport in the single-molecule junctions. Our work also provides guidance in how to experimentally identify the nature of gold–sulfur interface in the single-molecule junction with spin-polarized transport.

Spin-Polarized Transport and Optoelectronic Properties of a Novel-Designed Architecture with a Porphyrin-Based Wheel and Organometallic Multidecker Sandwich Complex-Based Axle

A novel “wheel-and-axle” architecture (c-P6)m/(FeBz)n, with (c-P6) denoting the wheel formed by six porphyrin-based segments and (FeBz)n the axle formed by the 1D iron benzene multidecker complex, is designed, and its electronic structure, transport property, and linear photoresponse are investigated. (c-P6)m/(FeBz)n shows a spin-polarized transport property. The spin filter efficiency of (c-P6)m/(FeBz)n can be > 90%, suggesting it is a very good candidate for spin filters. Furthermore, a distinct NDR feature is observed for (c-P6)m/(FeBz)n so it is can be used for making electronic switches and oscillators. Under linear light, both the wheel and axle of (c-P6)m/(FeBz)n exhibit a distinct polarized photoresponse character. The magnitude of the photoresponse can be tuned by the photon energy or by the l bias voltage. An off–on–off switch is observed within the considered photon energy range, showing potential application for optical switches. All these fascinating properties of (c-P6)m/(FeBz)n make the new 1D material especially attractive for electronic and optoelectronic devices.

First principles investigation of the spin transport properties in graphene-porphine-graphene nanojunctions

The spin-dependent transport properties of a porphine molecule linking with two zigzag-edged graphene nanoribbon (ZGNR) electrodes have been investigated by using the density of functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. The device shows clearly negative differential resistance (NDR), large tunnelling magnetoresistance (TMR) of 103 magnitude and nearly 100% perfect spin filter efficiency (SFE) properties, respectively. What’s more, the projected density of states (PDOS), molecular projected self-consistent Hamiltonian (MPSH) eigenvalues and transmission eigenstates were carried out to discuss the transport properties of the ZGNR/Porphine/ZGNR nanojunction. These results suggest that the ZGNR/Porphine/ZGNR device is a promising candidate for multi-functional spintronic devices.

Highly Efficient and Tunable Filtering of Electrons' Spin by Supramolecular Chirality of Nanofiber-Based Materials.

Organic semiconductors and organic-inorganic hybrids are promising materials for spintronic-based memory devices. Recently, an alternative route to organic spintronic based on chiral-induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical π-conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin-filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both "up" and "down" orientations of filtered spins can be obtained in a single enantiopure system via the temperature-dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure-property relationship for spintronic applications.

Spin-Polarized Electron Transfer in Multilayers with Different Types of Rough Interfaces

In this study, the influence of interfacial roughness of multilayers structures on spin filter tunneling is investigated at low biases as a function of interfacial roughness type. The results show that the roughness causes the reduction of resonant tunneling, maximum achievable tunneling magneto resistance (TMR), and spin-filtering efficiency of tunneling structures. Based on the numerical results, decreasing the electronic devices’ efficiency is straight related to roughening of interfacial. Additionally, the effect of different temperatures is investigated on the spin polarization (SP). The values of SP decreased sensibly by increasing the temperature.

Enhanced robustness of half-metallicity in VBr3 nanowires by strains and transition metal doping.

The study of half-metallic behavior for transition metal tribromide nanowires is of great significance to the basic research and application in spintronics. We have theoretically investigated the spin transport properties of half-metallic VBr3 nanowires sandwiched between Au(100) electrodes. For the VBr3 nanowire with a length of 24.75 Å, the spin-filter efficiency (SFE) achieves 100% when the applied bias is less than 0.4 V. The robustness of half-metallic behavior in VBr3 nanowires is investigated by strains and transition metal doping. The bias range of VBr3 nanowires exhibiting perfect SFE is extended by tensile strains, while it becomes narrower under compressive strains. For VBr3 nanowires doped with Co and Cr, the bias range with ideal SFE is significantly broadened at a low doping concentration. In particular, the VBr3 nanowire doped with 1 Cr atom exhibits perfect SFE in an extremely wide bias range of 0-1.0 V. Our results show that the robustness of half-metallicity of VBr3 nanowires can be improved by tensile strains or certain doping, which provides promising ways for further design of high-performance spintronic devices.

Spin filtering with Mn-doped Ge-core/Si-shell nanowires.

Incorporating spin functionality into a semiconductor core–shell nanowire that offers immunity from the substrate effect is a highly desirable step for its application in next generation spintronics. Here, using first-principles density functional theory that does not make any assumptions of the electronic structure, we predict that a very small amount of Mn dopants in the core region of the wire can transform the Ge–Si core–shell semiconductor nanowire into a half-metallic ferromagnet that is stable at room temperature. The energy band structures reveal a semiconducting behavior for one spin direction while the metallic behavior for the other, indicating 100% spin polarization at the Fermi energy. No measurable shifts in energy levels in the vicinity of Fermi energy are found due to spin–orbit coupling, which suggests that the spin coherence length can be much higher in this material. To further assess the use of this material in a practical device setting, we have used a quantum transport approach to calculate the spin-filtering efficiency for a channel made out of a finite nanowire segment. Our calculations yield an efficiency more than 90%, which further confirms the excellent spin-selective properties of our newly tailored Mn-doped Ge-core/Si-shell nanowires.

High spin-filtering and magnetoelectric properties of one-dimensional transition metal.bis(dithiolene) structures

The electric, magnetic and transport properties of new one dimensional π-conjugated transition metal.bis.dithiolene (1D-TM.b.DT) structures, were investigated based on density functional theory (DFT) and nonequilibrium Green's function formalism (NEGF). Mn, Co and Ni atoms are considered as TMs in the 1D-TM.b.DT structures. The three 1D-TM.b.DT structures are found to be in square planar geometry. dx2−y2 and dz2 orbitals of the considered TMs remain non-bonding however other d orbitals make π and σ bonds with p orbitals of sulfur atoms. The 1D-Ni.b.DT is a metal and has similar α- and β-spin behaviors, thus has the highest transmission in both spin states. However, the 1D-Co.b.DT and 1D-Mn.b.DT are half-metal with a 100% spin-filtering efficiency at the Fermi level. In addition, 1D-Mn.b.DT has a high magnetic moment of 3.002μB per unit cell. The results indicate that 1D-Co.b.DT and 1D.Mn.b.DT can be suggested as prominent spin-filtering devices.

Spin magnetism of graphene nanoribbon modulated by triangular boron nitride flake

The electronic properties and spin magnetism of graphene nanoribbon doped with triangular BN flake are systematically investigated. For the spin polarized state, the size of the triangular BN flake could tailor the spin magnetism of devices. More importantly, it will intrigue property of bipolar magnetic semiconducting with the spin-filtering efficiency (SFE) nearly reaching 100% and keep good transport property when the triangular BN flake is large enough. Although the triangular vacancies also present a bipolar magnetic semiconducting property with a higher SFE, the transmission probability drops dramatically and the stability is no more than those of triangle BN flake.

Nonmagnetic single-molecule spin-filter based on quantum interference

Key spin transport phenomena, including magnetoresistance and spin transfer torque, cannot be activated without spin-polarized currents, in which one electron spin is dominant. At the nanoscale, the relevant length-scale for modern spintronics, spin current generation is rather limited due to unwanted contributions from poorly spin-polarized frontier states in ferromagnetic electrodes, or too short length-scales for efficient spin splitting by spin-orbit interaction and magnetic fields. Here, we show that spin-polarized currents can be generated in silver-vanadocene-silver single molecule junctions without magnetic components or magnetic fields. In some cases, the measured spin currents approach the limit of ideal ballistic spin transport. Comparison between conductance and shot-noise measurements to detailed calculations reveals a mechanism based on spin-dependent quantum interference that yields very efficient spin filtering. Our findings pave the way for nanoscale spintronics based on quantum interference, with the advantages of low sensitivity to decoherence effects and the freedom to use non-magnetic materials.

Design of spin-filtering devices with rectifying effects and negative differential resistance using armchair phosphorene nanoribbon

In the present work, we employ density functional theory in combination with non-equilibrium Green function to investigate the spin transport properties of six types of armchair phosphorene nanoribbons (APNRs) devices, i.e., M1–M6, in which the left and right electrodes are doped with different transition metal (TM) atoms such as Fe, Co, and Ni atoms. The M1, M3, and M5 devices are obtained by substituting (Co and Ni), (Fe and Co), (Fe and Ni) atoms, respectively, with the first atom (for example, Co in M1) placed in the center of the left and the other one (for example, Ni in M1) placed in the center of right electrode. Furthermore, the M2, M4, and M6 devices are obtained by moving the substitution sites of (Co and Ni), (Fe and Co), and (Fe and Ni) near the edge of NRs, respectively. In this study, APNRs with the widths of n = 7, 8, and 9 are chosen to study the effects of various widths of APNRs on their electronic transport properties. The current–voltage characteristics, rectification ratio and spin-filtering efficiency are calculated for the assumed devices. Our findings demonstrate that M3 and M5 devices in all considered widths represent 100% spin-filtering efficiency in dual-orientation of positive and negative bias. Besides, spin-dependent rectifying behavior is observed in our assumed systems. Also, all devices reveal spin-dependent negative differential resistance. Based on the obtained results, these phenomena can be manipulated by moving the positions of TM atoms from the center to the edge of NRs or changing the width of APNRs. Overall, our research may be helpful in designing multifunctional spintronic devices.

Magneto thermoelectric properties of transition metal.bis(dithiolene) wires

Inspired by recent success in experimental synthesis of π-conjugated transition metal.bis.dithiolene (TM.b.DT) wires, their charge- and spin-dependent thermoelectric properties have been investigated using density functional theory (DFT) and nonequilibrium Green's function formalism (NEGF). Mn, Co and Ni are considered as TMs in this study. The metal Ni.b.DT wire has the highest electrical conductance versus chemical potential because of having both α and β spin contribution to the transport. Whereas, because of their perfect spin-filtering efficiency at the Fermi level, the Co and Mn half-metal wires have high spin conductance and spin thermopower. In addition, the final results of charge- and spin-dependent thermoelectric efficiency suggests Co.b.DT and Mn.b.DT wires for promising devices in spin-filtering, charge- and spin-thermoelectric sciences.

Rational design of magnetic semiconductors of longitudinal silicene/III-V compound heteronanoribbons

The generation and transportation of spin-polarized current in silicon-based materials represent one of the most attractive topics in spintronics. Here, we present a strategy of designing magnetic semiconductors based on zigzag silicene covalently bonded with honeycomb III-V compound nanoribbons (i.e., ZSi/AlPNR and ZSi/GaAsNR). By tuning the edge states of the heteronanoribbons, high spin-filtering efficiencies can be achieved as a result of the opposite transverse electric fields induced by the III (Al, Ga) or V (P, As) atoms connected to the Si atoms at the interfaces. Importantly, the Si-III interfaced heteronanoribbons are found to be more suitable for spin-filtering applications. It is also demonstrated that the spin polarization and spin-filtering efficiency are robust and independent of the width of the heteronanoribbons.

Rationally Designed High-Performance Spin Filter Based on Two-Dimensional Half-Metal Cr2NO2

Summary Half-metals are promising candidates for designing efficient spin filters owing to their unique electronic structures, which show the electrical conductivity for spin-up states and a band gap for spin-down states. Herein, designing excellent ultrathin spin filters by using half-metal two-dimensional Cr2NO2, which has a Curie temperature of 566 K, is demonstrated based on first-principles calculations. Our results reveal that the required energy to overturn the spin arrangement of bilayer Cr2NO2, from parallel to anti-parallel states, is quite low. The bilayer Cr2NO2 maintains its half-metal behavior while sandwiched within the Au/Cr2NO2/Au heterojunction. Owing to the half-metal characteristic, the total current of Au/Cr2NO2/Au can reach a value of 15 nA per primitive cell under a low voltage of 10 mV in parallel states. The magnetoresistance ratio is 9,333% at low voltage. The robust half-metal behavior, high Curie temperature, and two-dimensional structure make Cr2NO2 an ideal ultrathin spin-filtering material with high switching ratio and low energy consumption.

Enhanced spin-dependent thermopower in a double-quantum-dot sandwiched between two-dimensional electron gases**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274101, 51362031, and 11675023), the Innovation Development Fund of China Academy of Engineering Physics (CAEP) (Grant No. ZYCX1921-02), the Presidential Foundation of CAEP (Grant No. YZ2015014), the Initial Project of University of Electronic Science and Technology of China,

We study the spin-dependent thermopower in a double-quantum-dot (DQD) embedded between the left and right two-dimensional electron gases (2DEGs) in doped quantum wells under an in-plane magnetic field. When the separation between the DQD is smaller than the Fermi wavelength in the 2DEGs, the asymmetry in the dots’ energy levels leads to pronounced quantum interference effects characterized by the Dicke line-shape of the conductance, which are sensitive to the properties of the 2DEGs. The magnitude of the thermopower, which denotes the generated voltage in response to an infinitesimal temperature difference between the two 2DEGs under vanishing charge current, will be obviously enhanced by the Dicke effect. The application of the in-plane magnetic field results in the polarization of the spin-up and spin-down conductances and thermopowers, and enables an efficient spin-filter device in addition to a tunable pure spin thermopower in the absence of its charge counterpart.

Endohedral Fullerene Fe@C28 Adsorbed on Au(111) Surface as a High-Efficiency Spin Filter: A Theoretical Study.

We present a theoretical study on the adsorption and spin transport properties of magnetic Fe@C28 using Ab initio calculations based on spin density functional theory and non-equilibrium Green’s function techniques. Fe@C28 tends to adsorb on the bridge sites in the manner of C–C bonds, and the spin-resolved transmission spectra of Fe@C28 molecular junctions exhibit robust transport spin polarization (TSP). Under small bias voltage, the transport properties of Fe@C28 are mainly determined by the spin-down channel and exhibit a large spin polarization. When compressing the right electrode, the TSP is decreased, but high spin filter efficiency (SFE) is still maintained. These theoretical results indicate that Fe@C28 with a large magnetic moment has potential applications in molecular spintronics.

Perfect negative differential resistance, spin-filter and spin-rectification transport behaviors in zigzag-edged δ-graphyne nanoribbon-based magnetic devices

Many layered graphene and graphene-like two-dimensional carbon materials have been successively proposed in theory and experiment owing to their exceptional properties and potential applications. By employing the first-principles study, we find that a new graphene-like material, δ-graphyne, whose properties of the zigzag-edged nanoribbon are very similar to those ones of zigzag-edged graphene case. The band structure clearly exhibits a metallic property in non-magnetic state. And we can see a visible spin splitting within the ferromagnetic state, however, spin degeneracy within the antiferromagnetic state. To this end, here we propose a molecular junction based on the zigzag-δ-graphyne nanoribbon symmetrically with additional phenyl rings at both edges. The computational results imply that the device has many good electron transport performances, such as negative differential resistance, spin-filtering and rectification effects, and so on. In particular, the spin-filtering efficiency can be up to 99%, and the maximum of the rectification ratio reaches up to 106%. The mechanisms for these effects are revealed and discussed in terms of the band structure, spin-resolved electron transmission spectrum, the local density of state and the transmission pathway. Therefore, our research results can provide a basis for the experimental preparation of the δ-graphyne-based junction.

Spin-filter effect by introducing an edge vacancy on armchair graphene nanoribbons with oxygen termination

We studied the spin transport properties of an oxygen-terminated armchair graphene nanoribbon (O-AGNR) with a single edge vacancy via an ab-initio calculation based on density functional theory combined with the nonequilibrium Green’s function technique. We found that spin-filtering occurs regardless of the ribbon width by introducing an edge vacancy on one edge side of the O-AGNR because the four energy bands mainly consisted of the oxygen p orbital, such that the up-spin states were localized on one edge side and the down-spin states were on the other edge side. Moreover, we showed that the absolute value of spin-filtering efficiency reached ∼1.00 at certain values of the chemical potential; additionally, the direction of spin polarization can be switched by controlling the chemical potential. These results suggest that O-AGNRs have potential applications in spintronics devices.

Anti-Resonance in a Laterally Coupled Triple-Quantum-Dot Chain

Employing the nonequilibrium Green’s function technique, electron transport characteristics through a laterally coupled triple-quantum-dot chain are investigated. The conductance versus electron energy is numerically calculated. Two anti-resonance points change into two anti-resonance bands with increasing the size of triple-quantum-dot chain. The widths of two anti-resonance bands are the same and can be regulated via adjusting the inter-dot tunneling coupling strength in the main chain or its attachment. Spin-polarized window emerges due to Zeeman splitting as an external magnetic flux is introduced. Our studies demonstrate that the system can be used as a quantum switch or an efficient spin filtering.