Spin Transfer Processes Research Articles

Unlocking Ultrafast Spin Transfer in Single-Magnetic-Center-Decorated Triangulene Systems.

Triangulene, as a typical open-shell graphene fragment, has attracted widespread attention for nanospintronics, promising to serve as building blocks in spin-logic units. Here, using ab initio calculations, we systematically study the laser-induced ultrafast spin-dynamic processes on triangulene nanoflakes, decorated with a transition-metal atom. The results reveal a competition between the induced magnetic center and the carbon edge of the triangulene, resulting in the coexistence of dual spin-density-distribution patterns on such single-magnetic-center systems, thus opening up possibilities of complex spin-dynamic scenarios beyond the spin flip. Interestingly, no matter what direction the spin points to, it is possible to achieve reversible spin-transfer processes using the same laser pulse. Increasing the pool of elementary processes to contain not only spin-direction-dependent but also spin-direction-independent scenarios allows for more versatile spin-logic operations, including classical handling of information and quantum computing. In the present work, we suggest downscaling nanospintronic devices by integrating triangulene-based nanostructures.

Using single and double laser pulses on the molecular Ni4@C48H36 system to design integrated nanospintronic units.

The accomplishment of long-distance spin transfer scenarios between several magnetic centers is a big challenge for building and supporting spin-logic units for developing future all-optical magnetic unit operations. Using high-level quantum chemistry theory CCSD and EOM-CCSD, we systematically study the ultrafast laser-induced spin-dynamics process on a carbon-based material, to which four magnetic centers are attached. We show that the CCSD method with the 6-31G basis set calculation is sensitive to the C-Ni bond length. The spin density distribution, which is computed using EOM-CCSD with LanL2DZ+ECP calculations, Mulliken population analysis, including spin-orbit-coupling (SOC) and a magnetic field, fulfills the requirements for achieving spin dynamics processes. Different local spin-flip and spin-transfer processes are accomplished within the subpicosecond regime. The impact of the propagation direction of the laser pulse by switching their polar and the azimuthal angles in spherical coordinates on the spin dynamics processes is analyzed. Double laser pulses with time delay δt ≥ 200 × FWHM yield in a realistic magnetic field gradient selectively a lateral resolution, which corresponds to distances smaller than the CMOS scale (2 nm in 2024) while our system size is comparable to the CMOS scale. Here Λ and V processes with two quasi-degenerate intermediate levels are used. We propose a model of an integrated spin-logic processor created from an array of individual spin-logic blocks, which are realized by four magnetic centers Ni. The findings of this study demonstrate the enormous potential of using laser-induced spin dynamics as the fundamental mechanism for future molecular magnetic technology.

Ultrafast control of laser-induced spin-dynamics scenarios on two-dimensional Ni3@C63H54 magnetic system.

The concept of building logically functional networks employing spintronics or magnetic heterostructures is becoming more and more popular today. Incorporating logical segments into a circuit needs physical bonds between the magnetic molecules or clusters involved. In this framework, we systematically study ultrafast laser-induced spin-manipulation scenarios on a closed system of three carbon chains to which three Ni atoms are attached. After the inclusion of spin-orbit coupling and an external magnetic field, different ultrafast spin dynamics scenarios involving spin-flip and long-distance spin-transfer processes are achieved by various appropriately well-tailored time-resolved laser pulses within subpicosecond timescales. We additionally study the various effects of an external magnetic field on spin-flip and spin-transfer processes. Moreover, we obtain spin-dynamics processes induced by a double laser pulse, rather than a single one. We suggest enhancing the spatial addressability of spin-flip and spin-transfer processes. The findings presented in this article will improve our knowledge of the magnetic properties of carbon-based magnetic molecular structures. They also support the relevant experimental realization of spin dynamics and their potential applications in future molecular spintronics devices.

Control of dephasing in spin qubits during coherent transport in silicon

One of the key pathways toward scalability of spin-based quantum computing systems lies in achieving long-range interactions between electrons and increasing their interconnectivity. Coherent spin transport is one of the most promising strategies to achieve this architectural advantage. Experimental results have previously demonstrated high-fidelity transportation of spin qubits between two quantum dots in silicon and identified possible sources of error. In this theoretical study, we investigate these errors and analyze the impact of tunnel coupling, magnetic field, and spin-orbit effects on the spin transfer process. The interplay between these effects gives rise to double dot configurations that include regimes of enhanced decoherence that should be avoided for quantum information processing. These conclusions permit us to extrapolate previous experimental conclusions and rationalize the future design of large-scale quantum processors.

Laser-induced ultrafast spin-transfer processes in non-linear zigzag carbon chain systems.

We combine the high-level quantum chemistry theory CCSD and EOM-CCSD together with local and global Λ processes to investigate the details of the laser-induced ultrafast spin manipulation scenarios in non-linear zigzag carbon chain systems Ni2@C32H32 and Ni2@C36H36. The spin density distribution, which is calculated on each many-body state using a Mulliken population analysis, fulfills the requirements to accomplish the spin dynamics processes. Various spin-flip and spin-transfer scenarios are accomplished. All the spin-dynamics processes can be achieved within subpicosecond times. Under the influence of a magnetic field, we find that the spin-transfer scenarios are preserved, while the local spin-flip scenario on a Ni atom can be significantly inhibited depending on the strength of the magnetic field. The impact of the propagation direction of the laser pulse on the spin dynamics processes by varying their polar and azimuthal angles in spherical coordinates is investigated. Additionally, we find that double laser pulses successfully induce the spin-transfer processes. Our outcomes underline the significant potential of carbon chain systems as building blocks for developing future all-optical integrated logic processing units.

Two-qubit logic gates based on the ultrafast spin transfer in π-conjugated graphene nanoflakes

Ultrafast optical spin control allows gate operations to be performed within picosecond time scale, orders of magnitude faster than microwave or electrical control. Here, using high-level quantum chemical computations, we suggest several two-qubit logic gates based on the optically induced ultrafast spin-flip and spin-transfer processes on rhombic graphene nanoflakes (Co4-GNF). It is demonstrated that Co atoms chemisorbed on π-conjugated C atoms can significantly influence the spin properties of the system. The spin density of different energy levels is distributed on different Co atoms, thus opening channels for successful spin-transfer processes between the Co atoms. Reversible local spin-flip processes on each Co atom as well as global spin-transfer processes between the Co atoms are realized. By carefully combining the various spin-dynamics processes achieved in our Co4-GNF structure, both classical (OR, AND, NAND) and quantum (CNOT, SWAP) binary logic gates are constructed. All spin manipulation processes finish at the subpicosecond time scale with fidelities above 96%. This theoretical design, which is exclusively based on laser-induced ultrafast spin dynamics, represents a novel implementation of qubit manipulation and can thus pave the way towards the construction of nano magnetic logic circuits and spintronic devices.

Exchange scaling of ultrafast angular momentum transfer in 4f antiferromagnets

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum1–3, materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons4 (the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4f antiferromagnets and systematically vary the 4f occupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices.

Strain-promoted reversible spin transfer in rhombic graphene nanoflakes

The strain-promoted reversible spin flip and spin transfer on double-magnetic-center graphene nanoflakes (Ni2&GNFs) are investigated with first-principles quantum chemical computations. We find that applying strain along the two diagonals of the rhombic graphene nanoflakes can significantly lead to the redistribution of spin density, and can therefore dominate the spin-flip and spin-transfer processes. The applied strain effectively changes the spin-density localization of the system, thus opening a channel for successful spin-transfer processes in all distorted structures. Especially, simultaneous spin-flip and transfer processes are achieved when the tensile strain is applied along the short diagonal, and some of the spin-transfer and spin-flip-transfer scenarios achieved on the stretched Ni2&GNF are reversible. This reversible strain-assisted spin dynamics provides a feasible yet efficient way to control the spin magnetism of GNFs, which could increase the functionality and the flexibility of the integrated spin-logic in real straintronic devices.

Two-Qubit Logic Gates Based on the Ultrafast Spin Transfer in π-Conjugated Graphene Nanoflakes

Ultrafast optical spin control allows gate operations to be performed within a picosecond timescale, orders of magnitude faster than microwave or electrical control. Here, using high-level quantum chemical computations, we suggest a two-qubit logic gate based on the optically induced ultrafast spin-flip and spin-transfer processes over rhombic graphene nanoflakes (Co4-GNF). It is demonstrated that the π-conjugated Co atoms can significantly influence the spin properties of the system. The spin density is distributed on different Co atoms in different energy levels, in this way opening a channel for successful spin-transfer processes between the Co atoms. Thus, the reversible local spin-flip processes on each Co atom and global spin-transfer processes between the Co atoms are realized. Importantly, based on various spin-dynamics processes achieved in Co4-GNF structures, both classical (OR, AND, NAND) and quantum (CNOT, SWAP) two-qubit logic gates are constructed, and the spin manipulation process can be completed in subpicosecond timescale with fidelity above 96%. This theoretical design based on the laser-induced ultrafast spin dynamics provides a new implementation of qubit manipulation, which could pave the way towards the construction of nano magnetic logic circuits and spintronic devices.

Phase diagram, stability and magnetic properties of nonlinear excitations in spinor Bose–Einstein condensates

We present the phase diagram, the underlying stability and magnetic properties as well as the dynamics of nonlinear solitary wave excitations arising in the distinct phases of a harmonically confined spinor F = 1 Bose–Einstein condensate. Particularly, it is found that nonlinear excitations in the form of dark–dark–bright solitons exist in the antiferromagnetic and in the easy-axis phase of a spinor gas, being generally unstable in the former while possessing stability intervals in the latter phase. Dark–bright–bright solitons can be realized in the polar and the easy-plane phases as unstable and stable configurations respectively; the latter phase can also feature stable dark–dark–dark solitons. Importantly, the persistence of these types of states upon transitioning, by means of tuning the quadratic Zeeman coefficient from one phase to the other is unravelled. Additionally, the spin-mixing dynamics of stable and unstable matter waves is analyzed, revealing among others the coherent evolution of magnetic dark–bright, nematic dark–bright–bright and dark–dark–dark solitons. Moreover, for the unstable cases unmagnetized or magnetic droplet-like configurations and spin-waves consisting of regular and magnetic solitons are seen to dynamically emerge remaining thereafter robust while propagating for extremely large evolution times. Interestingly, exposing spinorial solitons to finite temperatures, their anti-damping in trap oscillation is showcased. It is found that the latter is suppressed for stronger bright soliton component ‘fillings’. Our investigations pave the wave for a systematic production and analysis involving spin transfer processes of such waveforms which have been recently realized in ultracold experiments.

First-principles study of ultrafast spin dynamics in Fe<i><sub>m</sub></i>B<sub>20</sub> (<i>m</i> = 1, 2) clusters

In this study, we use first-principles calculations to investigate the geometry, the electronic and the magnetic structure as well as to propose the laser-induced ultrafast spin dynamics on the tubular FeB<sub>20</sub> and Fe<sub>2</sub>B<sub>20</sub> clusters. Our results show that the FeB<sub>20</sub> is a stable configuration when its Fe atom gets preferably adsorbed inside the B<sub>20</sub> tube, while the Fe<sub>2</sub>B<sub>20</sub> is more stable configuration when one of its two Fe atoms is located inside and the other outside the boron tube. In the latter cluster, due to the higher number of d states introduced by the additional magnetic atom, the density-of-states in the low-energy region becomes higher, thus leading to richer spin dynamics. The different local geometries of the two Fe atoms lead to a multitude of many-body states with high degree of spin-density localization. Based on the calculated ground state and excited states and by using suitably tailored laser pulses we achieve ultrafast spin-flip and spin crossover scenarios for both structures. Besides, the spin-flips reach a high fidelity (above 89.7%) and are reversible, while the crossovers have lower fidelity (below 78%) and are irreversible. We also propose an ultrafast spin-transfer process from Fe2 to Fe1 for Fe<sub>2</sub>B<sub>20</sub>. The present investigation, in which we predict various ultrafast spin dynamic taken by magnetic atoms absorbed inside and outside of tubular boron clusters, is expected to provide significant theoretical guidance for the future experimental implementation and the potential applications of the relevant spin logic functional devices.

Theory of chemical bonds in metalloenzymes XXIV electronic and spin structures of FeMoco and Fe-S clusters by classical and quantum computing

Fundamental principles for theoretical understanding and elucidation of structure and reactivity of iron-sulfur (Fe-S) Fe n S m (n, m=2∼8) clusters are investigated and elucidated on the theoretical and experimental grounds. To this end, the nature of chemical bonds of these clusters is investigated by three methods; (1) the spin Hamiltonian model for analysis of EPR results, (2) broken-symmetry (BS) hybrid density functional theory (HDFT) methods for full geometry optimisations and elucidations of complete active spaces (CAS) for one-electron transfers reactions and (3) beyond HDFT methods such as CAS configuration interaction (CI) and MR CI for high precision energy calculations on classical and quantum computers. Theoretical concepts revealed are applied for elucidation of the mechanism of nitrogen fixation with FeMoco (Fe7MoS9C) cluster, indicating an important role of proton-coupled (PC) one electron spin transfer (OEST) processes instead of radical reaction mechanisms.

Detection of radiation torque exerted on an alkali-metal vapor cell

We have developed a torsion balance to detect the rotation of a cell containing spin-polarized gaseous atoms to study angular momentum transfer from gaseous atoms to solid. A cesium vapor cell was hung from a thin wire in a vacuum chamber, and irradiated from the bottom with circularly polarized light tuned to the D2 transition to polarize cesium atoms in the cell. By varying the light helicity at the resonance frequency of the torsion balance, we induced forced rotational oscillation of the cell and detected radiation torque exerted on the cesium vapor cell through the cesium atoms inside. The torque was particularly large when both hyperfine levels of cesium atoms were optically pumped with application of a longitudinal magnetic field. Further detailed study will provide new insights into spin-transfer processes at the gas-solid interface.

Spin Hall magnetoresistance in heterostructures consisting of noncrystalline paramagnetic YIG and Pt

The spin Hall magnetoresistance (SMR) effect arises from spin-transfer processes across the interface between a spin Hall active metal and an insulating magnet. While the SMR response of ferrimagnetic and antiferromagnetic insulators has been studied extensively, the SMR of a paramagnetic spin ensemble is not well established. Thus, we investigate herein the magnetoresistive response of the as-deposited yttrium iron garnet/platinum thin film bilayers as a function of the orientation and the amplitude of an externally applied magnetic field. Structural and magnetic characterization shows no evidence for the crystalline order or spontaneous magnetization in the yttrium iron garnet layer. Nevertheless, we observe a clear magnetoresistance response with a dependence on the magnetic field orientation characteristic for the SMR. We propose two models for the origin of the SMR response in paramagnetic insulator/platinum heterostructures. The first model describes the SMR of an ensemble of noninteracting paramagnetic moments, while the second model describes the magnetoresistance arising by considering the total net moment. Interestingly, our experimental data are consistently described by the net moment picture, in contrast to the situation in compensated ferrimagnets or antiferromagnets.

Towards X-ray transient grating spectroscopy.

The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using ∼3 keV X-ray pulses.

Ultrafast spin dynamics in double-magnetic-center endohedral fullerene Y<sub>2</sub>C<sub>2</sub>@C<sub>82</sub>-<i>C</i><sub>2</sub>(1)

Spin switching and spin transfer are essential prerequisites for designing the spin-logic devices based on endohedral fullerenes. In this paper by combining the theoreticalΛ-process model with a self-designed genetic algorithm, we are able to theoretically observe spin-switching and spin-transfer scenarios on the subpicosecond time scale in the endohedral fullerene Y<sub>2</sub>C<sub>2</sub>@C<sub>82</sub>-<i>C</i><sub>2</sub>(1) from first principles. The results show that the geometry of the optimized enclosed Y<sub>2</sub>C<sub>2</sub> cluster is consistent with the experimental data. There exists a certain repulsive force between the external C<sub>82</sub>-<i>C</i><sub>2</sub>(1) cage and the encaged cluster. However, the whole system still maintains its integral cage structure due to the excellent stability of the fullerene. In the Y<sub>2</sub>C<sub>2</sub>@C<sub>82</sub>-<i>C</i><sub>2</sub>(1) system, it is found that the spin density is highly localized on the two Y atoms and only minimally distributed on the carbon cage. By analyzing the spin-density distribution and the evolution of the spin expectation values as influenced by the laser pulses, it is found that global spin switching can be achieved on the two Y atoms, while spin transfer between the two Y atoms actually results from the redistribution of the spin density among the two magnetic centers and the carbon cage under the action of the optimized laser pulses. The achieved spin-switching scenario completes within about 1000 fs and its fidelity reaches 97.8%, while the obtained spin-transfer process completes within 200 fs and its fidelity reaches 95.1%. The electron absorption spectra of the system verify that optical transitions are possible between the main intermediate states and the initial and final states involved in the spin-switching and spin-transfer scenarios. Therefore, by analyzing the electron absorption spectra corresponding to the initial and final states, the energy of the laser pulses adopted for the studied spin-dynamics process can be predicted, and the spin transferability can be evaluated. In addition, it is found that the smaller the detuning between the required energy difference and the applied laser pulse energy is, the greater the probability for spin switching/transfer scenarios becomes. The present results reveal the mechanisms of the laser-induced ultrafast spin dynamics in Y<sub>2</sub>C<sub>2</sub>@C<sub>82</sub>-<i>C</i><sub>2</sub>(1) and can provide a theoretical basis for designing the spin-logic devices on realistic endohedral fullerenes.

The Effect of Polymer Structures on Complete Degradation: A First-Principles Study.

The widespread application of hydrocarbon polymer materials has spurred an increasing interest in the study of their dissociation mechanism, which is related to key issues such as environmental protection. In this work, the last‐step dissociation characteristics of carbon chain polymers were investigated. By using density functional theory, we considered all possible structures, including three typical normal linkage polymers and four typical abnormal linkage ones. In these structures, it can be found that the energy barrier required for the complete degradation of chain‐end saturated and unsaturated polymers are in the range of 3.42 to 4.78 eV and 0.35 to 1.31 eV, respectively. It shows that the unsaturated polymer is easier to degrade. Interestingly, as for three linkages of the polymer, the calculated results further suggest that the energy barrier of head‐to‐head, head‐to‐tail, and tail‐to‐tail linkages of the polymer dissociating to produce the monomer increase, no matter if the chain‐end is saturated or not. Therefore, we form a regular understanding of how to achieve the complete degradation of the polymer. In addition, analyses of the bond characteristics and electronic structures agree with the results of the energy barrier measurements. Meanwhile, the spin population analysis presents an obvious net spin transfer process in depolymerization reactions. We hope that the current results can provide a basic insight into polymer degradation.

Speed and efficiency of femtosecond spin current injection into a nonmagnetic material

We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within $\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface.

Spin Transfer in Polymer Degradation of Abnormal Linkage

The degradation of polymer materials plays an important role in production and life. In this work, the degradation mechanism of poly-α-methylstyrene (PAMS) tetramers with abnormal linkage was investigated by using density functional theory (DFT). Calculated results indicate that the head-to-head and the tail-to-tail reactions needed to overcome the energy barriers are about 0.15 eV and about 1.26 eV, respectively. The broken C–C bond at the unsaturated end of the chain leads to the dissociation of alpha-methylstyrene (AMS) monomers one by one. Furthermore, the analyses of bond characteristics are in good agreement with the results of energy barriers. In addition, the spin population analysis presents an interesting net spin transfer process in depolymerization reactions. We hope that the current theoretical results provide useful help to understand the degradation mechanism of polymers.

Depolymerization of Free-Radical Polymers with Spin Migrations.

The mechanism of depolymerization is one of the most essential issues in chemical engineering and materials science. In this work, we investigate the depolymerization reactions of three typical free-radical poly(alpha-methylstyrene) tetramers by using first-principles density functional theory. The calculated results show that these reactions all need to overcome the energy barriers in the range of 0.58 to 0.77 eV, and that breaking the C-C bond at the chain end leads to the dissociation of alpha-methylstyrene monomers from the polymers. Electronic-structure analysis indicates that the reactions occur easily at the CR3 unsaturated end, and that the frontier molecular orbitals that participate in the reactions are mainly localized at the unsaturated ends. Meanwhile, spin population analysis presents the unique net spin-transfer process in free-radical depolymerization reactions. We hope the current findings can contribute to understanding the free-radical depolymerization mechanism and help guide future experiments.