Antiparallel Spin Configuration Research Articles

Helicity dependent cross sections for the photoproduction of π0π± pairs from quasi-free nucleons

Photoproduction of π0π±-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon beam, determined the double polarization observable E. From these results the spin-dependent cross sections σ1/2 and σ3/2, corresponding to the anti-parallel and parallel spin configurations of the beam photon and target nucleon, have been derived. The measurements were performed at the Mainz MAMI accelerator with tagged, circularly-polarized photon beams produced via bremsstrahlung from longitudinally polarized electron beams. The reaction products were detected with an almost 4π solid-angle covering calorimeter composed of the Crystal Ball and TAPS detectors, supplemented by plastic scintillation detectors for charged particle identification. The results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged ρ mesons, and are compared to recent model results.

Creating a three-dimensional intrinsic electric dipole on rotated CrI3 bilayers

Two-dimensional (2D) materials are being explored as a novel multiferroic platform. One of the most studied magnetoelectric multiferroic 2D materials are antiferromagnetically coupled (AFM) ${\mathrm{CrI}}_{3}$ bilayers. Neglecting magnetism, those bilayers possess a crystalline point of inversion, which is only removed by the antiparallel spin configuration among its two constituent monolayers. The resulting intrinsic electric dipole on those bilayers has a magnitude no larger than 0.04 pC/m, it points out of plane, and it reverts direction when the Ising-like-chromium spins are flipped (toward opposite layers versus away from opposite layers). The combined presence of antiferromagnetism and a weak intrinsic electric dipole makes this material a two-dimensional magnetoelectric multiferroic. Here, we remove the crystalline center of inversion of the bilayer by a relative ${60}^{\ensuremath{\circ}}$ rotation of its constituent monolayers. This process enhances the out-of-plane intrinsic electric dipole tenfold with respect to its magnitude in the nonrotated AFM bilayer and also creates an even stronger and switchable in-plane intrinsic electric dipole. The ability to create a three-dimensional electric dipole is important because it enhances the magnetoelectric coupling on this experimentally accessible 2D material, which is explicitly calculated here as well.

Transport properties of As-F-based molecular magnetic tunnel junctions

By combining the density functional theory with non-equilibrium Green's function method, we theoretically design and investigate the spin-dependent transport properties of two molecular magnetic tunnel junctions (MMTJs) with the AsFNRs embedded in zigzag graphene nanoribbons in parallel and antiparallel spin configurations. The findings reveal that the MMTJs have distinct transport characteristics depending upon the orientations of the AsFNRs. Changing the spin configurations leads to two different net spin currents, and the maximal order of the tunneling magnetoresistance magnitude can reach 107%. In addition, a negative differential resistance (NDR) effect is found in our MMTJs. Our findings provide suitable candidates of MMTJs for the future spintronic devices.

Multifunctional 2D g-C4N3/MoS2 vdW Heterostructure-Based Nanodevices: Spin Filtering and Gas Sensing Properties.

Two-dimensional (2D) magnetic materials are the key to the development of the new generation in spintronics technology and engineering multifunctional devices. Herein, the electronic, spin-resolved transmission, and gas sensing properties of the 2D g-C4N3/MoS2 van der Waals (vdW) heterostructure have been investigated by using density functional theory with non-equilibrium Green's function method. First, the g-C4N3/MoS2 vdW heterostructure demonstrates ferromagnetic half-metallicity and superior adsorption capacity for gas molecules. The spin-dependent electronic transport of the g-C4N3/MoS2-based nanodevice is obviously regulated by parallel or anti-parallel spin configuration in electrodes, leading to perfect single-spin conduction behavior with a nearly 100% spin filtering efficiency, a negative differential resistance effect, and other interesting electrical transport phenomena. Moreover, g-C4N3/MoS2 exhibits directional dependency and strong transport anisotropic behavior under bias windows, indicating that the electric current propagates more easily through the vertical direction than the horizontal direction. The physical mechanisms are revealed and analyzed by presenting the bias-dependent transmission spectra in combination with the projected local device density of states. Finally, the g-C4N3/MoS2-based gas sensor is more sensitive to CO, NO, NO2, and NH3 molecules with the chemisorption type. The strong chemical adsorption leads to the formation of electrons on the local scattering center and ultimately affects the transport properties, resulting in the maximum gas sensitivity reaching 6.45 for NO at the bias of 0.8 V. This work not only reveals that the g-C4N3/MoS2 vdW heterostructure with high anisotropy, perfect spin filtering, and outstanding gas sensitivity is a promising 2D material but also provides an insight into the further application in futuristic electronic nanodevices.

Ultrafast optical control of surface and bulk magnetism in magnetic topological insulator/antiferromagnet heterostructure

Optical control of the magnetic properties in topological insulator systems is an important step in applying these materials in ultrafast optoelectronic and spintronic schemes. In this work, we report the experimental observation of photo-induced magnetization dynamics in the magnetically doped topological insulator (MTI)/antiferromagnet (AFM) heterostructure composed of Cr-(Bi,Sb)2Te3/CrSb. Through proximity coupling to the AFM layer, the MTI displays a dramatically enhanced magnetism, with robust perpendicular magnetic anisotropy. When subjected to intense laser irradiation, both surface and bulk magnetism of the MTI are weakened by laser-induced heating of the lattice, however, at the surface, the deleterious heat effect is compensated by the strengthening of Dirac-hole-mediated exchange coupling as demonstrated by an unconventional pump-fluence-dependent exchange-bias effect. Through theoretical analyses, the sizes of exchange coupling energies are estimated in the MTI/AFM bilayer structure. The fundamentally different mechanisms supporting the surface and bulk magnetic order in MTIs allow a novel and distinctive photo-induced transient magnetic state with antiparallel spin configuration, which broadens the understanding of the magnetization dynamics of MTIs under ultrashort and intense optical excitation.

Spin-tunable thermoelectric performance in monolayer chromium pnictides

Historically, finding two-dimensional (2D) magnets is well known to be a difficult task due to the instability against thermal spin fluctuations. Metals are also normally considered poor thermoelectric (TE) materials. Combining intrinsic magnetism in two dimensions with conducting properties, one may expect to get the worst for thermoelectrics. However, we will show this is not always the case. Here, we investigate the spin-dependent TE properties of monolayer chromium pnictides ($\mathrm{Cr}X$, where $X=\text{P}$, As, Sb, and Bi) using first-principles calculations of electron- and phonon-energy dispersion, along with the Boltzmann transport formalism under an energy-dependent relaxation time approximation. All the $\mathrm{Cr}X$ monolayers are dynamically stable and they also exhibit half metallicity with ferromagnetic ordering. Using the spin-valve setup with an antiparallel spin configuration, the half metallicity and ferromagnetism in monolayer $\mathrm{Cr}X$ enable the manipulation of spin degrees of freedom to tune the TE figure of merit $(ZT)$. At an optimized chemical potential and operating temperature of 500 K, the maximum $ZT$ values ($\ensuremath{\approx}0.22$, 0.12, and 0.09) with the antiparallel spin-valve setup in CrAs, CrSb, and CrBi improve up to almost twice the original values ($ZT\ensuremath{\approx}0.12$, 0.08, and 0.05) without the spin-valve configuration. Only in CrP, which is the lightest species and less spin polarized among $\mathrm{Cr}X$, the maximum $ZT$ $(\ensuremath{\approx}0.34)$ without the spin-valve configuration is larger than that $(\ensuremath{\approx}0.19)$ with the spin-valve one. We also find that, at 500 K, all the $\mathrm{Cr}X$ monolayers possess exceptional TE power factors of about 0.02--0.08 W/m ${\mathrm{K}}^{2}$, which could be one of the best values among 2D conductors.

Structural, electronic and magnetic properties of inverse spinel NiFe2O4: DFT + U investigation

We have used ab initio density functional theory (DFT) to explore the structural, mechanical, electronic and magnetic properties of inverse spinel NiFe2O4. The ultra-soft pseudopotential method parametrized under generalized gradient approximation (GGA) for the exchange-correlation functional with Hubbard correction (GGA + U) was applied. The equilibrium lattice parameter was estimated to be 8.4620 Å and the 3 independent elastic constants C11, C12 and C44 were found to be 304.12 GPa, 144.45 GPa and 57.46 GPa, respectively. GGA + U formalism could successfully explain the semiconducting and ferrimagnetic behaviour of NiFe2O4. The spin-polarized electronic density of states reveals that the p−d hybridization between O and Fe atoms is responsible for the semiconducting nature of NiFe2O4. The ferrimagnetic behaviour of inverse spinel NiFe2O4 can be explained through the anti-parallel spin configuration of tetrahedrally and octahedrally occupied Fe atoms. The resultant magnetic moment is thus solely contributed by the octahedrally occupied Ni atoms. Spin-polarized band structure calculations reveal indirect band gap of 1.6 eV for spin-up channel and direct band gap of 2.2 eV for spin-down channel.

Tunable giant magnetoresistance ratio in bilayer CuPc molecular devices.

We investigate the influence of the distance between the buffer layer and the central molecule on the electrical transport, spin-filter transport, magnetoresistance effects and thermoelectric properties of a bilayer CuPc molecular device with V-shaped zigzag-edged graphene nanoribbon (VZGNR) electrodes by combining density functional theory and the non-equilibrium Green's function. The results show that the spin-dependent total conductance and spin filter efficiency of the bilayer CuPc molecular device reach a maximum with a parallel spin configuration (PC) when the carbon atom at the edge of the electrode is in the center of the carbon atom at the edge of the bilayer molecules due to the stronger coupling interaction between the double-layer molecules and the leads. Moreover, the spin polarization of the bilayer CuPc molecular device is reversed at certain distances; there is a minimum spin filter efficiency (SFE) of −99.93448% and a maximum SFE of 97.91% observed in the anti-parallel spin configuration (APC) of the device and there is a minimum SFE of −26.03175% and a maximum SFE of 99.99996% observed with the PC at zero bias. The SFE oscillates with increasing considered bias voltage in the PC and APC when the distances are d = 0 Å and d = −1.06 Å, and a negative differential resistance (NDR) effect was observed. For the PC and APC, there is a giant magnetoresistance (MR) effect and the MR ratio exceeds 5.21 × 107% (99.9996%), and the MR ratio oscillates with increasing considered bias voltage when d = 0 Å. The MR ratio could be reserved by applying a certain bias voltage. These transport behaviors can be well understood by analyzing the transmission spectra, projected density of states and scattering states. There are pure spin Seebeck coefficients and pure charge Seebeck coefficients at certain temperatures when the distances are certain values, which means that the corresponding temperature differences could produce pure spin current and pure charge current, respectively. Our results provide new ideas for designing ultrahigh-performance spintronic molecular devices.

Spin-resolved band structures and transport properties of δ-graphyne nanoribbon with bilateral or unilateral fluorination

Among the graphyne family, δ-Graphyne is a new typical representative for instance. In this article, we have studied the spin-splitting phenomena in the band structures and transport properties of zigzag δ-graphyne nanoribbron (ZδGYNR) with edge fluorination. Our results show that the spin-splitting can be observed obviously in the ZδGYNRs with unilateral or bilateral edge fluorinations. Moreover, some distinct physical effects, such as spin-filtering, rectification and negative differential resistance, can be found in the proposed device models. Particularly, the efficiency of spin-filtering can approach to 100% in the antiparallel spin-configuration of the bilateral fluorination model, which would be capable of leading to promising applications in spintronics and nanodevices.

Ultrahigh Spin Filter Efficiency, Giant Magnetoresistance and Large Spin Seebeck Coefficient in Monolayer and Bilayer Co-/Fe-/Cu-Phthalocyanine Molecular Devices.

The spin related electrical and thermoelectric properties of monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices in a parallel spin configuration (PC) and an anti-parallel spin configuration (APC) between the V-shaped zigzag-edged graphene nanoribbon electrodes and the center bilayer MPc molecules are investigated by combining the density functional theory and non-equilibrium Green’s function approaches. The results show that there is an ultrahigh spin filter efficiency exceeding 99.99995% and an ultra-large total conductance of 0.49996G0 for FePc-CoPc molecular devices in the PC and a nearly pure charge current at high temperature in the APC and a giant MR ratio exceeding 9.87 × 106% at a zero bias. In addition, there are pure spin currents for CuPc and FePc molecular devices in the PC, and an almost pure spin current for FePc molecular devices in the APC at some temperature. Meanwhile, there is a high SFE of about 99.99585% in the PC and a reserved SFE of about −19.533% in the APC and a maximum MR ratio of about 3.69 × 108% for the FePc molecular device. Our results predict that the monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices possess large advantages in designing high-performance electrical and spintronic molecular devices.

Theory of giant magnetoresistance caused by magnetic-ion pairs embedded in paramagnetic metals

To clarify a mechanism of a giant magnetoresistance (MR), we study the resistivity of a paramagnetic metal in which a magnetic-ion pair is embedded as impurities. In this pair, magnetic ions at the positions ${\mathbf{r}}_{1}$ and ${\mathbf{r}}_{2}$, respectively, are assumed to have localized spins ${\mathbf{S}}_{1}$ and ${\mathbf{S}}_{2}$, which are called $d$ spins. Conducting electrons, on the other hand, are assumed to have a light mass, being called $s$ electrons. The $s$ electrons are scattered by both the $d$ spins through the $s\ensuremath{-}d$ exchange interaction. Since interferences occur in the wave functions of electrons scattered by both the $d$ spins, transition probabilities between different electronic states are shown to depend strongly on $\mathbf{R}={\mathbf{r}}_{2}\ensuremath{-}{\mathbf{r}}_{1}$ and the spin configuration in the pair $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$. The steady state of the $s$ electron system under an external electric field is obtained on the basis of the Boltzmann equation and the relaxation-time approximation. By using the obtained relaxation times, the resistivity of the metal is calculated to find its $\mathbf{R}$ and $({\mathbf{S}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathbf{S}}_{2})$ dependencies. MRs are shown to appear when an antiparallel spin configuration realized by the Ruderman-Kittel-Kasuya-Yoshida interaction is changed to be a parallel one by a magnetic field. The maximum MR ratio ${[({\ensuremath{\rho}}_{\mathrm{ap}}\ensuremath{-}{\ensuremath{\rho}}_{\mathrm{p}})/{\ensuremath{\rho}}_{\mathrm{p}}]}_{\mathrm{MR}}$ is found to give a giant MR (GMR) of the order of ${10}^{2}\phantom{\rule{4pt}{0ex}}%, {\ensuremath{\rho}}_{\mathrm{ap}}$ and ${\ensuremath{\rho}}_{\mathrm{p}}$ being the resistivities in the metals with the antiparallel and parallel spin pairs, respectively. These results are extensively applied to the GMR in the metallic magnetic multilayers by taking into account possible magnetic-ion pairs in their nonmagnetic layers. This application shows that Mott's two-fluid model can exhibit a GMR of magnetic multilayers also in the case of the current in planes (CIP).

Tunneling magnetoresistance in ferromagnet/organic-ferromagnet/metal junctions*Project supported by the National Natural Science Foundation of China (Grant Nos. 11974215, 11704230, 11674197, and 11874242), the Natural Science Foundation of Shandong Province, China (Grant No. ZR2019MA043), and the Taishan Scholar Project of Shandong Province, China.

Spin-dependent transport in ferromagnet/organic-ferromagnet/metal junctions is investigated theoretically. The results reveal a large tunneling magnetoresistance up to 3230% by controlling the relative magnetization orientation between the ferromagnet and the central organic ferromagnet. The mechanism is explained by distinct efficient spin-resolved tunneling states in the ferromagnet between the parallel and antiparallel spin configurations. The key role of the organic ferromagnet in generating the large magnetoresistance is explored, where the spin selection effect is found to enlarge the difference of the tunneling states between the parallel and antiparallel configurations by comparing with the conventional organic spin valves. The effects of intrinsic interactions in the organic ferromagnet including electron–lattice interaction and spin coupling with radicals on the magnetoresistance are discussed. This work demonstrates a promising potential of organic ferromagnets in the design of high-performance organic spin valves.

Spin-dependent transport in all-carbon multifunctional spintronic device

By using density functional theory and non-equilibrium Green’s function method, we investigate the spin-dependent transport properties of an all-carbon spintronic device consisting of a perylene molecule linked to two symmetrical ferromagnetic zigzag-edge graphene nanoribbon (ZGNR) electrodes via carbon atomic chains, where the magnetization of ZGNR electrodes are modulated by applying an external magnetic field. The device exhibits spin filtering and negative differential resistance (NDR) effect in parallel spin configuration. Bipolar spin filtering, spin rectifying and NDR effect are revealed in antiparallel spin configuration. And there is a giant magnetoresistance effect between parallel and antiparallel spin configurations. We discuss these interesting transport mechanisms, and suggest that the proposed all-carbon device holds promise in high-performance multifunctional nanoelectronic devices.

Engineering of carbon-based superlight spin filter with negative differential resistance

Based on density functional theory and non-equilibrium Green's function, we investigate the edge hydrogenation and oxidation effects on the spin transport of devices consisting of a zigzag C2N nanoribbon (ZC2NNR) embedded in zigzag graphene nanoribbons in parallel (P) and antiparallel (AP) spin configurations. The results show that device with edge hydrogenation exhibits dual spin filtering effect in AP spin configuration and obvious negative differential resistance in both P and AP spin configuration. By substituting oxygen for hydrogen as passivation atoms of ZC2NNR, the spin filtering efficiency is as high as 100% in the P spin configuration, and the negative differential resistance is largely enhanced with a peak to valley ratio in excess of 4×103. Our theoretical studies suggest that zigzag C2N nanoribbon modulated by edge substitution has great potential in the design of future multifunctional spin devices.

Tunable induced magnetic moment and in-plane conductance of graphene in Ni/graphene/Ni nano-spin-valve-like structure: A first principles study

This study theoretically investigated the magnetic properties and electronic structure of a graphene-based nano-spin-valve-like structure. Magnetic nickel layers on both sides of the graphene were considered. A spin-polarized generalized-gradient approximation determined the electronic states. In an energetically stable stacking arrangement of graphene and two nickel layers, the anti-parallel spin configuration of the underlayer and overlayer magnetic moments had the lowest energy, which is in agreement with previous experimental studies. The spin density mapping and obtained band-structure results show that when the upper and lower Ni(111) slabs have an anti-parallel (parallel) magnetic-moment configuration, the carbon atoms of sublattices A and B will have an antiferromagnetic (ferromagnetic) spin configuration. A band gap at the Dirac cone was open when the alignment had an anti-parallel configuration and closed when the alignment had a parallel configuration. Therefore, the in-plane conductance of the graphene layer depends on the magnetic alignment of the two nickel slabs when the Fermi level is adjusted at the Dirac point. Both the magnetic properties and electronic structures of the Ni/graphene/Ni nanostructure cause the system to be a new prospective spintronic device showing controllable in-plane magnetoresistance.

Spin transport investigation of two type silicene nanoribbons heterostructure

Based on first principles calculation method, we design and investigate the spin transport properties of two type heterostructures based on zigzag silicene nanoribbons (ZSiNRs). The first one consists of hydrogen-terminated ZSiNR (ZSiNR-H) and Rx-terminated ZSiNR (ZSiNR-Rx), here, Rx = O, S, P. The rectification behavior can be observed for heterostructures consisting of ZSiNR-H and ZSiNR-O (ZSiNR-S). The second one can be fabricated with a ZSiNR-Rx central scatter region between two ZSiNR-H electrodes. The results show that this device could maintain its good spin filtering effect for ZSiNR-O model in parallel (P) and antiparallel (AP) spin configuration with large bias range. Then we further investigate the spin-dependent transport with various length of ZSiNR-O region, and find that better spin filtering effect (near 100% spin polarization) can be observed for longer ZSiNR-O region. ZSiNR-S model show analogous spin filtering effect. However, neither rectification behavior nor spin filtering effect arise for ZSiNR-P models.

Ab-initio calculation of the magnetic properties of P and As doped SnO2

Magnetic properties of SnO2 has been theoretically studied by systematically doping various p-block elements (viz., B, C, N, Al, Si, P, S, Ga, Ge, As) at oxygen site. Density functional theory has been applied to calculate the spin polarized density of states, magnetic moment in all the systems. A significant amount of magnetic moment has been found in B, N, P and As doped SnO2 systems. The primary source of magnetism in all the cases is the p-orbital electrons of the dopant atom along with nearby oxygen atoms. Spin-spin interaction study has been performed in all the cases at two different doping distances. Gibbs free energy of parallel spin and anti-parallel spin configuration has been compared to get the stable magnetic ordering. Among all the dopants only P and As atoms when doped at short position gives a stable ferromagnetic ordering at room temperature. In all other cases, either the spin-spin interaction is very low or the anti-parallel ordering of spin is more favored.

Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect.

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (XD) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of XD states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the XD emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe2 on a gold substrate, we demonstrate ~6 × 105-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 103 of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

Role of Hund's splitting in electronic phase competition inPb1−xSnxTe

We study the effect of Hund's splitting of repulsive interactions on electronic phase transitions in the multiorbital topological crystalline insulator ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Te}$ when the chemical potential is tuned to the vicinity of low-lying type-II Van Hove singularities. Nontrivial Berry phases associated with the Bloch states impart momentum dependence to electron interactions in the relevant band. We use a multipatch parquet renormalization-group (RG) analysis to study the competition of different electronic phases, and we find that if the dominant fixed-point interactions correspond to antiparallel spin configurations, then a chiral $p$-wave Fulde-Ferrell-Larkin-Ovchinnikov state is favored, otherwise none of the commonly encountered electronic instabilities occurs within the one-loop parquet RG approach.

Thermal spin filtering effect and giant magnetoresistance of half-metallic graphene nanoribbon co-doped with non-metallic Nitrogen and Boron

Ab initio calculations based on density functional theory and non-equilibrium Green's function are performed to investigate the thermal spin transport properties of single-hydrogen-saturated zigzag graphene nanoribbon co-doped with non-metallic Nitrogen and Boron in parallel and anti-parallel spin configurations. The results show that the doped graphene nanoribbon is a full half-metal. The two-probe system based on the doped graphene nanoribbon exhibits various excellent spin transport properties, including the spin-filtering effect, the spin Seebeck effect, the single-spin negative differential thermal resistance effect and the sign-reversible giant magnetoresistance feature. Excellently, the spin-filtering efficiency can reach nearly 100% in the parallel configuration and the magnetoresistance ratio can be up to −1.5 × 1010% by modulating the electrode temperature and temperature gradient. Our findings indicate that the metal-free doped graphene nanoribbon would be a promising candidate for spin caloritronic applications.